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2
Do We Understand What Is Driving Climate
Change?
This chapter summarises our understanding of the causes and effects of the
recent observed changes in climate. In particular, it addresses whether, and to
what extent, human activities (emissions of greenhouse gases) can be held
responsible for the observed changes and to what extent the changes can also
be explained by natural causes.
The structure and key material in this module is based almost entirely on the
Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate
Change (IPCC), in particular the Working Group I and II Reports (WGI chapters
1, 2, 6, 8, and 9 and WGII chapter 1) and the Synthesis Report (SYR Topic 2).16
Since the AR4, further research advances have been made. In particular,
recent studies explore the causes for changes in parameters other than
temperature (especially rainfall), incorporate more recent observations of rapid
changes (eg, of Arctic sea ice) and their link with global changes, and update the
link between observed impacts of climate change and the degree to which
greenhouse gases can be linked to those impacts. I refer to some of the more
recent papers to illustrate this continuing growth in knowledge, but this is not a
comprehensive summary of recent work.
Chapter contents
2.1
Understanding Earth’s energy balance – a brief history........................................36
2.1.1 Earliest efforts to understand the Earth’s energy balance and temperature.....36
2.1.2 Fundamental lessons learnt from early studies................................................37
2.2
Cause of rising greenhouse gas concentrations........................................................40
2.2.1 Changes in greenhouse gas concentrations .....................................................40
2.2.2 Sources of greenhouse gases and causes of their recent increases ..................42
2.3
Changes in Earth’s energy balance...........................................................................44
2.3.1 Greenhouse gases ............................................................................................45
2.3.2 Aerosols and clouds.........................................................................................46
2.3.3 Reflectivity of the Earth’s surface (albedo).....................................................47
2.3.4 Volcanic eruptions...........................................................................................47
2.3.5 Solar radiation .................................................................................................48
2.3.6 Changes in Earth’s energy balance from all known natural and human
factors ..............................................................................................................48
2.4
Attribution of observed climate change ...................................................................50
2.4.1 ‘Fingerprinting’ the causes of recent climate change ......................................50
Vertical pattern of warming.............................................................................54
16 AR4 comprises four volumes: the Working Group I, II, and III Reports (IPCC, 2007a (WGI), 2007b
(WGII), 2007c (WGIII)) and the Synthesis Report (IPCC, 2007d (SYR)). Each report has a
Summary for Policymakers (SPM), and each Working Group report has a Technical Summary (TS).
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Climate Change 101 – An Educational Resource
2.4.2
2.4.3
2.4.4
2.5
Geographical pattern of warming and the land–ocean difference ...................54
Temporal warming pattern and the role of aerosols ........................................54
Attribution of the increase in global average temperature...............................55
Attribution of regional temperature changes ...................................................56
Attributing other changes in the climate system..............................................57
Sea-level rise ...................................................................................................57
Temperature and precipitation extremes .........................................................57
Changes in wind patterns.................................................................................58
Changes in patterns and longer-term trends in rainfall....................................58
Attributing observed impacts of climate change .....................................................58
Boxes in chapter
Box 2.1: What determines the concentrations of greenhouse gases in the atmosphere?...........43
Box 2.2: Climate models – can they reproduce and explain real climate change?.................51
Box 2.3: Can we attribute individual storms and other extremes to human causes?..............58
Box 2.4: Recent studies investigating the causes for changes in rainfall and Arctic
sea ice ...........................................................................................................................59
2.1
Understanding Earth’s energy balance – a brief
history
Chapter 1 demonstrated that the world has been warming significantly over the past
100 years, and that the magnitude and rate of this warming is unusual at least in the
past 1,300 years. It also showed that many natural systems and human societies have
already felt the effects of this warming. This, of course, raises the question: do we
understand what has been driving these climate changes? More specifically, have
humans caused the observed climate change and the impacts that are related to those
changes?
Before we embark on answering these questions, we need to discuss an even
more fundamental issue: why does the Earth have the specific temperature that it
does? What processes determine the Earth’s temperature?
2.1.1
Earliest efforts to understand the Earth’s energy balance
and temperature
The first person to try to calculate the temperature the Earth ought to have was the
French scientist Joseph Fourier in 1827. In doing so, he stumbled across a surprise.
At that time, it was already well known that all bodies emit electromagnetic
radiation, and that the frequency and intensity of this radiation depends on the
temperature of the body. The sun, for example, has a temperature of several thousand
degrees Celsius at its surface, and hence the radiation it gives off has a very high
frequency and intensity – it is the visible light that brightens our days. Colder bodies,
such as you and me and the Earth’s surface, give off much less radiation at a lower
frequency in the so-called infrared region of the electromagnetic spectrum. This
radiation is invisible to the human eye, but in large enough quantities and at the right
frequency you can feel it as heat, because the infrared energy is absorbed by your
skin and warms you. (WGI 1.4)
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Do We Understand What Is Driving Climate Change?
Every body radiates energy, so cools down unless this loss of energy is
compensated. The Earth’s loss of energy is compensated by the energy it receives
from the sun in the form of visible light, but individual human bodies also exchange
energy through infrared radiation with bodies around us.
This simple energy balance allows us to calculate the average temperature that
the Earth ought to have – we simply take the total amount of energy that the Earth
absorbs from the sun, and equate this with the energy that the Earth is losing by
radiating infrared energy outward into space. If those two terms roughly balance, the
Earth will have a steady average temperature. The reason why it is generally colder
during the night than during the day is that the Earth does not receive solar energy
during the night, but it still radiates infrared energy outward, hence its temperature
goes down during the night.
When Fourier did this calculation, he found the result did not match reality: he
calculated that the Earth should have a temperature of about minus 15°C rather than
the observed plus 15°C (in the global yearly average). Something was missing, and
he suspected that there might be something in the atmosphere that blocked infrared
radiation and prevented the Earth from losing as much heat as he had assumed in his
calculation (Walker and King, 2008).
A few decades later, in 1859, the Irish scientist John Tyndall did a series of
experiments with an artificial mix of gases. He wanted to test whether the
atmosphere absorbs infrared radiation, and whether this might be the reason why the
Earth was warmer than expected. He found that ‘pure’ atmosphere, consisting only
of oxygen and nitrogen, did nothing to block infrared radiation, but when he added a
small amount of water vapour, carbon dioxide (CO2), and methane (which he
regarded as ‘impurities’), his artificial atmosphere blocked the infrared radiation. He
had discovered what is now known as the natural greenhouse effect: very small
amounts of certain trace gases in the atmosphere absorb infrared radiation, so keep
the Earth warm by preventing some of its heat from escaping into space (WGI 1.4).
Several decades later again, the Swedish scientist Svante Arrhenius was the first to
suggest that any substantial alterations of the concentration of CO2 in the atmosphere
should lead to a noticeable change in the average temperature of the Earth. He
surmised that changes in the concentration of CO2 might have led to the advance and
retreat of the ice ages, though we now know that the relationship between ice ages and
CO2 is more complex than he had assumed. Arrhenius was also the first to suggest that
the increasing use of coal for Europe’s industrialisation should lead to a noticeable
increase in the atmospheric concentration of CO2, and that an eventual doubling of the
CO2 concentration should increase the average temperature of the Earth by 5–6°C
(Christiansen, 1999). British scientist Stuart Callendar’s more complex calculations
gave an estimated warming of 2°C for doubling of CO2 concentrations. This range of
values is surprisingly close to the current best estimate of 3°C, with a range from about
2°C to 4.5°C (see chapter 3). (WGI 1.4)
2.1.2
Fundamental lessons learnt from early studies
This very brief historical overview shows two important things about the Earth’s
climate and the potential for climate change.
The first is that the Earth’s temperature is indeed controlled by a balance between
incoming energy from the sun (much of which is in the form of visible light) and
outgoing energy from the Earth (in the form of invisible infrared radiation). The
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Climate Change 101 – An Educational Resource
second is that we cannot even begin to understand this balance without considering
the role of greenhouse gases and other particles or clouds that influence the transfer
of energy through the Earth’s atmosphere. If greenhouse gases did not exist, or if
they had no warming influence on the Earth’s temperature, you and I might be sitting
shivering in an ice cave. More likely, we would not exist at all because the planet
would have been permanently frozen for the past billion years or more. Figure 2.1
shows this basic understanding of the Earth’s energy balance in a schematic form.
Given these facts, it is more than just a plausible assumption that a change in the
concentration of greenhouse gases will lead to a change in the average temperature of
the Earth, and related climate changes. It would be rather surprising and a major
scientific puzzle, if it did not make any difference to the world’s climate when we
change the amount of infrared radiation that can escape by altering the concentration
of greenhouse gases, even though their current concentrations are responsible for a
warming effect of as much as 30°C compared with a ‘pure’ atmosphere without any
greenhouse gases.
But the key question of course remains: are increasing greenhouse gas
concentrations really the cause of the recent observed climate changes? How much
have greenhouse gas concentrations increased, and can we be sure this increase has
resulted from human activities? How large is the warming effect resulting from this
increase, and how does it compare with other changes that can also affect the
climate, in particular, changes in the output of the sun?
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Do We Understand What Is Driving Climate Change?
Figure 2.1: Schematic illustration of the Earth’s energy balance
Note: Visible sunlight is partially reflected by clouds, aerosols, and atmospheric gases
(about 20%) and partially by the Earth’s surface (about 10%). The remaining 70% is
absorbed and warms the Earth’s surface and the atmosphere. In turn, the Earth
(including oceans) and atmosphere emit heat radiation. Greenhouse gases absorb part of
this heat radiation in the atmosphere and transmit it back to the Earth’s surface, thus
keeping the Earth’s surface warmer than it would otherwise be, while another part of this
heat radiation eventually escapes into space. The balance between incoming and outgoing
radiation determines the temperature of the Earth’s surface.
Source: Based on WGI FAQ 1. 3 Figure 1.1.
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Climate Change 101 – An Educational Resource
2.2
Cause of rising greenhouse gas concentrations
2.2.1
Changes in greenhouse gas concentrations
The concentration of greenhouse gases for many thousands of years in the past can
be determined with high accuracy from the analysis of ice cores. As snow
accumulates over the large ice sheets of Greenland and Antarctica and becomes
compressed into ice, tiny bubbles of air are trapped inside the ice. Each year, new
layers get added. By drilling long cores hundreds of metres into the massive ice
sheets, we can analyse the concentration of greenhouse gases within these air bubbles
that are trapped in successive layers of snow. Because these gases remain in the
atmosphere for a long time (from decades to many centuries), they are mixed within
the atmosphere and spread around the globe within a few years. Their concentrations
are, therefore, virtually the same no matter where they are measured. Ice cores from
the polar regions, therefore, give us very good information about the global
abundance of greenhouse gases in earlier times. (WGI 6.2, 6.4)
Analysis of such ice cores, together with more recent direct measurements, shows
that the concentrations of some key greenhouse gases that are emitted by human
activities, namely CO2, methane (CH4), and nitrous oxide (N2O), have risen
significantly since large-scale industrialisation (also known as the ‘industrial
revolution’) started in Europe in about 1750. The most significant rise occurred
during the 20th century, with CO2 concentrations in particular growing at an everaccelerating pace. Figure 2.2 shows the concentrations of those gases in the
atmosphere for the past 10,000 years. Ice cores going further back in time show that
CO2 and CH4 are now at higher concentrations than at any time in at least the past
650,000 years17 – long before Homo sapiens first walked the Earth (WGI 2.3, 6.4).
We can see from Figure 2.2 that over the past 10,000 years, apart from slow
variations, the concentrations of CO2, CH4, and N2O were relatively constant until
about two centuries ago. Natural sources of CO2 are the weathering of rocks and the
decay of organic material including soils; CO2 is slowly removed from the
atmosphere by photosynthesis, the uptake of CO2 by the ocean, and the gradual
burial of carbon-containing compounds in sediments. Natural sources of CH4 are
anaerobic microbial processes mainly in swamps and wetlands, and in ruminant
animals, while N2O is naturally produced by the microbial break-down of nitrogen
compounds in soils. Both CH4 and N2O are removed from the atmosphere by a series
of chemical reactions. (WGI FAQ 2.1, 7.2, 7.3, 7.4)
17 The same may be true for N2O, but we do not have continuous data to be able to make such a
claim. A recent ice core going back 800,000 years shows that the concentrations of CO2 and CH4
are currently the highest even for this more extended time-frame (Loulergue et al, 2008; Luthi
et al, 2008).
40
Figure 2.2: Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide over the past 10,000 years and since 1750
Source: WGI Figure SPM.1.
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Do We Understand What Is Driving Climate Change?
Note: The large panels show changes over the past 10,000 years. The inset panels show
changes since 1750. Measurements are from ice cores (different symbols for different
studies) and recent direct atmospheric measurements (solid lines).
Climate Change 101 – An Educational Resource
2.2.2
Sources of greenhouse gases and causes of their recent
increases
The fairly steady concentrations of CO2, CH4, and N2O in the millennia before
industrialisation indicate that their natural sources and respective removal processes
(also called ‘sinks’) were roughly in balance – neither suddenly increased nor
decreased at a significant scale (see also Box 2.1; WGI FAQ 10.3).
Human activities release large quantities of these gases in addition to their natural
sources. CO2 is mainly emitted from the burning of fossil fuels, but also when forests
are cleared and burned and the carbon stored in trees is released back into the
atmosphere. Some emissions also come from the chemical processes involved in
cement and lime manufacture. CH4 is produced by human activities such as rice
paddies (which require wet soils), the farming of ruminant animals (such as cattle and
sheep) for human consumption, and the burning of fossil fuels and other industrial
processes. CH4 is also released from landfills and as a by-product of oil extraction. The
main human sources of N2O are agricultural activities, which involve the spreading of
nitrogen-based fertilisers and animal manure, but a small fraction also comes from
chemical manufacturing processes. Both CH4 and N2O are also released when forests
are burnt. The concentration changes for each of these gases since the pre-industrial
period are summarised in Table 2.1. (WGI FAQ 2.1, 7.2, 7.3, 7.4; WGIII 1.3)
Table 2.1: Concentrations of carbon dioxide (CO2), methane (CH4), and nitrous
oxide (N2O) in pre-industrial and modern times
Approximate
concentrations
before
industrial
period
Concentrations
in 2005
Percentage
increase from
pre-industrial
period to
2005 (%)
Update:
concentrations
in 2008
CO2
280 ppm
379 ppm
35
384 ppm
CH4
715 ppb
1,774 ppb
248
1,785 ppb
N2O
270 ppb
319 ppb
18
321 ppb
Note: ppb = parts per billion; ppm = parts per million. These data are preliminary and the
Intergovernmental Panel on Climate Change has not assessed them.
Source: Values for 2008 are from Dr David Hofmann, National Oceanic and Atmospheric
Administration, Earth System Research Laboratory (www.esrl.noaa.gov/gmd/aggi).
How can we be sure the observed increases in the concentrations of these gases
are due to human activities? First, the numbers broadly match – if we add the
estimated additional emissions from human activities to natural emissions and
removal processes, we can calculate how much concentrations should have increased
since the industrial revolution, and this agrees fairly well with the observed change.
But there is additional evidence: the atomic composition of CO2 that is produced
from the burning of fossil fuels is subtly different from that produced from the
burning of biomass. This is because the CO2 molecules contain different carbon
isotopes that can tell us the age of the carbon that has been burnt. The isotopic
‘fingerprint’ we see from the different sources of CO2 in the atmosphere matches
very well what we would expect given the amount of fossil fuels we know is burnt
every year around the world, which is much older than CO2 produced from the
burning of biomass. In addition, the burning of fossil fuels consumes oxygen, and
even though the change is very small, scientists have been able to measure a small
decrease in oxygen concentrations that matches what we would expect, if the
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Do We Understand What Is Driving Climate Change?
additional CO2 is produced from fossil fuels and the burning or decay of forests. This
latter method proves that the additional CO2 did not come from some magically
undetected volcanic eruptions or other natural sources. Finally, all three greenhouse
gases show marginally higher concentrations in the northern than in the southern
hemisphere, which is what one would expect because most emissions come from the
more populated northern hemisphere (WGI 2.3, 7.2, 7.3, 7.4, FAQ 7.1).
We are, therefore, confident that human activities are the cause for the increasing
concentrations, even though the contribution from some specific sources may be less
certain. One example is the uncertainty about recent changes in sources of CH4: the
concentration of CH4 grew almost exponentially during the second half of the 20th
century, but grew little from the early 1990s (WGI 2.3, 7.4). We do not yet know
whether this is because direct human sources have reduced (some speculate that the
closure of leaky gas pipes in the former Soviet Union could have played a role), or
because natural sources such as wetlands have changed their emissions (due to a
natural reduction in water levels in some regions or drainage by human activities).
There is no evidence that natural sinks for CH4 (mostly chemical reactions in the
atmosphere) would have increased sufficiently to explain the reduced growth in
concentrations (WGI 7.4).
Box 2.1: What determines the concentrations of greenhouse gases in the
atmosphere?
The concentration of any gas in the atmosphere is determined by the balance
between its sources (emissions) and its sinks (processes that remove the gas
from the atmosphere through chemical reactions within the atmosphere or in
contact with vegetation, soils, and water, and the weathering of rocks). The
slower the removal processes for a gas, the smaller the percentage of the gas
that is removed naturally from the atmosphere every year, and the longer it
takes for the concentration of a gas to decline if human emissions stop.
For gases such as carbon dioxide (CO2) and nitrous oxide (N2O), natural
processes remove only a small fraction of these gases from the atmosphere
every year, so some fraction of an emission made today will remain in the
atmosphere for several hundreds of years. This is particularly true for CO2,
which is removed from the atmosphere through a variety of processes: of
every quantity of CO2 that is emitted today, about 50% will be removed from
the atmosphere within 30 years; it takes another few centuries to remove
another 30%; and the remaining 20% of the original emission will remain in
the atmosphere for many thousands of years. (WGI 2.3, 7.2)
The reason CO2 is removed relatively rapidly initially, but then only much
more slowly, is that CO2 does not get destroyed through chemical reactions
(as do methane (CH4) and N2O); it only gets redistributed between the
atmosphere, ocean and biosphere through a variety of processes. The
biosphere and oceans absorb part of an initial emission of CO2 relatively
quickly, but then over time these reservoirs emit some of the CO2 back into
the atmosphere. This means that a small fraction of an emission (though not
necessarily the same molecules!) remains in the atmosphere almost forever;
the only significant long-term removal process is the geological process of
burying carbon in sediments. (WGI 2.3, 7.2)
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Climate Change 101 – An Educational Resource
The atmospheric concentrations of these long-lived greenhouse gases will
continue to grow as long as the rate of emissions is greater than their slow
natural removal processes. If human emissions of these gases were kept
constant, or even if they were reduced by as much as 50%, the
concentrations of these gases would still continue to rise. Human emissions
would have to be reduced to a very small fraction of current emissions to stop
concentrations from growing. If we wanted to reduce the concentration of
these gases in the atmosphere, human net emissions would have to be zero.
Even so, for CO2 it would take tens of thousand of years for its concentrations
to decay back to their original (pre-industrial) levels (WGI 7.3, FAQ 10.3;
Figure 2.3).
By comparison, the lifetime of CH4 in the atmosphere is much shorter
(about 12 years): its natural sinks are relatively fast, so remove a
significant fraction of the gas from the atmosphere every year. As a result,
if emissions of CH4 were reduced significantly, its atmospheric
concentrations would decline within a few years; if human emissions were
stopped entirely, its concentration would approach pre-industrial levels
within a few decades (WGI 7.4, FAQ 10.3; Figure 2.3).
Figure 2.3: Relationship between emissions and concentrations of shortand long-lived greenhouse gases
Note: The figure shows changes in concentrations for constant emissions and 50%
and 100% emissions reductions for the very long-lived greenhouse gas carbon
dioxide (CO2) (left panel) and the shorter-lived greenhouse gas methane (CH4)
(right panel).
Source: Based on WGI FAQ 10.3 Figure 1.
2.3
Changes in Earth’s energy balance
We started this chapter by looking at the basic energy balance of the Earth. We now
need to look in more detail at how Earth’s energy balance could be changing, and
what we know about the different components. We then need to ask whether any or a
combination of these components could explain the observed global warming
discussed in chapter 1.
Increasing greenhouse gas concentrations are one obvious candidate to change
the Earth’s energy balance. Another way would be changes in solar radiation
received by the Earth, which could be caused by a change in the sun or by a change
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Do We Understand What Is Driving Climate Change?
in the fraction of sunlight the Earth absorbs. For the latter, aerosols play an important
role. Aerosols are tiny particles that are so light and small that they remain suspended
in the air for weeks, if not years. They are produced by fire and the burning of fossil
fuels (eg, smoke stacks and exhaust pipes) and chemical reactions of gases within the
atmosphere, but are also naturally created by wind-blown dust and large volcanic
eruptions. Most aerosols act as tiny mirrors suspended in the air; they reflect some
sunlight directly back into space, so prevent solar energy from reaching the Earth.
However, some aerosols are very large and dark, so absorb rather than reflect solar
radiation and contribute to the heating of the atmosphere (WGI 2.4).
Another way to alter the amount of sunlight that the Earth absorbs is to change
the reflectivity of the Earth’s surface. For example, snow reflects sunlight directly
back into space without absorbing it, whereas dark soil absorbs most of the light and
its energy (WGI 2.5).
All of these processes, in principle, have the potential to change the energy
balance between incoming and outgoing radiation, so could lead to a change in the
Earth’s average temperature. This change in energy balance is expressed as ‘radiative
forcing’ (WGI 2.2). We now discuss the best estimates for the radiative forcing we
can attribute to each of these factors in turn, and see how they compare in strength.
2.3.1
Greenhouse gases
For greenhouse gases, we can calculate the warming effect (their radiative forcing)
associated with a change in the concentration of any specific gas, based on its
infrared absorption properties, its concentration, and its vertical distribution in the
atmosphere (the latter is important because the effectiveness with which greenhouse
gases absorb infrared radiation varies with temperature and, therefore, with altitude).
All of these individual properties can be measured (WGI 2.3).
Apart from CO2, CH4, and N2O, another important group of greenhouse gases are
halocarbons. Most halocarbons do not occur naturally but are produced specifically
for industrial and domestic cleaning, insulation, and refrigeration processes (such as
chlorofluorocarbons, hydrochlorofluorocarbons, and hydrofluorocarbons). A further
important greenhouse gas is ozone. Ozone has a dual function: on one hand, it
protects life on Earth by absorbing ultraviolet light from the sun, but it is also a
greenhouse gas and hence any changes in ozone concentrations alter the Earth’s
energy balance. Ozone occurs naturally, but human activities have increased ozone
concentrations near the ground, because it is part of the mix of gases that makes up
urban air pollution. At the same time though, human activities have decreased ozone
in the stratosphere by releasing gases (notably chlorofluorocarbons) that destroy
ozone in this part of the atmosphere. (WGI 2.3)
The overall warming effect from an increase in the concentrations of greenhouse
gases needs to take an important feedback mechanism into account, and that is the
response of water vapour to changing atmospheric temperatures. Water vapour is an
important natural greenhouse gas. Its lifetime in the atmosphere is very short, typically
only several days: water vapour condenses and is removed from the atmosphere as rain
or snow; on the other hand, water on the Earth’s surface can evaporate and turn into
water vapour. The concentration of water vapour is, therefore, highly variable in space
and time, but importantly, it also depends on temperature: warm air can hold more
water vapour than can cold air. This is why the air in Antarctica is generally extremely
dry, but the air in the tropics is very humid. If an increase in the concentration of
another greenhouse gas leads to a small rise in the Earth’s temperature, this increase in
45
Climate Change 101 – An Educational Resource
temperature will then increase the average concentration of water vapour, which means
the total absorption of infrared radiation by the atmosphere increases even more and
adds to the original radiative forcing (WGI 2.8, 7.2, 8.6, 9.2).
This feedback is not included in calculations of the radiative forcing of other
greenhouse gases, but it needs to be taken into account when calculating how much
warmer the Earth will become when greenhouse gas concentrations are increased by
any given amount – essentially it is a ‘get two for the price of one’ warming effect.
Incidentally, an increase in the atmospheric concentration of water vapour has
already been observed, broadly consistent with the warming that has taken place over
the past few decades (WGI 3.4, 3.9). Direct emissions of water vapour by human
activities have very little effect on the global climate because water vapour is so
quickly removed again from the atmosphere (WGI 2.5).
Altogether, we have very high confidence that human activities have significantly
increased the warming effect from greenhouse gases, mainly through the direct
emission of the greenhouse gases CO2, CH4, N2O, and halocarbons. Quantitative
estimates of the warming effect from each gas or group of gases are shown in
Figure 2.4 (in section 2.3.6).
2.3.2
Aerosols and clouds
The radiative forcing associated with aerosols is more complex than that from
greenhouse gases because aerosols can affect the Earth’s energy balance in several
ways.
First, aerosols can directly block sunlight from reaching the ground by reflecting
it back into space. This direct effect equates to a cooling of the Earth. But aerosols
can also influence cloudiness, because the tiny particles encourage the condensation
of water vapour, which influences the process by which clouds are formed and tends
to make clouds brighter; in addition, aerosols are also known to influence the lifetime
of clouds. The effect of changes in cloudiness on the Earth’s energy balance can go
in either direction: low clouds tend to cool the Earth because they block sunlight, but
very faint high clouds can have the opposite effect, because they act as an additional
blanket that prevents heat from the Earth from escaping into space. In addition, very
large and dark aerosols can also absorb sunlight directly (rather than reflect it), so
contribute to the warming rather than cooling of the atmosphere (WGI 2.4, 7.5).
On balance, when we combine all these different effects there is good evidence
that aerosols have an overall cooling effect on the Earth. The magnitude of this
cooling effect is much less certain compared with how well we can quantify the
warming from greenhouse gases (WGI 2.4, 2.9).
Human activities can directly influence cloudiness in other ways. For example,
aircraft traffic leaves clouds called contrails criss-crossing the sky. Best estimates
indicate that, globally, the effect of contrails is small, but some observations suggest
it can influence climate at a regional scale (WGI 2.6).
It is worth noting that clouds can also change in response to radiative forcing and
the resulting changes of atmospheric temperatures. Such possible changes in cloud
cover constitute an important feedback mechanism that can increase or reduce the
overall temperature change that would result from increasing greenhouse gas
concentrations alone (if we get more faint high-level clouds in response to
greenhouse-induced warming, the Earth would warm even more; if we get more
thick low-level clouds in response to warming, the Earth would warm less).
46
Do We Understand What Is Driving Climate Change?
Averaged across the globe, cloud feedback is estimated to be positive from a wide
range of model simulations. However, its magnitude is highly uncertain; in fact, one
of the key uncertainties in projections of future climate change. (WGI 8.6)
2.3.3
Reflectivity of the Earth’s surface (albedo)
The reflectivity of the Earth’s surface (albedo) has been affected in some parts of the
world in significant ways during the past two centuries. One important factor was
land clearance, where relatively dark forest cover has been replaced with brighter,
and, therefore, more reflective, agricultural land or savannah, which leads to a net
cooling effect. Other measurable changes came from the deposit of carbon aerosols
from the burning of fossil fuels on snow, which reduces the snow’s reflectivity and
makes it absorb more sunlight, and thus contributes a warming effect (WGI 2.5). The
combined influence of these two competing effects is generally believed to be
negative (ie, a net cooling of the Earth) and much smaller than that from aerosols, but
the relative uncertainties for the estimated radiative forcing are large (see Figure 2.4
in section 2.3.6).
Note that long-term changes in snow cover or sea ice could also affect the Earth’s
energy balance. If snow or sea ice melt, a highly reflective and, therefore, cooling
surface is replaced with darker vegetation, bare ground, or deep blue ocean that can
more readily absorb solar radiation. However, snow cover and sea ice are not
changed directly by human activities (at least not on a global scale) but only in
response to rising temperatures. Therefore, it is not considered a direct radiative
forcing of the climate system, but a feedback of the system to such forcing. Current
best estimates for the magnitude of this feedback suggest it may be relevant in the
higher latitudes of the northern hemisphere, where the melting process may further
enhance the rapid melting of Arctic sea ice and may accelerate the loss of small
glaciers and snow fields (WGI 8.3, 8.6, 9.2, 9.5).
2.3.4
Volcanic eruptions
Some large and explosive volcanic eruptions can spew major amounts of sulphuric
acid high into the atmosphere. This sulphuric acid can then form a thin layer of
sulphate aerosols that can persist for several years before eventually sinking into the
lower parts of the atmosphere where they dissolve in water droplets and get washed
out with the rain (WGI 2.7). The formation of a sulphate aerosol layer can have a
temporary but significant cooling effect, because the sulphate aerosols block some of
the sunlight and prevent it from warming the Earth’s surface. Whether a specific
volcanic eruption has such an effect depends on the size and direction of the
eruption, and whether it consists mainly of ash or lava.
The cooling effect related to such eruptions is indeed visible in global average
temperatures, which can dip for a few years following an eruption. The size of the
dip depends on the size of the eruption – the largest recent volcanic explosions have
led to cooling of a few tenths of a degree for a couple of years. The effect of volcanic
eruptions on temperature is necessarily transient though, because it lasts only as long
as the aerosols remain in the atmosphere. For this reason, and because very large
eruptions happen only infrequently (which is a good thing if you live near a volcano),
it is not meaningful to assign an average radiative forcing to volcanoes. It is
important though to keep in mind that their radiative forcing is always negative (ie,
volcanoes always cool the Earth’s atmosphere) even if it lasts only for a few years at
a time (WGI 2.7).
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2.3.5
Solar radiation
The sun is the source for virtually all of the Earth’s energy (apart from nuclear
processes that are happening inside the Earth’s molten core). Hence, if we want to
understand climate change we need to look closely at how the sun’s radiation could
have changed.
The sun undergoes a well-known 11-year cycle in which its output varies by
about 0.1–0.15% around the longer term mean. This cycle has been measured from
satellites with relatively high precision since 1978, which means we have very good
data for more than two complete cycles. There is a clear correlation between this
solar cycle and the temperature high up in the atmosphere (about 40 km above the
Earth’s surface), and limited evidence for related smaller temperature variations of
perhaps a few tenths of a degree Celsius in the lower atmosphere. However, the sun
systematically cycles back to where it started every 11 years, and these short-term
ups and downs cancel over time-scales of decades or more. A more important factor
for climate change, therefore, would be longer-term changes in average solar
radiation. The available satellite data suggest that overall solar irradiance since 1978
changed only very little, by 0.01% or less (WGI 2.7).
Longer-term studies of changes in solar radiation have to rely on reconstructions
based on proxy data because no direct long-term measurements are available. There
are large uncertainties and differences between different reconstructions and models
of the sun’s activity over time, but most agree that average solar radiation probably
increased during the first half of the 20th century, but changed only a little during the
second half. Before the 20th century, reconstructions of solar activity are even less
reliable but agree on slow variations with minima just after 1800 and during the 17th
century (the so-called Maunder minimum) (WGI 2.7).
Taken altogether, the available studies suggest that the sun’s radiation, and hence
radiative forcing, increased during the first half of the 20th century, and may have
varied more during earlier times, but high-precision measurements show that the
sun’s average irradiance changed only very little during the second half of the 20th
century. This means solar changes cannot be responsible for the strong warming that
has taken place over the past 50 years. The total radiative forcing associated with
solar changes since the pre-industrial period (ie, since about 1750), based on
observations and reconstructions of solar activity, is shown in Figure 2.4 (in
section 2.3.6). (WGI 2.7, 2.9)
Other mechanisms by which the sun could influence the Earth’s climate have
been proposed, such as changes in high-energy particles from the sun that could
affect cloudiness on the Earth and thereby its climate. However, the detailed physical
mechanism behind this proposed influence remains speculative and relies largely on
historical correlations based on proxy data (WGI 2.7). Some studies have suggested
that changes in particle flux might be able to explain a large part of the observed
warming, but the most recent data from satellites have shown that particle flux has
not changed sufficiently to be able to account for the observed warming (Bard and
Delaygue 2007; Pierce and Adams, 2009).
2.3.6
Changes in Earth’s energy balance from all known natural
and human factors
The radiative forcing exerted by each of the factors discussed above can be compared
quantitatively, and with consideration of their respective uncertainties, to gain a
picture of the dominant warming and cooling influences on the Earth’s climate since
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Do We Understand What Is Driving Climate Change?
the beginning of the industrial period (about 1750). This comparison is shown in
Figure 2.4. Note that the figure does not show potential feedbacks of the climate
system to this radiative forcing (such as snow melting and resulting change in
albedo, or change in cloudiness in response to a warming of the atmosphere). These
feedbacks could affect the energy balance of the climate system further and are
included in model projections of future changes (see chapter 3) and in analyses of the
likely causes of observed changes (see section 2.4).
Figure 2.4 shows that the main direct influences of human activities on the
energy balance of the climate system are from greenhouse gases and aerosols. The
warming effect from greenhouse gases is larger and has smaller uncertainties, than
the cooling effect from aerosols and the forcing from all other human influences.
Based on this assessment, we are very confident that human activities since the
industrial revolution overall had a warming effect on the climate, mainly through the
increasing concentrations of greenhouse gases (WGI 2.9).
Figure 2.4: Global average radiative forcing in 2005 relative to 1750 from
human and natural causes
Note: The bars give best estimates, and the error bars indicate 5–95% uncertainty
ranges. The right-most bar shows the total net radiative forcing from all known human
activities, which can be compared with the estimated forcing from changes in solar
activity. Note that radiative forcing from volcanic eruptions would always be negative
(ie, lead to cooling) but is not shown because of its episodic nature.
Source: Based on WGI Figure SPM.2.
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In comparison, the warming effect from solar changes is much smaller and, as
discussed earlier, most of it occurred before the middle of the 20th century. The other
main natural forcing of the climate, which is the cooling from aerosols emitted by
large volcanic eruptions, is not included in Figure 2.4 due to its episodic nature. Over
the second half of the 20th century, the combined natural influences on climate (solar
irradiance and volcanic eruptions) would have exerted an overall cooling effect: solar
irradiance changed only little (if at all) during the past few decades, but some large
volcanic eruptions added significant cooling episodes (Mt Agung in 1963–64,
El Chichon in 1982, and Mt Pinatubo in 1991). (WGI 2.9)
2.4
Attribution of observed climate change
So, where are we at? Based on the evidence presented in chapter 1, the world has
been warming during the 20th century, and this warming is unusual. Based on the
evidence presented in this chapter so far, we can compare the different ways in which
the energy balance of the climate system could change, and have found that human
activities have had a warming influence on the climate that is considerably greater
than any warming from increased solar radiation. In addition, over the past 50 years,
the sum of all natural factors (solar activity plus volcanic eruptions) would have had
a cooling rather than warming effect. Can we, therefore, conclude that human
activities are responsible for the observed warming?
2.4.1
‘Fingerprinting’ the causes of recent climate change
The strong warming influence of increasing greenhouse gas concentrations, and the
cooling influence of natural changes in solar radiation and volcanic activities over
the past 50 years makes a strong case, but this evidence alone is not sufficient. To be
able to attribute observed warming to human activities, we need not only a plausible
mechanism but also to demonstrate that the evolution of temperature over space and
in time matches what should happen if greenhouse gases are the main cause of the
warming. In addition, we need to rule out that the same warming and its pattern in
space and time could not have equally been caused by other factors such as changes
in solar radiation (perhaps amplified by some unknown mechanism) or natural
climate patterns. In other words, we need to look at tell-tale ‘fingerprints’ of
warming patterns from different causes to determine which processes can be held
responsible for the observed warming and which processes cannot (WGI 9.1, 9.2).
It is clear that we cannot study these issues simply by comparing global warming
or cooling influences, but we need detailed climate models that can simulate the effect
of these external factors on the Earth’s temperature in space and time. This is where
climate models become crucial tools of study. Box 2.2 explains what climate models
are and how they work, and how we know that they work (WGI 8.1, 8.2, FAQ 8.1).
The climate models discussed in Box 2.2 identify several key ‘fingerprints’ that
all point to greenhouse gases having been the dominant cause of warming over the
past 50 years. These fingerprints are the spatial pattern of warming vertically within
the atmosphere and across geographical regions, and the evolution of warming
during the 20th century. (WGI 9.1)
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Box 2.2: Climate models – can they reproduce and explain real climate
change?
A key criticism levelled against climate scientists is that they rely on computer
simulations of the global climate and do not deal with the real world. Of course,
we do not have the luxury of experimenting with two real planets, where we fill
the atmosphere of one Earth with greenhouse gases and compare its climate
with that of another identical Earth without greenhouse gases. We are limited
to one experiment in the real world only, and this is the one where we are
filling the atmosphere with greenhouse gases.
Consequently, if we want to understand how the real climate works, we have
to construct mathematical models of its operation, because only in a computer
system can we turn greenhouse gases on or off, or change solar radiation, and
see what happens – hopefully before it is too late to decide on any
corresponding changes in our one and only real world.
We have considerable confidence that climate models allow us to understand
the drivers of climate change and provide credible estimates of future changes,
particularly at the geographical scale of continents and above. This confidence
comes from three sources.
First, all models follow well-established principles such as the conservation
of energy and other fundamental physical quantities. Tyndall and Arrhenius
investigated the basic principle that greenhouse gases warm the climate long
before the first computers were built. Climate models are not made up from
observed correlations (which could be spurious and cannot tell us what is a
cause and what is an effect), but they essentially integrate basic laws of
physics, chemistry, and biology that are all well known and tested over time.
The development of climate models has a long history; forecasts of the impact
of increasing greenhouse gas emissions have been developed since the 1960s
(Manabe and Bryan, 1969).
Secondly, current climate models generally provide good representations
of the current climate: not just average temperature, but geographical
differences and the changes of temperature between seasons, broad patterns
of precipitation, and typical patterns of climate variability (Figure 2.5;
WGI 8.3, 8.4).
Thirdly, climate models can reproduce key features of observed climate
changes, which gives us confidence that they can quantitatively simulate the
effect of external changes on the Earth’s climate. Models can be tested for their
performance against these criteria. It might be possible to construct an
erroneous model that matched the real world in one particular aspect purely by
chance (eg, only in terms of global average temperature), but the chance of
such an accidental agreement becomes smaller the more climate parameters
and patterns are successfully reproduced for different regions of the world
(WGI 8.2, FAQ 8.1).
The most complex models are atmosphere–ocean general circulation models
(AOGCMs). These models divide the atmosphere, ocean, land surface, and sea
ice into many individual small boxes, calculate key physical and chemical
processes within each box, and then calculate how each box interacts with its
neighbours and in response to external changes such as solar radiation. These
models can simulate complex patterns of the existing climate, for example the
broad distribution of precipitation (see Figure 2.5) or broad patterns of regional
climate variability (WGI 8.3, 8.4).
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Figure 2.5: Observed and model-simulated pattern of average annual
precipitation, 1980–1999
Note: The top panel shows observed precipitation patterns. The bottom panel shows
the average pattern reproduced by a large number of models assessed by the
Intergovernmental Panel on Climate Change. Note that temperature is better
simulated than precipitation, but precipitation shows sharper regional differences.
Source: WGI Figure 8.5.
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The main limitation of AOGCMs is that they require enormous computer
resources. This limits the number of different simulations that can be
performed with such models and to test the influence that small changes in
specific parameters might make. For this reason, models with reduced
complexity have been constructed; these models represent complex physical or
chemical processes using simplified equations, but require fewer computing
resources than AOGCMs. As a result, they can explore the effect of many
different variations in external drivers as well as internal mechanisms within
the climate system. AOGCMs are generally considered the most reliable models
for understanding what caused historic climate changes and making
quantitative projections of future changes. Less complex models (so-called
simple climate models and earth models of intermediate complexity) have been
used to run large numbers of tests and to make projections that cover not only
one or two but many centuries into the future. These simpler models are
generally calibrated against the more complex AOGCMs to ensure they
successfully reproduce the large-scale features of the more complex and
realistic models (WGI 8.8).
Despite advances in model development and performance, significant
limitations remain. Many of these limitations are due to the still limited spatial
resolution of models: small-scale processes (eg, eddies in ocean currents and
regional climate fluctuations) can, nonetheless, influence larger climate
patterns. A point always comes when models can no longer simulate every
detail but have to represent a complex process using simplified equations, and
it makes a difference how exactly such processes are represented (WGI 8.4,
8.5, 8.7). As a result, climate models are not perfect in their representation
of the current climate in all its features, and they give slightly different
projections of how much the world is likely to warm in response to greenhouse
gas emissions. However, all models that are able to reproduce the key
features and patterns of current climate and recent climate changes are
unanimous that the world will warm further to a significant degree, if
greenhouse gas concentrations increase.
The final (and some would argue, most important) test of climate models is to
assess how past forecasts of climate change compare with the warming that has
recently been observed. Since 1990, the IPCC has used climate models to
project warming trends for the immediate next few decades. We now have more
than one and a half decades to look back on since the first such projections were
made. Climate models the IPCC assessed suggested warming of 0.15–0.3°C per
decade from 1990. The observed warming trend since 1990 has been roughly
0.2°C per decade. This gives us further confidence in terms of the models’ ability
to simulate the warming influence of greenhouse gases and the response of the
climate system to such a warming influence over time (WGI 1.1, 3.2,
Figure 1.1). Incidentally, this agreement between models and observations also
suggests that statements in the popular press such as ‘climate change is
happening faster than expected’ are not entirely supported by the facts. To be
clear, there are some aspects of the climate system where changes have been
happening faster than expected (eg, recent reductions in Arctic sea ice), but the
most fundamental measure of climate change, the rate of global average
temperature change (see chapter 1), has been resolutely keeping pace with
model projections that include rising greenhouse gas concentrations.
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Vertical pattern of warming
The first fingerprint is the vertical pattern of warming. Climate models show that if
warming is caused by increasing solar radiation, we would expect the entire
atmosphere to heat up. By contrast, increasing greenhouse gases should lead to
greatest warming in the so-called troposphere (the layer of the atmosphere that
extends from the ground to about 15 km above the Earth), but cooling in the
stratosphere (the layer above the troposphere, about 15–50 km above the Earth).
Observations since 1950 show indeed a significant cooling of the stratosphere. Some
of this cooling can be explained by ozone depletion, but this alone is not sufficient to
explain the total amount of cooling that has been observed. This vertical pattern very
strongly points to greenhouse gases, rather than solar radiation, playing a key role in
the observed warming during the second half of the 20th century.18 (WGI 9.2)
Geographical pattern of warming and the land–ocean difference
The second fingerprint is that high-latitude land regions in the northern hemisphere
warmed more strongly than other regions, and all land regions warmed more than the
adjacent oceans. This pattern corresponds well to the warming effect we would
expect from greenhouse gases, whereas no known natural climate pattern (such as El
Niño or ocean circulation changes) leads to such a strong excess warming over land
compared with the ocean. A related fingerprint is the stronger warming of the
northern compared with the southern hemisphere, which also indicates that natural
climate patterns cannot be the main source of the observed warming (WGI 9.2).
Temporal warming pattern and the role of aerosols
The third fingerprint is the warming over time: the global average temperature
increased during the first half of the 20th century, then changed only a little or even
cooled slightly until it started rising again rapidly from about 1970. Do we
understand this sequence? Aerosol concentrations increased strongly during the
middle of the 20th century, resulting from the rapid expansion of industrial output by
the United States and Europe. During the past few decades, aerosol pollution in
industrialised countries has declined and is now mainly coming from developing
countries in southern and eastern Asia, but global aerosol concentrations have fallen.
This suggests the cooling effect of aerosols was responsible for the flattening or
slight decline in global average temperatures between 1950 and 1970. The reduction
in global aerosol concentrations, combined with a steady increase in global
greenhouse gas concentrations, explains the steady increase in temperatures from
1970. Changes in solar activity are likely to have contributed to the warming in the
first half of the 20th century, but the combined influence of solar activity and
volcanic eruptions should have led to a cooling of the atmosphere during the second
half of the century, not the observed warming since about 1970. (WGI 2.9, 9.3)
18 The reason why we expect this vertical pattern of warming and cooling from greenhouse gases is
similar to the way a blanket works: it keeps you warm underneath, but it is colder on top of the
blanket because it prevents heat from your body from escaping. By contrast, if you sit close to a
fire that warms you from the outside, the blanket around you warms up as much (or even more)
than you do. Note that this is only a rough analogy, the real atmosphere is a bit more complex!
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2.4.2
Attribution of the increase in global average temperature
Figure 2.6 puts all the factors together and shows the observed warming at the
Earth’s surface, the warming expected from natural factors only, and the warming
expected if greenhouse gases and aerosols are included. The observed rates of
warming over the globe, over land only, and over the ocean only match the pattern
and rates of warming that are expected if we include greenhouse gases and aerosols
with natural drivers. In contrast, no model has been able to reproduce the observed
temperature change over the past 50 years and its geographical pattern using only
natural external forcings, and no known mode of natural internal variability can
explain the observed global-scale warming. (WGI 9.7)
Figure 2.6: Comparison of observed and modelled warming for land, ocean, and
the world as a whole
Note: The panels on the left compare observed annual global average temperatures with
a series of model simulations that include (top) and exclude (bottom) the effects resulting
from human activities (mainly emissions of greenhouse gases and aerosols). The panels
on the right show smoothed results for the world as a whole and separately for land and
ocean areas.
Source: Based on WGI Figure SPM.4, Figure 9.5.
Considering all these lines of evidence, the IPCC concluded that there is a
probability of at least 90% that most of the observed warming over the past 50 years
is due to human activities in the form of greenhouse gas emissions (WGI 9.7, SPM).
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Some important uncertainties remain in this attribution. Perhaps the most
significant one is the magnitude of the cooling effect of aerosols compared to the
warming effect of greenhouse gases. All models agree that cooling from aerosols has
offset some of the warming that would have occurred from greenhouse gases alone.
This leaves it open whether a lot of warming from greenhouse gases may have been
offset by a lot of cooling from aerosols, or whether little warming has been offset by
little cooling. This uncertainty does not affect the basic conclusion that most of the
warming is due to the combined effect of human activities, because only greenhouse
gases can explain the existence of the warming at all.
This uncertainty about the role of aerosols has important implications with regard
to the interplay between efforts to control climate change and to reduce air pollution.
Since we cannot be sure just how much aerosols are currently cooling the planet, we
are not sure how much more the world would warm, if the concentrations of aerosols
were to be reduced by efforts to improve air quality (eg, by placing filters into smoke
stacks to reduce aerosol emissions). One study suggests that temperatures could go
up by as much as 0.8°C if hypothetically all aerosols produced by human activities
were removed from the atmosphere. The resulting warming could happen within a
space of years because aerosols have a much shorter lifetime than greenhouse gases,
and thus any reductions in aerosol emissions almost immediately result in changes in
aerosol concentrations (WGI 2.5, Box 7.4, 7.6, 10.7).
2.4.3
Attribution of regional temperature changes
The studies we have discussed so far conclude that, at the global scale, human
activities are responsible for most of the warming over the past 50 years. We can
even identify a significant warming influence from human activities over each
continent (except Antarctica, which has limited observational data and the warming
signal over the past 50 years is not as strong as over other continents) (WGI 9.7,
SPM). Nonetheless, a recent study claims to have demonstrated a human influence
on changes even in Antarctica’s temperature (Gillett et al, 2008).
However, there is an important limit to the attribution of regional temperature
changes. Even though most of the global average warming over the past 50 years is
due to greenhouse gases, this does not mean that any recent warming over a very
small region (eg, a small country, mountain range, or river catchment) is necessarily
also due to greenhouse gases. The reason for this limitation is that at smaller spatial
scales and over shorter periods, natural climate variability is a lot larger than the
global long-term average. It is very difficult to warm the planet as a whole over
several decades through a natural climate pattern, simply because the additional
energy has to come from somewhere – we cannot warm the atmosphere as well as
the ocean, and lose ice cover, all at the same time through a natural fluctuation.
However, it is possible to warm only a part of the planet for a brief period through a
natural climate pattern that shifts energy from one part of the globe to another. Such
shifts indeed routinely happen in natural climate patterns such as the so-called El
Niño Southern Oscillation or the North Atlantic Oscillation. Even though the
observed warming of the entire world over the past 50 years cannot be due to natural
climate fluctuations, warming over individual geographical regions smaller than
about the size of a continent and over much shorter periods certainly can.
In addition, feedback mechanisms also become more important at subcontinental
scales. For example, we already discussed that large-scale changes in land use or
melting of snow are unlikely to have affected the climate significantly on the global
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scale, but regionally, such effects can make a difference. Therefore, while the
warming trend in any particular location may be consistent with a global warming
trend, we cannot necessarily and unequivocally attribute every recent local
temperature increase to the influence of greenhouse gases.
Conversely, the fact that some (very few) regions of the planet have been cooling
during the past few decades or experienced significant brief cold snaps does not
disprove that the global average warming is due to human activities. It simply shows
that, at the regional and local scale, natural climate variability and feedback
mechanisms can be greater than the underlying global warming trend. It is, therefore,
entirely possible to observe cooling over limited regions and time spans even though
the world as a whole is on average getting warmer.
2.4.4
Attributing other changes in the climate system
So far we have discussed the influence of greenhouse gases only on average
temperature. However, we can also test for human influences on observed changes in
other important climate variables such as precipitation, sea level, and climatic
extremes. The principles by which such tests are conducted are the same: we need to
compare whether greenhouse gases can explain the observed change, but also
whether other natural factors could not equally have contributed to or caused the
observed changes.
The available studies show that human emissions of greenhouse gases are indeed
not only raising the Earth’s temperature but are also contributing to many other
changes in the climate system. Identifying the causes of changes in other climate
variables is generally harder than for temperature because the natural variability of,
for example, rainfall, is greater and because data records are often shorter. Also, the
detailed mechanisms leading from greenhouse gases to the change in question may
be more complex than for temperature. Consequently, the confidence with which we
can attribute some of the observed changes to human activities is generally lower
than for the global average temperature change, and has to rely on expert opinion as
well as statistics and model simulations.
Sea-level rise
It is at least 90% probable that human influences have contributed to the sea-level
rise during the latter half of the 20th century. This should not surprise us, given that
more than half of the observed sea-level rise is due to thermal expansion of the ocean
waters, and that the oceans have absorbed most of the additional energy that has been
added to the climate system (see chapter 1).
Temperature and precipitation extremes
The probability is at least 66% that human activities are the main cause of extreme
hot nights having become hotter and more frequent, while the coldest days and nights
have also become warmer and thus less frequent. It is more likely than not (ie, a
chance of more than 50%) that the risk of heat waves increased due to human
activities, and also the risk of drought and the frequency of extremely heavy
precipitation since the 1970s. Although models have not yet been able to quantify the
contribution of human activities, most experts believe it is also more likely than not
that greenhouse gas emissions have contributed to the increased intensity of tropical
cyclones. (See Box 2.3.)
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Box 2.3: Can we attribute individual storms and other extremes to human
causes?
Whenever a newspaper reports about a recent storm or heat wave, reporters
often ask: is it because of climate change?
Since the climate varies naturally and this variation can occasionally lead to
temperatures or storms that are far outside the long-term average, we
can rarely say that a particular event would not have happened anyway
without human influences. However, we can determine whether human
activities might have changed the probability that a particular event would
occur (WGI FAQ 9.1).
For example, the European heat wave of 2003 was probably the hottest
summer in at least the past 500 years. Model studies show that the rising
average temperatures in Europe have made it at least twice as likely that such
an extremely hot period would occur. This does not mean that it could not have
occurred even without greenhouse gases – but it does mean that the chance
that it did occur was greater because of greenhouse gas–induced warming, and
that we can expect such heat waves to occur more and more frequently in
future (Stott et al, 2004).
Similar considerations apply to the occurrence of tropical cyclones, floods, and
other extreme weather events. Human emissions of greenhouse gases change
the probabilities that such events occur, but we cannot determine whether any
particular event has been caused by human activities or is a natural
phenomenon – any individual storm, flood, or drought will virtually always be a
combination of natural variability and human influence (WGI FAQ 9.1).
Changes in wind patterns
There is at least a 66% chance that the effect of greenhouse gases contributed to
changes in wind patterns in mid-latitudes, which would have affected storm tracks
and temperature patterns. However, models suggest much smaller changes than what
has been observed, so we cannot claim that all of the changes we have seen are
necessarily due to human activities, or that these trends will continue into the future.
Changes in patterns and longer-term trends in rainfall
Finally, there is some evidence that greenhouse gas emissions and aerosols
contributed to the pattern of changes in rainfall over the 20th century (see Box 2.4).
However, the studies available during the IPCC’s assessment did not allow a
quantitative assessment of the contribution that greenhouse gases made to the
observed changes, and what might be due to natural variability.
2.5
Attributing observed impacts of climate change
Based on the evidence presented in this chapter, human emissions of greenhouse gases
and aerosols are very likely the main cause of the observed global warming over the
past 50 years, and contributed to some extent to other changes in climate. Based on the
evidence from chapter 1, we also know that the recent observed warming has had
noticeable effects on many ecosystems and species. Does it follow from these two
findings that we can blame greenhouse gas emissions for these effects?
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Box 2.4: Recent studies investigating the causes for changes in rainfall
and Arctic sea ice
Studies since the Fourth Assessment Report have further added to an
increasingly consistent picture of a human influence on not only temperature but
also other aspects of the climate system, in particular rainfall. Recent studies
suggest that the warming of the atmosphere due to human activities is affecting
moisture and the amount of water available for precipitation, which affects the
distribution of rainfall across the world and the likelihood of heavy rainfall
episodes (Santer et al, 2007; Willett et al, 2007; Zhang et al, 2007; Min et al,
2008). However, research is ongoing to examine whether observed changes are
proceeding at a rate consistent with predictions, and whether models that
include anthropogenic emissions of greenhouse gases successfully reproduce
recent observed changes (Wentz et al, 2007; Allan and Soden 2008).
Other recent studies investigate the degree to which human influences are
responsible for changes in Arctic sea ice, which has shown particularly
large decreases in 2007 and 2008 (see chapter 1). Studies are being carried
out to see whether local feedback mechanisms are now beginning to accelerate
the loss of ice, or whether the past two years may have been an example of
natural variability superimposed on a longer-term trend (Stroeve et al, 2007;
Stroeve et al, 2008a; Zhang et al, 2008a; Zhang et al, 2008b; Eisenman and
Wettlaufer 2009).
You may recall that almost 90% of the climate-related effects that have been
observed so far are consistent with the direction of change that would be expected
under regional warming trends. This very strong correlation holds on every continent
and most oceans (wherever effects have been documented). Given that greenhouse
gases have caused most of the global average warming over the past 50 years, we
simply cannot explain this statistical global-scale agreement between climate
changes and their effects without concluding that greenhouse gases have contributed
to these effects. Natural variability in temperatures or in natural systems alone would
have been very unlikely to produce such a consistent pattern of change across the
globe. In addition, several modelling studies have compared specific impacts with
the impacts expected with and without greenhouse gases and their effect on the
climate, and found that the agreement was better with than without greenhouse gases
(WGII 1.4, SYR 2.4).
We can, therefore, conclude that human emissions of greenhouse gases and
aerosols have indeed begun to affect not only the climate, but are also responsible at
the global scale for at least some of the observed effects of climate change on natural
systems (WGII SPM).
It is important to note that this does not mean that greenhouse gas emissions are
necessarily the cause behind every individual climate-related impact. The reason is
that most specific impacts (such as a change in flowering or migration date, the
shrinking of an individual glacier, or a shift in the distribution of a particular species)
occur over a fairly limited geographical region, and we have already noted the
difficulties of attributing temperature changes to human influences at particular
locations (such as a specific mountain range or river catchment). In addition, many
observational records of the effects of climate change to date extend over only
30 years or less, which makes it difficult to determine whether the effects are caused
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Climate Change 101 – An Educational Resource
by human influences on the climate or part of decadal fluctuations. Furthermore,
non-climate factors such as pollution, invasive species, and changes in land use (eg,
large-scale shifts from forestry to agriculture or urbanisation) could also play a role
for some specific observed impacts. (WGII 1.4, SYR 2.4)
Consequently, we can, at this stage, rarely ever prove that any particular observed
impact that is related to regional climate change is the result of human emissions of
greenhouse gases. Such changes might be consistent with a human influence, but we
cannot rule out that any particular local effect could have been caused by natural
variability either in climate or in the ecosystem in question, or that local non-climate
factors could have contributed to the change. However, we can demonstrate that
human greenhouse gas emissions have an overall influence on impacts around the
world, because there is no other way to explain the global-scale agreement between
increasing greenhouse gas concentrations, the pattern of global temperature
increases, and the documented effects that have followed increasing temperatures on
every continent and in most oceans. 19 (WGII 1.4, SPM) A recent update of this
analysis, using a more extended data set, confirmed the general findings regarding
global-scale impacts and emphasised that, as the amount of data on observed impacts
grows, we can also observe a discernible human influence on impacts at least in
some continents (Rosenzweig et al, 2008).
19 This may sound like academic hair-splitting – and I admit that it does indeed sound strange that
we should be able to detect a human influence on observed effects all around the world, but
nowhere in particular. Such a distinction becomes a lot less academic though when one considers
that already, various legal cases have been launched that try to hold countries or companies
responsible for some specific climate-related impacts that have already occurred or are occurring.
We may well see more and more of such efforts to bring scientific evidence on climate change
into courts of law. What often counts in such circumstances is not whether an observed impact is
consistent with a human cause through the emission of greenhouse gases, but whether we can rule
out that any other factors could not also have been a major cause; in other words, it’s the
difference between ‘is it plausible to think that he/she could have done it?’ and ‘guilty beyond
reasonable doubt’. Further research may reduce the geographical scale and timescales over which
we can establish the causes of climate changes and hence their impacts with greater confidence,
but for the time being this is an open research question.
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