Insensitive climate

Barry Brill points out the warmists don’t seem pleased at the recent good news about climate sensitivity (a new paper says it’s not going to warm catastrophically). Could it be because the warmists cry only crocodile tears about the forecast disaster and they’re actually looking forward to it? (It lets them control us.) – Richard Treadgold

— by Barry Brill, Chairman of the New Zealand Climate Science Coalition

Whether the future level of Anthropogenic Global Warming is dangerous or catastrophic or merely interesting turns entirely on how “sensitive” the climate is to carbon dioxide emissions.

The key sensitivity measure is the increase in temperature resulting from a doubling of the CO2 atmospheric concentration of 280ppm that is said to have existed in pre-industrial times.

Laboratory experiments have shown that sensitivity should be about 1°C before feedbacks – but the net impact of positive and negative feedbacks is the subject of incessant debate. Sceptics generally say negatives (especially clouds) are dominant so that the final outcome falls back to about 0.4°C. The majority view is that positives (especially water vapour) drive the outcome up to 3-4°C, while some catastrophists see runaway warming up to 6°C or above.

There is no consensus.

The science is not settled. The debate is not over.

But now, there is an intriguing new paper in the journal Science which calculates that a doubling of CO2 will most likely lead to a warming of 2.4°C, though the number could be as low as 1.7° or as high as 2.6°C.

“Our study shows that very high climate sensitivities are virtually impossible, suggesting that we still have enough time to deal with the problem and reduce carbon emissions, which could avoid the most severe impacts,” said Andreas Schmittner, a climate scientist at Oregon State University and the study’s lead author.

Eric Berger of the Houston Chronicle describes the research as follows:

“They compiled land and ocean surface temperature reconstructions from the Last Glacial Maximum about 20,000 years ago and created a global map of those temperatures. They then compared those values with those of climate models. There was a big difference, suggesting the models were overcooking the sensitivity of temperatures to carbon dioxide levels.

“If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought,” Schmittner said.

Predictably, Gavin Schmidt at RealClimate draws attention to the Schmittner team’s stated intention to test further models and says that one paper does not represent a new consensus. But it does move the bar, and presents a major hurdle for the catastrophists. Perhaps more importantly, it has drawn out the fact that mainstream ‘alarmists’ have rather more modest expectations than activists and the IPCC have led us to believe.

Berger marginalises the extremists

Is this significant? I asked Andy Dessler, a climate scientist at Texas A&M University, for his thoughts. His reply:

Ordinarily, when something is published in Science, you expect it to be a significant advance/revision of our prior knowledge. I’m not sure that’s the case here. The conclusion about the climate sensitivity is pretty consistent with what most climate scientists think, so I don’t think this paper will change what people think.

My sense is that most scientists consider the very high end of the sensitivity range (greater than 4°C) to be pretty unlikely (although it cannot be ruled out), and the most likely value for climate sensitivity is around, probably slightly below, 3°C.

In other words, I was not terribly worried about runaway climate change before this. After all, we know that the Earth’s had much higher CO2 in the past (and the temperatures were correspondingly much higher) and the Earth did not turn into Venus.

To me, the real effect of this paper will be to really impair the credibility of the more extreme environmentalists who have been saying the planet faces certain doom from climate change.

I am thinking about such efforts as Bill McKibben’s 350 campaign, in which he asserts that 350 ppm is the most important number in the world. Such environmentalists assert that the planet will warm as much as 6 Celsius degrees with a doubling of atmospheric carbon dioxide levels.

That’s a big number and doubtless would have catastrophic consequences for the planet. This is not in dispute. But scientists are now telling us this is not going to happen.
Clearly climate change is nonetheless worth being concerned about, and the levels of carbon dioxide and other greenhouse gases we’re putting into the atmosphere will have consequences now, and more serious consequences for our children.

But it is time to rein in the 6-degrees, world-is-ending rhetoric. This is a serious issue that will require serious solutions. Just as denial of any human role in a warming climate has no place in reality, neither does overstating the case for political action.

It seems appropriate that Dessler is quoted. Climate sensitivity is his special interest and he has recently locked horns with Roy Spencer over the impact of feedbacks. As it now turns out, his best guess is in the vicinity of the 2.4°C favoured by the Science paper. If we halve the difference between his estimate and Spencer’s (0.4°), we get approximately 1.5°C – a level which most scientists think would do more good than harm to the planet.

Do the maths

If the best estimate of the effect of doubling from 280ppm to 560ppm is 2.4°C, this will be the total change occurring between pre-industrial times (say 1750AD) and the doubling date.

CO2 concentrations have been increasing at about 2ppm pa in recent years to reach the current 388 level. If this continues unabated, the remaining 162mm to achieve doubling will take about 80 years – say 2100.

A rise of 2.4°C over 350 years would create a straight-line trend of about 0.7°C/century. That’s more or less what we’ve endured over the last hundred years without any great inconvenience, and we could certainly tolerate the same warming rate during the 21st century.

But that calculation doesn’t take account of the fact that increasing CO2 concentration has a logarithmic impact on temperature. So, if 40% (110/280ppm) of the doubling has occurred to date, about 75% of the AGW temperature effect has already happened. That leaves only 0.6°C still to come!

This might very well be the case. The IPCC admits it has no accurate estimate of the impact of human-generated aerosols, but all climate models assume they have significantly moderated AGW impacts for at least the last 50 years. And, while we don’t have any instrumental record of what the starting temperature was in the “pre-industrial” 18th century, all the proxies and historians tell us it was abnormally cold.

Whether the trend is logarithmic or arithmetic, it all augurs very well for the rest of the 21st century!

Some conclusions

The most recent “warmist” science (which accords with the mainstream view) puts the likely global impact of “business as usual” at only slightly above 2°C. That figure is gross – i.e., before the effect of aerosols – so the planet can probably squeeze under the target as long as most countries keep burning coal for their electricity needs.
However, even that can be seen as a worst-case scenario, because 75% of the AGW impact has probably already happened, leaving only a gross 0.6°C to come during the 21st century. And let’s remember that plenty of scientists don’t agree with the mainstream view regarding feedbacks, which account for most of that 2.4°C calculation.

Although the ‘dangerous’ level of 2°C warming seems to have little scientific backing, it is the figure provided to the G7 (when chairman Tony Blair insisted on having a number) as the point at which the costs of a warmer global atmosphere would clearly exceed the benefits.

Adopted at Copenhagen/Cancun/Durban, staying below the 2°C cut-off point has become the fervent prayer of climate worriers everywhere. In a sane world, the Schmittner paper ought to have triggered unconfined joy and public celebrations. Bob Brown should be delirious, and calling for more brown coal stations. There should be singing in the streets and “repeal carbon tax” parties.

It’s not happening. Perhaps people weren’t that worried after all.

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