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A Warming World
Kevin E. Trenberth

Top climate news 2005

Our planet is warming. The years 2002 to 2004 are the second, third and fourth warmest years since 1861 (1998 remains the warmest) and nine of the last 10 years (1995 to 2004) — the exception being 1996 — are among the 10 warmest years. Based on reconstructions of temperatures from proxy data, such as tree rings and ice cores, several studies have also concluded that northern hemisphere surface temperatures are warmer now than at any time in at least the last 1,000 years.

Scientists this year published work that reconciles past errors in temperature measurements of the lower atmosphere, or troposphere, the top boundary of which is shown here at the division between the orange- and blue-colored bands of atmosphere. The research shows an overall warming trend in the troposphere that matches ground measurements of temperatures. Courtesy of NASA Marshall Space Flight Center.

Global surface temperatures today are about 0.75 degrees Celsius warmer than at the beginning of the 20th century. Land regions have warmed the most, with the greatest warming in the winter and spring months over the northern hemisphere’s continents.

In the past few years, improvements have been made to both land-surface air temperature and sea-surface temperature coverage and daily data availability. One area of improvement relates to quantification of the effects of urbanization on the global temperature record and recognizing that the urban heat island effect is real. Another is the recovery and digitization of data from old oceanographic records, locked away in ship logs, that have filled in gaps in the past record.

The past year has also seen increased understanding of the temperature records from balloon-borne radiosondes and estimates of upper-air temperatures from satellites that have provided true global coverage since 1979. Initial analyses indicated that temperatures in the lower atmosphere showed little or no warming. Climate change skeptics have used this result to raise questions about both the reliability of the surface record and the cause of the surface warming, as temperatures on the surface and in lower atmosphere should match. New datasets in the past five years, including some recent work published by Carl Mears and Frank Wentz, Benjamin Santer et al., and Steven Sherwood et al. in Science, help resolve this issue (see Geotimes, October 2005). Flaws have been discovered in the original satellite temperature record associated with orbital changes and satellite drift, and new analyses indicate that the lower atmosphere has been warming similarly to the surface since 1979. The surface and upper-air records of temperature change can now be reconciled, and the overall vertical pattern of observed temperature change is consistent with that simulated by today’s climate models.

The number of daily warm extremes has increased, while the number of cold extremes has decreased, especially at night.
The warming described is also consistent with a body of other observations indicating a warming world. For example, the number of frost days in middle-latitude regions has decreased, principally due to an earlier last day of frost in spring. The number of daily warm extremes has increased, while the number of cold extremes has decreased, especially at night. Widespread increases in surface water vapor have been found since 1976, and the amount of total water vapor in the atmosphere has increased over the global oceans by 2 percent from 1988 to 2003, consistent in pattern and amount with changes in sea-surface temperatures and a fairly constant relative humidity.

Ocean temperatures have warmed at depth as well, and global sea levels have risen 15 to 20 centimeters over the 20th century, with some evidence of an accelerating rate in the past decade (3.0 centimeters). As the oceans warm, seawater expands and sea level rises, but nearly worldwide glacial melt also contributes to the increase. These changes provide fuel for tropical storms, such as Hurricane Katrina, and enhance associated heavy rains and flooding, likely contributing to the breaching of levees (see story, this issue).

Snow cover has decreased in many northern hemisphere regions, particularly in spring. Sea-ice extents have decreased in the Arctic, particularly in the spring and summer seasons. In the Antarctic, patterns of warming and cooling are related to changes in global atmospheric circulation. The warming of the Antarctic Peninsula region since the early 1950s is one of the largest and the most consistent warming signals observed: Large reductions in sea ice and in the size of the Larsen Ice Shelf have occurred, including the collapse of the Larsen B Ice Shelf in 2002, which Eugene Domack and co-workers recently reported in Nature to be the largest to have occurred there in the last 10,000 years.

Considerable progress has also been made in sorting out the role of changes in the sun, pollution (aerosols) and greenhouse gases, such as carbon dioxide. Aerosols, such as soot and sulfate particles (the milky white haze seen from airplanes), have short lifetimes (a week or so) as they are washed out of the atmosphere by rain, but their overall influence is to cool the climate and possibly change rainfall patterns. In contrast, greenhouse gases such as carbon dioxide, methane and nitrous oxide are not washed out. Many, such as carbon dioxide and nitrous oxide, have lifetimes of a century or longer. Hence they build up in the atmosphere over time, as observed. Carbon dioxide is now 32 percent higher than in preindustrial times, with half of the increase occurring since 1970, owing mainly to combustion of fossil fuels and deforestation. Greenhouse gas concentrations in the atmosphere are now higher than at any time in at least the last 750,000 years.

Climate model simulations have reliably shown that the global surface warming of recent decades is a response to the increased concentrations of greenhouse gases in the atmosphere. When the models are run without the human-made changes in atmospheric composition, they fail to capture the increase in global surface temperatures observed since the mid-1970s. But with anthropogenic forcings included, the models simulate the observed temperature record with impressive fidelity. These same model experiments also reveal that changes in solar luminosity account for much of the warming in the first half of the 20th century.

Uncertainties arise from shortcomings in our understanding of climate processes, and how to best represent those processes in models. In recent years, climate models have improved, as they are run at higher resolutions, and representations of many processes (such as those involving cloudiness) have become more realistic. Today’s best climate models are able to reproduce the climate of the past century, and simulations of the evolution of global surface temperatures over the past millennium are consistent with paleoclimate reconstructions. Hence they are useful tools for carrying out numerical climate experiments and making predictions.

Climate change is with us; we cannot stop it, although we can slow it down. It behooves us therefore to track how and why the climate is changing. New data will provide the basis for more detailed predictions of what will happen over the next few decades, using new and improved climate models, together allowing us to plan for the future in an informed way.
Trenberth, an atmospheric scientist, is head of the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colo. E-mail:

"Revisiting the satellite record," Geotimes, October 2005.
"Global climate affects storms?" Geotimes, December 2005.

Further Reading:
Barnett, T. P., D. W. Pierce, K. M. AchutaRao, P. J. Gleckler, B. D. Santer, J. M. Gregory, W. M., Washington, 2005: Penetration of Human-Induced Warming into the World's Oceans. Science, 309: 284-287
Brohan, P. et al., 2005: Uncertainty estimates in regional and global observed temperature changes , J. Geophys. Res. (submitted).
Emanuel, K., 2005: Increasing destructiveness of tropical cyclones over the past 30 years. Nature, 436, 686-688.
Hansen, J., L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S. Menon,T. Novakov, J. Perlwitz, G. Russell, G. Schmidt, N. Tausnev, 2005: Earth's Energy Imbalance: Confirmation and Implications. Science, 308: 1431-1435
Mears, C.A. and F.J. Wentz, 2005: The effect of diurnal correction on satellite-derived lower tropospheric temperature. Science, 309, doi: 10.1126/science.1114772 (online).
Peterson, T.C. and T.W. Owen, 2005: Urban heat island assessment: Metadata are important. J. Climate, 18, 2637-2646.
Rayner, N.A., et al., 2005: Improved analyses of changes and uncertainties in marine temperature measured in situ since the mid-nineteenth century. J. Climate, Submitted.
Santer, B.D., et al., 2005: Amplification of surface temperature trends and variability in the tropical atmosphere. Science, 309, doi: 10.1126/science.1114867 (online).
Sherwood, S., J. Lanzante, and C. Meyer, 2005: Radiosonde daytime biases and late 20th century warming. Science, 309, doi: 10.1126/science.1115640 (online).
Trenberth, K.E., J. Fasullo, and L. Smith, 2005: Trends and variability in column integrated atmospheric water vapor. Climate Dyn., 24, 741 758.
Trenberth, K. E., 2005: Uncertainty in hurricanes and global warming. Science, 308, 1753-1754.

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