21 March 2021

Climate science with the students

 I am teaching on our final year climate module again! And the way I go about it is to record lectures about the topics I find important, and then have a live session where I try to get some discussion going. An I tend to ask the students to prepare in some way. And there were two sessions I thought I'd discuss here. One was about climate sensitivity, and the other one was about a potential eruption of the Yellowstone super volcano.

For those who are unfamiliar with this concept; climate sensitivity is the globally averaged warming expected from a doubling of preindustrial atmospheric CO2 levels. And that is important, of course. We are on our way to reach that level; preindustrial values were 280 ppm so climate sensitivity tells us about what happens at 560 ppm, and we are already at 417 ppm on the day I write this. It will be crucial to know in how much trouble that will get us. And some of that is easy; an atmospheric physicist will have no trouble calculating how much global warming that would cause if the Earth was not a dynamic system. But the thing is, it is. There are all sorts of feedback mechanisms at play. The greenhouse effect will melt snow and ice, which reduces the planet's albedo, so increases global warming further. Higher temperatures lead to more moisture in the atmosphere, and water vapour is a greenhouse gas too. And so on. So finding out just how much warming an increase of 280 ppm CO2 causes is not straightforward.

I had asked the students to look up in article in which someone had come up with value for climate sensitivity. And then we discussed what the range was, how they had come to their conclusions, and whether there was a trend in that. And I am afraid there was.

In my lecture I had used three or four sources, and they had all come down to around 3 K. That is quite alarming as it is! But the students had found more recent sources, and some of the estimates went up to 5 or 6 K. They even found some old sources with values like that. And that is a lot more alarming still! And there didn't seem to be much of a trend in what methods they had used. Both modelling and using very long time series had ended up yielding results a lot higher than what I had seen before. Oh dear. I should make sure I keep my well-paid job so I can replace my gas boiler with a heat pump!

The other topic I wanted to mention here was Yellowstone; I suppose most people will know that this is a super volcano. And volcanoes might erupt. I wanted my students to figure out how bad that would be. And it wasn't particularly bad, really. Of course it will do a lot of damage. A bit of cold already caused a lot of grief in Texas. Imagine widespread ash fall! That will be a lot worse. But in the greater scheme of things, it will be minor.

We compared Yellowstone to 3 other organic events; the worst one was a generation of the Siberian traps, some 250 million years ago. This caused the biggest extinction event ever. And it was a much much much bigger event, and it was exacerbated by the regional geology. And reassuringly, the geology in the area of Yellowstone seems to be mainly rhyolite and granite, and these are quite inert. You can volcanically heat them but nothing will happen. Then we compared it to the Toba eruption. This is not very widely known, but it was the biggest eruption in the Quaternary. It happened some 75,000 years ago, so there already were people around, and we clearly didn't get extinct. This, by the way, was also a rhyolitic eruption and these are the more explosive kind. Yellowstone is an order of magnitude smaller, and is a hotspot volcano, so more a Hawaiian type eruption than an Indonesian one. And these are less severe. The magma will be less viscous with less gas in them, so the eruption will be more gloopy, and fewer sulphidic aerosols will make it into the stratosphere.

Then we had the Tambora eruption in 1815; it famously caused the "year without a summer" in Europe. This one was an order of magnitude smaller than Yellowstone. It is still a subduction volcano, so more likely to get aerosols in the stratosphere and cause mayhem. And additionally; all these Indonesian volcanoes which are close to the equator, will manage to get aerosols all around the globe, while Yellowstone will probably only affect the northern hemisphere directly. So altogether; if Yellowstone erupts I am sure it will be a big thing. But almost everything we found out about it was reassuring! And volcanoes work on rather long timescales. The chance that it will erupt in my lifetime is very small. So I will never find out exactly how disruptive it would be. But it was nice to do a climate session and not everything being doom and gloom!

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