What is an actually safe target for higher global temperatures? This post presents the basic case for an argument of just 0.5C, which of course is well below the number we have lately arrived at. It was written by David Spratt, based largely on the newly published work of James Hansen and a group of like-minded colleagues. Hansen is the best climate scientist I know of and Spratt is perhaps the most diligent student, who has been at this work for decades. My own knowledge is highly limited, but I like the way these people think so much that I want to be sure their best ideas and conclusions are incorporated into these letters. Please read this carefully, and then decide if it is right for you. If so, well, you are part of a small minority, and should find it a struggle to maintain a sense of optimism, but isn’t that better than any alternative that is less than realistic?
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For today, let’s spend a little time talking about their argument.
What is wrong with 1.0C, which we are now slipping past? Hansen says that this temperature is around the peak reached in the Eemian, which then allowed a sea level rise of 6 to 9 meters as ice sheet melting was given the time to fully proceed. Today we have just started getting the melting process underway, which may now be proceeding in a manner that is unstoppable. That is reason enough, right there, to quickly turn around and try our best to go back. May I add that the Eemian period is not known to have had a CO2 level above 300 ppm. It could still have become as warm as we are today (at just over 400 ppm) because of an existing period of greater solar insolation, which is cyclical, and also a lower level of albedo. A reduction of albedo could quite possibly be soon headed our way, for more than one reason, adding comparably to our warmth.
Using the same scale, the maximum pre-modern temperature for the Holocene was plus 0.5C, about 7000 years ago. At that time the CO2 level was still under 280 ppm while the air was no doubt much cleaner than it is today, causing a lesser amount of albedo. I would like to ask Hansen if he truly believes that 350 is actually safe for a downside target for CO2 ppm, in the full light of results from his historical studies. Whatever, there exists the difficult problem of how any low number can be reached in time to help.
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Notes on the rising discussion about geoengineering and its costs. Geoengineering, by a variety of means, can be employed to cool the Earth either by direct capture of CO2 from the air or by increasing the planet’s albedo without any need to remove CO2. The former generally has the most support from scientists but is also the most costly while the latter is cheaper, more likely to prove effective with existing technology, but more risky and only useful as a temporary stopgap in an emergency mode. The article has some rough numbers about the cost of direct air capture, as currently known and as potentially hoped for. The challenge exists to find something new and better.
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From Greenland, more evidence that today’s level of warming is enough to destabilize glacial ice in a way that will lead to much higher sea levels. Another degree of warming, or just half a degree, would surely speed things up, as Hansen observes in the opening story.
Carl