ENVIRONMENT
Climate Observations Substantiate Global Warming Models • Rising atmospheric carbon dioxide levels and receding alpine glaciers support projected global temperature increase Bette Hileman, C&EN Washington any people believe that a great deal of controversy surrounds the science of global warming. In reality, however, scientists in the field do agree on many aspects of global warming. For example, on the basis of a variety of evidence a consensus is emerging among researchers that human beings, primarily through their burning of fossil fuels, are already perturbing Earth's climate—defined as weather averaged across years and large regions. This consensus, and the evidence that supports it, are documented in the United Nations Intergovernmental Panel on Climate Change's (IPCC) latest report on the science of global warming. The 2,000-plus page report, written by about 500 scientists and reviewed by about 500 other experts, will be released next month. IPCC was established in 1988 under the auspices of the UN Convention on Climate Change to review the science of global warming and to advise some 70 countries on ways to mitigate and prevent it. IPCC issues a major report every five years. The report will be available at the Office of the U.S. Global Change Research Program, Washington, D.C. Climate experts agree that the average global air temperature has risen 0.3 to 0.6 °C over the past century. This finding is substantiated by other indicators—accelerated melting of alpine glaciers, a sea-level rise of 10 to 25 cm over the past 100 years, and coral bleaching caused by anomalously high sea-surface temperatures—that are all consistent with the increase in global
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air temperatures. And according to present indications, the average global temperature in 1995 is likely to be as high as or higher than in any year since record keeping began around 1860. These experts identify a number of other changes that have occurred in global and U.S. climate, some or all of which can be attributed to global warming. Nighttime temperatures have generally increased more than daytime temperatures. Climatic variability, or the frequency of extreme events, has increased in some regions, although it is not known whether these have risen on a worldwide scale. For example, heavy rainfall events in the U.S. have increased in intensity, and behavior of El Nino—the warming of the eastern equatorial Pacific that sometimes brings severe droughts to some regions and heavy rains to other regions around the world—has been unusual since 1976. These occurrences fit well with complex mathematical models of global climate change.
Almost all specialists agree that without drastic steps to curb greenhouse gas emissions, the average global temperature will increase 1 to 3.5 °C during the next century because effective levels of carbon dioxide are expected to double sometime between 2050 and 2100. This temperature range results from varied economic and population projections as well as climate sensitivity to greenhouse gases. Even with a change of 1 °C, the global rate of warming would be greater than it has been at any time in the past 10,000 years. Only a few experts expect the atmosphere to warm less than 1 °C by 2100, and virtually no scientist who has studied the issue expects global temperatures to decline during the next century. Moreover, the warming is predicted to continue, reaching much more elevated temperatures over the next several centuries, unless bold measures are taken to reduce greenhouse gas emissions.
Global mean temperature has been on the rise since 1880
Note: Zero is the 1951-80 average temperature. Source: NASA Goddard Institute for Space Studies
Even a 1 °C change would be significant. During the so-called Little Ice Age, a period lasting from 1500 to 1850 that was marked by extensive glacial advances in almost all alpine regions, the global temperature was only about 0.5 °C lower than it was in 1900. Temperatures before the advent of a global thermometer network are determined from a combination of ice core, ocean sediment, tree ring, fossil, and other geologic data. Ice cores drilled in Greenland and Antarctica and on some alpine glaciers give the most useful data. The annually deposited layers in the ice cores contain bubbles of air trapped at the time the ice was formed, providing a mechanism to determine atmospheric carbon dioxide concentrations. The layers in the ice cores also contain a record of the ocean's temperature in the form of the ratio of isotopically "heavy" water (enriched in 18 0) to "light" water in the ice. When the world's temperature is higher, more heavy water is evaporated preferentially. From the ocean temperature estimate, the air temperature for huge regions can be estimated. There is also a general consensus that higher temperatures projected for the next century will cause more frequent and intense heat waves, widescale ecological disruptions, a decline of agricultural production in the tropics and subtropics, and continued acceleration of sea-level rise. One broad area of agreement, as noted in the IPCC report, is that levels of greenhouse gases in the atmosphere, primarily carbon dioxide, methane, nitrous oxide, ozone, and halocarbons, have grown significantly since preindustrial times. During this period, the C0 2 level has risen 30% to nearly 360 ppm, methane 145% to more than 1,700 ppb, and nitrous oxide 15% to more than 300 ppb. Implementation of the Montreal Protocol on Substances That Deplete the Ozone Layer has led to a slowdown in the growth of the atmospheric level of some halocarbons. The level of chlorofluorocarbon-11 has plateaued and that of methyl chloroform has declined. Ozone has increased in the troposphere of the Northern Hemisphere but has been somewhat depleted in the stratosphere, primarily in midlatitude and polar regions. The rates of buildup of C0 2 and methane slowed in the early 1990s, but now both are increasing again. There is also broad agreement that
Key greenhouse gases have lifetimes of more than a decade C0 2
CH 4
N20
CFC-12 (CCI2F2)
HCFC-22 (CHCIF2)
CF4
-280 ppm
700 ppb
275 ppb
0
0
0
Concentration in 1994a
358 ppm
1,721 ppb
311 ppbb
503 pptb
110 ppt
70 pptb
Atmospheric lifetime (years)
50-200 c
12.2±3
120
102
12.1 ±2.4
50,000
Preindustrial concentration8
a Concentrations are measured by volume, b Concentration in 1992. Value for 1994 is not available, c No single lifetime for carbon dioxide can be defined because of the different rates of uptake by different sinks. Source: United Nations Intergovernmental Panel on Climate Change
stratospheric ozone depletion and aerosols—small particulates consisting of mainly sulfates from the burning of fossil fuels—act to cool Earth. However, aerosols are different than greenhouse gases because they have very short lifetimes (mostly less than two weeks) in the atmosphere, and rather than causing uniform global cooling, they affect primarily the subcontinental pattern of climate change. Because of their short life-
times, they cannot build up in the atmosphere and therefore, in the long run, cannot offset much of the warming from greenhouse gases. The loss of ozone in the lower stratosphere has a net cooling effect because ozone absorbs incoming solar radiation. However, these cooling effects would be more than offset by the expected doubling (over its preindustrial level) in the next century in the level of C0 2 in the
Atmospheric carbon dioxide has increased 30% since 1800
a Fossil fuel burning C0 2 emissions. Source: United Nations Intergovernmental Panel on Climate Change
Carbon dioxide concentrations over the past 1,000 years determined from ice core records (shown as symbols) appear to have fluctuated little until 1850. Since 1958, air measurements (shown as purple line) taken at Mauna Loa, Hawaii, have supplemented the ice core data. The smooth black curve is based on a 100year running mean. The inset of the period from 1850 onward shows COz emissions in gigatons (billions of metric tons) per year attributed to burning fossil fuels (shown as a blue line).
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Higher temperatures in Antarctica have led to disintegration of some ice shelves One of the important unknowns in global warming science is how the Antarctic ice sheet, which covers a land area as large as the continental U.S. and Mexico combined, would respond to a much warmer world. This ice cap, with an average thickness of 2,500 meters, would
raise the sea level by 74 meters if it were to melt completely. Antarctica drew notice this year when National Oceanic & Atmospheric Administration satellite images captured part of the pack ice disintegrating on the Antarctic Peninsula (which ex-
Antarctic Peninsula James Ross Island Prince Gustav Channel
Larsen Ice Shelf
Jan. 14, 1995
Antarctic Peninsula James Ross Island Prince Gustav Channel
Larsen Ice Shelf
Giant iceberg
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tends outward toward the southern tip of South America). Between Jan. 14 and Feb. 27, an ice shelf that formerly blocked the Prince Gustav Qiannel between James Ross Island and the peninsula broke up. During the same period, the northern section of the Larsen Ice Shelf also disintegrated, leaving a plume of debris extending 200 km east. In addition, a Rhode Island-sized chunk of ice broke off the Larsen Ice Shelf farther south. This iceberg, although large, was a fairly small component of what has happened overall recently in Antarctica, says David G. Vaughan, glaciologist at the British Antarctic Survey, Cambridge, England. Other ice shelves around the Antarctic Peninsula are also in retreat Over the whole of Antarctica, temperatures have increased an average of about 1 °C in 50 years, Vaughan says. But on the Antarctic Peninsula, temperatures have risen an average 2.5 °C since record keeping began there in the 1950s. This change could be a local manifestation of global warming, Vaughan says, or "it's possible it has nothing to do with global warming." In any event, these warmer temperatures have probably caused the disintegration of the ice shelves around the peninsula, he explains. Scientists would also like to know whether the entire Antarctic ice sheet melted during the Pliocene Epoch, 3 million to 4 million years ago, when global temperatures were higher than they are today. Determining the degree of melting that occurred then would give some indication about how much of the Antarctic ice sheet would melt if global warming proceeds unabated, because without curbs on greenhouse gas emissions global warming is expected to eventually produce a Pliocene-like climate. However, scientists don't even know whether the Antarctic ice sheet is getting thicker or thinner overall, Vaughan says. "We're completely in the dark. We really have no idea what goes into the ice sheet or what comes out" It is not known how much snow falls, what remains, what sublimes, what is blown away, or how much ice is lost through calving of icebergs from the ice shelves. It is known that a lot of ice is lost from the undersides of the ice shelves (which extend over seawater), but that is not being well measured yet, nor are the smaller icebergs leaving the ice shelves being counted in a comprehensive way. However, satellite altimetry data now being collected should indicate whether the ice sheet is getting thicker or thinner, Vaughan says.
model is able to simulate the lag in greenhouse warming caused by deepocean circulation. Mahlman says that despite the criticisms leveled at the models, they work well in many respects. "They capture a lot of the fundamental physics of how the atmosphere works, how the daily temperature range works, how the seasonal variations from summer to winter A boulder at the western margin of the Quelccaya Ice Cap in the tropical Andes of Peru in 1978 work, how ice-age climates work, and (left) and in 1995. The extensive retreat of the ice cap in that area has been caused by regional how natural variability works," he says. warming. If the current warming trend persists, the ice cap could be in danger of being lost. The models simulate the large-scale features of the current climate reasonatmosphere. Because many of the greenThe atmospheric models were devel- ably well, and they have accurately rehouse gases remain in the atmosphere oped from those used for weather fore- produced some climate features of the for a long time (C0 2 and nitrous oxide casting. "It's a fairly well-developed sci- distant past. They also predicted very persist from decades to centuries) their ence," says Jerry D. Mahlman, director well the amount of cooling that resulted radiative forcing—their tendency to of the National Oceanic & Atmospheric from the Mount Pinatubo eruption in warm Earth—persists for periods that are Administration's (NOAA) Geophysical mid-1991. The sulfates and other aerolong compared with human life spans. Fluid Dynamics Laboratory in Prince- sols from Pinatubo, a volcano in the Even if nations decide to stabilize the glo- ton, N.J. "The models use the same Philippines, cooled the climate noticebal C0 2 level at a relatively high 1,000 mathematics that drive today's numer- ably in 1992 and 1993 almost exactly as ppm, C0 2 emissions eventually would ical weather and hurricane prediction predicted in the model run by James E. Hansen, director of the National Aerohave to be cut below 1990 levels because models." C0 2 's atmospheric lifetime is so long. Conceptually, the atmospheric mod- nautics & Space Administration's GodThe basic theory behind the greenhouse els divide Earth's surface into rectan- dard Institute for Space Studies, New effect is well established. Natural green- gles or grid points roughly 500 km on York City. When the models account for the house gases in the atmosphere, primarily a side, within which cells are stacked water vapor and carbon dioxide, raise about 20 layers deep. The flow of atmo- cooling from aerosols, they also reproEarth's average temperature about 33 °C spheric gases from each of these cells duce fairly accurately the global warming that has taken place over the past higher than it would be if these gases into adjacent cells is calculated. were not present. These gases allow solar Equations governing the transfer of century. Even more important, Hansen radiant energy to pass through the atmo- electromagnetic radiation through a het- says, when aerosols are included the sphere to be absorbed at Earth's surface, erogeneous gaseous medium then relate geographic pattern of temperature but trap in the lower atmosphere much incoming solar radiation with changes in changes more closely matches the of the radiant heat emitted from the sur- the content of each cell and the amount changes in the real world. These tests of face back toward space. Since the green- of radiation reaching Earth's surface. the models have given climatologists house effect is a permanent part of the The exchange of radiative energy and more confidence in their results. climate system, warming from higher water between the atmosphere and variThe models all project that climate than natural levels of greenhouse gases ous surfaces—such as snow, ice, oceans, change, once begun, will continue for should be called an "enhanced green- clouds, and vegetation—is included, as hundreds of years. Taken together, they house effect." is a simple representation of the upper indicate that temperatures will rise 1 to The current scientific debate sur- ocean. Supercomputers are used to solve 3.5 °C by 2100, with an associated searounding global warming focuses on the equations that estimate wind pat- level rise of 15 to 95 cm. The current how sensitive Earth's climate is to an en- terns, temperature, sunlight, relative hu- temperature projections for 2100 are hanced greenhouse effect and whether midity, and precipitation for each grid somewhat lower than those in the 1990 IPCC report, primarily because the the resulting feedbacks will strengthen point on the globe. or diminish the warming from elevated Over the past five years or so, several models now account for the cooling eflevels of greenhouse gases. atmospheric general circulation models fect of sulfates and other aerosols. The To determine how enhanced levels of have been coupled to an ocean general models indicate that warming in genergreenhouse gases affect Earth's climate, circulation model. The ocean model rep- al will be greater over land than over modeling researchers employ three- resents various layers of the ocean to its the oceans and that the maximum dimensional numerical models that calcu- full depth and circulation among and warming will occur in high northern latlate global temperature and precipitation between those layers. When cold high- itudes in winter. But in the high-latitude at specified levels of C 0 2 and other density saline water sinks to the ocean Southern Ocean (Antarctic Ocean) and greenhouse gases in the atmosphere. The depths in polar regions, the ocean acts to the northern North Atlantic, there will families of the more complex models are slow down the rate of warming in the be little warming, perhaps even a coolatmospheric general circulation models atmosphere by storing heat contained in ing due to the effect of changes in ocean and atmospheric general circulation mod- salty surface water that moves from the circulation. els coupled to ocean general circulation tropics to sink in polar regions. ConseFurthermore, most models project that models of comparable complexity. quently, a coupled ocean-atmosphere with global warming, the increase in NOVEMBER 27,1995 C&EN
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ENVIRONMENT mean surface temperatures will be more pronounced during the cold season; that precipitation at mid to high latitudes will increase, especially during the cold season; that droughts will be more severe and longer lasting, particularly during the warm season; that nighttime temperatures will increase more than daytime temperatures during the warm season; that a greater portion of warm season precipitation will come in heavy showers or thunderstorms rather than in gentler, longer lasting rainfalls; and that the day-to-day variability of temperatures will decline for mid to high latitudes. These are changes that are now being observed, if not globally, at least in many regions. But after many decades, Hansen says, "models show that daytime warming will be almost as great as nighttime warming/' Thomas R. Karl, senior scientist at NOAA's National Climatic Data Center in Asheville, N.C., has analyzed weather data for the U.S. (excluding Alaska and Hawaii) over this century to see if the observed changes are consistent with what models predict for global greenhouse warming. Karl created what he calls a greenhouse climate response index (GCRI), which is a measure of how well climate data fit what models indicate for global warming. GCRI is the arithmetic average of four indicators: the percent of the U.S. with much above normal minimum temperatures, the percent of the U.S. with much above normal precipitation during the cold season, the percent of the U.S. in extreme drought in the warm season, and the percent of the U.S. with a much greater than normal proportion of precipitation derived from extreme one-day events. Karl found that since 1976, GCRI values have been higher than the average GCRI for previous years in the century, meaning that "the late-century changes in the U.S. climate are consistent with the general trends anticipated from a greenhouse-enhanced atmosphere." "There is only a 5 to 10% chance that the increase in GCRI results from natural variability," he says, which means there is a 90 to 95% chance that the U.S. climate is already being affected by enhanced levels of greenhouse gases. Although the country's contiguous states cover only 2% of the globe, the changes in this region are similar to what has happened in many other countries in the Northern Hemisphere, Karl 22
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says. Scientists are now calculating precise GCRI indexes for these countries. One of the more important uncertainties in climate modeling is how the cloud system reacts in response to increases in the levels of greenhouse gases. In general, high clouds act as a greenhouse and warm the climate, while low clouds, by reflecting sunlight back to space, tend to cool the system. Overall, clouds averaged together globally now have a net cooling effect of -15 watts per square meter, says Jeffrey T. Kiehl, senior scientist at the National Center for Atmospheric Research, Boulder, Colo. With enhanced greenhouse gases, clouds could change in such a way that they cool Earth more than they do today—in other words, greenhouse warming could result in more low-level clouds—or they could change so they cool Earth less. Another drawback of the current climate models is their inability to project climate for small regional areas. This is partly because they provide average readings for large areas of Earth's surface. Finer resolution would require much more computer power. But when it becomes possible to obtain finer resolution, it will become necessary to incorporate into the models much more information about land cover, land use changes, and vegetation. "When you get to smaller scales, the uncertainties in land surface processes and their physics and biology start to come back to haunt you," Mahlman says. In the long run, to gain more certainty in global climate predictions, much more needs to be known about how global warming will affect the biosphere, especially terrestrial carbon storage. In general, warming is expected to increase carbon storage in areas where moisture and nutrients are sufficient. In those areas, trees will grow faster. But over decades to centuries, climate change will also alter the global distribution of ecosystems. For example, forests would spread into tundra and could increase the warming and the carbon storage there. But trees could die rapidly in other areas that are subject to drought, and their deaths could introduce a large pulse of carbon into the atmosphere. There has been a lot of speculation recently about whether more frequent hurricanes and more intense and longer lasting El Ninos are related to global warming. "Until our models become a little more certain, it's difficult to conjecture whether hurricanes would increase
Mahlman: quantitative predictions difficult
Hansen: models closely match real world
or decrease with global warming," Karl says. "On a theoretical basis, there has been some work suggesting stronger hurricanes," he adds. A warmer sea surface is the primary feature of global warming that might cause more significant hurricanes, he explains, but ocean circulation changes may counter the effects of this added warmth. Since 1976, El Ninos have been stronger, more frequent, and more persistent than they were earlier in the century. "Some of the models suggest that stronger, more frequent El Ninos would be a tendency in a warmer world," Karl says. But there is no consensus in the scientific community about how these would change. Scientists are doing a lot of research to see if they can establish a cause-effect relationship between global warming and the change in the pattern of El Ninos.
Γη the western Caribbean off the coast den jumps as regional temperatures of Belize, the Cook Islands, and in the shifted 5 to 10 °C over less than two de Philippines, massive coral bleaching has cades. Measurements of oxygen isotopes been observed since 1983. Coral reef and dust in Greenland ice cores and bleaching results from the expulsion of studies of glaciers in the Chilean Andes symbiotic zooxanthellae algae from the and in New Zealand's Alps show that coral reefs. The algae provide reefs with some of these sudden temperature shifts most of their color, carbon, and ability to were felt globally. deposit limestone. Wallace S. Broecker, a geochemist at Raymond L. Hayes, professor of anat Lamont-Doherty Geological Observato omy at Howard University, Washing ry at Columbia University, believes the ton, D.C., has investigated coral bleach sudden shifts were caused by changes in ing and found that it occurs when the the ocean circulation, primarily in the ocean temperature exceeds 30 °C for North Atlantic. He speculates that the more than two weeks as shown by engine for these leaps was an abrupt NOAA satellites. This is only about 1 °C change in deep-ocean circulation—what higher than the normal maximum of 28 he calls the ocean's great conveyor sys to 29 °C. Once bleached, coral does not tem—which is governed by the temper receive adequate nutrients or oxygen, ature and salt content of the water. This Hayes says. If water temperatures re deep-current system moves 20 times turn to normal the following year, cor more water than all the world's rivers al reefs may recover, but if the thermal combined. For example, in the Atlantic, stress is repeated year after year, the warm salty upper waters flow north reefs may die. ward reaching the vicinity of Greenland, "I consider coral bleaching on tropical where the Arctic air cools them and they coral reefs as an early warning of detri sink to the ocean depths. mental changes attributed to global cli Broecker hypothesizes that as Earth mate change/' Hayes says. El Nino was coming out of the last glacial period, events could be the cause of some coral water from the fast-melting Laurentide bleaching, he explains, but if Earth con ice sheet that covered northern North tinues to warm from greenhouse gases, America came down the St. Lawrence much more bleaching can be expected. River into the North Atlantic. This water Another graphic indicator of a warm shut down deep-water convection in the er globe is the accelerated retreat of al ocean water between Norway and Ice pine glaciers. "In the tropics, every gla land that takes saline water at the sur cier that we have any data on is retreat- face to the depths as it cools. Heat ener ing," says Lonnie G. Thompson, professor gy carried by the Gulf Stream is the ma of geological sciences at Ohio State Uni jor reason northern Europe is warm versity, Columbus. "And where we have compared with other regions at the same time-lapse data, the rate of retreat is ac latitude. So a shutdown in this circula celerating." In Venezuela, three glaciers tion would plunge Europe into sudden have completely disappeared since 1972. cold and London would have a climate In the temperate zones, the majority of like Siberia. "Even though a shutdown glaciers are retreating, even as a few do of the Gulf Stream is a low-probability continue the traditional cyclic pattern of event," Broecker says, "its consequences would be so massive that it is something advance and retreat. Peru is particularly concerned about we should think about." the accelerating melting of glaciers in the There is some evidence that this North Cordera Blanca region of the Andes, Atlantic circulation has begun to slow. Thompson says. The glaciers there pro Compared with the 1970s, the salinity of vide irrigation water to the coastal desert the Norwegian Sea has decreased and and provide water for rivers that are deep-water production there has virtually dammed for hydroelectric power. Loss ceased. However, because there are no of glaciers would harm both power pro records of the extent of deep-water pro duction and agriculture for the country. duction before 1970, there is no way to tell It used to be thought that as climate changed in the past—when Earth came out of ice ages, for example—it hap This article is available on the World pened very slowly. But over the past de Wide Web at http://pubs.acs.org. Click on "What's New" or "Hot cade, and especially during the past five Articles." years, evidence has been accumulating that climate sometimes has made sud
whether this slowdown is part of a natu ral variation in the climate or whether it is caused by greenhouse warming. There is a broad perception that the science of global warming is much less certain than the science of stratospheric ozone depletion. However, Mahlman, who has done research in both areas, says the level of uncertainty surround ing the ozone problem is not much dif ferent than it is for global warming. What distinguishes the two problems is the number of responsible entities. "For ozone depletion, you can blame about 12 companies, more or less," Mahlman says, "but for global warming you can blame about 5 billion people." Consequently, because it is much more difficult to do something about global warming, "people demand more cer tainty than they required of the science of ozone depletion." More research is needed but is unlike ly to be funded, at least by the U.S. In fiscal 1995, federal agencies spent a total $1.8 billion on global change research. A somewhat larger amount was requested for 1996, but Congress has proposed sub stantial cuts in the programs of some agencies. For example, the House passed a 25% cut in NASA's Mission to Planet Earth, a major part of its research effort. For NOAA climate and air quality re search, the House passed a 41% cut in the request, and the Senate proposed a 33% cut. At press time, it was still not clear how large the final reductions would be because some of the appropriations bills were still in conference and were still sub ject to a presidential veto. Furthermore, the Environmental Protection Agency, whose research has focused on the effects of global change, has decided to drasti cally reduce its participation in the U.S. Global Change Research Program. Γη a recent talk, John H. Gibbons, direc tor of the White House Office of Science & Technology Policy, quoted the British novelist and historian C P. Snow: "A sense of the future is behind all good pol itics. Unless we have it, we can give noth ing—either wise or decent—to the world." Gibbons went on to say that "in creasing our understanding of the cli mate system and the impact of human activities is a necessary part of this en deavor to gain a sense of the future." Without research, mankind will not un derstand the impact of its activities or how to stop its "uncontrolled global ex periment"—elevating the levels of green house gases in the atmosphere. Π NOVEMBER 27,1995 C&EN
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