Paris climate aim ‘still achievable’

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The 2015 Paris agreement’s ambitious goal of limiting global warming to 1.5C remains within reach, a study suggests.

The study is one of several to address the “carbon budget”, which – among other things – determines how much CO2 the planet can emit and still reach a given limit for global warming.

It indicates the 2015 target, perceived by some as tough, could be met with very stringent emissions cuts.

It used computer models that project climate behaviour into the future.

The aim of the Paris deal was “holding the increase in global average temperature to well below 2C above pre-industrial levels and pursuing efforts to limit temperature increase to 1.5C.”

But scientists admit they were taken by surprise by the ambition of this aim in the agreement.

The results of the work with computer models have been published in Nature Geoscience. This type of work, which involves projecting the behaviour of the climate into the future, necessarily contains uncertainties.

But the study authors say: “Pursuing ‘efforts to limit the temperature increase to 1.5C’ is not chasing a geophysical impossibility”.

Co-author Michael Grubb, from University College London, said: “This paper shows that the Paris goals are within reach, but clarifies what the commitment to ‘pursue efforts to limit the temperature increase to 1.5C’ really implies.”

Those commitments would require strengthening the nationally determined contributions (NDCs) – the pledges to cut emissions contained in the Paris agreement.

Analysis by David Shukman, BBC Science Editor:

The climate models are exaggerating. The predictions are too alarmist. The Tuvaluans and other islanders are safer than we thought. These are among the conclusions that some might reach from this latest work. In reality, nothing is quite that straightforward. The models are simulated approximations of possible futures. Inevitably they are going to be at least slightly adrift of reality, either in the amount of warming or its timing.

They come with caveats and margins of error. In many ways, it’s remarkable that these computer constructs are even roughly on track. And models designed to come up with very broad potential outcomes for the end of the century may not be fine-tuned enough to give more detailed forecasts year-by-year.

The authors themselves are anxious that their research is not misunderstood. The need for urgent action to reduce emissions is unchanged, they say. It’s just that the most ambitious of the Paris Agreement targets is not as unachievable as many once thought, that there is time to act, though the task remains a monumental one.

Scientists agree urgent action will be needed to tackle the effects of rapid temperature increase over the next century.

And a study earlier this year suggested the allowable carbon budget had probably been overestimated.

It said the “pre-industrial baseline” used to benchmark present day warming was probably older than the Intergovernmental Panel on Climate Change (IPCC) had assumed.

Therefore, the degree of warming since that baseline was probably greater than had been believed.

Method challenged

On Twitter, one of the authors of that report, Prof Michael Mann, said the latest research, published in Nature Geoscience, “doesn’t account for [the] pre-industrial baseline issue we examined”.

He added: “There is some debate about precise amount of committed warming if we cease emitting carbon immediately. We’re probably very close to 1.5C.”

But Prof Pierre Friedlingstein, from the University of Exeter, one of the authors of the Nature Geoscience study, said: “Previous estimates of the remaining 1.5C carbon budget based on the IPCC Fifth Assessment [Report] were around four times lower, so this is very good news for the achievability of the Paris targets.

“The Fifth Assessment did not specifically address the implications of the very ambitious 1.5C goal using multiple lines of evidence as we do here.

“The ambition of Paris caught much of the science community by surprise.”

Meanwhile, another study in Nature Geoscience, by Gunnar Myhre, from the Center for International Climate and Environmental Research, in Oslo, and colleagues, suggests the greenhouse effect caused by human-induced CO2 emissions is now half-way to doubling compared with pre-industrial conditions.

Although the concentrations themselves have not yet reached the halfway mark, they describe this as an iconic watermark.

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