Good discussion on the last blog. Plus, an old man learned a new definition of "troll". A number of ideas were brought up--how warm it is in Greenland this year, snow cover, and the difficulty of extracting a climate signal from natural variability. I deliberately put in that figure with the "regime change". Today, in class, I was asked about the most credible criticisms of the climate change consensus. Definitely at the top of the list are modes of variability--have we accounted for them properly? Are there modes we have not yet discovered? We will return this, especially when we get to the hurricane question. (I know, you love hurricanes.) Still, I want to stay a little closer to classic climate for a while.
Back on February 11, 2007 the following figure was posted in one of the responses. Figure 1.
From Marian Koshland Science Museum
Variations of Northern Hemisphere Surface Temperature and Carbon Dioxide for the past 350,000 years.
There's a little glitch in the transition from the last ice age to the current temperate period. This glitch is called the Younger Dryas; it's labeled at the bottom of the figure. There has been a lot of study of the Younger Dryas, especially in the spirit of trying to understand "abrupt" climate change. The current leading theory, and recall that scientific theories are testable (not conjecture), is that the Younger Dryas was related to melting of the Greenland ice sheet, freshening of ocean, and disrupting the Gulf Stream. That glitch is definitely a period of significant cooling during a warming trend--a super example of the problem of accounting for dynamical variability.
The Lamont Doherty Earth Observatory has a good web page on abrupt climate change
, with, of course, references to the primary literature. This figure is a blown up version of the Younger Dryas. Figure 2.
From Lamont Doherty.
Variation of temperature and snowfall accumulation during the Younger Dryas. Note time goes back as you go to the right on the "x" axis.
You can see that about 15,000 years ago it was warming up, then it started getting colder again. As it was getting warming, however, the snow pack was increasing. This demonstrates one of the balance issues that we face studying climate change. The air holds more moisture, so it precipitates, and if it is cold enough to snow; it snows - it might even snow more. This observation led scientists to think for many years that warming might build the ice sheets--there might be a feedback loop that moderated the melting of the ice sheets. There may be some range of temperatures where this is true.
Blogs are supposed to be short, not a lecture, perhaps with a little dramatic tension. So the next blog I will talk about some of the recent work of Richard Alley at Penn State, which describes the physics behind the recent observed melting. Also, we have those legitimate questions of "snow trends"--think of that not only in terms of mid-winter snow cover, but in terms of springtime snow cover.