Science and Policy News

Study Questions Accuracy of Reconstructions of Past Climate

The Northern Hemisphere’s climate during the past 1,000 years may have been much more variable than suggested by recent analyses of tree-rings, ice cores, and other indirect sources of data on past temperatures. A new study suggests that the statistical method used in previous reconstructions of past climate is flawed, because it underestimates—perhaps by a factor of two or more—fluctuations in temperature that occurred over decades or centuries.

Hans von Storch of the GKSS Research Center (Geesthacht, Germany) and colleagues tested the ability of this statistical method to reproduce known temperature changes in the “virtual world” of a climate model. The researchers first used the model to produce a simulated temperature record for the past 1,000 years. They then generated virtual data for tree rings, ice cores, and other indirect data sources by adding statistical noise to the model’s simulated temperatures, mimicking the noise inherent in real-world data (such as the influence of changes in moisture and pest outbreaks on the width of tree rings). The team then applied the statistical method used in previous analyses to see how accurately it could reproduce the model’s simulated temperature record from the virtual tree ring and ice core data. They found good agreement for the past 100 years or so, but large disparities over longer timescales.

The new findings raise the possibility that the current warming trend may not be as unusual or unprecedented as previously thought, and that the climate’s natural variability may be greater than most recent studies have assumed. However, the authors emphasize that their results do not challenge conclusions that rising concentrations of greenhouse gases have contributed to the warming of the 20th century.

The study was published in the 30 September 2004 issue of ScienceExpress.

Climate Change Expected to Lead to Stronger Hurricanes

A comprehensive new computer modeling study suggests that hurricanes will become more intense as the climate warms, with stronger winds and heavier rainfall. The study projects an average 6 percent increase in maximum hurricane winds by the year 2080, along with an 18 percent increase in the rate of precipitation within 60 miles (100 kilometers) of the storm’s core. The increase in intensity amounts to roughly a half step in the 5-category hurricane scale. These specific projections are based on the assumption that carbon dioxide concentrations in the atmosphere will increase by 1 percent per year (compounded) over the next 80 years, which is higher than the current rate of about 0.6 to 0.7 percent per year.

The study’s authors do not expect the changes in hurricane intensity to be detectable “for decades to come,” but warn that there may be a gradually increasing risk of highly destructive category 5 storms over the course of this century.

The study’s basic findings are consistent across nine different climate models and a range of characterizations of physical processes in a hurricane model, bolstering the conclusions. Previous studies based on input from one climate model had also shown a tendency toward stronger hurricanes in warmer climates, but it was unclear how much of this effect was due to assumptions in the model.

The authors, Thomas R. Knutson of NOAA’s Geophysical Fluid Dynamics Laboratory (Princeton, New Jersey) and Robert Tuleya of Old Dominion University (Norfolk, Virginia) did not explore whether climate change would affect the frequency of hurricanes. Past research on this question has been inconclusive, with conflicting results. Knutson and Tuleya’s findings are reported in the 15 September 2004 issue of the Journal of Climate (vol. 17, pp. 3477-3495).

Antarctic Glaciers Surge When Ice Shelves Collapse

Two teams of researchers have provided conclusive evidence that Antarctic ice shelves can act as “brakes” on the land-based glaciers behind them, slowing their flow toward the sea. When the ice shelves collapse, the glaciers surge forward, dumping more ice into the ocean.

The breakup of ice shelves does not contribute directly to sea level rise, because the ice shelves are already floating on water. But the new findings indicate that the collapse of ice shelves may lead to higher sea levels by allowing the flow of glaciers to accelerate.

When the Larsen B ice shelf collapsed in 2002, researchers observed nearby glaciers flowing up to eight times faster than they did prior to the breakup. The research teams used RADARSAT and Landsat measurements to monitor changes in the glaciers’ rate of flow. Three of the glaciers behind the collapsed ice shelf flowed eight times faster in 2003 than they did in 2000, slowing moderately in late 2003. Two glaciers increased their rate of flow by a factor of two in early 2003 and by a factor of three by the end of 2003, compared with their flow in 2000. Nearby glaciers in areas where the ice shelf remained intact showed no significant changes. The glaciers in the Larsen area are not large enough to have a noticeable effect on sea level rise, but glaciers to the south are much bigger and could affect sea levels if nearby ice sheets were to collapse.

The findings are reported in the September 22, 2004 issue of Geophysical Research Letters.

Study Projects Longer, Stronger Heat Waves

A new climate modeling study projects that heat waves in North America and Europe will become more intense, more frequent, and longer lasting in the decades ahead. In the United States, the increase in the severity of heat waves will be greatest in the West and the South.

In their model, Gerald Meehl and Claudia Tebaldi of the National Center for Atmospheric Research found that an increase in the atmospheric concentration of greenhouse gases intensifies an atmospheric circulation pattern that researchers have observed during past heat waves in Europe and North America. As the pattern becomes more pronounced, severe heat waves occur in the Mediterranean region and the southern and western United States.

In light of the severe impacts of recent heat waves in Chicago (1995) and Paris (2003), the researchers also examined how those cities might be affected in the future. The average annual number of heat waves in the Chicago area would increase by 25 percent, and in Paris by 31 percent, according to the study. The researchers note that the northwestern United States, France, Germany, and the Balkans would also become more susceptible to severe heat waves, potentially leading to more serious impacts because these areas are currently less well adapted to heat waves than are cities such as Chicago. Meehl and Tebaldi’s study is reported in the 13 August 2004 issue of Science (vol. 305, pp. 994-997).

Reef Corals May Adapt to Climate Change

Reef-building corals and certain species of single-celled algae live in a symbiotic relationship, each depending on the other for survival. The corals provide protection and a nutrient-rich environment for the algae, and the algae provide the corals with food. Researchers have known for years that higher ocean temperatures can cause corals to eject or digest the algae, a phenomenon known as coral bleaching, raising concerns that coral ecosystems would be endangered by a warmer climate. But two new studies, published in the 12 August 2004 issue of Nature (vol. 430, pp. 741-742) report that some species of corals may be adapting to long-term increases in temperature by hosting heat-tolerant strains of algae.

Andrew Baker of the Wildlife Conservation Society and colleagues surveyed reefs in the far eastern Pacific, the Indian Ocean, the Red Sea, and the Persian Gulf, and found that reefs that had been devastated by severe bleaching are recovering with corals containing a particular type of algae that is resistant to bleaching and usually common in warmer-water environments. The study indicates that these reefs will be more resistant to future bleaching events, and is the first to show that corals may be able to adapt to climate change by shifting the types of algae they contain.

Rob Rowan of the University of Guam found that corals of the genus Pocillopora in Guam associate with these heat-tolerant algae as well as with another less-tolerant strain. He showed that it is the heat tolerance of the algae that determines how corals respond to warming waters, and suggests that the heat-tolerant strains have evolved as high-temperature specialists. Together, the two papers indicate that the ability of corals to associate flexibly with multiple types of algae is key to understanding their survival during periods of rapid climate change.

Ocean Absorb Half of Industrial CO2 Emissions

The world’s oceans have absorbed about 48 percent of the past two centuries’ worth of carbon dioxide (CO2) emissions from burning fossil fuels and cement manufacturing, according to a new study. The ocean sink represents a significant brake on the pace of climate change, and the amount absorbed to date is only about a third of the carbon dioxide that the oceans are expected to be capable of soaking up over the long term. However, the researchers warn that the oceans may absorb CO2 more slowly in the future due to changes in ocean chemistry. In addition, another new study reports that the oceans’ surface waters are becoming less alkaline as they absorb more CO2, leading to potentially “profound” adverse effects on marine life and ecosystems.

In the first study, Christopher Sabine of the National Oceanic and Atmospheric Administration (NOAA) and colleagues analyzed ocean chemistry data collected from nearly 10,000 stations during two large international research programs in the 1990s. Their estimate of the size of the ocean sink for CO2 is the most comprehensive and rigorous to date. While the atmospheric concentration of CO2 has climbed from 280 to 380 parts per million (ppm) since 1800, the authors calculate that it would have climbed to more than 435 ppm if it weren’t for the oceans’ uptake of the gas. Their results also show that terrestrial ecosystems have acted as a net source of CO2 over the past 200 years, releasing more of the gas to the atmosphere than they have taken up. The team’s findings are reported in the 16 July 2004 issue of Science (vol. 305, pp. 367-371).

The second study, led by NOAA scientist Richard Feely and published in the same issue of Science (pp. 362-366), investigated the rates at which calcium carbonate is dissolving in ocean surface waters and discussed the impact of reduced alkalinity on marine life. As pH levels drop, corals and other shell-forming organisms have a harder time drawing carbonate from the water to produce their shells. Studies have estimated that the rate at which marine organisms produce shells would decline by as much as 25-45 percent if atmospheric CO2 levels were to rise to 800 ppm.

Cooling Stratosphere Affects Satellite Temperature Trends

A strong cooling of the stratosphere during the past 20 years may explain why satellites show a smaller global warming trend than that measured at the Earth’s surface. Reporting in the 6 May 2004 issue of Nature (vol. 429, pp. 55-58), Qiang Fu of the University of Washington and colleagues say that the satellite record of temperature trends in the lower atmosphere has effectively been contaminated by the cooling influence of the stratosphere. When that influence is removed, the satellite trend is more consistent with the surface temperature record.

The satellite data channel that is used to measure temperatures in the lower atmosphere picks up about 15 percent of its signal from the stratosphere, which has cooled at a rate of 0.5 to 0.9ºC (0.9-1.6ºF) per decade since 1979. Using data from another channel, whose signal comes almost entirely from the stratosphere, the researchers were able to quantify the stratospheric signal and remove it from the lower atmospheric data. The resulting trend is a warming of 0.09-0.18ºC (0.16-0.32ºF) per decade from 1979-2001. Previous analyses of the satellite data showed a warming of only 0.01-0.1ºC. (0.02-0.18ºF) per decade. The warming trend measured at the Earth’s surface during the period was 0.17ºC (0.3ºF) per decade.

Climate Change May Threaten Greenland Ice Sheet

Jonathan Gregory of the University of Reading, UK, and colleagues used climate models to estimate how Greenland’s temperature would change during the next 350 years under each of the 35 emission scenarios developed by the Intergovernmental Panel on Climate Change. Under 24 of the 35 scenarios, summertime warming in Greenland exceeded 2.7°C (4.9°F), a threshold beyond which the ice sheet would contract even if snowfall increases. Previous studies have concluded that the ice sheet would eventually melt away entirely if the annual average warming exceeds 3°C (5.4°F). Under most of the scenarios, summertime warming exceeded 3°C by 2100-2200.

High-latitude regions such as Greenland are expected to warm more rapidly than the world as a whole, because the loss of snow and sea ice amplifies the warming by reducing the amount of sunlight that is reflected back to space.

Although some recent studies have found an increase in summertime melting of the ice sheet over the past several decades, a new analysis of Greenland temperatures indicates that summers have actually been getting cooler on average since 1940. Reporting in the March, 2004 issue of Climatic Change (vol. 63, pp. 201-221), Petr Chylek of Los Alamos National Laboratory and colleagues document a rapid warming in Greenland between 1920 and 1930, followed by a cooling trend along the coast of about 1°C (1.8°F) since 1940. Coastal temperatures warmed from 1992 to 1998, but the overall trend since 1940 is still toward cooling. The cooling is even more pronounced further inland, at the summit of the ice sheet, where the summertime average temperature has declined at the rate of 2.2°C (3.9°F) per decade since records began in 1987.

Because the warming in the 1920s occurred before greenhouse gases began rising rapidly, the researchers conclude that the region’s climate is naturally highly variable. They suggest that Greenland’s temperature trends may be strongly influenced by the cyclic climate phenomenon known as the North Atlantic Oscillation. If they are correct, predictions of the ice sheet’s fate would need to consider this relationship in conjunction with the effects of human-induced climate change.

Carbon Dioxide Growth Rate Increases

Daily average CO2 concentrations measured at Mauna Loa. Data shown in gray are preliminary. SOURCE: NOAA, Climate Monitoring and Diagnostics Laboratory.

The rate of increase of carbon dioxide in the atmosphere accelerated in the past two years, according to scientists with the National Oceanic and Atmospheric Administration (NOAA). However, the researchers caution that it is still too early to tell whether the growth rate is picking up on a long-term basis.

During 2003, the gas’s concentration rose by approximately 2.4 parts per million (ppm), compared with an average rate of 1.8 ppm per year over the past decade. The growth rate for 2002 was 2.6 ppm/year.

Carbon dioxide concentrations measured at Mauna Loa Observatory on the island of Hawaii reached a record high of 379 ppm in mid-March. CO2 concentrations are normally high at this time of year because global respiration from plants outpaces photosynthesis. After mid-May, the concentration of CO2 declines steeply as a burst of plant growth draws the gas out of the atmosphere. This seasonal increase and decrease can be seen clearly in the graph.

Outer Atmosphere Cooling and Thinning

A new study reports that the outermost layer of the atmosphere has become less dense during the past 35 years, most likely as a result of cooling due to increased atmospheric carbon dioxide levels.

Known as the thermosphere, this layer of the atmosphere begins about 90 kilometers (60 miles) above the Earth’s surface, and is the region where many communications satellites maintain their orbits. Although the atmosphere is extremely thin in the thermosphere, it still has enough density to exert a drag force on satellites, causing their orbits to slowly decay over time.

John Emmert of the Naval Research Laboratory (Washington, DC) and colleagues used data about the changing orbits of 27 space objects that had been in orbit for at least 35 years to determine how much drag was acting on them at any given time. For all 27 of the objects studied, their results indicate a long-term decrease in the density of the thermosphere, amounting to about 10 percent during the past 35 years.

Computer models simulating the effects of increasing greenhouse gases in the Earth’s atmosphere project a similar decrease in the thermosphere’s density. Although carbon dioxide traps heat in the lower layers of the atmosphere, it has a cooling effect in the higher layers where it acts to more effectively radiate heat back to space. As the upper atmosphere cools, it contracts, bringing lower-density gas closer to the surface of the Earth, and resulting in a decline in density at any given height in the atmosphere.

A decrease in the density of the thermosphere will have mixed blessings, the scientists say. Although it will allow orbiting satellites to stay in orbit longer, the amount of potentially damaging space debris will increase since it will also be able to stay in orbit for a longer period.

The research appears in the 5 February 2004 issue of the Journal of Geophysical Research.

Climate Change May Be Greater Threat to Biodiversity Than Habitat Loss

An international team of 19 researchers studying more than 1,100 plant and animal species around the world suggests that, based on their data and assumptions, nearly one-quarter of these species could become extinct if climate trends follow middle-of-the-road projections. The researchers acknowledge significant uncertainties in their analysis, and describe their study as a “first pass” at estimating the effects of climate change on global extinctions.

The team identified endemic species (species that do not occur outside a region) in several regions of the globe, including parts of Australia, Brazil, Europe, Mexico, and South Africa. For each species, the researchers identified the present “climate envelope” (temperature, precipitation, seasonality, etc.) in which the species occurs. They then used projections of climate change (minimal, moderate, and maximum) to determine where the same climate conditions might occur in the year 2050.

The scientists then used the species-area relationship, a well-documented relationship between the size of an area and the number of species it contains, to determine the potential for species extinction under varying scenarios of climate change. Their analysis suggests that with minimal climate warming, about 18 percent of the species studied would become extinct; with moderate climate warming, about 25 percent would become extinct; and with maximum warming, some 35 percent would become extinct. Since there may be delays in the effects of warming on some species, these percentages reflect the number of species that would be “committed to extinction” but not necessarily extinct by the year 2050.

The research is published in the 8 January 2004 issue of Nature (vol. 427, pp. 145-148).

Soot-Darkened Snow Exacerbates Warming

Tiny particles of soot that collect on snowflakes as they fall through the atmosphere may play a “substantial role” in global warming, according to a new study. The soot absorbs solar energy, reducing the amount of sunlight reflected back to space by snow and ice. The authors, James Hansen and Larissa Nazarenko of the NASA Goddard Institute for Space Studies, estimate that soot reduces the albedo (reflectivity) of snow and ice by about 3 percent in Northern Hemisphere land areas, 1.5 percent in the Arctic, and 0.6 percent in Greenland. That’s enough to account for a quarter of the global warming observed in the past century.

Sooty snow also may be a factor in the widespread melting of ice and glaciers, the authors say. Soot causes snow and ice to age more rapidly than they would otherwise, leading to an earlier and longer melt season for glaciers. Furthermore, as the ice melts, soot accumulates on the surface and makes the ice darker, causing it to melt faster. On a glacier, the added meltwater seeps to the base of the ice, providing lubrication that speeds the glacier’s movement and disintegration.

A soot content of only a few parts per billion is needed to reduce the albedo of snow by 1 percent. Diesel fuel is the largest source of soot in industrialized countries; biofuels are an important source in developing countries.

Hansen and Nazarenko note that their findings “do not alter” the conclusion that greenhouse gases have been the main cause of recent global warming and will be the predominant cause in the future. But the new research suggests that soot may play a greater role in warming than previously thought. The study was published online in the December 29, 2003 issue of the Proceedings of the National Academy of Sciences, at http://www.pnas.org/cgi/content/abstract/2237157100v1.

Researchers See “No Doubt” of Human Influence on Climate

Two leading atmospheric scientists say there is “no doubt” that human activities are changing the composition of the atmosphere, and that increases in greenhouse gas concentrations are the dominant cause of modern climate change. In a review article in the 5 December 2003 issue of Science (vol. 302, pp. 1719-1723), Thomas Karl of the National Climatic Data Center and Kevin Trenberth of the National Center for Atmospheric Research conclude that, in the absence of new policies to mitigate climate change, “substantial future climate change is guaranteed.” Karl and Trenberth report that the world is 90 percent likely to warm by 1.7° to 4.9°C (3.1° to 8.9°F) by the end of this century. The authors’ conclusions are based on existing, published studies.

Karl and Trenberth note that while there is “considerable uncertainty” about the rate, magnitude, and duration of future climate change, the human influence on climate will eventually become “overwhelmingly large” compared with natural changes. “We are venturing into the unknown with climate,” they write, “and its associated impacts could be quite disruptive.”

Satellites Document Arctic Warming

Most of the Arctic warmed considerably in the 1990s compared with the 1980s, according to new research from NASA’s Goddard Space Flight Center. Researcher Josefino Comiso used satellite measurements of Arctic surface temperatures collected between 1981 and 2001 for his analysis. Comparing the satellite data with longer-term surface measurements, Comiso found that the rate of warming during the last 20 years was eight times larger than that of the past 100 years.

The satellite data show that, per decade, the North American Arctic warmed by 1.06 ºC (1.91 ºF), Eurasia warmed by 0.5 ºC (0.9 ºF), and Greenland showed a slight cooling trend of 0.09 ºC (0.16 ºF).

Over sea ice, annual temperatures climbed by 0.33 ºC (0.59 ºF) per decade, while during the summer months temperatures rose by 1.22 ºC (2.2 ºF) per decade. Rising temperatures over sea ice lengthened the season of ice melting by 10-17 days per decade, and may help explain recent findings that sea ice cover in the Artic is declining by 9 percent per decade, reaching record low levels in 2002. Scientists suggest that this reduction in sea ice cover could lead to even greater warming in the Arctic, as open water absorbs more incoming solar radiation, and hence heat, than does ice.

The research appears in the November 1, 2003 issue of the Journal of Climate (vol. 16, pp. 3498-3510). For more information, visit: http://www.gsfc.nasa.gov/topstory/2003/1023esuice.html.

Largest Arctic Ice Shelf Breaks Up

A team of scientists reports that the largest ice shelf in the Arctic, the Ward Hunt Ice Shelf on the north coast of Canada’s Ellesmere Island, broke in two during the period 2000 to 2002. Derek Mueller and Warwick Vincent of Laval University (Quebec, Canada) and Martin Jeffries of the University of Alaska Fairbanks discovered the breakup while working on the ice shelf, and used satellite images to trace the progression of the fissure through time.

The break in the ice shelf drained most of the freshwater from Disraeli Fjord on northern Ellesmere Island. Created by an ice shelf dam at the fjord’s mouth, this rare “epishelf lake” had consisted of a thick layer of freshwater lying above seawater, supporting a unique Arctic ecosystem. The depth of the freshwater layer had been declining gradually since it was first measured in 1967, but it dropped dramatically from 92 feet in 1999 to less than 10 feet in 2002.

At Alert, the nearest weather monitoring station (approximately 100 miles east of the ice shelf), air temperatures have increased by more than 0.7 ºF per decade since 1967. Based on temperatures measured at the ice shelf itself, the researchers estimate that the average July temperature during 1967-2002 was 34.3 ºF, well above the 32 ºF threshold that has led to the breakup of ice shelves at the other end of the world, in Antarctica. Other factors that could have contributed to the breakup of the Ward Hunt Ice Shelf include changes in ocean temperature, salinity, and flow patterns; changes in local hydrology; or changes in the ice shelf’s surface heat budget.

The team’s findings will be published in a forthcoming issue of Geophysical Research Letters.

Climate Change Has Little Effect on Central European Floods

Although global climate change is expected to increase the risk of extreme floods, evidence from the two largest rivers in Central Europe suggests a more complicated relationship. Heavy winter flooding in the Elbe and Oder rivers actually has decreased during the past 80-150 years, while summer floods show no trend.

According to Manfred Mudelsee of the University of Leipzig, Germany, the decrease in winter flooding may be due partly to a decline in deep cold spells, which has prevented the formation of thick river ice that can form dams and cause heavy flooding when the ice breaks up in spring. Warmer winters also may have reduced the occurrence of frozen soils, which contribute to flooding because they absorb little water. Mudelsee and his colleagues published their findings in the 11 September 2003 issue of Nature (vol. 425, pp. 166-169).

The authors note that while extreme floods occurred in central Europe during the summers of 1997 and 2002, there is no evidence for recent upward trends in their rate of occurrence. Furthermore, they found that regional deforestation and the expansion of reservoirs have had a “negligible” impact on the frequency of flooding.

Northern Hemisphere Climate Warmest in 2,000 Years

A new reconstruction of past temperatures at the Earth’s surface suggests that late 20th century temperatures in the Northern Hemisphere were higher than at any time in the past 2,000 years. A lack of suitable data for the Southern Hemisphere limited the conclusions that the researchers could make about temperatures south of the Equator or for the globe as a whole.

Michael Mann of the University of Virginia and Philip Jones of the University of East Anglia (UK) used surrogate temperature data inferred from ice cores, lake sediments, tree rings, and fossilized shells for their reconstruction. Data for the Northern Hemisphere covered eight distinct regions, while data from the Southern Hemisphere covered five. Mann and Jones rigorously standardized and compared the data with instrumental records for the same region during the past century.

In general, Northern Hemisphere temperatures remained well below the 1961-1990 average throughout the past 2,000 years. The reconstruction does show a “Medieval warm period” in the Northern Hemisphere from about AD 800-1400. “This warmth is, however, dwarfed by late 20th century warmth which is observed to be unprecedented at least as far back as AD 200,” the authors state.

The research is reported in the 1 August 2003 issue of Geophysical Research Letters (vol. 30, no. 15, pp. 1820-1823).

Effects of Land-Use Change on Temperature Record May Have Been Underestimated

Temperatures in the lower atmosphere, as measured by balloons and more recently satellites, show smaller warming trends over the past 50 years than those measured at the Earth’s surface. A new study suggests that the influence of urbanization, agriculture, and other land-use changes on surface-based weather stations could play a significant role in the discrepancy between surface and atmospheric temperature trends. If correct, the findings imply that land-use changes could account for a greater portion of the global warming observed to date in the surface record than previously estimated.

Eugenia Kalnay and Ming Cai of the University of Maryland compared temperatures measured at nearly 2,000 weather stations across the United States over the past 50 years with temperatures inferred from balloons and satellites. Unlike the surface record, the data from satellites and balloons showed no significant differences in trends between rural and urban weather stations. Based on this and other evidence, the authors speculate that the influence of land-use changes may account for most of the difference between the two data sets.

Kalnay and Cai conclude that as much as half of the surface data’s observed trend toward a smaller daily temperature range (due primarily to warmer nighttime minimums) could be attributed to urbanization and other land-use changes, especially changes in agriculture. This estimate is twice as high as previous estimates of the effect of land-use change. The authors suggest that one reason their estimate is higher is that previous estimates did not include the effects of agricultural development on temperature.

Kalnay and Cai now plan to do a follow-up study to see if the differences between the two data sets correlate with actual land use changes.

The new findings are reported in the 29 May 2003 issue of Nature (vol. 423, pp. 528-531).

Satellite Studies Conflict on Warming

Two new analyses of satellite data on temperatures in the lower 4.8 miles of the atmosphere have come up with conflicting results. Both studies find that the atmosphere has warmed since the satellites began collecting data in 1979, but they disagree sharply on the amount of warming that has occurred.

The discrepancy prolongs a more than decade-long debate over whether satellites, with their global coverage, provide a more accurate picture of the world’s temperature trends than do surface records from weather stations and ships. The debate also has raised questions about the reliability of climate models, which project stronger warming in the middle troposphere than shown by the satellite data.

One of the new studies, headed by John Christy of the University of Alabama in Huntsville, found that the middle troposphere warmed at a rate of 0.04 ºF per decade from 1979-2001, an upward revision of 0.02 ºF from an estimate that Christy and his colleagues published in 2000. The new estimate, reported by Christy’s team in the May issue of the Journal of Atmospheric and Oceanic Technology (vol. 20, pp. 613-629), is based on the addition of new data and the application of improved correction factors to the satellite readings.

Another group of researchers, led by Carl Mears of Remote Sensing Systems, a research firm based in Santa Rosa, California, performed a comprehensive reanalysis of the same satellite data, starting from scratch to develop their own independent quality controls and correction factors. Mears’s team estimates that the middle troposphere warmed by 0.17 ºF per decade between 1979 and 2001—a rate four times higher than that reported by Christy’s group. Mears and his colleagues have submitted their findings to the Journal of Climate.

Which team is right? According to a forthcoming paper in Science by Benjamin Santer of Lawrence Livermore National Laboratory and colleagues (including Mears and his team), the Mears data set agrees more closely with computer model simulations and with the surface temperature record. But Christy counters that his group’s data set correlates very closely—within 94 to 98 percent—with temperature data for the middle troposphere gathered from weather balloons. The next phase in the debate may center on the accuracy of the balloon data.

Human Influence Seen on Changes in Air Pressure

Widespread regional changes in barometric pressure during the past 50 years may be linked to emissions of greenhouse gases and sulfate aerosols, according to a report in the journal Nature (20 March 2003, vol. 422, pp. 292-294). The finding could have important implications for projections of future climate change, because changes in air pressure can affect the patterns of air circulation in the atmosphere. Changes in circulation alter regional precipitation patterns, temperatures, winds, and the frequency and severity of storms.

Nathan Gillett of the University of Victoria, British Columbia, and colleagues analyzed trends in air pressure measured at sea level around the world between 1948 and 1998. The data show that barometric pressure declined over the Arctic, Antarctic, and North Pacific, while increasing over the subtropical North Atlantic, southern Europe, and North Africa.

The researchers then studied the output of four climate models to see if the models reproduced similar regional trends in pressure given the changes that have occurred in greenhouse gases and sulfate aerosols during the past five decades. All four models reproduced the trends, leading the researchers to conclude that human activity had a “detectable influence” on sea-level air pressure during the second half of the 20th century.

However, the models greatly underestimated the magnitude of the trends, predicting much smaller changes in pressure than those that actually occurred. The authors conclude that climate scientists will need to resolve this discrepancy in order to make more realistic projections of regional climate change.

Past items in the news include:

• Administration launches "Climate VISION" on February 14, 2003. For more information see the fact sheet on Climate VISION.

• National Research Council Reports on Climate Change. See the report. (304K pdf)

Available on a standing basis:

• Speeches. Provides a selection of notable speeches related to climate change and global warming.

• Inside the Greenhouse. Focusing on innovative state and local programs to reduce greenhouse gas emissions; Inside the Greenhouse is a quarterly, electronic publication produced by the EPA’s State and Local Climate Change Program.