There has been considerable discussion with respect to whether the large number of cold waves and snow this winter in the Northern Hemisphere and last winter in the Southern Hemisphere (e.g. see Dot Earth and the several excellent posts on this subject at Watts Up With That, Climate Audit and ICECAP). They have accurately described this very unusual weather over vast areas of the Earth including the recent sharp cooling in the troposphere. This is clearly climate variability that has not been accurately captured by even the seasonal weather prediction models, much less the longer term global climate models.
Global Warming, of course, is just a subset of the more general issue of climate variability and change. Nonetheless, the political focus has been on global warming using the global average surface temperature trend as the metric.
Climate Science has repeatedly emphasized, however, that the proper way to assess global warming (or cooling) is with diagnoses of global average ocean content; e.g. see
A Litmus Test For Global Warming - A Much Overdue Requirement
The recent cooling in the atmosphere does not demonstrate that global warming has stopped for multi-year time periods. It does, however, highlight a major failing in that the multi-decadal global model predictions (and even the seasonal weahter prediction models) have predicted no such behavoir.
Moreover, as reported on Climate Science, global warming requires a more-or-less monotonic increase in the accumulation of heat (in Joules) within the climate system. The use of a global average surface temperature, regardless if it is increasing or decreasing is an inadequate and inaccurate metric of global warming as the heat is not only a function of temperature but also mass over which the heat change occurs! This is why the ocean is the dominate reservoir of heat content change.
With respect to the change in upper ocean heat content, as reported on a Climate Science weblog on February 15 2008, the paper
Willis, J. K., D. P. Chambers and R. Steven Nerem, 2008: Assessing the Globally Averaged Sea Level Budget on Seasonal and Interannual Time Scales. Journal of Geophysical Research-Oceans (in press),
reports on no upper (700m) ocean warming since 2004.
Thus while we cannot state that the recent widely distributed cold waves or overall cooling of the troposphere are evidence of the end of global warming over decadal and longer time scales, we can state that global warming has not occurred in the last 4 years. This is a major issue for both climate science and for policymakers, as only those who blindly (or deliberately) ignore the scientific evidence can still accept the 2007 IPCC conclusions as settled science.