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Suggested Citation:"Q&A." National Academy of Sciences. 2014. Climate Change: Evidence and Causes. Washington, DC: The National Academies Press. doi: 10.17226/18730.
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15Evidence & Causes Q& A n How does climate change affect the strength and frequency of floods, droughts, hurricanes, and tornadoes? Earth’s lower atmosphere is becoming warmer and moister as a result of human-emitted greenhouse gases. This gives the potential for more energy for storms and certain severe weather events. Consistent with theoretical expectations, heavy rainfall and snowfall events (which increase the risk of flooding) and heatwaves are generally becoming more frequent. Trends in extreme rainfall vary from region to region: the most pronounced changes are evident in North America and parts of Europe, especially in winter. Attributing extreme weather events to climate change is challenging because these events are by definition rare and therefore hard to evaluate reliably, and are affected by patterns of natural climate variability. For instance, the biggest cause of droughts and floods around the world is the shifting of climate patterns between El Niño and La Niña events. On land, El Niño events favour drought in many tropical and subtropical areas, while La Niña events promote wetter conditions in many places, as has happened in recent years. These short-term and regional variations are expected to become more extreme in a warming climate. There is considerable uncertainty about how hurricanes are changing because of the large natural variability and the incomplete observational record. The impact of climate change on hurricane frequency remains a subject of ongoing studies. While changes in hurricane frequency remain uncertain, basic physical understanding and model results suggest that the strongest hurricanes (when they occur) are likely to become more intense and possibly larger in a warmer, moister atmosphere over the oceans. This is supported by available observational evidence in the North Atlantic. Some conditions favourable for strong thunderstorms that spawn tornadoes are expected to increase with warming, but uncertainty exists in other factors that affect tornado formation, such as changes in the vertical and horizontal variations of winds. 13

16 Clim ate Cha nge n Q& A How fast is sea level rising? Long-term measurements of tide gauges and recent satellite data show that global sea level is rising, with best estimates of the global-average rise over the last two decades centred on 3.2 mm per year (0.12 inches per year). The overall observed rise since 1901 is about 20 cm (8 inches) [Figure 6]. This sea-level rise has been driven by (in order of importance): expansion of water volume as the ocean warms, melting of mountain glaciers in most regions of the world, and losses from the Greenland and Antarctic ice sheets. All of these result from a warming climate. Fluctuations in sea level also occur due to changes in the amounts of water stored on land. The amount of sea level change experienced at any given location also depends on a variety of other factors, including whether regional geological processes and rebound of the land weighted down by previous ice sheets are causing the land itself to rise or sink, and whether changes in winds and currents are piling ocean water against some coasts or moving water away. The effects of rising sea level are felt most acutely in the increased frequency and intensity of occasional storm surges. If CO 2 and other greenhouse gases continue to increase on their current trajectories, it is projected that sea level may rise by a further 0.5 to 1 m (1.5 to 3 feet) by 2100. But rising sea levels will not stop in 2100; sea levels will be much higher in the following centuries as the sea continues to take up heat and glaciers continue to retreat. It remains difficult to predict the details of how the Greenland and Antarctic Ice Sheets will respond to continued warming, but it is thought that Greenland and perhaps West Antarctica will continue to lose mass, whereas the colder parts of Antarctica could start to gain mass as they receive more snowfall from warmer air that contains more moisture. Sea level in the last interglacial (warm) period around 125,000 years ago peaked at probably 5 to 10 m above the present level. During this period, the polar regions were warmer than they are today. This suggests that, over millennia, long periods of increased warmth will lead to very significant loss of parts of the Greenland and Antarctic Ice Sheets and to consequent sea level rise. 14 Figure 6. Observations show that the global average sea level has risen by about 20 cm (8 inches) since the late 19th century. Sea level is rising faster in recent decades; measurements from tide gauges (blue) and satellites (red) indicate that the best estimate for the average sea level rise over the last two decades is centred on 3.2 mm per year (0.12 inches per year). The shaded area represents the sea level uncertainty, which has decreased as the number of gauge sites used in the global averages and the number of data points have increased. Source: Shum and Kuo (2011)

17Evidence & Causes Q& A n What is ocean acidification and why does it matter? Direct observations of ocean chemistry have shown that the chemical balance of seawater has shifted to a more acidic state (lower pH) [Figure 7]. Some marine organisms (such as corals and some shellfish) have shells composed of calcium carbonate which dissolves more readily in acid. As the acidity of sea water increases, it becomes more difficult for them to form or maintain their shells. CO 2 dissolves in water to form a weak acid, and the oceans have absorbed about a third of the CO 2 resulting from human activities, leading to a steady decrease in ocean pH levels. With increasing atmospheric CO 2 , the chemical balance will change even more during the next century. Laboratory and other experiments show that under high CO 2 and in more acidic waters, some marine species have misshapen shells and lower growth rates, although the effect varies among species. Acidification also alters the cycling of nutrients and many other elements and compounds in the ocean, and it is likely to shift the competitive advantage among species, with as-yet-to-be-determined impacts on marine ecosystems and the food web. 15 figure 7. As CO 2 in the air has increased, there has been an increase in the CO 2 content of the surface ocean (upper box), and a decrease in the seawater pH (lower box). Source: adapted from Dore et al. (2009) and Bates et al. (2012). 1990 1995 2000 2005 2010 pH 8.06 8.07 8.08 8.09 8.10 8.11 Surface Ocean pH Bermuda Hawaii Atmospheric CO2 concentration (ppm) Surface Ocean pCO2, Bermuda (μatm) Surface Ocean pCO2, Hawaii (μatm) 320 340 360 380 400 330 350 370 390 pC O 2 o r CO 2 co nc en tra tio n Year

18 Clim ate Cha nge n Q& A How confident are scientists that Earth will warm further over the coming century? Very confident. If emissions continue on their present trajectory, without either technologi- cal or regulatory abatement, then warming of 2.6 to 4.8 °C (4.7 to 8.6 °F) in addition to that which has already occurred would be expected by the end of the 21st century. Warming due to the addition of large amounts of greenhouse gases to the atmosphere can be understood in terms of very basic properties of greenhouse gases. It will in turn lead to many changes in natural climate pro- cesses, with a net effect of amplifying the warming. The size of the warming that will be experienced depends largely on the amount of greenhouse gases accumulating in the atmosphere and hence on the trajectory of emissions [Figure 8]. If the total cumulative emissions since 1870 are kept below about 1 trillion (million million) tonnes of carbon, then there is a two-thirds chance of keeping the rise in global average temperature since the pre-industrial period below 2 °C (3.6 oF). However, over half this amount has already been emitted. Based just on the established physics of the amount of heat CO 2 absorbs and emits, a doubling of atmospheric CO 2 concentration from pre-industrial levels (up to about 560 ppm) would by itself, without amplification by any other effects, cause a global average temperature increase of about 1 °C (1.8 °F). However, the total amount of warming from a given amount of emissions depends on chains of effects (feedbacks) that can individually either amplify or diminish the initial warming. The most important amplifying feedback is caused by water vapour, which is a potent greenhouse gas in the atmosphere as warmer air can hold more moisture. Also, as Arctic sea ice and glaciers melt, more sunlight is absorbed into the darker underlying land and ocean surfaces causing further warming and further melting of ice and snow. The biggest uncertain factor in our knowledge of feedbacks is in how the properties of clouds will change in response to climate change. Other feedbacks involve the carbon cycle. Currently the land and oceans together absorb about half of the CO 2 emitted from human activities, but the capacities of land and ocean to store additional carbon are expected to decrease with additional warming, leading to faster increases in atmospheric CO 2 and faster warming. Models vary in their projections of how much additional warming to expect, but all such models agree that the overall net effect of feedbacks is to amplify the CO 2 — only warming by a factor of 1.5 to 4.5. 16 figure 8. If emissions continue on their present trajectory, without either technological or regulatory abatement, then the best estimate is that global average temperature will warm a further 2.6 to 4.8 °C (4.7 to 8.6 °F) by the end of the century (right). The figure on left shows projected warming with very aggressive emissions reductions. The figures represent multi-model estimates of temperature averages for 2081-2100 compared to 1986–2005. Source: IPCC AR5

19Evidence & Causes Q& A n Are climate changes of a few degrees a cause for concern? Yes. Even though an increase of a few degrees in global average temperature does not sound like much, global average temperature during the last ice age was only about 4 to 5 °C (7 to 9 °F) colder than now. Global warming of just a few degrees will be associated with widespread changes in regional and local temperature and precipitation as well as with increases in some types of extreme weather events. These and other changes (such as sea level rise and storm surge) will have serious impacts on human societies and the natural world. Both theory and direct observations have confirmed that global warming is associated with greater warming over land than oceans, moistening of the atmosphere, shifts in regional precipitation patterns and increases in extreme weather events, ocean acidification, melting glaciers, and rising sea levels (which increases the risk of coastal inundation and storm surge). Already, record high temperatures are on average significantly outpacing record low temperatures, wet areas are becoming wetter as dry areas are becoming drier, heavy rainstorms have become heavier, and snowpacks (an important source of freshwater for many regions) are decreasing. These impacts are expected to increase with greater warming and will threaten food production, freshwater supplies, coastal infrastructure, and especially the welfare of the huge population currently living in low-lying areas. Even though certain regions may realise some local benefit from the warming, the long-term consequences overall will be disruptive. 17 What are scientists doing to address key uncertainties in our understanding of the climate system? Science is a continual process of observation, understanding, modelling, testing and prediction. The prediction of a long-term trend in global warming from increasing greenhouse gases is robust and has been confirmed by a growing body of evidence. Nevertheless, understanding (for example, of cloud dynamics, and of climate variations on centennial and decadal timescales and on regional-to-local spatial scales) remains incomplete. All of these are areas of active research. Comparisons of model predictions with observations identify what is well-understood and, at the same time, reveal uncertainties or gaps in our understanding. This helps to set priorities for new research. Vigilant monitoring of the entire climate system—the atmosphere, oceans, land, and ice—is therefore critical, as the climate system may be full of surprises. 18 continued

20 Clim ate Cha nge n Q& A Together, field and laboratory data and theoretical understanding are used to advance models of Earth’s climate system and to improve representation of key processes in them, especially those associated with clouds, aerosols, and transport of heat into the oceans. This is critical for accurately simulating climate change and associated changes in severe weather, especially at the regional and local scales important for policy decisions. Simulating how clouds will change with warming and in turn may themselves affect warming, remains one of the major challenges for global climate models, in part because many cloud processes occur on scales smaller than the current models can resolve. Greater computer power may enable some of these processes to be resolved in future-generation models. Dozens of groups and research institutions work on climate models, and scientists are now able to analyse results from essentially all of the world’s major Earth-System Models and compare them with each other and with observations. Such opportunities are of tremendous benefit in bringing out the strengths and weak- nesses of various models and diagnosing the causes of differences among models, so that research can focus on the relevant processes. The differences among models allow estimates to be made of the uncertainties in projections of future climate change, and in understanding which aspects of these projections are robust. Studying how climate responded to major changes in the past is another way of checking that we understand how different processes work and that models are capable of performing under a wide range of conditions. Why are computer models used to study climate change? The future evolution of Earth’s climate as it responds to the present rapid rate of increasing atmospheric CO 2 has no precise analogues in the past, nor can it be properly understood through laboratory experiments. As we are also unable to carry out deliberate controlled experiments on Earth itself, computer models are among the most important tools used to study Earth’s climate system. Climate models are based on mathematical equations that represent the best understanding of the basic laws of physics, chemistry, and biology that govern the behaviour of the atmosphere, ocean, land surface, ice, and other parts of the climate system, as well as the interactions among them. The most comprehensive climate models, Earth-System Models, are designed to simulate Earth’s climate system with as much detail as is permitted by our understanding and by available supercomputers. The capability of climate models has improved steadily since the 1960s. Using physics-based equations, the models can be tested and are successful in simulating a broad range of weather and climate variations, for example from individual storms, jet stream meanders, El Niño events, and the climate of the last century. Their projections of the most prominent features of the long-term human-induced climate change signal have remained robust, as generations of increasingly complex models yield richer details of the change. They are also used to perform experiments to isolate specific causes of climate change and to explore the consequences of different scenarios of future greenhouse gas emissions and other influences on climate.

21Evidence & Causes Q& A n Are disaster scenarios about tipping points like ‘turning off the Gulf Stream’ and release of methane from the Arctic a cause for concern? Results from the best available climate models do not predict abrupt changes in such systems (often referred to as tipping points) in the near future. However, as warming increases, the possibilities of major abrupt change cannot be ruled out. The composition of the atmosphere is changing towards conditions that have not been experienced for millions of years, so we are headed for unknown territory, and uncertainty is large. The climate system involves many competing processes that could switch the climate into a different state once a threshold has been exceeded. A well-known example is the south-north ocean overturning circulation, which is maintained by cold salty water sinking in the North Atlantic and which involves the transport of extra heat to the North Atlantic via the Gulf Stream. During the last ice age, pulses of freshwater from the ice sheet over North America led to slowing down of this overturning circulation and to widespread changes in climate around the Northern Hemisphere. Freshening of the North Atlantic from the melting of the Greenland ice sheet is however, much less intense and hence is not expected to cause abrupt changes. As another example, Arctic warming could destabilise methane (a greenhouse gas) trapped in ocean sediments and permafrost, potentially leading to a rapid release of a large amount of methane. If such a rapid release occurred, then major, fast climate changes would ensue. Such high-risk changes are considered unlikely in this century, but are by definition hard to predict. Scientists are therefore continuing to study the possibility of such tipping points beyond which we risk large and abrupt changes. 19

22 Clim ate Cha nge n Q& A If emissions of greenhouse gases were stopped, would the climate return to the conditions of 200 years ago? No. Even if emissions of greenhouse gases were to suddenly stop, Earth’s surface temperature would not cool and return to the level in the pre-industrial era for thousands of years. If emissions of CO 2 stopped altogether, it would take many thousands of years for atmospheric CO 2 to return to ‘pre-industrial’ levels due to its very slow transfer to the deep ocean and ultimate burial in ocean sediments. Surface temperatures would stay elevated for at least a thousand years, implying extremely long-term commitment to a warmer planet due to past and current emissions, and sea level would likely continue to rise for many centuries even after temperature stopped increasing [Figure 9]. Significant cooling would be required to reverse melting of glaciers and the Greenland ice sheet, which formed during past cold climates. The current CO 2 -induced warming of Earth is therefore essentially irreversible on human timescales. The amount and rate of further warming will depend almost entirely on how much more CO 2 humankind emits. 20 figure 9. If global emissions were to suddenly stop, it would take a long time for surface air temperatures and the ocean to begin to cool, because the excess CO 2 in the atmosphere would remain there for a long time and would continue to exert a warming effect. Model projections show how atmospheric CO 2 concentration (a), surface air temperature (b), and ocean thermal expansion (c) would respond following a scenario of business-as-usual emissions ceasing in 2300 (red), a scenario of aggressive emission reductions, falling close to zero 50 years from now (orange), and two intermediate emissions scenarios (green and blue). The small downward tick in temperature at 2300 is caused by the elimination of emissions of short-lived greenhouse gases, including methane. Source: Zickfeld et al., 2013

Evidence & Causes Q& A n B1 Greenhouse gases affect Earth’s energy balance and climate The Sun serves as the primary energy source for Earth’s climate. Some of the incoming sunlight is reflected directly back into space, especially by bright surfaces such as ice and clouds, and the rest is absorbed by the surface and the atmosphere. Much of this absorbed solar energy is re-emitted as heat (longwave or infrared radiation). The atmosphere in turn absorbs and re-radiates heat, some of which escapes to space. Any disturbance to this balance of incoming and outgoing energy will affect the climate. For example, small changes in the output of energy from the Sun will affect this balance directly. If all heat energy emitted from the surface passed through the atmosphere directly into space, Earth’s average surface temperature would be tens of degrees colder than today. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, act to make the surface much warmer than this, because they absorb and emit heat energy in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm [Figure B1]. Without this greenhouse effect, life as we know it could not have evolved on our planet. Adding more greenhouse gases to the atmosphere makes it even more effective at preventing heat from escaping into space. When the energy leaving is less than the energy entering, Earth warms until a new balance is established. figure b1. Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, absorb heat energy and emit it in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm. Adding more greenhouse gases to the atmosphere enhances the effect, making Earth’s surface and lower atmosphere even warmer. Image based on a figure from US EPA. THE GREENHOUSE EFFECT Some solar radiation is reflected by Earth and the atmosphere Some of the infrared radiation passes through the atmosphere. Some is absorbed by greenhouse gases and re-emitted in all directions by the atmosphere. The effect of this is to warm Earth’s surface and the lower atmosphere. Infrared radiation is emitted by Earth’s surface Some radiation is absorbed by Earth’s surface and warms it Earth‘s Surface Atmosphere The Basics of Climate Change

B2 Clim ate Cha nge basics of clim ate change Greenhouse gases emitted by human activities alter Earth’s energy balance and thus its climate. Humans also affect climate by changing the nature of the land surfaces (for example by clearing forests for farming) and through the emission of pollutants that affect the amount and type of particles in the atmosphere. Scientists have determined that, when all human and natural factors are considered, Earth’s climate balance has been altered towards warming, with the biggest contributor being increases in CO 2 . Human activities have added greenhouse gases to the atmosphere The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased significantly since the Industrial Revolution began. In the case of carbon dioxide, the average concentration measured at the Mauna Loa Observatory in Hawaii has risen from 316 parts per million (ppm)1 in 1959 (the first full year of data available) to 396 ppm in 2013 [Figure B2]. The same rates of increase have since been recorded at numerous other stations worldwide. Since pre-industrial times, the atmospheric concentration of CO 2 has increased by 40%, methane has increased by about 150%, and nitrous oxide has increased by roughly 20%. More than half of the increase in CO 2 has occurred since 1970. Increases in all three gases contribute to warming of Earth, with the increase in CO 2 playing the largest role. See page B3 to learn about the sources of human emitted greenhouse gases. Scientists have examined greenhouse gases in the context of the past. Analysis of air trapped inside ice that has been accumulating over time in Antarctica shows that the CO 2 1 that is, for every million molecules in the air, 316 of them were CO 2 Figure B2. Measurements of atmospheric CO 2 since 1958 from the Mauna Loa Observatory in Hawaii (black) and from the South Pole (red) show a steady annual increase in atmospheric CO 2 concentration. (The measurements are made at remote places like those because they are not greatly influenced by local processes, so therefore are representative of the background atmosphere.) The small up and down saw-tooth pattern reflects seasonal changes in the release and uptake of CO 2 by plants. Source: Scripps CO 2 Program Year CO 2 / p pm

B3Evidence & Causes basics of clim ate change concentration began to increase significantly in the 19th century [Figure B3], after staying in the range of 260 to 280 ppm for the previous 10,000 years. Ice core records extending back 800,000 years show that during that time, CO 2 concentrations remained within the range of 170 to 300 ppm throughout many ‘ice age’ cycles — see page B4 to learn about the ice ages — and no concentration above 300 ppm is seen in ice core records until the past 200 years. Figure B3. CO 2 variations during the past 1,000 years, obtained from analysis of air trapped in an ice core extracted from Antarctica (red squares), show a sharp rise in atmospheric CO 2 starting in the late 19th century. Modern atmospheric measurements from Mauna Loa are superimposed in blue. Source: figure by Eric Wolff, data from Etheridge et al., 1996; MacFarling Meure et al., 2006. 250 300 350 400 1000 1200 1400 1600 1800 2000 Atmospheric measurements (ML) Law Dome Year CO 2 / p pm Learn about the sources of human-emitted greenhouse gases ■■ Carbon dioxide (CO 2 ) has both natural and human sources, but CO 2 levels are increasing primarily because of the combustion of fossil fuels, cement production, defor- estation (which reduces the CO 2 taken up by trees and increases the CO 2 released by decomposition of the detritus), and other land use changes. Increases in CO 2 are the single largest contributor to global warming. ■■ Methane (CH 4 ) has both human and natural sources, and levels have risen significantly since pre-industrial times due to human activities such as raising livestock, growing paddy rice, filling landfills, and using natural gas (which is mostly CH 4 , some of which may be released when it is extracted, transported, and used). ■■ Nitrous oxide (N 2 O) concentra- tions have risen primarily because of agricultural activities such as the use of nitrogen-based fertilisers and land use changes. ■■ Halocarbons, including chloro- fluorocarbons (CFCs), are chem- icals used as refrigerants and fire retardants. In addition to being potent greenhouse gases, CFCs also damage the ozone layer. The production of most CFCs has now been banned, so their impact is starting to decline. However, many CFC replacements are also potent greenhouse gases and their concen- trations and the concentrations of other halocarbons continue to increase.

B4 Clim ate Cha nge basics of clim ate change Measurements of the forms (isotopes) of carbon in the modern atmosphere show a clear fingerprint of the addition of ‘old’ carbon (depleted in natural radioactive 14C) coming from the combustion of fossil fuels (as opposed to ’fewer’ carbon coming from living systems). In addition, it is known that human activities (excluding land-use changes) currently emit an estimated 10 billion tonnes of carbon each year, mostly by burning fossil fuels, which is more than enough to explain the observed increase in concentration. These and other lines of evidence point conclusively to the fact that the elevated CO 2 concentration in our atmosphere is the result of human activities. Climate records show a warming trend Estimating global average surface air temperature increase requires careful analysis of millions of measurements from around the world, including from land stations, ships, and satellites. Despite the many complications of synthesising such data, multiple independent teams have concluded separately and unanimously that global average surface air temperature has risen by about 0.8 °C (1.4 °F) since 1900 [Figure B4]. Although the record shows several pauses and accelerations in the increasing trend, each of the last three decades has been warmer than any other decade in the instrumental record since 1850. Going further back in time before accurate thermometers were widely available, temperatures can be reconstructed using climate-sensitive indicators (‘proxies’) Learn about the ice ages Detailed analyses of ocean sediments, ice cores, and other data show that for at least the last 2.6 million years, Earth has gone through extended periods when temperatures were much lower than today and thick blankets of ice covered large areas of the Northern Hemisphere. These long cold spells, lasting in the most recent cycles for around 100,000 years, were interrupted by shorter warm ‘interglacial’ periods, including the past 10,000 years. Through a combination of theory, observations, and modelling, scientists have deduced that the ice ages* are triggered by recurring variations in Earth’s orbit that primarily alter the regional and seasonal distribution of solar energy reaching Earth. These relatively small changes in solar energy are reinforced over thousands of years by gradual changes in Earth’s ice cover (the cryosphere), especially over the Northern Hemisphere, and in atmospheric composition, eventually leading to large changes in global temperature. The average global temperature change during an ice-age cycle is estimated as 5 °C ± 1 °C (9 °F ± 2 °F). *Note that in geological terms Earth has been in an ice age ever since the Antarctic Ice Sheet last formed about 36 million years ago. However, in this document we have used the term in its more colloquial usage indicating the regular occurrence of extensive ice sheets over North America and northern Eurasia.

B5Evidence & Causes basics of clim ate change in materials such as tree rings, ice cores, and marine sediments. Comparisons of the thermometer record with these proxy measurements suggest that the time since the early 1980s has been the warmest 30-year period in at least eight centuries, and that global temperature is rising towards peak temperatures last seen 5,000 to 10,000 years ago in the warmest part of our current interglacial period. Many other impacts associated with the warming trend have become evident in recent years. Arctic summer sea ice cover has shrunk dramatically. The heat content of the ocean has increased. Global average sea level has risen by approximately 20 cm (8 inches) since 1901, due both to the expansion of warmer ocean water and to the addition of melt waters from glaciers and ice sheets on land. Warming and precipitation changes are altering the geographical ranges of many plant and animal species and the timing of their life cycles. In addition to the effects on climate, some of the excess CO 2 in the atmosphere is being taken up by the ocean, changing its chemical composition (causing ocean acidification). Figure B4. Earth’s global average surface temperature has risen as shown in this plot of combined land and ocean measurements from 1850 to 2012, derived from three independent analyses of the available data sets. The top panel shows annual average values from the three analyses, and the bottom panel shows decadal average values, including the uncertainty range (grey bars) for the black (HadCRUT4) dataset. The temperature changes are relative to the global average surface temperature, averaged from 1961−1990. Source: IPCC AR5, data from the HadCRUT4 dataset (black), UK Met Office Hadley Centre, the NCDC MLOST dataset (orange), US National Oceanic and Atmospheric Administration, and the NASA GISS dataset (blue), US National Aeronautics and Space Administration.

B6 Clim ate Cha nge basics of clim ate change Many complex processes shape our climate Based just on the physics of the amount of energy that CO 2 absorbs and emits, a doubling of atmospheric CO 2 concentration from pre-industrial levels (up to about 560 ppm) would, by itself, cause a global average temperature increase of about 1 °C (1.8 °F). In the overall climate system, however, things are more complex; warming leads to further effects (feedbacks) that either amplify or diminish the initial warming. The most important feedbacks involve various forms of water. A warmer atmosphere generally contains more water vapour. Water vapour is a potent greenhouse gas, thus causing more warming; its short lifetime in the atmosphere keeps its increase largely in step with warming. Thus, water vapour is treated as an amplifier, and not a driver, of climate change. Higher temperatures in the polar regions melt sea ice and reduce seasonal snow cover, exposing a darker ocean and land surface that can absorb more heat, causing further warming. Another important but uncertain feedback concerns changes in clouds. Warming and increases in water vapour together may cause cloud cover to increase or decrease which can either amplify or dampen temperature change depending on the changes in the horizontal extent, altitude, and properties of clouds. The latest assessment of the science indicates that the overall net global effect of cloud changes is likely to be to amplify warming. The ocean moderates climate change. The ocean is a huge heat reservoir, but it is difficult to heat its full depth because warm water tends to stay near the surface. The rate at which heat is transferred to the deep ocean is therefore slow; it varies from year to year and from decade to decade, and helps to determine the pace of warming at the surface. Observations of the sub-surface ocean are limited prior to about 1970, but since then, warming of the upper 700 m (2,300 feet) is readily apparent. There is also evidence of deeper warming. Surface temperatures and rainfall in most regions vary greatly from the global average because of geographical location, in particular latitude and continental position. Both the average values of temperature, rainfall, and their extremes (which generally have the largest impacts on natural systems and human infrastructure), are also strongly affected by local patterns of winds. Estimating the effects of feedback processes, the pace of the warming, and regional climate change requires the use of mathematical models of the atmosphere, ocean, land, and ice (the cryosphere) built upon established laws of physics and the latest understanding of the physical, chemical and biological processes affecting climate, and run on powerful computers. Models vary in their projections of how much additional warming to expect (depending on the type of model and on assumptions used in simulating certain climate processes, particularly cloud formation and ocean mixing), but all such models agree that the overall net effect of feedbacks is to amplify warming.

B7Evidence & Causes basics of clim ate change Human activities are changing the climate Rigorous analysis of all data and lines of evidence shows that most of the observed global warming over the past 50 years or so cannot be explained by natural causes and instead requires a significant role for the influence of human activities. In order to discern the human influence on climate, scientists must consider many natural variations that affect temperature, precipitation, and other aspects of climate from local to global scale, on timescales from days to decades and longer. One natural variation is the El Niño Southern Oscillation (ENSO), an irregular alternation between warming and cooling (lasting about two to seven years) in the equatorial Pacific Ocean that causes significant year-to-year regional and global shifts in temperature and rainfall patterns. Volcanic eruptions also alter climate, in part increasing the amount of small (aerosol) particles in the stratosphere that reflect or absorb sunlight, leading to a short-term surface cooling lasting typically about two to three years. Over hundreds of thousands of years, slow, recurring variations in Earth’s orbit around the Sun, which alter the distribution of solar energy received by Earth, have been enough to trigger the ice age cycles of the past 800,000 years. Fingerprinting is a powerful way of studying the causes of climate change. Different influences on climate lead to different patterns seen in climate records. This becomes obvious when scientists probe beyond changes in the average temperature of the planet and look more closely at geographical and temporal patterns of climate change. For example, an increase in the Sun’s energy output will lead to a very different pattern of temperature change (across Earth’s surface and vertically in the atmosphere) compared to that induced by an increase in CO 2 concentration. Observed atmospheric temperature changes show a fingerprint much Learn more about other human causes of climate change In addition to emitting greenhouse gases, human activities have also altered Earth’s energy balance through, for example: ■■ Changes in land use. Changes in the way people use land — for example, for forests, farms, or cities — can lead to both warming and cooling effects locally by changing the reflectivity of Earth’s surfaces (affecting how much sunlight is sent back into space) and by changing how wet a region is. ■■ Emissions of pollutants (other than greenhouse gases). Some industrial and agricultural processes emit pollutants that produce aerosols (small droplets or particles suspended in the atmosphere). Most aerosols cool Earth by reflecting sunlight back to space. Some aerosols also affect the formation of clouds, which can have a warming or cooling effect depending on their type and location. Black carbon particles (or ‘soot’) produced when fossil fuels or vegetation are burned, generally have a warming effect because they absorb incoming solar radiation.

B8 Clim ate Cha nge basics of clim ate change closer to that of a long-term CO 2 increase than to that of a fluctuating Sun alone. Scientists routinely test whether purely natural changes in the Sun, volcanic activity, or internal climate variability could plausibly explain the patterns of change they have observed in many different aspects of the climate system. These analyses have shown that the observed climate changes of the past several decades cannot be explained just by natural factors. How will climate change in the future? Scientists have made major advances in the observations, theory, and modelling of Earth’s climate system; and these advances have enabled them to project future climate change with increasing confidence. Nevertheless, several major issues make it impossible to give precise estimates of how global or regional temperature trends will evolve decade by decade into the future. Firstly, we cannot predict how much CO 2 human activities will emit, as this depends on factors such as how the global economy develops and how society’s production and consumption of energy changes in the coming decades. Secondly, with current understanding of the complexities of how climate feedbacks operate, there is a range of possible outcomes, even for a particular scenario of CO 2 emissions. Finally, over timescales of a decade or so, natural variability can modulate the effects of an underlying trend in temperature. Taken together, all model projections indicate that Earth will continue to warm considerably more over the next few decades to centuries. If there were no technological or policy changes to reduce emission trends from their current trajectory, then further warming of 2.6 to 4.8 °C (4.7 to 8.6 °F) in addition to that which has already occurred would be expected during the 21st century [Figure B5]. Projecting what those ranges will mean for the climate experienced at any particular location is a challenging scientific problem, but estimates are continuing to improve as regional and local-scale models advance. Figure B5. The amount and rate of warming expected for the 21st century depends on the total amount of greenhouse gases that humankind emits. Models project the temperature increase for a business-as-usual emissions scenario (in red) and aggressive emission reductions, falling close to zero 50 years from now (in blue). Black is the modelled estimate of past warming. Each solid line represents the average of different model runs using the same emissions scenario, and the shaded areas provide a measure of the spread (one standard deviation) between the temperature changes projected by the different models. All data are relative to a reference period (set to zero) of 1986-2005. Source: IPCC AR5 6.0 4.0 2.0 −2.0 0.0 (o C ) Global average surface temperature change 1950 2000 2050 2100 Historical Aggressive emissions reductions “Business as usual” emissions

B9Evidence & Causes This document explains that there are well-understood physical mechanisms by which changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and are still increasing rapidly, that climate change is occurring, and that most of the recent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further climate change is inevitable; if emissions of greenhouse gases continue unabated, future changes will substantially exceed those that have occurred so far. There remains a range of estimates of the magnitude and regional expression of future change, but increases in the extremes of climate that can adversely affect natural ecosystems and human activities and infrastructure are expected. Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limit emissions of greenhouse gases and hence the magnitude of climate changes; they can wait for changes to occur and accept the losses, damage and suffering that arise; they can adapt to actual and expected changes as much as possible; or they can seek as yet unproven ‘geoengineering’ solutions to counteract some of the climate changes that would otherwise occur. Each of these options has risks, attractions and costs, and what is actually done may be a mixture of these different options. Different nations and communities will vary in their vulnerability and their capacity to adapt. There is an important debate to be had about choices among these options, to decide what is best for each group or nation, and most importantly for the global population as a whole. The options have to be discussed at a global scale, because in many cases those communities that are most vulnerable control few of the emissions, either past or future. Our description of the science of climate change, with both its facts and its uncertainties, is offered as a basis to inform that policy debate. Conclusion

B10 Clim ate Cha nge n ack nowledgements Acknowledgements The following individuals served as the primary writing team for this document: ■■ Eric Wolff FRS (UK lead), University of Cambridge ■■ Inez Fung (NAS, US lead), University of California, Berkeley ■■ Brian Hoskins FRS, Imperial College London and University of Reading ■■ John Mitchell FRS, UK Met Office ■■ Tim Palmer FRS, University of Oxford ■■ Benjamin Santer (NAS), Lawrence Livermore National Laboratory ■■ John Shepherd FRS, University of Southampton ■■ Keith Shine FRS, University of Reading ■■ Susan Solomon (NAS), Massachusetts Institute of Technology ■■ Kevin Trenberth, National Center for Atmospheric Research ■■ John Walsh, University of Alaska, Fairbanks ■■ Don Wuebbles, University of Illinois This document was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the Royal Society and the National Academy of Sciences. The reviewers provided comments and suggestions, but were not asked to endorse the views of the writing team, nor did they see the final draft before its release. We wish to thank the following individuals for their review of this report: ■■ Richard Alley (NAS), Department of Geosciences, Pennsylvania State University ■■ Alec Broers FRS, Diamond Light Source and Bio Nano Consulting (Former President of the Royal Academy of Engineering ) ■■ Harry Elderfield FRS, Department of Earth Sciences, University of Cambridge ■■ Joanna Haigh FRS, Imperial College London ■■ Isaac Held (NAS), NOAA Geophysical Fluid Dynamics Laboratory ■■ John Kutzbach (NAS), Center for Climatic Research, University of Wisconsin ■■ Jerry Meehl, National Center for Atmospheric Research ■■ John Pendry FRS, Imperial College London ■■ John Pyle FRS, Department of Chemistry, University of Cambridge ■■ Gavin Schmidt, NASA Goddard Space Flight Center ■■ Emily Shuckburgh, British Antarctic Survey ■■ Gabrielle Walker, Journalist ■■ Andrew Watson FRS, University of Exeter Oversight of the review process was provided by representatives of the Royal Society Council (John Pethica FRS) and the NAS Council (Jeremiah Ostriker, NAS). Staff assistance was provided by Nancy F. Huddleston, Laurie Geller, Sally Tyldesley and Tracey Elliott. Sincere thanks to the Raymond and Beverly Sackler US-UK Scientific Forum for support of this activity.

3Evidence & Causes For more detailed discussion of the topics addressed in this document (including references to the underlying original research), see: ■■ IPCC 2013, Climate change 2013: The physical science basis. Fifth Assessment Report (AR5) Working Group 1. visit site ■■ NRC 2010, America’s climate choices: Advancing the Science of Climate Change. visit site ■■ NRC 2011, Climate stabilization targets: Emissions, Concentrations, and Impacts over Decades to Millennia. visit site ■■ NRC 2013, Abrupt impacts of climate change: Anticipating Surprises. visit site ■■ Royal Society 2010, Climate Change: A Summary of the Science. visit site Much of the original data underlying the scientific findings discussed here are available at: ■■ metoffice.gov.uk/hadobs ■■ www.cru.uea.ac.uk ■■ cdp.ucar.edu ■■ climatedataguide.ucar.edu ■■ iridl.ldeo.columbia.edu ■■ cdiac.ornl.gov ■■ ncdc.noaa.gov ■■ esrl.noaa.gov/gmd/ccgg/trends ■■ scrippsco2.ucsd.edu ■■ hahana.soest.hawaii.edu/hot THE NATIONAL ACADEMY OF SCIENCES (NAS) was established to advise the United States on scientific and technical issues when President Lincoln signed a Congressional charter in 1863. The National Research Council, the operating arm of the National Academy of Sciences and the National Academy of Engineering, has issued numerous reports on the causes of and potential responses to climate change. Climate change resources from the National Research Council are available at nas-sites.org/americasclimatechoices. THE ROYAL SOCIETY is a self-governing Fellowship of many of the world’s most distinguished scientists. Its members are drawn from all areas of science, engineering, and medicine. It is the national academy of science in the UK. The Society’s fundamental purpose, reflected in its founding Charters of the 1660s, is to recognise, promote, and support excellence in science, and to encourage the development and use of science for the benefit of humanity. More information on the Society’s climate change work is available at royalsociety.org/policy/climate-change For further reading

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Climate Change: Evidence and Causes is a jointly produced publication of The US National Academy of Sciences and The Royal Society. Written by a UK-US team of leading climate scientists and reviewed by climate scientists and others, the publication is intended as a brief, readable reference document for decision makers, policy makers, educators, and other individuals seeking authoritative information on the some of the questions that continue to be asked.

Climate Change makes clear what is well-established and where understanding is still developing. It echoes and builds upon the long history of climate-related work from both national academies, as well as on the newest climate-change assessment from the United Nations' Intergovernmental Panel on Climate Change. It touches on current areas of active debate and ongoing research, such as the link between ocean heat content and the rate of warming.

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