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If The MWP Was Global, What Does That Tell Us About Climate Change Now?

A new paper is out saying that the Medieval Warm Period and the subsequent Little Ice Age extended to the Antarctic Peninsular. Most interesting for of course we’re seeing effects of the current warming there too. Leaving aside the boring details of what they actually did we get this:

This ikaite record qualitatively supports that both the Medieval Warm Period and Little Ice Age extended to the Antarctic Peninsula.

To give a little background: we know very well that Europe was as warm if not warmer than it is now some 800, 900 years ago. Grapes grew further north than they do now for example, Greenland (the warmth extended that far at least) could be used in part to grow wheat and so on. So many have been saying, well, if it was this warm or warmer before, what’s all the fuss about this climate change then?

The answer has always been, well, so far as we know that warming was local, not global, and now we’re having global warming which is a very different thing. This research brings that explanation into question.

It doesn’t, however, thus mean that we have nothing to worry about even as it means that we have less to worry about for a rather complicated reason.

That the Earth has temperature cycles isn’t a surprise, we all know there were Ice Ages. So that we had a time when temperatures were higher than now and everything was just dandy doesn’t mean that it will all remain dandy if we carry on in our current manner.

However, the great unknown of climate change science is “climate sensitivity”. This is how much temperature will rise given a doubling of atmospheric CO2 (technically, CO2-equivalent, converting all the methane etc to one handy unit). We know how much will come directly: 0.7 of a degree. That isn’t something to worry overmuch about.

The question then becomes, well, what happens then: when the ice melts and albedo lowers, when warmer weather and higher CO2 increase plant growth, what happens to clouds and so on through a list of hundreds of possible feedbacks. We don’t even know if some of these will be positive or negative, will further increase temperatures or reduce them and we certainly don’t know what the cumulative effect will be.

Currently the best guesses are in the 2 to 4.5 degree range but these are indeed guesses. Well informed guesses, being done with a variety of methods by very good scientists trying to get at the truth but they are guesses. And the most important information underlying them is, well, what happened previous times the Earth got warmer? Did it carry on doing so? Or did it get so far and stop or even retreat?

Which is where the MWP comes in. If it was a truly global phenomenon then we’ve more evidence (only more evidence, nothing conclusive at all) that rises in temperature similar to what we’re seeing don’t, always, carry on. Sometimes at least they stop of their own accord: could be solar action, could be feedbacks.

From which the takeaway point is that perhaps climate sensitivity is lower than currently thought and thus climate change is less dangerous than currently thought.

Although there’s one terribly important word we have to add here: maybe.