Geotimes Untitled Document Feature
A Warming World Kevin E. Trenberth Top
climate news 2005
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 hemispheres
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 todays
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
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.
Todays 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
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: email@example.com.
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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
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