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Have we entered a new phase of climate change?

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In just over a week, if all goes as planned, a colossal report on the state of the global climate will emerge from the UN’s Intergovernmental Panel on Climate Change.

This is the sixth analysis of its kind in 31 years and, like the other five, it will be a sweeping scientific assessment of how and why the planet is warming. Yet this report will be different. 

It will arrive as the impact of a shifting climate seems brutally apparent, not just on remote Himalayan glaciers or Arctic sea ice, but right in front of frightened human eyes.

In the past four weeks alone, wildfires virtually burnt a Canadian village off the map after it shattered the national record with heat of 49.6C. Floodwaters tore through German towns like a tsunami, tossing cars like corks. Terrified Chinese subway passengers stood in chest-high water as nearly a year’s worth of rain fell in three days.

Much of this was predicted. Scientists have warned for years that a warming climate would lead to more weather extremes. Yet the frequency and severity of these events raise unsettling questions: could we be entering a period of non-linear climate change, where temperatures and extreme events do not increase smoothly as expected but instead come suddenly, more often and perhaps more powerfully? And if we are, how would we know?

The short answer is that scientists are divided about whether a more dangerous phase of non-linear change has begun. “I don’t think it’s correct to conclude that’s what we’re seeing, though I have seen people arguing this,” says Michael Mann, director of the Earth System Science Center at Pennsylvania State University. “It’s not so much that climate change itself is proceeding faster than expected — the warming is right in line with model predictions from decades ago. Rather, it’s the fact that some of the impacts are greater than scientists predicted.”

One of the most striking effects came at the end of June as a prolonged heatwave scorched western parts of Canada and the US Pacific Northwest. Records that had stood for decades were smashed by as much as 5C.

“That’s just sort of staggering,” says Brian Hoskins, chair of the Grantham Institute for Climate Change at Imperial College London. “For many years I’ve said that the projections from the climate models are what we get if we’re lucky, because their behaviour is very smooth. If you take the output from models, then that heatwave should not have happened.”

Geert Jan van Oldenborgh, a climate researcher at the Dutch national weather service, says the record North American heat has “shaken the confidence of a lot of climate researchers”. “It means that the assumption that we had about how heatwaves react to a gradual increase in global warming may not be correct,” he says.

Van Oldenborgh co-leads the World Weather Attribution group of scientists who concluded this month that the North American heatwave would have been “virtually impossible” without human-caused climate change. He and colleagues are now planning wider research that will look at whether there is any evidence to suggest the climate is in fact starting to change globally in a non-linear way. Could it be, for instance, that changes in the jet stream or the migration of drought zones are triggering shifts we do not yet understand?

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This work is notable, considering the role of the scientific phenomenon that researchers such as Harvard historian Naomi Oreskes have called ESLD — Erring on the Side of Least Drama. Climate scientists have been relentlessly accused of fear-mongering and alarmism. But as Oreskes and her colleagues wrote in a 2012 paper, “core scientific values of objectivity, rationality and dispassion” have led to conservative projections about the impact of climate change, even in IPCC assessments.

This has not stopped the study of exceedingly dramatic concepts such as “tipping points” or thresholds that, once crossed, lead to drastic changes such as the loss of the West Antarctic ice sheet or Amazon rainforest. Indeed, a leaked draft of the new IPCC report suggests it may cover such shifts in more detail than past assessments have.

Despite, or perhaps because, these scenarios are so bleak, some of the scientists focusing on them have begun to offer more hopeful ideas.

The logic of a tipping point means it could also set off the irreversible advance of electric cars, renewables and other decarbonising measures, says Tim Lenton, director of the Global Systems Institute at the University of Exeter: “That’s the glimmer of hope.” 

In July 2021, however, it still feels distinctly remote.

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