Satellite measurements

Given that there are possible errors in the direct measurements of temperature, it is useful to compare them with satellite measurements, which provide a completely independent record, less influenced by urbanization issues. Of course, satellites have not been taking measurements for as long as the surface record has existed, and they too are subject to their own errors—difficulties both in calibration and in accounting for the fact that the orbit of a satellite tends to decay over time, potentially contaminating the results.3 Nevertheless, the combination of satellite and surface observations gives a rather powerful check on temperature increase of the past few decades, as shown in figure 7.2.

Satellite measurements are taken with a microwave sounder, which measures the microwave radiation in several bands, as well as an infrared sounder, which makes similar measurements in the infrared band. The brightness in each band is sensitive to both the temperature and the amount of water vapor in the atmosphere, but by measuring in multiple bands a temperature profile of the atmosphere can be constructed. The temperature trends measured by the satellites agree well with those directly measured at the surface and with those in the lower atmosphere measured by radiosondes (instruments carried on weather balloons). The surface measurements show a trend of 0.16°C per decade over the past three decades, and the satellite

Figure 7.2. Top: Lower troposphere temperature as measured by various satellites and by radiosondes; the gray shading indicates the spread between all measurements. Bottom: Surface temperature records from NOAA, NASA, and UKMO, with gray shading again indicating the spread. Records are monthly means, smoothed with a seven-month running mean filter, and are relative to 1979-1997 mean. Adapted from Solomon et al., 2007.

El Nino yAi

Lower Troposphere vj

VT

El Chichón 1

Pinatubo 1

vH

1960 1970 1980 1990 2000

1960 1970 1980 1990 2000

measurements show trends of between 0.14°C and 0.18°C per decade, depending on the particular method used to obtain temperatures from the brightness measurements.

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