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CO2 doubling and beyond

11 Mar 2020 | Science Notes

Suppose in a wild worst-case scenario, instead of just doubling atmospheric CO2 over pre-industrial times, we somehow doubled it again (going to 4 times the starting level) and again (eight times the starting level). There probably isn’t enough fossil fuel in the ground to do so and, as we have previously noted, realistic business-as-usual scenarios wouldn’t get us even to the low end of this scenario. But suppose anyway. Runaway greenhouse effect? Planet cooks? Well, no. As we saw in our Simple Physics video, the warming effect of additional CO2 tapers off as more and more is added to the atmosphere. A new study using climate models unsurprisingly confirms this effect, but then finds something very unexpected: Past a certain CO2 level, cooling feedbacks due to cloud formation get strong enough to more than offset the new warming and create intervals where big CO2 increases make the planet cooler than pre-industrial times. So now what do you put on your protest sign?

If you watch our Climate Sensitivity video you’ll know that most models predict an “ECS” or “Equilibrium Climate Sensitivity” figure of about 3 degrees C warming for any doubling of atmospheric CO2, whereas data-driven estimates mostly predict under 2 degrees. The authors of the new study report that their model projects that ECS is about 2 degrees. So if it were to double from the 18th-century figure, and then double it again, which in real world terms would take hundreds of years if it were even possible, they still only get around 2 degrees additional warming, meaning 4 degrees total compared to preindustrial times.

While that number might surprise people panicked over “tipping points” where the warming supposedly accelerates uncontrollably in the presence of more CO2, the real scientific surprise came when they doubled it again in their model. The climate got a bit warmer, but it also started to cycle between cloudy, cool periods and sunny, warm periods. What’s more, the swings between the two were large enough that the mean temperature change became about zero instead of heading to infinity and beyond or, more soberly, going up by another 2 degrees.

The scientists don’t offer these results as predictions, of course. What they take away from it is that there are complicated cloud feedback processes that models have difficulty representing, and that changing the assumptions in the models a little bit causes big changes in the resulting warming patterns. And since those assumptions are themselves very often loose approximations, even leaps of faith, the result is that the uncertainty bars dwarf the results.

It’s not surprising. We’ve already shown that computing the basic effect of greenhouse gases is very complicated. As this new study reminds, us, trying to get the feedbacks sorted out is far from settled science.

In fact, we suggest you write “Chaos theory is scary” on that protest sign. Then go read a book on it.

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