Global Warming: a Science Overview

MacCracken 1 Global Warming: A Science Overview By Michael C. MacCracken ossil fuels (i.e., coal, oil, and natural Human Activities are Changing gas) provide about 85% of the world’s Atmospheric Composition by energy, sustaining the world’s Increasing the Concentrations of standard-of-living and providing much Radiatively Active (Greenhouse) of the power for transportation, generation of Gases and Particles electricity, home heating, and food production. Compared to other sources of energy, fossil Observations from global measurement stations fuels are relatively inexpensive, transportable, and reconstructions of the composition of the safe, and abundant. At the same time, their use atmosphere in the past clearly indicate that contributes to environmental problems such as human activities are increasing the atmospheric air pollution and acid rain, which are being concentrations of carbon dioxide (CO2), addressed through various control efforts, and to methane (CH4), nitrous oxide (N2O), and of long-term climate change, which governments various halocarbons (HCFCs and, until very have begun to address through adoption of the recently, CFCs). These gases are collectively UN Framework Convention on Climate Change referred to as greenhouse gases because of their negotiated in 1992. warming influence on the climate. Drawing primarily from international While these gases occur naturally, records assessment reports (see references for reports by going back many thousands of years indicate the Intergovernmental Panel on Climate Change that the present concentrations are well above (IPCC)), this paper summarizes six key elements natural levels. The history of emissions versus of the science of climate change (often referred concentrations, analyses of carbon isotopes, and to simply as “global warming” although the other scientific results all confirm that these projected changes involve changes in many changes are occurring as a result of human variables in addition to a rise in global average activities rather than because of natural temperature). These results are presented as processes. For example, the CO2 concentration context for considering the challenges of both is currently about 370 parts per million by limiting long-term warming and adapting to the volume (ppmv), which is about 30% above its warming that will occur as a result of past use of preindustrial value of about 280 ppmv (see fossil fuels and the inevitable future use over Figure 1b). This increase has been due primarily coming decades. to the combustion of fossil fuels and secondarily to the release of carbon occurring in the clearing The Potential Impacts of Climate Change on Transportation 2 Global Warming: A Science Overview Figure 1. Carbon emissions, CO2 concentrations, and temperature change over the past millennium. (a) Over the past 1000 years, but especially since 1850, the emissions of carbon dioxide (expressed in terms of carbon emitted) have grown from near zero to over 7 GtC/year (billions of tonnes of carbon per year); of this total, approximately 6 GtC/yr are from combustion of coal, oil, and natural gas and approximately 1 GtC/yr are from net changes in land use. (b) As a result of the CO2 emissions, its concentration has increased from about 280 ppmv to about 370 ppmv over the past 1000 years, especially since 1850. (c) Reconstruction of the global-scale record of temperature departures from the 1961-90 average (primarily from proxy records from the Northern Hemisphere) suggests a relatively slow and steady cooling of about 0.2ºC (0.4ºF) that extended over most of the last 1000 years; beginning in the late 19th century and continuing through the 20th century, an unusually rapid warming of about 0.6ºC (1.0ºF) has taken place (from NAST, 2000, which contains primary references). The Potential Impacts of Climate Change on Transportation MacCracken 3 of forested land and the plowing of soils for acidification of precipitation known popularly agriculture (see Figure 1a). The CH4 as “acid rain”). Because of their relatively short concentration is up over 150%. Its increase is lifetime in the atmosphere, emissions must be due primarily to emissions from rice agriculture, quite substantial for global concentrations to ruminant livestock, biomass burning, landfills, build up to the level that will have a long-term and fossil fuel development, transmission, and climatic influence that is as great as for the combustion. The concentrations of many greenhouse gases with their longer atmospheric halocarbons are entirely new to the lifetimes. However, concentrations can become atmosphere—many of these compounds are quite high in particular regions, and the solely a result of human activities. The pollution effects that result can cause regional persistence (or lifetimes) of the excess disturbances of the climate. For example, contributions of these gases in the atmosphere aerosols lofted in southern Asia are suspected to range from decades (for CH4) to centuries (for be contributing to the diminishment of the CO2 and some halocarbons) to thousands of monsoon. years (for some perfluorocarbons). Thus, with ongoing emissions, the excesses of their Although natural processes can also affect concentrations above natural levels are likely to the atmospheric concentrations of gases and persist for many centuries. aerosols, observations indicate that this has not been an important cause of changes over the Human activities are also contributing to an past 10,000 years. Thus, it is well-established increase in the atmospheric concentrations of that human activities are the major cause of the small particles (called aerosols), primarily as a dramatic changes in atmospheric composition result of emissions of sulfur dioxide (SO2), soot, since the start of the Industrial Revolution about and some various organic compounds. The 200 years ago. emissions of these human-induced aerosols result primarily from use of fossil fuels Increasing the Concentrations of (primarily from coal combustion and diesel and Greenhouse Gases will Warm the 2-stroke engines) and from biomass burning. Planet and Change the Climate Once in the atmosphere, these compounds can be transformed or combined in various ways. From laboratory experiments, from study of the For example, SO2 is transformed into sulfate atmospheres of Mars and Venus, from aerosols that create the whitish haze common observations and study of energy fluxes in the over and downwind of many industrialized atmosphere and from space, and from areas. This haze tends to exert a cooling reconstructions of past climatic changes and influence on the climate by reflecting away solar their likely causes, it is very clear that the radiation. Soot aerosols can combine with atmospheric concentrations and distributions of organics and form mixed aerosols that can exert radiatively active gases play a very important warming or cooling influences. Changes in land role in determining the surface temperature of cover, especially where this leads to the Earth and other planets. Figure 2 provides a desertification, can also lead to increased lofting schematic diagram of the energy fluxes that of particles into the atmosphere. Dust lofted in determine the Earth’s temperature (and climate). this way generally has a cooling influence on the climate while also decreasing visibility; dust Of the solar radiation reaching the top of the can also be carried to intercontinental scales as a atmosphere, about 30% is reflected back to result of long-distance transport. space by the atmosphere (primarily by clouds) and the surface; about 20% is absorbed in the Of critical importance is that the typical atmosphere (primarily by water vapor, clouds, lifetime of aerosols in the atmosphere is less than 10 days (for example, sulfate and nitrate compounds are often rained out, causing the The Potential Impacts of Climate Change on Transportation 4 Global Warming: A Science Overview Figure 2. Schematic diagram of the Earth’s greenhouse effect, with arrows proportional in size to the fluxes of energy by the particular process (NAST, 2000). Of incoming solar radiation, about 30% is reflected back into space by clouds and the surface, about 20% is absorbed in the atmosphere, and about 50% is absorbed at the surface. Most of the infrared (heat) radiation emitted by the surface is absorbed in the atmosphere and the atmosphere in turn then emits about 90% of this amount back to the surface, adding to its heat gain from the Sun. The extra energy at the surface is used to evaporate water or heat the near surface atmosphere. In the atmosphere, the extra energy it receives from the Sun, from absorbed infrared radiation, from latent heating released during precipitation, and from sensible heating, is emitted to space to balance the net solar radiation absorbed by the surface and atmosphere. and aerosols), and about 50% is absorbed at the However, the Earth’s atmosphere is not surface. For a system to come to a steady state transparent to infrared radiation, being able to temperature, the energy absorbed must be recycle some of it in a way that creates a balanced by radiation that is emitted away as warming influence. This warming effect occurs infrared (or heat) radiation. Were the Earth’s because much of the infrared radiation emitted atmosphere transparent and its surface a simple by the surface and by the greenhouse gases and radiator of energy to space, the Earth’s average low clouds in the atmosphere is absorbed by surface temperature would equilibrate at close to various radiatively active gases in the 0oF (-18ºC), given the current reflectivity of the atmosphere. For example, less than 10% of the Earth-atmosphere system. Such a temperature infrared radiation emitted by the surface gets would be much too cold to sustain life as we through directly to space without being know it. absorbed. A significant fraction of the absorbed energy is radiated back to the surface by the atmosphere’s greenhouse gases and clouds, The Potential Impacts of Climate Change on Transportation MacCracken 5 providing additional energy to warm the surface.

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