As the COP28 climate summit in Dubai, United Arab Emirates, kicked off this week, the meeting鈥檚 president Sultan Al Jaber said that keeping the key 1.5掳C climate target 鈥渨ithin reach鈥� was . But realistically, barring a catastrophic financial collapse, a massive meteor impact or all-out nuclear war, it now seems inevitable that the global average surface temperature will exceed 1.5掳C above the preindustrial level sometime around 2030.
To be clear, our failure to stay under 1.5掳C will have catastrophic consequences for people around the world. But it also presents a scientific problem 鈥� under the definition currently accepted by most climate scientists, we wouldn鈥檛 know that we have passed the limit until around 2040, as it requires looking back over an extended period to get an average figure.
That means much time could be wasted arguing about whether we have passed the limit instead of about how we cool the world back down, says Richard Betts at the UK鈥檚 Met Office. That is why he and his colleagues have proposed a solution. 鈥淵ou need a definition agreed in advance so that when we get there we can say, 鈥極K, right, let鈥檚 just get on with dealing with this鈥�,鈥� he says.
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The goal of keeping warming below 1.5掳C has its origins in the 2015 Paris agreement, when countries pledged not just to keep the rise in the global average surface temperature well below 2掳C above pre-industrial levels, but also to try to limit the increase to 1.5掳C.
However, the Paris agreement failed to define what a 1.5掳C limit means in practice. Instead, it was left to climate scientists to come up with a definition. One issue is the baseline 鈥� what was the pre-industrial temperature? Researchers have settled on the average temperature between 1850 and 1900. There was already some human-caused warming before then, but not enough temperature measurements were being made around the world to be sure of how much.
Another problem is that institutions, including NASA and the UK Met Office Hadley Centre, calculate the average global surface temperature in different ways, largely due to the way they handle the gaps in data in places such as the poles. But the differences are small and can be addressed by combining different records.
These records show that there have already been days and months when the average global surface temperature exceeded 1.5掳C above that of 1850 to 1900. It is touch-and-go as to whether 2023 will be the first year with an average 1.5掳C rise, says Betts. It could happen next year, if not this year.
But that wouldn鈥檛 mean we have passed the 1.5掳C limit. Climate is generally defined as the average weather over 20 years, so there is wide agreement among climate scientists that the limit won鈥檛 be passed until the 20-year average exceeds 1.5掳C. 鈥淭he date that we cross that is the midpoint of that 20 years. If you try and do a 20-year average now, you can only look at the last 20 years,鈥� says Betts. 鈥淪o your time of crossing is 10 years out of date.鈥�
One alternative is to use a running average. The trouble with this is that if there was a very hot period followed by cooler years, we could end up in a situation where 鈥渁ctually we haven鈥檛 exceeded 1.5掳C as we thought鈥�, says Betts.
In other words, to avoid a big delay in knowing when we have passed the limit, we need some kind of way of predicting the future 鈥� and we have exactly that in the form of climate model projections, realised Betts.
His proposal is to work out the 20-year average 10 years ahead, by combining the previous 10 years of temperature measurements around the world with what climate models predict for the next 10 years.
Because the model projections have so far closely matched the observed warming, there is good reason to think this approach will be very close to the 20-year average that would be calculated 10 years later based on observations alone. According to this method, the world had warmed 1.26掳C by the start of 2023.
鈥淭he important bit about our approach is it鈥檚 fully consistent with existing definitions,鈥� says Betts, so he is hopeful the proposal will be widely accepted. He is calling for the Intergovernmental Panel on Climate Change to do a special report on the issue.
When we do pass 1.5掳C, the focus needs to be on adaptation, limiting future warming and getting the temperature back down, says Betts. 鈥淵ou don鈥檛 want to be wasting time and being distracted by arguing about whether you鈥檙e at 1.5掳C or not.鈥�
Nature
