|
 July
21, 2000 -- The question sounds like a Zen
koan: How could the globe be warming and not warming at the
same time?
That's the riddle posed to climatologists by satellite and radiosonde
data which show that while the Earth's surface has been warming
over the past decades, the lowest layer of the atmosphere shows
a weaker warming trend.
The measurements are surprising, because computer simulations
of the world's climate predict that the two lowest layers of
the atmosphere -- which together form the "troposphere"
-- should be warming faster than the Earth's surface.
Above: Satellite measurements show a highly complex
temperature structure throughout Earth's atmosphere. Temperature
measurements at the Earth's surface indicate a warming trend,
whereas satellite measurements show both warming and cooling
-- depending on where you look. [more
information]
"I think it points out that the atmosphere is more complex
than the computer models currently simulate," says Dr. Roy
Spencer, senior scientist for climate studies at the Global Hydrology
and Climate Center (GHCC) at NASA's Marshall Space Flight Center.
"However, it does not by itself substantially alter the
expectation that some amount of global warming will occur in
the future."
Spencer and Dr. John Christy, a professor of atmospheric
science at the University of Alabama in Huntsville, are trying
to account for the unexpected temperature patterns. By explaining
the contrary behavior of atmospheric and surface-level temperature
trends, they hope to improve computer models used to simulate
the world's climate. This would provide a better picture of how
severe or mild global warming will be over the next century.
"I believe the data bolster the traditional scientific skepticism
one must have when discussing predictions of the future,"
Christy said.
A recent National Research Council report states: "For the
time period from 1979-1998, it is estimated that on average,
over the globe, surface temperature has increased by 0.25 to
0.4 degrees C and lower to mid-tropospheric temperature has increased
by 0.0 to 0.2 degrees C." These are stated as ranges because
of measurement uncertainties in each. Current climate models
predict that the layer of the atmosphere called the "lower
troposphere" -- which extends from the surface to an altitude
of about 5 miles -- would be warming at a slightly faster
rate than the surface.
But satellite measurements of temperatures in the lower troposphere
over the last 21 years don't agree with that prediction. Collected
by the National Oceanic and Atmospheric Administration's TIROS-N
series of weather satellites, the data show only a slight net
warming of 0.05 degrees Celsius per decade.
Above: Monthly temperature deviations from a seasonally
adjusted average for the lower troposphere, which is Earth's
atmosphere from the surface to 8 km, or 5 miles up. The temperature
in this region is strongly influenced by oceanic activity, particularly
the "El Niño" and "La Niña"
phenomena. A stronger-than-usual warming trend in 1998-99 was
associated with a strong El Niño event, while the slight
cooling in 1999-2000 coincides with the ongoing La Niña
phase. The overall trend in the lower tropospheric data is approximately
steady: the temperature increases by approximately +0.047oC
per decade. [more
information]
What could possibly be causing these unexpected trends? Right
now, there are only theories.
"Stratospheric ozone depletion,
unknown effects related to the major volcanic eruptions and the
infrared effect of aerosols have all been bounced around as ideas,
but none has had serious work done on them," Spencer said.
Some scientists suspect that the record of surface temperature
warming has been exaggerated by the so-called "asphalt effect,"
creating unrealistically high expectations for the warming of
the troposphere.
Thermometers used to calculate the average surface temperature
are usually located in areas easily accessible by people. In
industrial countries, the thermometers are most often found at
airports. It is not clear what fraction of the observed warming
of the Earth's surface is due to the influence of "urban
heat islands" on the measurements.
 "I
believe there are still urban warming biases in the global thermometer
record that are exaggerating the global warming signal,"
Spencer said. "I don't think it will be possible to remove
these biases since virtually all thermometer sites have experienced
changes in their microclimate due to (humanity's) activities."
Right: Asphalt is replacing trees in many urban areas,
causing local temperatures to rise. Some scientists wonder if
the Urban Heat Island effect might
lead to over-estimates of global surface temperatures.
The satellites, on the other hand, sweep over almost the entire
globe as they take their measurements, covering about 95 percent
of the Earth's surface. Oceans and continents, forests and factories
are all incorporated into the satellite figures.
Most of the current work at GHCC focuses on improving and expanding
the body of data to provide a clearer, more detailed picture
of the long-term temperature patterns of the atmosphere.
For example, Christy is trying to expand the atmospheric temperature
record to before 1979 -- which is when the first of the TIROS-N
satellites was put in orbit -- by using data from radiosonde
balloons. Potentially, the balloon data could extend the record
back to the late 1950s.
Also, a new version of the temperature sensors used by the satellites
will improve the detail of the measurements taken. The new Advanced
Microwave Sounding Unit (AMSU) sensors can detect temperatures
in the upper stratosphere, which is the atmospheric layer above
the highest layer that the older sensors could measure. The new
sensors can also distinguish between more sub-layers of the troposphere
-- the layer where most weather occurs.
The first AMSU sensor was launched into space in May of 1998
aboard the NOAA-15 satellite, and data from that sensor are already
being incorporated into the daily temperature updates produced
by Spencer. Another AMSU sensor is scheduled to launch in late
August aboard the NOAA-16 satellite, and the Aqua satellite and
the European Space Agency's MetOp series of polar-orbiting satellites
will also bear the sensors.
"(AMSU sensors are) what's going to be providing our temperature
information from satellites for the foreseeable future,"
Spencer said.
Above: This global surface temperature map is a sample
of AMSU-A remote sensing data available online in near real time.
For more information, visit the AMSU-A
web site at the Global Hydrology and Climate Center. (Note:
The black vertical regions are areas not yet covered by the satellite
in this realtime snapshot.)
The improved temperature record will guide efforts to refine
computer models of the world's climate so that the behavior of
the models more closely resembles the observed behavior of the
atmosphere.
Current models suffer from several shortcomings.
For example, clouds are not well represented by the models. The
resolution of current models is too coarse for features as small
as clouds, Spencer said. Yet clouds clearly play a crucial role
in climate due to their influence on humidity, precipitation
and albedo (the percentage of solar energy reflected back into
space as light).
"The role of clouds is still regarded as one of the biggest
uncertainties in global warming predictions," Spencer said.
The ability of plants to remove carbon dioxide from the atmosphere
and the role of soils have only recently been added to the models,
and scientists aren't confident yet of how the models portray
these factors, Spencer said.
"While we know that vegetation takes up some of the carbon
dioxide we generate from burning of fossil fuels, how that sink
of carbon will change in the future is still pretty uncertain,"
Spencer said.
Climate models are also limited by the computing power available.
"The global models would be much better if computers were
much faster," Spencer said. "Instead, a lot of approximations
are made to make the models simple enough to do climate simulations
over the whole globe.
"Unfortunately," Spencer continued, "we know that
many of the processes that are crudely represented are quite
non-linear, and so have the potential to respond in unexpected
ways."
The Global Hydrology and Climate Center is a joint venture
between government and academia to study the global water cycle
and its effect on Earth's climate. Jointly funded by NASA and
its academic partners, and jointly operated by NASA's Marshall
Space Flight Center in Huntsville, Ala., and the University of
Alabama in Huntsville, the Center conducts research in a number
of critical areas. |
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