Guest Post by Dr. Roger Pielke Senior, University of Colorado
Image: NASA Earth Observatory. This map shows temperature anomalies for the Russian Federation from July 20–27, 2010, compared to temperatures for the same dates from 2000 to 2008. The anomalies are based on land surface temperatures observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Areas with above-average temperatures appear in red and orange, and areas with below-average temperatures appear in shades of blue. Oceans and lakes appear in gray.
There has been considerable discussion of the heat wave in Russia and of the floods in Pakistan and China as to whether these events are from global warming. Examples of this in the media include
Will Russia’s Heat Wave End Its Global-Warming Doubts? By Simon Shuster / Moscow
Climate change whips up floods, fire and ice by Brian Sullivan and Madelene Pearson
The second article starts with the text
CLIMATE change has been blamed for floods that have killed thousands and left millions homeless from Pakistan to North Korea, fires and a heatwave in Russia that have left 5000 dead and disrupted global food markets, and a severe tropical storm threatening Bermuda.
and includes the statements
The weather drew comment from officials and activists at international climate change talks in Bonn.
One US delegate said Russia’s heatwave and the recent floods that have devastated Pakistan are ”consistent with the kind of changes we would expect to see from climate change and they will only get worse unless we act quickly”.
A new article in the Economist
has a more complete discussion for these weather events. Excerpts from the article includes the text
“The immediate cause of the problems is the behaviour of the jet stream, a band of high-level wind that travels east around the world and influences much of the weather below it. Part of the jet stream’s meandering is tied to regular shifts of air towards and away from the pole, called Rossby waves. The Rossby waves set up wiggles in the jet stream, wiggles which, left to themselves, would move westward. Since the jet stream is flowing eastward, though, the net effect of the Rossby waves varies. When the waves are short, they go with the jet’s flow and the resultant wiggling heads downstream to the east. When they are long they go against the flow, and the jet’s wiggling is transmitted upstream to the west. In between, there is a regime in which the waves move neither west nor east, and the weather stays put.”
Part of the straightforwardness of that analysis is that it treats all the previous years equally. When instead Dr van Oldenborgh takes into account that there has been a general warming trend over those past 60 years the heatwave starts to look less improbable—more like the sort of thing you might expect every century. As the warming trend continues in the future, the chances of such events being repeated more frequently will get higher. A single heatwave cannot be said to have been caused by global climate change; but what is known about climate change says such heatwaves are now more probable than they were.
The intensity of this heatwave has been remarkable. It is hotter than at any time in the instrumental record. According to an analysis by Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute a straightforward comparison of the temperatures seen this summer with those of the past 60 years suggests that a large patch of Russia is experiencing temperatures which might be expected only once every 400 years or so. Some places within that patch are hotter than might be expected over several millennia.
In a world where greenhouse warming gets stronger, the tropics expand—an effect the beginning of which has already been observed. The paths of the jet streams to the north and south of the tropics will change in response to this. What that means for the interactions between jet streams and Rossby waves that lead to blocking, though, is unclear. Tony Lupo, an atmospheric scientist from the University of Missouri, has been looking at the question with some Russian colleagues. He says their climate modelling provides some reason to believe blocking effects might become more common in a warmer world, but also less forceful.
The attribution of the heat wave to atmospheric blocking this summer is a scientifically sound conclusion. The heat can occur from
- the advection of hot air from lower latitudes on the west side of a warm core anticyclone
- from compressional warming due to sinking air in the troposphere associated with the warm core anticyclone
- from a larger portion of solar insolation going into sensible versus latent surface heating as result of dry soils and stressed vegetation that occurs due to the absence of rainfall associated with the core of these anticyclones
- from added heating of the atmosphere from the absorption of solar insolation by aerosols from forest fires that occur in this dry environment.
[for a discussion of warm core anticyclones, see
Pielke Sr., R.A. 2002: Synoptic Weather Lab Notes. Colorado State University, Department of Atmospheric Science Class Report #1, Final Version, August 20, 2002.]
However, the statements that the tropics have expanded in recent years and the probabilities that such heat waves are becoming more common has not yet convincingly been made.
Indeed we looked at this issue for the heat wave in Europe in 2003 in the paper
Chase, T.N., K. Wolter, R.A. Pielke Sr., and Ichtiaque Rasool, 2006: Was the 2003 European summer heat wave unusual in a global context? Geophys. Res. Lett., 33, L23709, doi:10.1029/2006GL027470
where we found that the 2003 heat anomaly was particularly extreme near the surface (perhaps due to dry soil) but less anomalous in the rest of the troposphere. Our conclusions were confirmed in
Connolley W.M. 2008: Comment on “Was the 2003 European summer heat wave unusual in a global context?” by Thomas N. Chase et al. Geophys. Res. Lett., 35, L02703, doi:10.1029/2007GL031171.
We updated our analysis in
Chase, T.N., K. Wolter, R.A. Pielke Sr., and Ichtiaque Rasool, 2008: Reply to comment by W.M. Connolley on ‘‘Was the 2003 European summer heat wave unusual in a global context?’’Geophys. Res. Lett., 35, L02704, doi:10.1029/2007GL031574.
In the Chase et al 2008 paper we reported that
Figure 1 updates Chase et al.  through 2006 for 2.0 and 3.0 SD levels and adds to our original conclusion that 2003 was not very unusual in terms of the spatial coverage of extreme depth-averaged temperatures.
However, the addition of three additional summers (2004– 2006) to the time series, all of which appear to be relatively warm, now indicates the possible emergence of an upward trend as suggested in previous work [Stott et al., 2004]. For example 2.0 SD warm anomalies now appear to have an upward trend (p = 0.05) though this trend should be viewed with caution because of the small sample size and the dominant effect of data points at the end of the series. The rise in 3.0 SD anomalies comparable to the 2003 heat wave is, however, still insignificant (p = 0.16) and so the increased probability of such extremes with time suggested by Stott et al.  is not yet apparent.
Tom Chase will be updating this analysis through August 2010 in early September when the data becomes available. Then, instead of qualitative claims about an expanding tropics and a greater frequency of heat waves, actual climate data will be available to quantify whether or not the claims made concerning the tropospheric temperature anomalies are robust or not.
We have certainly seen a warm troposphere this year. The July lower tropospheric temperature anomalies were presented in my August 5 2010 post and the global spatial plot is reproduced below
The heat wave in western Russia is clear in the data along with a substantial warm anomaly in eastern Russia and part of China, as are smaller warm anomalies in other locations worldwide. Only Antarctica has a large negative anomaly [although interestingly, Pakistan has a modest below average lower tropospheric temperature anomaly]
This warmth presents an opportunity in the coming months to assess whether this is really related to a long term global warming related effect, or is due to some other aspects of the climate system (perhaps as modified by spatially heterogeneous forcing due to human activity including land use change and aerosols).
If it is a long term global warming signature, than the global average tropospheric warm anomaly will persist when the blocking pattern is removed. If, however, the lower tropospheric temperatures cool to or below their long term average and this heat cannot be found in the oceans, long term global warming cannot be the culprit. I will report on this early in 2011.