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Earlier this week I posted on uncertainty in climate model projections, raising the question of whether lack of certainty made the future more or less "scary" - and what it should mean for policymaking.

As would have been expected, I guess, your responses varied greatly - from Maurizio Maurabito's

"Richard's words clearly undermine the mitigation side of climate change...there is not enough detail to know what is the impact going to be, where it is going to hit and when."

... to Stefane's question:

"What do those who are adamant on not even trying to accept the possibility that humans may have been damaging the environment and on a greater scale, the Earth really want to do?
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"Most probably, in their opinion, we just should keep on polluting, killing and destroying everything that gets in the way?"

Still working without a time machine that can visit the future and tell us, there is another way of trying to evaluate climate impacts.

It consists of looking at what's happened in the Earth's past and attempting to calculate what that tells us about the future.

Especially for anyone who takes computer models to be GIGO-flop processors - Garbage In, Garbage Out - this should be an attractive option, depending as it does on real hard facts that can be taken from the real physical world around us (even if interpreting these facts is tricky – see below).

In this week's Science, Jeffrey Kiehl from the in Boulder, US, - and presents a vision scarier than the projections of computer models.

(Science lives behind a paywall, but chunks of the article along with its references.)

As he outlines, if the atmospheric CO2 concentration keeps rising as it has, by the end of the century it could well have risen to around 1,000 parts per million - about four times the level before the Industrial Revolution.

Take the time machine back about 50 million years, and [pdf link], leavened with cooler, lower CO2 interludes.

But how much warmer?

You can find a number of analyses concluding that typically, it was a few degrees warmer than now (I like the visual clarity of ).

But via calculations involving several recently-published papers, Jeff Kiehl sets out why temperatures could at times have been much hotter than that - a global mean 15C above the present, with concomitantly higher sea temperatures that would certainly challenge the life prospects for many current marine organisms.

As the author summarises things:

"Thus, if atmospheric CO2 reaches 1,000ppmv, then human civilization will face another world, one that the human species has never experienced in its history."

The reason why these large temperature rises could occur is, he explains, down to long-term feedback processes such as the melting of icecaps and vegetation changes, which materialise over centuries or millennia.

These aren't typically included in computer models, aiming as they generally do to project over a century or so at most.

As with computer models, interpreting palaeological evidence isn't straightforward.

For one thing, the Earth hasn't kept records perfectly tailored to our current research needs.

Secondly, and deriving from that imperfection, interpretations vary between scientists.

Thirdly, Planet Earth isn't exactly the same as it was 30 million years ago - partly because seven billion humans have done much to change it besides enhancing the greenhouse effect, such as reducing forest cover.

So deducing the future from the past wouldn't be straightforward, even if the dataset were perfect.

But as with the glacier projections I flagged up in the earlier post, what this analysis raises is a question of risk.

Our generation isn't looking at bequeathing our children or grandchildren a 15C warmer world - it'd be many generations after that down the line, if it comes at all.

But do we want to take that risk, when the real world evidence can be interpreted so as to project such an outcome - a true hothouse?

Or are we ok with that? Do the doubts in the interpretations mean we can sleep easy on a bed of business-as-usual?

Once more, over to you...

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NB: as JaneBasingstoke points out below (hat tip), a technical issue meant the bottom half of this article and comments were not visible to IE users. That's fixed now. Apologies. RB

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