Global Warming

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Encyclopedia of Nonlinear Science

Few modern scientific concerns have achieved such notoriety as the possibility of relatively rapid anthropogenic global warming through increased CO₂ emissions. This complex problem became a matter of considerable public attention during the 1980s, and during the 1990s, the first attempt was made at international management of the challenge (the Kyoto Protocol under the United Nations Framework Convention on Climatic Change). However, scientific awareness of CO₂-induced climatic change is not new, and the underlying physical processes were understood from the beginning of studies concerning the absorption of radiation by the atmosphere. Later research resulted in a deeper understanding of the dynamics of the biospheric carbon cycle, and global atmospheric circulation models have been adopted, and adapted, for assessing the future course of tropospheric CO₂ levels. In spite of all of these advances, much remains unclear and uncertain.

Several years before his death in 1830, the French mathematician Joseph Fourier concluded that the atmosphere acts like the glass of a greenhouse, letting light through and retaining the invisible rays emanating from the ground (Fourier, 1822). In modern scientific terms, the atmosphere is highly (though not perfectly) transparent to incoming (shortwave) solar radiation, but it is a strong absorber of certain wavelengths in the outgoing (longwave) infrared spectrum produced by the reradiation of absorbed sunlight.

John Tyndall was the first scientist to study this process in detail by measuring the absorptive properties of air and its key constituent molecules (water vapor and about a dozen different compounds). He used a sensitive galvanometer to measure the electric current passing through gases irradiated by heat. In 1861, Tyndall concluded that water vapor accounts for most of the atmospheric absorption and hence “every variation of this constituent must produce a change in climate. Similar remarks would apply to the carbonic acid diffused through the air…” (Tyndall, 1861). The next major contribution to the field came just before the end of the 19th century when Svanté Arrhenius offered the first calculations of the global surface temperature rise resulting from naturally changing atmospheric CO₂.

Arrhenius’s conclusions contained all of the key qualitative modern results. He found that geometric increases of CO₂ will produce a nearly arithmetic rise in surface temperatures, that the warming will be smallest near the equator and highest in polar regions, that the Southern hemisphere will be less affected, and that the warming will reduce temperature differences between night and day (Arrhenius, 1896). His quantitative results also resembled those of today’s best global climatic models: he predicted that the increase in average annual temperature will be about 50°C in the tropics and just over 6°C in the Arctic. All of these findings applied to natural fluctuations of atmospheric CO₂: Arrhenius concluded (correctly) that future anthropogenic carbon emissions would be largely absorbed by the ocean and (incorrectly, as he grossly underestimated future fossil fuel combustion) that the accumulation would amount to only about 3 ppm in half a century.

The link between CO₂ and climate change was resurrected in 1938 by George Callendar who calculated a more realistic temperature rise with doubling of CO₂ concentrations (1.5°C rise) and documented a slight global warming trend of 0.25°C for the preceding half a century (Callendar, 1938). In his later writings, he also recognized the importance of carbon emissions from land-use changes. In 1956, Gilbert Plass performed the first computerized calculation of the radiation flux in the main infrared region of CO₂ absorption (Plass, 1956). His results (average surface temperature rise of 3.6°C with the doubled atmospheric CO₂) were published a year before Roger Revelle and Hans Suess summarized the problem with continuing large-scale fossil fuel combustion in such a way that the key sentence has become a citation classic:

An almost instant response to this concern was the setting up of the first two permanent stations for the measurement of background CO₂ concentrations, at Mauna Loa in Hawai’i and at the South Pole. Accumulating measurements began showing a steady rise of atmospheric CO₂ at these two remote locations, but, once again, attention to the problem of potential global warming eased during the 1960s and began to grow only in the aftermath of OPEC’s two sudden oil price hikes during the 1970s.

By the late 1960s, improvements in computer capabilities made it possible to run the first three-dimensional models of global atmospheric circulation and use them to simulate the effects of higher CO₂ levels. Most of these simulations looked at possible effects arising from the doubling of preindustrial CO₂, that is, after reaching levels around 600 ppm. Initial simulations indicated a 2.93°C rise with the doubling of the CO₂ level to 600 ppm (Manabe & Wetherald, 1967). Increases in computing power (subject to Moore’s famous law) and better understanding of interactions between the atmosphere, oceans, and the terrestrial biosphere has led to increasingly more realistic models of global climate. Another important refinement was the inexplicably delayed consideration of other greenhouse gases (above all, of CH₄, N₂O, and chlorofluorocarbons) whose combined radiative forcing is now slightly higher than that of carbon dioxide (about 1.5 and 1.4 W m⁻²).

By the late 1990s, the best models coupled the atmosphere’s physical behavior with changes on land and in the ocean, and with simulations of some key features of carbon and sulfur cycles and of atmospheric chemistry (Houghton et al., 2001). At the same time, even our best numerical models still represent the atmosphere with a relatively coarse grid and are incapable of reproducing the intricacies and multiple feedbacks that determine the course and the rate of climate change.

One of the most important sources of potential error in the climate models is the treatment of clouds. The best general circulation models represent fairly well some essential gross features of global atmospheric physics but their iterative calculations are done at such widely spaced points of three-dimensional grids that it is impossible to treat cloudiness in a realistic manner. And yet clouds are key determinants of the planetary radiation balance because they have, on balance, a pronounced net cooling effect. Because clouds account for about half of the Earth’s albedo (the fraction of incident radiation that is reflected), even relatively small changes in their properties could have an appreciable effect on the course of global warming.

Other unresolved matters include the response of terrestrial biota (Will carbon sequestration take place mostly in short- or long-lived tissues or in soil?), marine algae (especially their role in forming clouds), sudden releases of methane (rising temperatures may lead to catastrophic emissions from methane hydrates), and effects of orbital and solar influences (particularly a very high correlation between the solar cycles shorter than the 11-year mean and higher average land temperature of the Northern Hemisphere, and a link between mid-atmospheric temperature and changing intensity of radiation over the sunspot cycle). If the global forecasts are uncertain, regional predictions are particularly questionable. The most complex coupled models now provide reasonably reliable simulations of climate down to the sub-continental level but their results still have unacceptably large variations on regional scales.

Indirect or proxy markers (such as isotopic and trace chemical analysis on tree rings, ice, or sediment cores) make it clear that a substantial decline of atmospheric CO₂ preceded the most extensive and longest lasting (some 70 million years, or Ma) glaciation of the entire Phanerozoic era that began about 330 Ma ago. Approximate reconstruction of CO₂ levels for the past 300 Ma—since the formation of the Pangea whose eventual break-up led to the current distribution of oceans and land masses—indicates, first, a pronounced rise (about five times the current level during the Triassic period), followed by a steep decline (Berner, 1998; Figure 1, top). Boron-isotope ratios of planktonic foraminifer shells point to CO₂ levels above 2000 ppm 60–50 Ma ago (with peaks above 4000 ppm), followed by an erratic decline to less than 1000 ppm by 40 Ma ago, and relatively stable and low (below 500 ppm) concentrations ever since the early Miocene 24 Ma ago (Pearson & Palmer, 2000; Figure 1, bottom).

Reliable record of atmospheric CO₂ is available only for the past 420,000 years thanks to the analyses of air bubbles from ice cores retrieved from Antarctica and Greenland. Preindustrial CO₂ levels never dipped below 180 ppm and never rose above 300 ppm (Raynaud et al., 1993; Petit et al., 1999; Figure 2) and their oscillations are highly positively correlated with changing temperatures. But these correlations are not a proof of a clear cause-and-effect relationship as there are no obvious lead-lag sequences. Other recent paleoclimatic studies actually found signs of decoupling of atmospheric CO₂ and global climate during the Phanerozoic eon and particularly during the early to middle Miocene, when a warm period coexisted with low CO₂ levels (Veizer et al., 2000; Pagani et al., 1999). These findings confirm the complexity of climate change where cause and effect are difficult to assign: atmospheric CO₂ may have

been a primary climate driver but the evidence is not conclusive (Kump, 2002). The most likely pacemaker during the Pleistocene period was small changes in the Earth’s orbit around the Sun; massive methane releases from gas hydrates and volcanic activity must be also considered.

During the time between the rise of the first high civilizations (5000–6000 years ago) and the beginning of the fossil fuel era, atmospheric CO₂ levels had fluctuated within an even narrower range of 250–290 ppm. Subsequent anthropogenic emissions pushed atmospheric concentrations of CO₂ to a high of 370 ppm by the year 2000. Paleoclimatic studies of the Northern Hemisphere during the last millennium show

warming periods during the 12th and 18th centuries and pronounced cooling during the 15th century (the Little Ice Age). A demonstrable cooling trend between the late 18th and the early 20th centuries was followed by an unprecedented rate of warming that has brought the average planetary temperature to levels higher than at any time during the past 1000 years (Figure 3).

The most extensive studies of the existing global record of surface temperatures have detected long-term planetary warming of, respectively, 0.5°C and 0.78°C since the middle of the 19th century (Jones et al., 1986; Hansen & Lebedeff, 1988). Changes in measurement techniques (different thermometers), station locations (from downtowns to suburbs) and station environment (increasing urban heat island effect), and until very recently, highly inadequate coverage of large areas of the Southern Hemisphere complicate the interpretation of this shift, which has distinct spatial patterns with areas of more pronounced warming and regions of slight cooling. However, the most recent (post-1976) spell of warming has been almost global and the 1990s were the warmest decade since the beginning of the

instrumental record in the 1850s and the warmest ten years of the millennium in the Northern Hemisphere.

According to the general circulation models, the warming should have been more pronounced. The best explanation of the discrepancy between the models and the actual temperature record is that the warming was partially counteracted by sulfate aerosols. The combined direct and indirect effect of all greenhouse gases resulted in a total anthropogenic forcing of about 2.8 W m² by the late 1990s (Hansen et al., 2000; Figure 4). This is equal to a little more than 1% of solar radiation reaching the ground.

If the atmospheric warming was primarily the function of radiative forcing, then the level of greenhouse gas emissions would be the key variable. Recent emission scenarios for CO₂ alone offer a very large range of concentrations, 540–970 ppm, by the year 2100. CH4 levels may range even wider, from just above 1500 ppb to more than 3600 ppb. The aggregate radiative forcing may thus be anywhere between 4 and 9 W m⁻² and the climate sensitivity would then range between 1.5°C and 4°C with 2.2−3°C considered to be the most likely by the latest IPCC assessment. Broad consensus from the latest generation of models foresees that this climatic change would cool the stratosphere while raising the tropospheric temperatures in a distinct spatial pattern, with the warming more pronounced on the land (and during nights) and with increases of about two to three times the global mean in higher latitudes in winter than in the tropics, and greater in the Arctic than in the Antarctic.

There are many effective ways to slow down the greenhouse gas emissions and reduce their environmental impact. Most significantly, the affluent countries could largely retain their quality of life while reducing their energy and material consumption by at least a third. While the means are available, the will to act, nationally and internationally, is mostly absent. Global warming is a complex natural process but its anthropogenic enhancement calls for a fundamentally moral solution that runs against some basic human propensities: consume less and do so more efficiently.

VACLAV SMIL

See also Atmospheric and ocean sciences; Forecasting; General circulation models of the atmosphere

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