February 7, 2008

More Satellite Musings

About a month ago, we ran a piece reflecting back on the behavior of the satellite-derived temperature history of the earth’s lower atmosphere for the past 10 years of so. We commented that the two major realizations of the temperature history of the lower atmosphere—one derived by researchers at the University of Alabama-Huntsville (UAH) and the other by researchers at Remote Sensing Systems (RSS)—seemed to be drifting apart in recent months. Well, it has since been determined that a slight computational error involving the data for 2007 had been introduced in the RSS routines, and this error has now been corrected (see here for more detail) so all is now well again in the world of satellite-derived global temperatures—or it is?

Yes and no.

Yes, if you are interested in the inner workings of how the satellite data are combined to produce temperature values of the atmosphere and how the two different research teams use slightly different methodologies to produce their slightly different temperature histories, but overall which are by and large very consistent. In other words, things are working correctly again. Figure 1 shows the record of monthly temperature anomalies in the lower atmosphere averaged for the globe for the RSS and the UAH data sets during the period January 1979 through January 2008. Notice the very close correspondence. Figure 2 shows the actual difference between the two data sets (UAH data minus RSS data)—and these differences are well understood. Comparing Figure 2 with the lower panel of Figure 1 from our January 8th posting and you will see that the large discrepancy that used to be present at the end of the record (in 2007) has been resolved.

Figure 1. Global average temperature history (January 1979 through January 2008) of the lower troposphere as produced by researchers at the University of Alabama-Huntsville (UAH, blue line) and from Remote Sensing Systems (RSS, red line).

Figure 2. Difference between the data depicted in Figure 1 (UAH minus RSS).

But the answer to ‘whether all is right’ is ‘emphatically, no!’ if you are looking to the satellite-derived temperature datasets to find evidence of a strong warming signal in global temperatures over the past several years. In fact, both the RSS and the UAH temperature records show that January 2008 was below the long term (1979-1998) average for the month. This is the first time since January 2000 (exactly 8 years ago) that both records were colder than normal. Figure 3 shows both records from January 2001 through January 2008 and illustrates that over this time period—more than 7 years, during which time global emissions of carbon dioxide have increased by probably 15-20% (the actual data are only available through 2004, but the emissions growth from 2001 to 2004 was over 11% and we doubt things have slowed down any, see here for data )—there has been absolutely no warming whatsoever in the lower atmosphere. In fact, global temperatures were 0.63ºC cooler in January 2008 than they were in January 2007. Such year-to-year variability is not particularly unusual (note that a similar drop occurred in the middle of 2004), but it is interesting nonetheless in that it certainly doesn’t lend itself to thinking that the lack of warming during the past 7 years is letting up anytime soon.

Figure 3. Global average temperature history during the past seven years (January 2001through January 2008) of the lower troposphere as produced by researchers at the University of Alabama-Huntsville (UAH, blue line) and from Remote Sensing Systems (RSS, red line). There is no overall temperature change during this period.

We can’t imagine that the global temperatures will stay down forever, but the last 7+ years does provide a clear example that the rate of temperature change is not simply going up and up and up. In fact, the rate of change seems to be slowing.

Stay tuned to see how global temperatures develop over the coming months. Things look to be getting interesting.

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