| Themes > Science > Earth Sciences > Hydrology, Meteorology, Climatology > Generalities > Effect of Global Water Cycle On Weather and Climate > What Microwaves Teach Us About the Atmosphere |
Mankind's impact on the global climate and whether pollution from modern energy use is indeed warming the Earth have become important issues for national and international policy makers. Political pressure and public sentiment are based on complex data sets that, alone, cannot tell the whole story. The ultimate question is whether our climate is becoming warmer because of the slow build-up in atmospheric greenhouse gas concentrations. The answer is not clear, because much of what we know about global climate change in inferred from historical evidence of uncertain quality. Reliable ground-based measurements by scientific instruments have been made just in this century. These measure conditions only at the location of each instrument, and they are usually land-based, although 75% of the Earth is covered with water. We have been able to take precise, direct measurements only in the last four decades, and not until the advent of precision spaceborne instruments in the 1970s were we able to measure global temperatures at a range of altitudes across the entire atmosphere. |
GHCC scientists have compiled
two decades of data showing how atmospheric temperature has behaved over
the entire globe. All matter emits microwave radiation that varies with
its temperature, among other factors. Microwave sensors on weather
satellites can take more than 60,000 temperature measurements of oxygen in
the atmosphere, from the surface to about 10 km (6 mi) altitude. The story
that these measurements tell is more complex than simply saying the Earth
is warming or cooling. Temperatures in the lower troposphere (the portion
of the atmosphere where we live) have shown a series of ups and downs
since 1979, mostly in a ±0.4oC band, with negligible trends over that
period. This contrasts with surface thermometers that show warming during
the same period of time. The 1997-98 El Nino caused strong lower
tropospheric warming in late 1997, and record warmth in February 1998.
Satellite measurements of the lower stratosphere reveal two marked warm
periods (as much as 1.5oC warmer), caused by sulfuric acid aerosols
deposited in this layer by the eruptions of two large volcanoes. These two
warm periods are superimposed upon a strong cooling trend over the 19-year
period that has been attributed to ozone depletion in the lower
stratosphere. In 1997, record low stratospheric temperatures were
recorded. This is an ongoing research effort that will have greater impact
as more data are collected and analyzed.
 Microwave measurements from satellites are also being utilized to measure rainfall. Because oceanic rainfall is particularly difficult to measure by conventional methods, additional data from the satellites have been utilized since 1979 to monitor oceanic precipitation and temperatures. Studies of tropical rainfall from these data have revealed large increases during El Nino years, such as 1997-98. Future satellite data from planned instruments, such as the Advanced Microwave Scanning Radiometer, will fuel further research into such processes. These efforts will help us better understand how the atmosphere adjusts to climate perturbations, such as increasing levels of greenhouse gases.  |
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