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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). 35 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) 36 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 37 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? 38 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. 39 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. 41 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 42 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) 43 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 44 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). 47 Climate Change 101 – An Educational Resource 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 48 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. 49 Climate Change 101 – An Educational Resource 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) 50 Do We Understand What Is Driving Climate Change? 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). 51 Climate Change 101 – An Educational Resource 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. 52 Do We Understand What Is Driving Climate Change? 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. 53 Climate Change 101 – An Educational Resource 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! 54 Do We Understand What Is Driving Climate Change? 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). 55 Climate Change 101 – An Educational Resource 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 56 Do We Understand What Is Driving Climate Change? 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.) 57 Climate Change 101 – An Educational Resource 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? 58 Do We Understand What Is Driving Climate Change? 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 59 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. 60