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."
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"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.
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.Web Links
Global Hydrology and Climate Center -- a joint venture between government and academia to study the global water cycle and its effect on Earth's climate
Measuring the Temperature of Earth From Space Even with needed corrections, data still don't show the expected signature of global warming
December 1997 is Coldest Month on Record in the Stratosphere
Is Earth's Temperature Up or Down or Both? Scientist's investigate reasons for temperature trend "disagreements" between layers of the atmosphere
schematic of layers of Earth's atmosphere
Java tool for global atmospheric temperature maps examine atmospheric temperature features
The Global Hydrology and Climate Center
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