Today, just for fun, and following all the uproar described in yesterday’s letter, I am going to describe a different way to think about climate sensitivity, or what to expect for temperature change when there is a doubling of CO2 in the atmosphere. All by itself, if nothing else changes but CO2, careful laboratory measurements have shown that you should get about a 1.0 or 1.1C increase. Anything more than that would be due to other things that increase. A whole suite of other greenhouse gases, methane in particular, are considered very likely to move higher in tandem with CO2, causing a combined impact of about 1.5C. That amount of warming will then cause an increase in water vapor large enough to double the effect, all the way up to 3.0C. Water vapor is the most powerful greenhouse gas but unlike the others its presence is extremely transient. Through changes in the evaporation rate it will immediately serve to amplify the warming or cooling effect created by any original source, of any type, by way of doubling that effect.
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That is the fundamental basis of CO2 sensitivity, but only the beginning. There are always other things to consider, which can be lumped under the heading of “boundary conditions.” The sun’s output is one of them, but changes are typically minor, and completely independent of what CO2 is doing. Albedo, on the other hand, Earth’s way of reflecting sunlight, can have a huge effect, and the way it changes may or may not closely coordinate with CO2 changes. The ice ages provide an ample demonstration of close coordination, with albedo (plus the way it affects water vapor) known to produce at least half of the temperature changes that occurred during the cycles, both up and down. Earth’s albedo is also affected in a large way by changes in the structure of cloud formations, by the passage of certain kinds of reflective aerosols, and by the kinds of things that are either growing or not growing on land. (Think of how bright desert sand is, for example.) Changes in the way the temperature of ocean water is distributed, especially on the surface, can also have a long-lasting effect on air temperatures. These are all boundary conditions that will have their own ebb and flow, often independent of CO2 but with an effect on temperatures that is always amplified by the usual water vapor feedback.
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The conclusion that can be drawn from these observations is that there is no point in talking about “equilibrium climate sensitivity” without factoring in all of the changes in boundary conditions that can be anticipated during the time it takes for equilibrium to be reached. Those conditions will differ from one era to the next, which means if you want to anchor sensitivity to CO2 change you will always come up with a different figure, one that is customized to whatever can be determined about the totality of the forthcoming situation. That’s where a lot of cutting edge research is taking place, searching for the best possible answers in an unprecedented situation.
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How hot was 2017? Four leading agencies that do the measuring have now made their reports. Two of them say last year was the second warmest on record and two say it was third. They all have different ways of estimating Arctic temperatures because of the difficulty involved in making direct measurements. No matter what, 2017 was the warmest ever without the presence of any gains due to an El Nino, which were significant in both 2015 and 2016. This story also notes that “It has been 33 years since the last month that the globe was cooler than normal, according to NOAA.”
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