Contrary Thermometers

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" (Heat islands are created when city growth alters the urban fabric by substituting manmade asphalt roads and tar roofs and other features for forest growth) 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."

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.