Download Climate Change Science

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This section provides an introduction to basic concepts of climate change science
such as weather, climate, the greenhouse effect, climate forcings and natural climate
fluctuations. It then discusses main causes and elements of anthropogenic (human
caused) climate change, including observed and projected changes in the surface
temperature. The section concludes with a short discussion of climate change
science, its importance and historical development.
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It is important to understand the difference between “weather” and “climate”. What is
happening in the atmosphere at any given time is considered “weather” (including e.g. wind
speed and direction, precipitation, barometric pressure, temperature, and relative
humidity). Weather changes in the short term (e.g. daily, weekly, monthly). Climate is average
weather and occurs over long time frames (e.g. 30 years). A common confusion between
weather and climate arises when scientists are asked how they can predict climate 50 years
from now when they cannot predict the weather a few weeks from now. The chaotic nature
of weather makes it unpredictable beyond a few days. Projecting changes in climate (i.e.,
long-term average weather) due to changes in atmospheric composition or other factors is a
very different and much more manageable issue. As an analogy, while it is impossible to
predict the age at which any particular man will die, we can say with high confidence that
the average age of death for men in industrialised countries is about 75.
IPCC (2007): Frequently Asked Questions - What is the Relationship between Climate Change
and Weather?
Further information:
The IPCC defines climate as follows: “Climate in a narrow sense is usually defined as the
average weather, or more rigorously, as the statistical description in terms of the mean and
variability of relevant quantities over a period of time ranging from months to thousands or
millions of years. The classical period for averaging these variables is 30 years, as defined by
the World Meteorological Organization. The relevant quantities are most often surface
variables such as temperature, precipitation and wind. Climate in a wider sense is the state,
including a statistical description, of the climate system.”
IPCC (2013). Climate Change 2013: The Physical Science Basis, Working Group I Contribution
to the IPCC Fifth Assessment Report, Glossary
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In a broader sense, climate is the status of the climate system which comprises the
atmosphere, the hydrosphere, the cryosphere, the surface lithosphere and the
biosphere. These elements all determine the state and dynamics of the Earth’s
climate. The graphic illustrates a number of both natural and human factors that
have an influence on the climate. An important mechanism within the climate
system is the greenhouse effect which is explained in the following slide.
IPCC (2007). Climate Change 2007: The Physical Science Basis
Further information:
The atmosphere is the envelope of gas surrounding the Earth. The hydrosphere is
the part of the climate system containing liquid water at the Earth’s surface and
underground (e.g. oceans, rivers, lakes…). The cryosphere contains water in its frozen
state (e.g. glaciers, snow, ice…). The surface lithosphere is the upper layer of solid
Earth on land and oceans supporting volcanic activity which influence climate. The
biosphere contains all living organisms and ecosystems over the land and in the
oceans.
WMO website
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The temperature of the Earth results from a balance between energy coming into the Earth
from the Sun (solar radiation) and the energy leaving the Earth into outer space. About half
the solar radiation striking the Earth and its atmosphere is absorbed at the surface. The
other half is absorbed by the atmosphere or reflected back into space by clouds, small
particles in the atmosphere, snow, ice and deserts at the Earth’s surface. Part of the energy
absorbed at the Earth’s surface is radiated back (or re-admitted) to the atmosphere and
space in the form of heat (or thermal) energy. The temperature we feel is a measure of this
heat energy. In the atmosphere, not all thermal radiation emitted by the Earth reaches outer
space. Part of it is absorbed and reflected back to the Earth’s surface by greenhouse gas
(GHG) molecules and clouds (the greenhouse effect) leading to a global average of around
14°C, well above the -19°C which would be felt without the natural greenhouse effect. The
concentrations of some GHGs, such as carbon dioxide (CO2), are significantly influenced by
humans, others, such as water vapor, are not.
WMO website.
Further information:
The two most abundant gases in the atmosphere, nitrogen (comprising 78% of the dry
atmosphere) and oxygen (comprising 21%), exert almost no greenhouse effect. Instead, the
greenhouse effect comes from molecules that are more complex and much less common.
Water vapour is the most important greenhouse gas, and carbon dioxide (CO2) is the
second-most important one. Methane, nitrous oxide, ozone and several other gases present
in the atmosphere in small amounts also contribute to the greenhouse effect.
IPCC (2007): Frequently Asked Questions - What is the Greenhouse Effect?
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Do you know how the greenhouse effect maintains the surface temperature of the
Earth? This Nasa’s Earth Observatory video explains it in a graphical way.
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The natural greenhouse effect is part of a balanced system of energy transfer and
transformation within the atmosphere, at the Earth’s surface and in the oceans. The
Earth’s climate remains largely stable because the energy received is equal to that
lost (the energy budget is balanced). However, there are a factors that have caused
major changes in the climate system. Since these factors drive or “force” the system
to change they are called "forcings”. The change in energy fluxes caused by these
drivers are quantified by radiative forcing (RF). Positive RF leads to surface warming,
negative RF leads to surface cooling.
During the last millennium, changes in the output of energy from the sun, volcanic
eruptions and increased concentration of greenhouse gases in the atmosphere have
been the most important forcings. Total radiative forcing has been positive and has
led to an up-take of energy by the climate system. The graphic shows that the
increase in the atmospheric concentration of CO2 since 1750 has become the largest
contributor to total radiative forcing.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers , p11
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Changes in the climate system due to climate forcings are not to be confused with
natural climate fluctuations. Indeed, even within a relatively stable period, the
systems that make up and influence the global climate still naturally fluctuate. These
fluctuations or “oscillations” as they are often called (because they oscillate between
two main states) can have a large affect on the climate, both locally and on a global
scale. One example is El Niño, La Niña and the El Niño Southern Oscillation (ENSO).
ENSO is a climate pattern that occurs, roughly every 5 years, accross the tropical
Pacific Ocean. El Niño (Spanish for little boy) describes extensive warming of the
ocean surface across the eastern and central equatorial Pacific lasting three or more
seasons (see red area near the equator on the left globe). When this oceanic region
switches to below normal temperatures, it is called La Niña (Spanish for little girl ,
see blue area near the equator on the right globe).
WMO website
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Since the beginning of the 20th century, scientists have been observing a change in the
climate that can not be attributed to any of the “natural” influences of the past only. This
change in the climate, also known as global warming, has occurred faster than any other
climate change recorded by humans.
The main cause of global warming is the increased concentration of greenhouse gases in the
atmosphere since the industrial revolution in the late 18th century. The increased amount of
gases which absorb and re-emit thermal radiation, has directly led to more heat being
retained in the atmosphere and thus an increase in global average surface temperatures.
The increase in temperature is also leading to other effects on the climate system. Together
these affects are known as anthropogenic (human caused) climate change.
WMO website
Further information:
Adding more of a greenhouse gas, such as CO2, to the atmosphere intensifies the
greenhouse effect, thus warming Earth’s climate. The amount of warming depends on
various feedback mechanisms. For example, as the atmosphere warms due to rising levels of
greenhouse gases, its concentration of water vapour increases, further intensifying the
greenhouse effect. This in turn causes more warming, which causes an additional increase in
water vapour, in a self-reinforcing cycle. This water vapour feedback may be strong enough
to approximately double the increase in the greenhouse effect due to the added CO2 alone.
IPCC (2007): Frequently Asked Questions - What is the Greenhouse Effect?
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This diagram gives an indication of the observed change in average surface
temperature between 1901 and 2012. It shows that almost the entire globe has
experienced surface warming.
According to the IPCC, the average global surface temperature has increased by
0.85°C over the period 1880 to 2012.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p3
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Scientists have not only observed past changes in the climate but also try to analyze
possible future changes. For this purpose they have developed a number of tools.
Just as an architect might build a scale model of a building to understand and predict
its behaviour, so too climate scientists can build a computer-based model of the
climate system to understand and predict its behaviour. One of the inputs to a
climate model are emission scenarios, which estimate future releases of greenhouse
gases and aerosols to the atmosphere based on assumptions concerning, for
example, future socioeconomic and technological developments. The outputs of a
climate model feed into a climate projection, i.e. a simulated response of the climate
system to a certain emission scenario. This dependence on emission scenarios
differentiates climate projections from climate predictions which are based on
conditions that are known at present and assumptions about the physical processes
that will determine future changes.
WMO website
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This figure illustrates the projected (i.e. future) change in average surface
temperature for two different scenarios. The projections are for the end of the 21st
century (2081-2100) and are given relative to 1986-2005. The projection on the left
is based on a scenario with relatively limited greenhouse gas emissions (RCP 2.6),
while the projection on the right is based on a scenario with very high greenhouse
gas emissions (RCP 8.5). RCP 2.6 projects an increase of 0.3 to 1.7°C in mean surface
temperature compared to preindustrial times, while RCP 8.5 projects an increase of
2.6 to 4.8°C by 2081-2100.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers
Further information:
Projections of changes in the climate system are based on a hierarchy of climate
models that range from simple climate to intermediate complex models, to
comprehensive climate models as well as Earth System Models. Based on a set of
scenarios of anthropogenic forcings, the modules simulate changes. The
Representative Concentration Pathways (RCPs) is a new set of scenarios that are
used for the 5th Assessment Report of the Intergovernmental Panel on Climate
Change (IPCC).
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Climate change has an impact on almost every aspect of our lives. Our ecosystems
suffer biodiversity and habitat loss and human systems like health will be negatively
impacted, for example by the spread of disease vectors like mosquitos. Climate
change also challenges us to rethink our urban systems (including transport and
buildings) and the way we do business (including green business opportunities). The
impacts of climate change might also result in conflict or force people to migrate (for
example from low-lying coastal areas).
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Scientists worldwide are trying to better understand how the climate is changing,
what changes we can expect in the future, and what role human activities play.
While there is a debate about the likelihood of certain changes and their causes,
broad scientific consensus exists that (1) warming of the climate system is
unequivocal, and (2) that human influence on the climate system is clear.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers
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Climate change science provides important information for decision-making at
various levels. For example, sound weather data and forecasts can help us determine
when the best time would be to harvest our lands. It can also help us plan
appropriately for emergency responses, in case of climate-related hazards, such as
cyclones. Climate models help to forecast long term climate scenarios and are
important for proactive planning.
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As far back as 1824, the French physicist Joseph Fourier is the first to describe the
Earth’s natural “greenhouse effect”. In 1861, the Irish physicist John Tyndall shows
that CO2 and H2O can cause changes in the climate. In 1895, Swedish chemist Svante
Arrhenius concludes that industrial-age coal burning will enhance the natural
greenhouse effect. In 1938 the British engineer Guy Callendar shows that
temperatures had risen over the previous century due to increased CO2
concentrations. The "Callendar effect" is widely dismissed. In 1958 the geochemist
Charles David Keeling was employed to continuously monitor CO2 levels in the
atmosphere, with an increase in Antarctica visible after only two years. During the
1970’s other greenhouse gases, CH4, N2O and CFCs, were widely recognized as
important anthropogenic greenhouse gases and in 1979 the First World Climate
Conference was held in Geneva, leading to the establishment of the World Climate
Programme. In 1988 the Intergovernmental Panel on Climate Change (IPCC) was set
up by the World Meteorological Organization (WMO) and the United Nations
Environment Program (UNEP). In 1990 the IPCC delivered its First Assessment Report
on the state of climate change, predicting an increase of 0.3 °C each decade in the
21st century.
IPCC (2007). Fourth Assessment Report, Chapter One - Historical Overview of
Climate Change Science
Zillman, J. (2009). A History of Climate Activities
Knight, M. for CNN (2008). A Timeline of Climate Change Science
BBC Website
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This section discusses the main driver of anthropogenic climate change, i.e. the
increased concentration of greenhouse gases in the atmosphere. The section starts
with an overview of the most important greenhouse gases emitted by humans. It
then discusses each gas in more detail, looking at (1) how significant the gas is in
terms of global warming, and (2) how its concentration in the atmosphere has
developed over time. The section concludes with a graph depicting the extent of the
human influence on the climate system.
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The Intergovernmental Panel on Climate Change (IPCC) has produced a video on the
physical science basis of climate change. It looks at evidence of how the climate has
changed in the past and present, what the causes of these changes are, and what
possible future scenarios exist.
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Greenhouse gases (GHGs) are trace gases in the atmosphere that absorb and emit long wave
radiation. They naturally blanket the earth and keep it at about 33° C warmer than it would be
without these gases in the atmosphere. The table features the seven most important greenhouse
gases as regulated under the Kyoto Protocol. The seven gases each have a different capacity to trap
heat in the atmosphere, or a so-called “global warming potential” (GWP). They all belong to the
group of long-lived greenhouse gases (LLGHGs), because they are chemically stable and persist in the
atmosphere over time scales of a decade to centuries or longer, so that their emission has a longterm influence on climate. Some of the GHGs occur naturally (e.g. CO2, CH4 and N2O) but increases in
their atmospheric concentrations over the last 250 years are due largely to human activities. Other
greenhouse gases are entirely the result of human activities (e.g. HFCs, PFCs, SF6 and NF3).
IPCC (2007). Fourth Assessment Report, Technical Summary – Changes in Human and Natural Drivers
of Climate
UNEP (2012). Emissions Gap Report
Further information:
Kyoto Protocol: The Kyoto Protocol sets out legally binding targets for developed countries to limit or
reduce their GHG emissions. It was adopted in 1997 and entered into force in 2005. Water vapor is
the most important greenhouse gas but since it is not produced by humans in any significant quantity,
we have no control over its concentration in the atmosphere. It is therefore not regulated under the
Kyoto Protocol.
Global Warming Potential: Carbon dioxide is the baseline unit to which all other greenhouse gases
are compared and therefore has a GWP of exactly 1. A GWP is calculated over a specific time interval
(commonly 20, 100 or 500 years), because some gases remain longer in the atmosphere than others.
For example, the 100 year GWP of methane is 25, which means that if the same mass of methane and
carbon dioxide were introduced into the atmosphere, that methane will trap 25 times more heat
than the carbon dioxide over the next 100 years.
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The most important anthropogenic GHG is carbon dioxide (CO2). It accounts for
around 64% of total radiative forcing due to LLGHGs. Carbon dioxide does not have a
specific lifetime because it is continuously cycled between the atmosphere, oceans
and land biosphere and its net removal from the atmosphere involves a range of
processes with different time scales. CO2 is primarily emitted as a result of burning of
fossil fuels, deforestation and forest degradation and iron and steel production.
Oceans and forests are the main sequesters of carbon i.e sinks that can absorb CO2
from the atmosphere. Carbon dioxide is the gas to which all other gases are
compared when speaking of Global Warming Potential. Emissions of other
greenhouse gases can be converted into CO2 equivalent emissions.
IPCC (2007). Fourth Assessment Report, Technical Summary – Changes in Human and
Natural Drivers of Climate
WMO (2013). Greenhouse Gas Bulletin
Further information:
The contribution of each greenhouse gas to radiative forcing is determined by its
global warming potential and the change in its concentration in the atmosphere over
time.
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CO2 levels in the atmosphere have been increasing steadily over the past 200 years.
This is due to the increased use of fossil fuels as well as an increase in deforestation,
both of which release vast amounts of CO2 into the atmosphere. Today’s
concentration of CO2 is believed to be the highest it has been in the past 800,000
years. CO2 concentrations increased by about 100 ppm (parts per million) since the
industrial revolution and exceeded the symbolic 400 ppm threshhold at several
atmosphere watch stations during 2012. Figure a) shows the increase in CO2
concentrations between 1984 and 2012. Figure b) shows the annual growth rate of
CO2 concentrations during the same time period.
WMO (2013). Greenhouse Gas Bulletin, p3
WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record
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The second most significant anthropogenic GHG is methane (CH4) which contributes
to approximately 18% of total radiative forcing due to LLGHGs. Approximately 40% of
methane is emitted into the atmosphere by natural sources (e.g. wetlands and
termites). About 60% comes from human activities (e.g. cattle breeding, rice
agriculture, fossil fuel exploitation, landfills and biomass burning). Methane is
mostly removed from the atmosphere by chemical reactions, persisting for about 12
years. Thus although methane is an important greenhouse gas, its effect is relatively
short-lived.
IPCC (2007). Fourth Assessment Report, Working Group I - The Physical Science Basis
WMO (2013). Greenhouse Gas Bulletin, p3
WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record
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This slide shows the atmospheric concentration of methane from 1984 to 2012 in
figure (a), while figure (b) shows the annually averaged growth rate. The
concentration of CH4 has more than doubled since pre-industrial times (from
approximately 700 parts per billion in 1750 to 1819 ppb in 2012 ). Since 2007,
atmospheric methane has been increasing again after a temporary period of
levelling-off.
WMO (2013). Greenhouse Gas Bulletin, p3
WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record
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Carbon in its various forms (such as CO2 and CH4) is continually recycled on Earth and
is never destroyed. The diagram illustrates the many ways in which carbon is
released and stored in the environment. Carbon can be stored over relatively short
periods in living organisms (i.e. plants and animals) or over thousands of years in the
oceans. It can also be stored over millions of years in rocks or fossils. The diagram
also shows how humans are affecting the carbon cycle. Before humans used fossil
fuels for energy, the carbon cycle was relatively balanced (i.e. the total amount
carbon in the atmosphere stayed constant). By removing carbon from the long term
storage underground (oil, gas, etc.) and putting it into the atmosphere, humans have
tipped the balance of the carbon cycle, which in turn affects the global climate.
Removing stored carbon through deforestation also further exacerbates this process.
UNEP (2009). Climate in Peril, p14
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Nitrous oxide is the third most significant GHG, contributing to about 6% of radiative
forcing due to LLGHGs. The primary human sources of N20 are fertilizer production
and use in agriculture and various industrial processes. It is estimated that N 20 stays
in the atmosphere for an estimated 114 years. Its impact on climate, over a 100-year
period, is 298 times greater than equal emissions of carbon dioxide. It also plays an
important role in the destruction of the stratospheric ozone layer which protects us
from the harmful ultraviolet rays of the sun.
IPCC (2007). Fourth Assessment Report, Working Group I - The Physical Science Basis
WMO (2013). Greenhouse Gas Bulletin, p3
WMO (2013). Greenhouse Gas Concentrations in Atmosphere Reach New Record
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The concentration of N2O has been steadily increasing over the past 30 years. Its
atmospheric concentration in 2012 was about 325.1 parts per billion, which is 20%
more than the pre-industrial level (270 ppb).
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Fluorinated gases are a family of man-made gases used in a range of industrial applications.
Sources include refrigerants, air-conditioning, solvents, aluminium and magnesium
production, etc. Many fluorinated gases have very high global warming potentials (GWPs)
relative to other greenhouse gases. That means small atmospheric concentrations can have
large effects on global temperatures. They can also have long atmospheric lifetimes, in some
cases, lasting thousands of years. Fluorinated gases are removed from the atmosphere only
when they are destroyed by sunlight in the far upper atmosphere. In general, fluorinated
gases are the most potent and longest lasting type of greenhouse gases emitted by human
activities. There are three main categories of fluorinated gases: hydrofluorocarbons (HFCs),
perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).
EPA website
Further information:
Hydrofluorocarbons (HFCs) are the most common group of F-gases. They are used in various
sectors and applications, such as refrigerants in refrigeration, air-conditioning and heat
pump equipment; as blowing agents for foams; as solvents; and in fire extinguishers and
aerosol sprays.
Perfluorocarbons (PFCs) are typically used in the electronics sector (for example for plasma
cleaning of silicon wafers) as well as in the cosmetic and pharmaceutical industry. In the past
PFCs were also used in fire extinguishers and can still be found in older fire protection
systems.
Sulphur hexafluoride (SF6) is used mainly as an insulating gas, in high voltage switchgear and
in the production of magnesium and aluminium.
EC website
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Some greenhouse gases are not included in the Kyoto Protocol because they are
already regulated under the Montreal Protocol on Substances that Deplete the
Ozone Layer which entered into force in 1989. The Montreal Protocol includes, for
example, chlorofluorocarbons (CFCs) which contribute about 12% to total radiative
forcing by LLGHGs. CFCs can stay in the atmosphere for more than 1,000 years. CFCs
have a global warming potential (GWP) that ranges between 4,750 and 14,400 (over
100 years time span). CFCs are used in the manufacture of aerosol sprays, blowing
agents for foams and packing materials, as solvents, and as refrigerants.
NOAA website
IPCC (2007). Fourth Assessment Report, Working Group I - The Physical Science Basis
WMO (2013). Greenhouse Gas Bulletin
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This slide shows the evolution of atmospheric concentrations of sulfur hexafluoride
(SF6) (graph on the left) and numerous CFC and F-gases (graph on the right).
Concentrations of CFCs, which are controlled by the Montreal Protocol, are in
decline. However, hydro-chlorofluorocarbones (HCFCs) and hydro-fluorocarbones
(HFCs), which are also potent greenhouse gases, are increasing at relatively rapid
rates.
WMO (2013). Greenhouse Gas Bulletin
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This figure illustrates the important impact of human activities on the climate. It shows the
contribution of different anthropogenic and natural factors to the observed temperature increase of
about 0.6°C since 1951 (black bar). The graph shows that GHGs such as carbon dioxide, methane and
nitrous oxide (green bar) are the main causes of the observed temperature change. The yellow bar
depicts the influence of aerosols (tiny particles found in the atmosphere) which have a negative
forcing effect (cooling) on the climate. In fact, aerosols and their interaction with clouds have offset a
substantial portion of positive radiative forcing by GHGs. Atmospheric aerosols are not to be
confounded with aerosol sprays which often contain GHGs and hence have a positive radiative forcing
effect.
Overall, human activity has led to positive radiative forcing (global warming) as indicated by the
orange bar. Radiative forcing due to changes in energy output from the sun and volcanic eruptions
only played a minor role in the reference period.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers , p11-12
Further information:
Atmospheric aerosols are able to alter climate in two important ways:
• They scatter and absorb solar and infrared radiation;
• They may change the microphysical and chemical properties of clouds and possibly their lifetime
and extent.
The scattering of solar radiation acts to cool the planet, while absorption of solar radiation by aerosols
warms the air directly instead of allowing sunlight to be absorbed by the surface of the Earth. The
direct radiative forcing summed over all aerosol types is negative. Aerosols also cause a negative
radiative forcing indirectly through the changes they cause in cloud properties.
WMO website
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This section describes some of the main observed changes in the climate since the
industrial revolution. It looks at changes in surface temperature, precipitation levels,
ocean warming and acidification, sea-level rise, Arctic sea ice extent, as well as
observed changes in physical and biological systems. The section concludes with a
discussion of whether the recent increase in extreme weather events (such as
cyclones and floods) can be attributed to anthropogenic climate change.
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This figure illustrates how the averaged land and ocean surface temperature has
changed between 1850 and 2012. The annual temperature average has always
varied, with cold and warm periods alternating. However, it is clear that each of the
last three decades has been successively warmer at the Earth’s surface than any
preceding decade since 1850.
The temperature increase is widespread across the world, but there are important
regional variations. Warming has been most marked in the northern Polar Regions.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers , pp 3-4
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Observations show that changes are occurring in the amount, intensity, frequency
and type of precipitation. These aspects of precipitation generally exhibit large
natural variability, and El Niño and other natural climate fluctuations have a
substantial influence. Over the past century, however, pronounced long-term trends
in precipitation amounts have been observed: significantly wetter in eastern North
and South America, northern Europe and northern and central Asia, but drier in the
Sahel, southern Africa, the Mediterranean and southern Asia. Moreover, widespread
increases in heavy precipitation events have been observed, even in places where
total amounts have decreased. The two maps show the observed precipitation
change from 1901 to 2010 and 1951 to 2010.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p3
IPCC (2007): Frequently Asked Questions – How Is Precipitation Changing?
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Ocean warming dominates the increase in energy stored in the climate system.
Oceans account for more than 90% of the energy accumulated between 1971 and
2010. 60% of the net energy increase is stored in the upper ocean (0-700 m) and
about 30% is stored in the ocean below 700 m. The ocean warming is largest near
the surface, am the upper 75 m warmed by 0.11°C per decade over the period 1971
to 2010.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p6
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About 30% of anthropogenic CO2 emissions has been absorbed by the oceans. This
leads to ocean acidification. The green curve in the figure shows the decreasing pH
of ocean surface water since the late 1980s. According to the IPCC, the pH of ocean
surface water has decreased by 0.1 since the beginning of the industrial era.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p10
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The rate of sea level rise since the mid-19th century has been larger than the mean
rate during the previous two millennia. Over the past century, global mean sea level
rose by 0.19m. Glacier mass loss and ocean thermal expansion from warming
together explain about 75% of the observed global mean sea-level rise since the
early 1970s.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p9
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Over the last two decades, the Greenland and Antarctic ice sheets have been losing
mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice has
continued to decrease in extent. The graph illustrates the decrease in summer sea
ice extent in the Artic between 1900 and 2010. The spatial extent has decreased in
every season since 1979.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p7
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This slide shows a number of changes that have been observed in physical (blue) and
biological (green) systems between 1970 and 2004. Biological changes include, for
example, species loss and ecosystem alterations. Physical changes include, for
example, changes in snow cover, changes in glacier density/coverage and run-off.
The percentages associated with the observations indicate how many of the
reported changes are due to a warming environment.
UNEP (2009). Climate in Peril
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Whenever an episode of extreme weather – heatwave, flood, drought, etc. – hits the
headlines, many people tend to blame human-induced climate change. But what
scientific evidence exist to explain individual extreme events, like a cyclone, with
global warming? First of all, determining whether a specific, single extreme event is
due to a specific cause, such as increasing greenhouse gases, is difficult, if not
impossible, for two reasons: 1) extreme weather events are caused by a combination
of factors, and 2) a wide range of extreme events is a normal occurrence even in an
unchanging climate.
At the same time, observations have shown a large increase in the number of strong
hurricanes globally since 1970. Specifically, the number of strong hurricanes
increased by about 75% since 1970. The IPCC points out that a trend towards longer
storm duration and greater storm intensity is closely correlated with tropical sea
surface temperature. This might be an indication of a causal link between global
warming and hurricane destructiveness. However, the high variability in tropical
storms and hurricanes over several decades and a lack of systematic high quality
observation before satellite observations make it difficult to detect long-term trends.
IPCC (2007): Frequently Asked Questions – Has There Been a Change in Extreme
Events?
UNEP (2009). Climate in Peril
WMO website
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This section presents projected future trends and impacts of climate change on
surface temperature, precipitation, ocean pH, sea-level and Arctic sea-ice extent.
The extent of these impacts depends on how anthropogenic emission levels will
develop over the next decades. Therefore, the section presents impacts under a lowemission scenario, as well as under a high-emission scenario. The section ends with
a discussion of cumulative CO2 emissions, and what different amounts of CO2
emissions would mean for the future climate.
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For the Fifth Assessment Report of the IPCC, the scientific community has defined a
set of four new scenarios, denoted Representative Concentration Pathways (RCPs).
These four RCPs include one mitigation scenario leading to a very low forcing level
(RCP2.6), two stabilization scenarios (RCP4.5 and RCP6), and one scenario with very
high greenhouse gas emissions (RCP8.5). The RCPs can thus represent a range of 21st
century climate policies, as compared with the no-climate-policy of the Special
Report on Emissions Scenarios (SRES) used in the Third and the Fourth Assessment
Reports.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers
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Global surface temperature change for the end of the 21st century is likely to exceed
1.5°C relative to pre-industrial levels for all RCP scenarios except RCP 2.6. It is likely
to exceed 2°C for RCP 6.0 and RCP 8.5. Since the Third IPCC Assessment Report
confidence has increased that a 1.5-2.5°C increase in global mean temperature
above pre-industrial levels poses significant risks to many unique and threatened
systems, including many biodiversity hotspots. Approximately 20-30% of species are
at increased risk of extinction if global average warming exceeds 1.5-2.5°C. In terms
of food security and human health, crop productivity for cereals in low latitudes
would decrease and the distribution of some disease vectors (like mosquitos
transmitting malaria) might change.
UNEP (2009). Climate in Peril, pp 27-29
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p18
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The IPCC projects that over the 21st century the contrast in precipitation between wet and
dry regions and between wet and dry seasons will increase. That means global warming
increases risks of both drought and floods. Extreme precipitation events are particularly
likely to become more intense and more frequent over most of the mid-latitude land
masses and over wet tropical regions.
The two maps illustrate projected changes in annual mean precipitation for the end of the
21st century under different scenarios. Under a low-emission scenario (RCP 2.6 – left map)
changes in annual mean precipitation will not exceed 20% compared to 1986-2005 levels.
However, under scenario RCP 8.5 (right map) significant changes in annual precipitation are
to be expected. The high latitudes and the equatorial Pacific Ocean will experience an
increase in annual mean precipitation, while in many mid-latitude and subtropical dry
regions, mean precipitation will decrease.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for Policymakers,
p18
IPCC (2007): Frequently Asked Questions – How is Precipitation Changing?
Further information:
As climate changes, several direct influences alter precipitation amount, intensity, frequency
and type. Warming accelerates land surface drying and increases the potential incidence and
severity of droughts, which has been observed in many places worldwide. However, a wellestablished physical law (the Clausius-Clapeyron relation) determines that the water-holding
capacity of the atmosphere increases by about 7% for every 1°C rise in temperature.
Because precipitation comes mainly from weather systems that feed on the water vapour
stored in the atmosphere, this has generally increased precipitation intensity and the risk of
heavy rain and snow events.
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By combining data from various climate models, used by the IPCC, this video shows
the expected changes in temperature and precipitation for the 21st century.
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Today the average ocean surface pH is about 8.1. Projections suggest a further
acidification of the oceans over this century due to increasing atmospheric carbon
dioxide concentrations. Under a low-emission scenario (RCP 2.6 – left map) ocean
acidification will be relatively limited. However, under scenario RCP 8.5 (see right
map) a reduction in average global surface ocean pH of between 0.30 and 0.32 units
is to be expected. This progressive acidification will harm marine creatures which
form shells, for instance corals, and the species which depend upon them.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p25
UNEP (2009). Climate in Peril, p30
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Global mean sea level will continue to rise during the 21st century (see graph). The
projected rise ranges between 0.26 m to 0.98 m depending on the scenario. Under
all RCP scenarios, the rate of sea level rise will exceed that observed during 1971 to
2010 due to increased ocean warming and increased loss of mass from glaciers and
ice sheets.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p23
48
During the 21st century the Arctic sea ice cover will continue to shrink and thin as
global mean surface temperature rises. Under scenario RCP 8.5 it is likely that the
Arctic Ocean will be nearly ice-free in the month of September before mid-century.
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, p23
49
Africa is one of the most vulnerable continents to climate change. Most of Africa will
see less precipitation, with only the east central region seeing an increase. By 2080,
an increase of 5 to 8% of arid and semi-arid land is projected under a range of
scenarios. Already by 2020, between 75 million and 250 million people will be
exposed to increased water stress due to climate change. Agricultural production,
including access to food is projected to be severely comprised. In some countries,
yields from rain-fed agriculture could be reduced by up to 50%. Sea level rise will
affect major cities in low-lying coastal areas, such as Alexandria, Cairo, Lomé,
Cotonou, Lagos and Massawa.
UNEP (2009). A Climate in Peril
IPCC (2007). Climate Change 2007: Synthesis Report, p11
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This slide details some of the impacts of climate change projected for the Asian
region. The light blue colour indicates permafrost regions that are at risk of thawing.
Glacier melt will increase flooding and rock avalanches, and affect water resources in
Tibet, India and Bangladesh, this will then cause decreased river flows and fresh
water availability as the glaciers recede. More than a billion people could have water
shortages by the 2050s. South East Asia, especially the heavily-populated mega
delta regions will be at risk from flooding. Around 30% of Asia’s coral reefs are likely
to be lost in the next 30 years due to multiple stresses and climate change. Changes
in rainfall will increase diarrheal diseases mainly associated with floods and
droughts. The green colour indicates the possible increase in malaria distribution.
UNEP (2009). A Climate in Peril
IPCC (2007). Climate Change 2007: Synthesis Report, p11
51
This slide presents climate change impacts projected for the Latin American region.
As a consequence of reduced precipitation and retreating glaciers, Latin America
may have less water, affecting human consumption, agriculture and energy
generation. Yields of food crops may diminish, with adverse consequences for food
security. Latin America also faces the risk of significant biodiversity loss through
species extinction in many tropical areas. Decreases in soil water are projected to
lead to gradual replacement of tropical forest by savanna in eastern Amazonia.
Another ecosystem situated in the Caribbean at threat are coral reefs, home to many
marine living resources. Sea-level rise will increase the risk of flooding in low-lying
areas, particularly the Caribbean.
UNEP (2009). A Climate in Peril
IPCC (2007). Climate Change 2007: Synthesis Report, p11
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Many small islands, e.g. in the Caribbean and the Pacific, will experience reduced
water resources to the point where they become insufficient to meet demand during
low-rainfall periods. Sea-level rise will lead to the infiltration of freshwater resources
by saltwater, making it undrinkable. Sea-level rise is also expected to exacerbate
inundation, storm surge, erosion and other coastal hazards, thus threatening vital
infrastructure, settlements and facilities that support the livelihood of island
communities. The deterioration in coastal conditions and coral bleaching will reduce
the value of these destinations for tourism.
IPCC (2007). Climate Change 2007: Synthesis Report, p12
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This diagram depicts four potential climate futures depending on what policies
governments adopt to cut emissions. It is based on the four scenarios used in the
2013 IPCC report on the physical science basis of climate change.
Cambridge University (2013). Climate Change: Action, Trends and Implications For
Business
54
The graph illustrates the linear relationship that exists between cumulative humaninduced CO2 emissions (horizontal axis) and increases in global mean surface
temperature (vertical axis). In other words, more CO2 emissions lead to higher mean
surface temperatures. Because we cannot tell exactly how much carbon dioxide we
will emit in the future, different scenarios exist for what global temperatures will be
at the end of this century. These scenarios range from 2°C (blue line – RCP 2.6) to
nearly 5°C (red line – RCP 8.5). In order to have a greater than two in three chance of
keeping global temperatures below 2°C (thereby avoiding ‘dangerous climate
change’), cumulative emissions of CO2 must not exceed 1,000 Gigatonnes of carbon
(GtC). However, as of 2011 more than half this amount, over 500 GtC, has already
been emitted (black line).
IPCC (2013). Climate Change 2013: The Physical Science Basis - Summary for
Policymakers, pp 25-26
Further information:
The horizontal axis on the bottom of the graph indicates cumulative emissions in
gigatonnes of carbon (GtC) while the top axis indicates emission levels in gigatonnes
of carbon dioxide (GtCO2). 1 GtC corresponds to 3.667 GtCO2.
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This section provides an overview of main source of scientific climate information,
relevant programmes and institutions.
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The IPCC is the leading international body which synthesizes and assesses climate
change knowledge. Established in 1988 by UNEP and the World Meteorological
Organization (WMO) the IPCC assesses all peer reviewed and published climate
change information. Awarded with the Nobel Peace prize in 2007 it calls on a
network of climate scientists, biologists, economists, sociologists etc. from every
continent on Earth to produce reports on the state of knowledge of climate change
science, analyze the social and economic impacts of climate change, and identify
possible adaptation and mitigation options.
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The IPCC publishes various reports that are relevant to the climate change regime.
Widely cited, the comprehensive assessment reports review the latest science on
climate change, its impacts, vulnerability and adaptation, as well as mitigation
options. In addition to these reports the IPCC also publishes special reports on
selected topics, for example: renewable energy sources, extreme events and
disasters, emission scenarios etc. It also produces methodological guidance
documents and technical papers.
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The WMO runs a number of global climate programmes that provide policy makers
and technical staff with information, necessary to successfully respond to climate
change. The World Climate Programme (WCP) aims to produce skillful climate
predictions and projections, develop operational structures to provide climate
services, and to develop an essential global observing system to meet climate
information needs. The WCP underpins the Global Framework for Climate Services
(GFCS) which aims to incorporate science-based climate information into planning,
policy and practice on the global, regional and national scales. Another major
programme is the Atmospheric Research and Environment Programme (ARE) which
co-ordinates and stimulates research on the composition of the atmosphere and
weather forecasting, focusing on extreme weather events and socio-economic
impacts. The Commission for Climatology (CCI) advises and guides the activities of
the WCP, while playing a key role in implementation of the GFCS.
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The GFCS is a global partnership of governments and organizations that produce and
use climate information and services. It seeks to enable researchers and the
producers and users of information to join forces to improve the quality and quantity
of climate services worldwide, particularly in developing countries. The presentation
provides an introduction to the main elements of the GFCS.
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The Global Atmosphere Watch (GAW) programme of WMO provides reliable
scientific data and information on the chemical composition of the atmosphere, its
natural and anthropogenic change, and helps to improve the understanding of
interactions between the atmosphere, the oceans and the biosphere. The backbone
of the GAW programme is a global network of more than 80 measurement stations
as seen on this map.
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WMO Regional Climate Centres (RCCs) are centres of excellence that create regional
products including long-range forecasts that support regional and national climate
activities. RCCs use data and products from Global Producing Centres for Long-range
Forecasts (GPCs), incorporating regional-scale information. An additional main
source of information for RCCs are data, products, know-how and feedback they
receive from National Meteorological and Hydrological Services (NMHSs).
Regional Climate Outlook Forums (RCOFs) bring together national, regional and
international climate experts, on an operational basis, to produce regional climate
outlooks based on input from national, regional, and global institutions. By bringing
together countries having common climatological characteristics, the forums ensure
consistency in the access to and interpretation of climate information. The
information provided by RCOFs is being applied to reducing climate-related risks and
supporting sustainable development.
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National Meteorological and Hydrological Services (NMHSs) provide weather,
climate, and water information to emergency managers, national and local
administrations, the public, and critical economic sectors. The NMHSs make a
significant contribution to safety, security, and economic well-being by observing,
forecasting, and warning of pending weather, climate, and water threats.
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