Midwestern Heat Wave: Attribute to global warming?

A Climate Central reporter tries to tease out the contribution of climate change to the unprecedented heat wave in the Midwest, mentioned above, but the attribution studies have yet to be done, and many questions remain. 

For instance, the extraordinary leap in temperatures this month in places like Michigan is due in part to the fact that oddly little snow was on the ground before the heat wave started. If we assume that global warming has no part of that oddity, then the leap in temperatures should be separated from our attempts to attribute the overall weather pattern to this or that cause.

But what if climate change contributed to the lack of snow in the first place?

Story doesn't really get into that. Story does point out that the "blocking high" that knocked the jetstream off its usual path may have no connection to climate chage. Marty Hoerling, the silver-haired dead of Southwestern climate change modelrs, argues that the blocking pattern was not caused by global warming. The story explains: : 

Such “blocking patterns” are often associated with temperature and precipitation extremes, and were present during the 2010 Russian heat wave and the 2003 European heat wave, as well. However, there is a lot of uncertainty about what causes blocking events or how global warming influences them.

Martin Hoerling, a research meteorologist at Earth Systems Research Lab, agreed with [Randall] Dole, saying that although global warming is likely playing a role in this event, it probably did not play a major one. “Meteorology, not climate change, is the main ingredient in the current March 2012 U.S. extreme warmth,” he wrote. Of climate change, he said, “. . . its contribution to the magnitude of current conditions (+30°F departures [from average]) is quite small (but not zero) indeed.”

But he's talking specifically about the dome of high pressure. On a broader scale, as Kevin Trenberth of the National Center on Atmospheric Research points out in a study published openly this month, global warming is contributing to all of this:

Anthropogenic global warming inherently has decadal time scales and can bereadily masked by natural variability on short time scales. To the extent that interactions arelinear, even places that feature below normal temperatures are still warmer than theyotherwise would be. It is when natural variability and climate change develop in the samedirection that records get broken. For instance, the rapid transition from El Niño prior to May2010 to La Niña by July 2010 along with global warming contributed to the record high seasurface temperatures in the tropical Indian and Atlantic Oceans and in close proximity toplaces where record flooding subsequently occurred. A commentary is provided on recentclimate extremes. The answer to the oft-asked question of whether an event is caused by climatechange is that it is the wrong question. All weather events are affected by climate changebecause the environment in which they occur is warmer and moister than it used to be.

Here's my question. Suppose a similar blocking high developed over the Midwest in August. How much hotter could it get? Surely not 30 degrees. But how much?

Someone must be thinking about this…perhaps I should ask Mr. Trenberth.