What worries one of the world’s leading climate scientists the most? Heatwaves – particularly the tendency of current models to underestimate the intensity of these bursts of deadly, searing temperature.

This is one of the “major mysteries” science still has to unravel, climatologist Robert Vautard said, even as researchers are able to pinpoint with increasing accuracy exactly how human fossil-fuel pollution is warming the planet and altering the climate.

“Today we have better climate-projection models, and longer observations with a much clearer signal of climate change,” said Vautard, one of the authors of an upcoming assessment by the United Nations’ panel of climate experts.

The assessment, the first of a trio of reports from the Intergovernmental Panel on Climate Change, will be released on Aug 9 at the end of meetings starting on Monday.

It focuses on the science underpinning the understanding of things like temperature increases, rising ocean levels and extreme weather events. This has progressed considerably since the last assessment in 2014, but so has climate change itself, with effects being felt ever more forcefully across the planet.

‘Phenomenal’ heat

Scientists now have a greater understanding of the mechanisms behind “extreme phenomena, which now occur almost every week around the world”, said Vautard, adding that this helps better quantify how these events will play out in the future.

In almost real time, researchers can pinpoint the role of climate change in a given disaster, something they were unable to do until very recently. Now, so-called “attribution” science means they can say how probable an extreme weather event would have been had the climate not been changing.

For example, within days of the extraordinary “heat dome” that scorched the western United States and Canada last month, scientists from the World Weather Attribution calculated that the heatwave would have been “almost impossible” without warming.

Despite these advances, Vautard said major mysteries remain. Scientists are still unsure what part clouds play “in the energy balance of the planet” and their influence on the climate’s sensitivity to greenhouse gases.

But it is “phenomenal temperatures”, like those recorded last month in Canada or in Europe in 2019, that preoccupy the climatologist. “What worries me the most are the heat waves and the thousands of deaths they cause,” said Vautard.

With rainfall, scientists have a physical law that says water vapour increases by 7% for every degree of warming, with intense precipitation increasing by about the same amount. But extreme heat is harder to predict.

“We know heatwaves are more frequent, but we also know our models underestimate the increasing intensity of these heatwaves, particularly in Europe, by a factor of two,” he said.

Tipping points

Climate models have come a long way, even since 2014, but there is still room for improvement to reduce these uncertainties.

“Before, we had models that represented the major phenomena in the atmosphere, in the oceans,” said Vautard.

Today the models divide the planet’s surface into grids, with each square around 10km. He said, however, that the “resolution of the models is not sufficient” for very localised phenomena.

The next generation of models should be able to add even more detail, going down to an area of about 1km. That would give researchers a much better understanding of “small-scale” events, like tornadoes, hail or storm systems.

But even on a global scale, some fundamental questions remain, especially pertaining to “tipping points”. These could be triggered, for example, by the melting of the ice caps or the decline of the Amazon rainforest, potentially swinging the climate system into dramatic and irreversible changes.

There are still a lot of uncertainties about tipping points, Vautard said, including what level of temperature rise might set them off.

They are currently seen as low-probability events, but he said it is still crucial to know more about them given the “irreversible consequences on the scale of millennia” that they could cause.

Another crucial uncertainty is the state of the world’s forests and oceans, which absorb about half of the carbon dioxide produced by humans. “Will this carbon sink function continue to be effective or not?” Vautard said.

If they stop absorbing carbon as has been found in areas of the Amazon, then more CO2 will accumulate in the atmosphere, raising temperatures even further. “It is a concern,” he added.