Recently NOAA wrote about the heat wave in western Russia, attributing it to "atmospheric blocking." They went on to say that greenhouse warming played no role in the high temperatures:
Despite this strong evidence for a warming planet, greenhouse gas forcing fails to explain the 2010 heat wave over western Russia. The natural process of atmospheric blocking, and the climate impacts induced by such blocking, are the principal cause for this heat wave. It is not known whether, or to what exent, greenhouse gas emissions may affect the frequency or intensity of blocking during summer. It is important to note that observations reveal no trend in a daily frequency of July blocking over the period since 1948, nor is there an appreciable trend in the absolute values of upper tropospheric summertime heights over western Russia for the period since 1900.
But something about this explanation doesn't smell right. Regional climate phenomena are always going to be conmplicated with many factors in play.
So I asked Kevin Trenberth, Head of the Climate Analysis Section at NCAR, and he says that global warming certainly played a role in the heat wave:
The "stalled jet stream" or really the so-called blocking pattern is merely a description of the atmospheric state or phenomenon, not a cause. The question is why is it like that? For that we seek to find systematic influences (what we call forcings) on the atmosphere that tends to lock it into one state. The main sources of such forcings are the mountains, land vs ocean, and the heating of the atmosphere. Only the latter changes. With El Nino or La Nina, the changes in sea surface temperatures change the areas where convection, thunderstorms, tropical storms etc, occur systematically. The heavy rains in those phenomena produce large heating of the atmosphere through the latent heat release: the release of the heat that went into evaporating the moisture in the first place is given up when the moisture condenses. It is that heating pattern that sets up unusual wave patterns and teleconnections in the atmosphere. It acts a bit like a rock in a stream of atmospheric air, with ripples up and downstream. In the case of the very active monsoon, there tends to be generally rising air and a lot of heat released in the rains, and some of that air was coming down over southern Russia. "What goes up, must come down". We can demonstrate a direct link between the anticyclone over Russia and the monsoon rains over southeast Asia. This is in addition to the waves in the jet stream.
Under normal circumstances, it is not unusual for this pattern to develop over Russia, but it normally lasts only a week or so. What is unusual is the persistence and duration of this, so that it lasted 5 weeks or so. Weather systems tend to wax and wane but the anticyclones that move through stall and strengthen systematically in the same region because of the influence from SE Asia through the overturning monsoon circulation and the associated wave patterns.
In this way, we can assign blame for the atmospheric pattern to that of the sea surface temperatures, and the current La Nina. The latter determines the pattern. The elevated SSTs in the Indian Ocean and Indonesian region arise because of global warming and the effects of the last El Nino, and bolster the amount of water vapor available for all the storms, resulting in the excessive rains and flooding.
So there is a chain of events here, and several things have come together to make it record breaking. But it is not unexpected, even if it is not predictable more than a couple of weeks in advance.
Global warming plays a role by 1) elevating the SSTs in the Indian Ocean and Indonesian region, where it contributes to the excessive moisture and rains that gave the flooding over Pakistan, India and China; and 2) In Russia by adding to the heat and drying, making the drought more intense, longer lasting, and with stronger and record breaking heat waves. These events would not have happened without global warming.