In June Christiana Figueres, the UN’s former climate chief who helped broker the Paris agreement in 2015, warned that the world has “three years to safeguard our climate”. It was a hyperbolic claim, even then. New research makes it seem even more of one today. An analysis published in Nature Geoscience on September 18th, by Richard Millar of Oxford University and his colleagues, suggests that climate researchers have been underestimating the carbon “budget” compatible with the ambitions expressed in Paris.
It may be possible for the world to emit significantly more carbon dioxide in the next few decades than was previously thought, and still keep global warming “well below” a 2°C rise above pre-industrial levels, which is what the agreement requires.
It is the total amount of carbon dioxide emitted, rather than the rate at which it is emitted, which determines how much greenhouse warming the world will undergo. This allows scientists to draw up budgets that quantify the total emissions associated with a given temperature rise. In the most recent report of the Intergovernmental Panel on Climate Change (IPCC) the carbon budget for a good chance of keeping global warming to 1.5°C—the preferred target of the Paris agreement—was 2.25trn tonnes of carbon dioxide since 1870. Estimates of the amount in fact emitted by the time of the Paris agreement were a smidgen over 2trn tonnes, and annual emissions at the moment are almost 40bn tonnes. This suggested that the total carbon budget would be spent by about 2020. Hence Ms Figueres’ alarm.
The calculations on which that IPCC budget was based—which were carried out in part by some of Dr Millar’s co-authors—depended on the use of a set of complex climate models called CMIP5 to replicate what had happened in the climate since 1870 and assess what might happen in the century to come. In the average of these models, which were run in the early 2010s, cumulative emissions took until a little after 2020 to top 2trn tonnes (ie, several years after they actually did so). Also, the temperature they predicted would prevail at that time was about 1.2°C above that of 1870 when, in actual fact, the temperature after 2trn tonnes had been emitted was only 0.9°C higher. In other words, the real world had seen slightly more carbon emitted, and less warming, than the models had suggested it should have.
The discrepancy caused Dr Millar to wonder what would happen if the CMIP5 results were shifted so that they centred on the known temperature and cumulative carbon-dioxide emissions for 2015 rather than 1870—if, in other words, you took a set of clocks with well-understood, if imperfect, inner workings and reset them all. The result was a sizeable increase in the budget. The world could emit about 750bn tonnes of carbon dioxide from 2015 onwards and still be “likely”—IPCC jargon for a two-thirds chance—to keep further warming below 0.6°C. Given that warming by 2015 was 0.9°C, this therefore defines the budget for staying below 1.5°C.
To confirm this, the researchers used a different type of computer model to look at futures in which emissions are reduced sharply from now on, the climate responds in the way it seems to have done in the past, and the overall temperature rise is limited to 1.5°C or less. They found it likely that this could be done with 920bn tonnes of post-2015 carbon-dioxide emissions. Given the uncertainties involved, that is a pretty close match to their other figure. Again, the carbon budget lasts a few decades, not just a few years.
These results are controversial. Those who have been sceptical about the case for strong action on climate change have fallen on them as evidence that models such as those used in CMIP5 have fundamental flaws. It is true that, as Dr Millar and his team point out, those models may have overestimated the cooling effects of some pollutants, and thus of the warming that would be “unmasked” when those cooling pollutants, such as sulphur from Chinese coal-fired power stations, were reduced.
At the same time, certain climate scientists have raised questions about the Oxford work. Some have doubts about the “resetting” of the CMIP5 results to the measured data for 2015. Another worry is that a number of different data sets claim to trace global temperature from the 1870s to now. The one Dr Millar and his colleagues used is that with the smallest increase. The set which shows the largest increase calculates the current temperature as being less than 0.4°C short of the 1.5°C target, rather than 0.6°C. Use this, and the new budget would be significantly smaller.
There are uncertainties, then, in both models and data, and no single study should be expected, of itself, to reset the world’s plans. Even if the Oxford paper’s new budgets were copper-bottomed truths, though, they would hardly provide the respite they might seem to. No one expected the constraints of the previous 1.5°C budget to be met, and meeting the new constraints would still be challenging.
Source: The Economist