Climate sensitivity is the number of degrees C that the earth's average temperature (of the atmosphere air and water on top of the "earth" per se) will increase with a doubling of "pre-industrial CO2" in the environment.
This is an important number ... and it is a number, and to save you the suspense, the number is about 3 ... because it tells us what the direct effects of the release of fossil Carbon (mainly in the form of CO2) from the burning of fossil fuels would be.
Here's the thing. Climate change denialists would like the number to be 1, or some other number lower than 3. Well, we would ALL like the number to be low, but those of us interesting in actual science and truth and such things mainly want to have a good estimate of this important value. Climate change denialists want to pretend that the number is lower than it is, regardless of what that number may be.
A while back, an unpublished study was leaked that seemed to indicate, taken at face value, that the Climate Sensitivity Number was about 1.9. The study is flawed, and as I said, unpublished (as far as I know). But this gave all the climate science deniers tingly feelings and there has been a fair amount of talk about how this backs up the obvious lack of warming over the last decade, global warming isn't real, etc. etc.
One of the more insidious forms of climate science denialism is the small number of writers, some editorial in nature, some bloggers, associated with mainstream publications like the New York Times or Forbes, who either don't really understand the science, or do understand it but don't care that it is science and not politics, that it is something that needs to be gotten right, and that if they make unsubstantiated claims about the science someone will notice and provide corrections. The Economist is one of these mainstream outlets that tends to pander to the business community and pushes out stuff that is just bad commentary. Recently, a piece in the economist got the whole "climate sensitivity" thing and made a number of rather embarrassing mistakes.
These mistakes have been corrected by Dana Nuccitelli and Michale Mann in "How The Economist Got It Wrong" on the ABC web site.
Go read that to get what The Economist messed up. Personally I find this morbidly humerous because all the actual economists I've ever known, and I've known quite a few, pride themselves on getting complex stuff like this right, but here, The Economist made errors you would not let a Middle School student do in a basic Earth Science project.
Anyway, there are two key things that Nuccitelli and Mann point out that relate to the bigger picture that I wanted to mention here. These have to do with both the question of the climate sensitivity factor and the idea, which is incorrect, that warming has stalled for a decade (or some other number of years).
1) There is a fair amount of internal variability in climate systems. For example, if you want to measure the amount of CO2 in the atmosphere and count how much we are adding, you can do that, but you have to account for the fact in the natural earth system, CO2 moves in and out of the atmosphere at several different scales of time (seasonally, over longer term oscillations, etc.) and you have to account for that. The unpublished paper failed to do so, but the point I want to make here is that climate scientists can in fact account for these things. I see a lot of people realizing that the climate system is complex and from this concluding that it is unknowable. It is actually complex and knowable. (See this for a peer reviewed paper related to the topic of variation, and this for recent work on the specific role volcanoes play.)
2) If you look at climate data longish term (over decades) the earth is warming and we are in a warm decade. If you look at only data for the earth's atmosphere over the last decade or so, and close one eye and tilt your head and kind of squint, and pretend to not notice that most of the years in this decade are warmer than any prior average value, then you might see a bit of a flattening off of temperature rise. It would be nice if this was true. It would mean that global warming has slowed down despite the release of lots of CO2 into the atmosphere (never mind that the rate of release over recent years has gone down because of the economy). However, if you measure the ocean and air together, you get a different picture. The heat that ends up on the earth because we circle the sun at 93 million miles or so warms both the air and the sea, and the two interact (and the ground, too, but mostly the air and the sea). In fact, most of the extra heat from global warming goes into the sea. You have to measure both. When you do, global warming does not look like it has stopped. Also, we have recently discovered that an alarming amount of heat may be building up in the deep, cold sea. This heat is important.
Global warming. It is real. And, real important, despite The Economist getting it wrong.
Please go have a look at Nuccitelli and Mann's piece, and in fact, spread it around. Tweet it, facebook it, G+ it, give it to your mom. It's important.
Graphic from EDF
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