What lessons from history's climate shifts?

Genoese map of the world, 1450 Image copyright SPL
Image caption Modern Europe is a very different place from that of the 1400s - so what impacts would climate change have?

Earlier this week, the journal Proceeedings of the National Academy of Sciences (PNAS) published a study on climate change that is at the same time scary, comforting, insightful and a statement of the obvious.

To be more accurate, I should probably say that the paper is capable of being interpreted in all of those ways, rather than risk implying that the authors intended to do more than run the numbers and see what popped up.

What they're talking about is climate change in Europe, specifically between 1500 and 1800 AD - a period that encompasses the so-called Little Ice Age.

It also encompasses a period that historian Eric Hobsbawm dubbed the General Crisis, when Europe was beset by a number of wars, inflation, migration and population decline.

So did the cold cause the chaos?

The method employed by David Zhang from the University of Hong Kong and his colleagues was basically to look for a chain of causality in changes in things such as temperature, crop yield, migration, famine, and war.

Image copyright SPL
Image caption Warmth, cooling - the 1500s-1800s saw both

On the one hand, their top line conclusion, contained in the paper's title, brooks little argument: "Climate change is the ultimate cause of large-scale human crisis".

Breaking that down, the chain of causality flowed from temperature changes through alterations in biological productivity to the impacts that might make up a "crisis" - war, famine, pestilence, migration and population loss.

"We conclude that climate change was the ultimate cause of human crisis in pre-industrial societies," they write.

So let me work backwards through the list of possible interpretations that I mentioned at the top of this post.

First, a statement of the obvious perhaps, because clearly in a pre-industrial society if you have a drastic change in climate (such as the rains not falling for several years) it's going to have a dramatic impact.

On the other hand, it's an insightful piece of work, because it tracks the chain of causality in a way that hasn't been done previously (at least as far as I can see) - producing what I believe is usually termed a "fine-grained" picture of events.

Why might it be thought of as scary? Pretty obviously, because if computer model projections are correct, major climate changes are coming within decades.

They might not be especially marked within Europe compared with Africa and other parts of the tropics.

But in a way, that's also the scary bit; because as has often been noted before, countries with the least developed infrastructures are in general more likely to be facing the biggest impacts.

Image copyright Other
Image caption It all starts with climate change...

Yet the word "development" may also give a dollop of comfort here and there.

And you can pick it up by looking at what's happening in Australia now.

As Jason Margolis from the BBC's The World co-production reports this week, the country is having to deal at the moment with a major drought, which could be here to stay - that bit's not certain, but a major regime shift to hotter and drier conditions is eminently possible.

The future is all to play for, but in Australia we are seeing indications of how a society can adapt to climate impacts - provided it has the wealth and infrastructure to do so.

One thing that I don't think can be considered comforting - though others may disagree - is that this PNAS analysis looked at a crisis caused by cooling, whereas in the near future it's warming to worry about.

As Australia among other countries shows, too little water as well as too much leads to trouble; and it surely matters not whether crop yields are cut by cold or heat.

The other big change since the 1600s is, of course, the number of people on the planet.

Dr Zhang's group notes that climatic shifts on the scale of a Little Ice Age would probably have had a small impact in North America because there was so much more land easily available for growing food.

With the human population set to top seven billion within a month or so, pushing up against the relatively small bits of forest and pristine nature we have left, that's a state of affairs that's been turned on its head the world over.

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