One of the most common climate change skeptic arguments against AGW is that short-term declines in globally averaged temperature completely refute arguments about the occurrence and causes of global warming. A new paper published in Geophysical Research Letters (Easterling and Wehner 2009) argues that short term periods of no-trend or even cooling (nested within longer term warming) are in fact predicted by Global Climate Models.
Abstract: Numerous websites, blogs and articles in the media have claimed that the climate is no longer warming, and is now cooling. Here we show that periods of no trend or even cooling of the globally averaged surface air temperature are found in the last 34 years of the observed record, and in climate model simulations of the 20th and 21st century forced with increasing greenhouse gases. We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming.
The reality of the climate system is that, due to natural climate variability, it is entirely possible to have a period as long as a decade or two of ‘‘cooling’’ superimposed on the longer-term warming trend due to anthropogenic greenhouse gas forcing. Climate scientists pay little attention to these short-term fluctuations as the short term ‘‘cooling trends’’ mentioned above are statistically insignificant and fitting trends to such short periods is not very meaningful in the context of long-term climate change. On the other hand segments of the general public do pay attention to these fluctuations.
It is easy to ‘‘cherry pick’’ a period to reinforce a point of view, but this notion begs the question, what would happen to the current concerns about climate change if we do have a sustained period where the climate appears to be cooling even when, in the end, the longer term trend is warming?
We highlight two periods in 2001–2010 and 2016–2031 [see bottom figure above]. Both of these periods show a small, statistically insignificant negative trend based on a simple least-squares trend line and there are other periods, such as the last seven years of this simulation, that show a similar lack of trend. This behavior occurs without any simulated volcanic eruptions or solar variability (natural forcing) that could result in a widespread cooling for some period of years and thus is presumed entirely due to natural internal variability. Climate models are often criticized for producing a more or less monotonic-type response to anthropogenic forcing in 21st century simulations. Part of this may be due to the lack of volcanic and solar forcing I the SRES scenarios of anthropogenic forcing increase for the 21st century and part could be due to the fact that largescale oscillatory climate features, such as the El Nino Southern Oscillation are not well simulated. However, even considering these criticisms, it is clear that the models can and do produce sustained multi-year periods of ‘‘cooling’’ embedded within the longer-term warming produced in the 21st century simulations.
Therefore, it is reasonable to expect that the natural variability of the real climate system can and likely will produce multi-year periods of sustained ‘‘cooling’’ or at least periods with no real trend even in the presence of long-term anthropogenic forced warming. Claims that global warming is not occurring that are derivedfrom a cooling observed over such short time periods ignore this natural variability and are misleading.
Easterling, D. R., and M. F. Wehner (2009), Is the climate warming or cooling?, Geophys. Res. Lett., 36, L08706, doi:10.1029/2009GL037810