A recent article by the BBC has given new life to an old myth. The article references conclusions from the UN’s World Meteorlogical Organization that the La Nina conditions responsible for a colder winter will continue through a large portion of the year, resulting in cooler temperatures. This conclusion along with a convenient timeframe has breathed new life into the old claim that global warming stopped in 1998. The argument goes that temperatures have either flatlined or been in decline for a decade.
The trick is to use 1998 as your reference point. The theory of anthropogenic global warming does not mean that natural variability and climate forcings cease to exist. Those forcings continue to influence the climate as always. Some of these forcings work in concert with AGW, pushing temperatures higher. Others work in opposition, pushing temperatures lower. In 1998, the planet was going through what has been termed “the El Nino of the century”. El Nino, and its counterpart La Nina, are names given to certain conditions related to ocean and wind currents around the equatorial Pacific Ocean, and these conditions have significant influence on global climate while they exist. El Nino conditions tend to drive temperatures higher, and the strength of the El Nino in 1998 drove global temperatures to the highest on record according to some measures.
By contrast, La Nina conditions tend to drive temperatures lower. As we are currently experiencing a rather strong La Nina, global temperatures in the latter half of 2007 were cooler, and these conditions are expected to continue through about half of this year.
These cooler temperatures have led some to claim that not only did global warming stop in 1998, that we are headed for a long term cooling trend. Many of these claims originate from satellite data of atmospheric temperatures from the University of Alabama at Huntsville (UAH). Charting that temperature data from 1998 to the present yields the graph in Figure 1.
Figure 1: Lower tropospheric temperature, 1998-2008
But, if you look at the same data over the entire thirty year period it has been available from 1978 to the present, the longer term trend of global temperature paints a very different picture, as seen in Figure 2.
Figure 2: Lower tropospheric temperature, 1978-2008
Alternately, a graph of temperature data since 1998 to the present also paints a picture that is very different from the claim that temperatures have leveled out or cooled in the past decade. This can be seen in Figure 3, which charts temperature data from 1999-2008.
Figure 3: Lower tropospheric temperature, 1999-2008
Taking these different perspectives, it becomes obvious that while individual years may be cooler, the long term trend of warming continues, and claims of a halt to warming could not be further from the truth.