Understanding the Causes of Global Climate Change

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1 FACT SHEET I: Attribution Environment Understanding the Causes of Global Climate Change Average air temperatures at the Earth s surface have increased by approximately 0.6 o C (1 o F) over the 20 th century. Analysis of the various human and natural influences on the global climate indicates that this warming cannot be explained without taking into account human emissions of greenhouse gases (GHGs). Furthermore, current analyses indicate that GHGs have been the dominant force driving temperature increases over the past 50 years. Introduction Recent decades have seen record-high average global surface air temperatures. The years 1998, 2002, and 2003 were the three warmest years recorded in the instrumental record (which dates back to the mid-1800s). In fact, all of the top 10 warmest years on record have occurred since These record warm years are the result of a century of global warming. Between 1900 and 2000, global surface air temperatures increased by o C ( o F). 1 Over the past 30 years, temperatures in the lower atmosphere, or troposphere, have warmed by o C ( o F) per decade. 2,3 This warming has resulted from numerous factors that influence climate. Some of these factors are natural, such as changes in solar radiation and volcanic activity. Others, particularly emissions of greenhouse gases (GHGs) and land-use changes, are human in origin. Determining their relative contributions to observed climate change is a difficult task. Nevertheless, the current scientific consensus is that, at least over the past 50 years, the major factor driving observed temperature increases has been human emissions of GHGs. 1 How have scientists arrived at this conclusion? Climate varies considerably over annual to millennial time-scales, and a change in global temperatures alone does not indicate human influence. The attribution of climate change to specific factors requires an analysis of the longterm temperature increases and the factors that could be responsible for those increases. Climate Over the Past Millennium Determining climate patterns over many centuries is difficult because of the absence of instrumental temperature records before the mid-1800s. A number of attempts at reconstructing 1

2 a temperature record of the northern hemisphere over the past 1000 years have used proxy indicators of temperature, such as tree rings, coral reefs, and ice cores. Despite significant uncertainties, the studies yield consistent results, indicating that the Northern Hemisphere was in a relative warm phase between 1000 and 1400 AD, followed by a prolonged cool phase until the 1800s. These studies also indicate that the last 50 years were probably warmer than any time period during the previous 1000 years (Figure 1), and recent data suggests that recent temperatures may be unprecedented over the past 2000 years. 5 The Factors Influencing Climate Climate varies considerably over time. This variability is a response to climate forcings factors that cause warming or cooling of the atmosphere. Over most of the Earth s history, such forcings have been exclusively natural, and include variability in solar radiation, the Earth s orbit, and the frequency or intensity of volcanic activity. However, since the industrial revolution, human activities have had an increasing influence on the global climate system. Greenhouse Gases. Collectively, GHGs have enhanced the greenhouse effect, contributing to global warming over the past century. Carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and tropospheric ozone, have all increased well above their pre-industrial concentrations. Various industrial GHGs that were previously absent from the atmosphere have also accumulated. Land Use Changes. Land-use changes have influenced the reflectivity of the Earth s surface, resulting in either a warming or a cooling depending on the change. Deforestation and agricultural activities have a net cooling effect because cleared land surfaces reflect more solar radiation than rough or forested lands. Increased urbanization causes localized warming at the Earth s surface, known as the Urban Heat Island (UHI) effect. Despite suggestions that UHIs have contributed significantly to recent warming, analysis of 20 th century temperature observations indicates that they have had a relatively small effect on global warming trends. On a global scale, the net effects of land-use change are believed to have caused a net cooling of global temperatures since the industrial revolution. Aerosols. Atmospheric particles, or aerosols, from industry and fossil fuel combustion also can cool or heat the Earth s surface and atmosphere. Sulfate aerosols reflect sunlight and contribute to cloud formation, both of which have a cooling effect. Black carbon aerosols (soot) warm the atmosphere by absorbing solar radiation, and have probably contributed significantly to observed global warming. 7 They also shade the Earth s surface, which has a minor cooling 2

3 effect. Although the exact magnitude of warming caused by black carbon aerosols is uncertain, some scientists have suggested that reductions in black carbon emissions as well as CO 2 may help slow the rate of global warming over the near-term. 8 Cosmic Rays. In recent years, the influence of cosmic rays on the formation of clouds has been considered as a potential factor in long-term climate change. 9 However, trends in cosmic rays and clouds over the 20 th century suggest that cosmic rays are not a major factor influencing the climate change that is currently being observed. 10 Collectively, some of these factors have had an important influence on the global climate over the past 50 years. 1 As indicated in Figure 2, the dominant force has been GHGs, which have had a net warming effect. Natural changes in solar radiation have contributed to warming as well, but these are minor relative to the effects of GHGs. Other factors appear to have had a cooling effect, but not large enough to offset warming caused by GHGs. Climate Change Over the 20 th Century There is uniform consensus that average surface air temperatures over the 20 th century increased between , decreased slightly between , and increased again from 1970 to the present, based on the instrumental temperature record. 1 The first step in determining the causes of climate change is to compare this pattern of warming with what is known about the various factors that have influenced climate over the 20 th century Although the burning of fossil fuels was commonplace as early as the 1700s, CO 2 concentrations were only about 295 parts per million (ppm) by 1900, just 7% higher than preindustrial levels of 270 ppm. The warming associated with this CO 2 increase is considered to be too small to account for the observed warming. However, natural factors, particularly increases in solar radiation and a decline in volcanic activity, are well correlated with temperature over this time period, indicating that the warming was predominantly due to these two factors The cooling observed between has been the cause of much speculation, because it is inconsistent with the larger picture of global warming over the 20 th century. By 1970, concentrations of CO 2 were approximately 330 ppm, 20% above preindustrial levels, yet cooling occurred. Again, natural factors appear to have played an important role. Solar radiation decreased between , partly explaining the observed cooling. In addition, rapid increases in human emissions of sulfate aerosols probably played a significant role by shading the Earth s surface from solar radiation. 3

4 1970-Present. Since 1970, atmospheric CO 2 has increased further to ~370 ppm, 30% above pre-industrial levels. By 2000, other GHGs, such as methane and nitrous oxide were 100% and 15%, respectively, above their pre-industrial levels. The warming caused by this substantial increase in GHGs has exceeded the cooling associated with sulfate aerosols. Sulfur emissions were declining in the United States and Europe by the late 20 th century due to air pollution regulations, although global emissions continue to increase. Solar radiation has been relatively stable over the late 20 th century, except for the 11-year cycle of sunspot activity, which is not sufficient to account for the observed warming. 1 Supporting Evidence from Climate Models Analysis of the factors that can force climate changes and corresponding temperature changes over the 20 th century provide strong evidence of a human influence on climate. Furthermore, statistical analyses detect human influences on climate that are distinct from natural variability. When only natural factors or human factors are included in climate models, results differ significantly from observations during certain time periods. Thus, climate models perform best at reproducing 20 th century climate observations when they include both human and natural factors. Furthermore, despite the importance of both natural and human factors in driving climate, human factors account for the majority of the observed increase in globally averaged surface air temperatures over the past 50 years. 1 Thus, recent trends in surface air temperatures and atmospheric temperatures are the result of a significant human influence, specifically increases in atmospheric GHG concentrations Conclusions The current state of knowledge regarding 20 th century temperature changes is clear. Temperatures of recent decades, at least in the northern hemisphere, are likely warmer than at any point during at least the previous millennium, and the probability that an unknown factor other than GHGs can account for this warming is low. Thus, despite the long-term natural variability of the climate system, current scientific evidence indicates there is a significant human influence on current climate trends. This human contribution to global warming is projected to grow increasingly strong in future decades as human emissions of long-lived GHGs continue to alter the composition of the atmosphere. 4

5 Figure 1. 4 Reconstructions of northern hemisphere average surface air temperatures over the past two millennia. Despite variability among the studies, they all show a period of relatively warm temperatures between AD, preceded and followed by cooler periods (the latter being the Little Ice Age). All of the reconstructions indicate that observed temperatures at the end of the 20th century are higher than at any earlier time. 5

6 Figure 2. 1 Changes in the global mean radiative forcing of the climate from The various factors presented in the figure are the result of human emissions of greenhouse gases, the release of aerosols from fossil fuels and volcanic activity, human land-use change, and solar radiation. Positive forcings have a warming influence on climate while negative forcings have a cooling influence. Although the radiative effect of a number of these factors remains uncertain, current estimates indicate GHGs are the dominant influence on the global climate. Figure 3. 6 Modeling of historical temperature changes using natural, anthropogenic, or all major climate forcings. The bold black line represents observed temperature changes and colored lines represent temperature 6

7 reconstructions from various climate models. The All Forcings results include an estimate of future warming (assuming a business-as-usual emissions scenario). The black boxes represent time periods where the agreement between model results and observations are particularly poor due to the failure to consider both natural and anthropogenic factors simultaneously. Conclusions The current state of knowledge regarding 20 th century temperature changes is clear. Temperatures, at least in the northern hemisphere, of recent decades are likely warmer than at any point during at least the previous millennium, and the probability that some as yet unidentified factor other than GHGs can account for this warming is low. Thus, despite the longterm natural variability of the climate system, current scientific evidence indicates there is a significant human influence on current climate trends. This human influence is projected to grow increasingly strong in future decades as human emissions of GHGs continue to alter the composition of the atmosphere. References 1 IPCC Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Houghton, J.T., et al. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY. 2 Christy, J.R., et al Error estimates of Version 5.0 of MSU/AMSU bulk atmospheric temperatures. Journal of Atmosphere and Ocean Technology 20: Santer, B.D., et al Influence of satellite data uncertainties on the detection of externally forced climate change. Science 300: Mann, M.E., et al On past temperatures and anomalous 20 th century warming. Eos 84: Mann,M.E., and Jones, P.D Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 2003GL Stott, P.A., et al External control of 20th century temperature by natural and anthropogenic forcings. Science 290: Jacobson, M.Z Strong radiative heading due to the mixing state of black carbon in atmospheric aerosols. Nature 409: Hansen, J., et al Global warming in the twenty-first century: an alternative scenario. Proceedings of the National Academy of Sciences 97: Shaviv, N. J., and Veizer, J Celestial driver of Phanerozoic climate? Geological Society of America Today 10 Carslaw, K.S., et al Cosmic rays, clouds, and climate. Science 298: Kaufmann, R.K., and Stern, D.I Cointegration analysis of hemispheric temperature relations. Journal of Geophysical Research 107:2000JD Gegerl, G.C., et al Detection of volcanic, solar, and greenhouse gas signals in paleo-reconstructions of Northern Hemispheric temperature. Geophysical Research Letters 30: GL Levitus, S. et al Anthropogenic warming of Earth s climate system. Science 292: Stott, P.A Attribution of regional-scale temperature changes to anthropogenic and natural causes. Geophysical Research Letters 30: 2003GL