WE MAY be in for more global warming than we hoped, 91av can reveal.
Over the past few years, a number of studies have concluded that a given level of carbon dioxide in the atmosphere produces less warming than previously thought. This rare good news on climate made headlines around the world.
But these studies were carried out towards the end of a period of little warming. Do the results still stand given the record warming in 2014, 2015 and 2016? To find out, 91av asked those behind the studies what would happen if the latest global temperature data was plugged into their models.
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One headline-making 2013 study had concluded that the immediate warming that would result from a doubling of CO2 in the atmosphere would be around 1.3 °C – significantly less than most previous estimates. If correct, this would mean we still have a chance of limiting warming to below the “dangerous” point of 2 °C despite soaring CO2 levels.
But this was before global temperatures shot past 1 °C above pre-industrial levels last year, as predicted by 91av in July 2015. If the 2013 study was repeated using that value, it would give an estimate for the immediate warming of 1.6 °C, says Piers Forster, one of the study’s authors based at the University of Leeds, UK.
If we assume that the average global surface temperature in 2016 will be a record-breaking 1.3 °C above the pre-industrial level, as expected, the estimate would be closer to 2.1 °C, Forster says. But that may be an overestimate due to the current spike in warming caused by a now-fading El NiÑo. “Maybe 1.6 °C is closer to the ‘truth’,” he says. Indeed, he points out that 1.6 °C is well within the uncertainty bounds of his 2013 study.
Even before the recent warming, several other studies had already concluded that the 2013 estimate of the immediate warming effect was too low. These suggested that Forster’s team underestimated how much warming has been masked by the cooling effect of other pollutants, such as sulphur aerosols, that we pump out alongside CO2.
In 2014, Drew Shindell of Duke University concluded that the immediate warming in response to a doubling of CO2 would be around 1.7 °C. A 2015 study by a team including Gavin Schmidt of the NASA Goddard Institute for Space Studies in New York got the same result. And this year, Trude Storelvmo of Yale University .
“Instead of a 1.3 °C rise for a doubling of CO2 in the atmosphere we may be looking at 1.8 °C or more“
Worse still, none of these later studies used any temperature data from after 2010. If they were repeated with the latest data, Shindell, Schmidt and Storelvmo all say the estimated value would be a little higher. “The estimate would probably be slightly higher if we had included data up to the present,” Storelvmo says. “We are in the process of doing that.” Only Shindell put a number on how much higher, guestimating that his approach would produce a value of 1.8 rather than 1.7 °C.
The reason these estimates are only slightly higher despite the record warming we have seen over the past couple of years is because the study methods were designed to minimise the effect of short-term temperature variations.
“Another few years of similar values will be another story,” Schmidt says. In other words, if rapid warming continues in the longer term – perhaps after a slight fall in 2017 due to the effects of La NiÑa – all estimates of climate sensitivity based on the warming over the past century would increase considerably.
Low warming fantasy
The bottom line is that low values for the immediate warming in response to a doubling of CO2 can now be ruled out. Similarly, low values for the warming in the decades following a doubling of CO2 can also be ruled out.
In its last report, the Intergovernmental Panel on Climate Change (IPCC) changed its estimate of warming after a doubling of CO2 from between 2 and 4.5 °C to between 1.5 and 4.5 °C, based on studies like Forster’s. This relatively minor change was seized upon by climate-change deniers as proof that the dangers of global warming had been exaggerated.
The next IPCC report will be revised back upwards, Shindell thinks. But the IPCC was right to lower it when it did, he says: its reports have to be based on the evidence available at the time.
While we can now rule out low estimates of climate sensitivity, Shindell says, we can’t rule out high estimates. “There’s a long tail of very high sensitivity that should dominate our thinking,” he says. When we buy house insurance, we take the worst-case scenarios – fires and floods – into account, Shindell points out. With climate change, we should also act based on the worst case scenarios.
Crunch time: double CO2 by 2050?
The immediate warming in response to a doubling of carbon dioxide levels in the atmosphere is known as the transient climate response. As other feedbacks kick in, such as changes in vegetation, warming continues.
Since pre-industrial levels of CO2 were around 280 parts per million, you might think a transient climate response of, say, 2 °C means the world will warm this much when CO2 levels pass 560 ppm. You’d be wrong.
That’s because CO2 isn’t the only greenhouse gas we are emitting: there is also methane, nitrous oxide and others. If their levels are translated into CO2 equivalents, the effective level of CO2 is already around 490 ppm, and on course to pass 560 ppm long before 2050.
However, we are also emitting aerosols, which are bad for our health but have a cooling effect. Countries such as China are trying to slash aerosol emissions to clean up their air, as Europe and the US have done already. If they succeed, warming will accelerate and the world will pass 2 °C above pre-industrial levels not long after the CO2 equivalent level passes 560 ppm; if aerosol pollution remains high it will take longer.
This article appeared in print under the headline “Earth’s sensitive side”
