Extreme heatwaves are due to humans — but what about storms?

Just a few weeks after a record-setting heatwave in June, scorching temperatures have returned to Western Europe. It was also really hot a few days ago in the eastern and central US, and now temperature records are being broken in California. Is human-induced climate change to blame?

That’s the wrong question, according to the 2016 National Academies of Science, Engineering and Medicine report on “Attribution of Extreme Weather Events in the Context of Climate Change.” An answer to such a query “cannot usually be provided in a deterministic sense because natural variability almost always plays a role”, so it’s better to ask something like: “Are events of this severity becoming more or less likely because of climate change?”

In the case of the European heatwave in June, the answer to that is much more likely. A quick-turnaround, 32-page analysis by 14 scientists affiliated with the World Weather Attribution project concluded on the basis of climate models and temperature data that such extreme heat events have become at least five times more likely in France than they were before humans burning fossil fuels started sending huge quantities of carbon dioxide (CO₂) into the atmosphere.

“However,” they added, “the observations show it could be much higher still, a factor of 100 or more.” This week’s extreme-heat repeat would seem to put an exclamation mark on that assertion.

In a handful of recent articles in the annual “Explaining Extreme Events from a Climate Perspective” special issue of the Bulletin of the American Meteorological Society, researchers have actually been willing to conclude outright that specific weather events could not have happened but for human-induced climate change.

These were the summer 2016 phenomena of record global heat, an Asian heatwave and a marine heatwave off the coast of Alaska, as well as a marine heatwave between Tasmania and New Zealand in the southern hemisphere summer of 2017/2018.

In general, heatwaves and cold snaps (which are becoming less frequent) are the weather events about which climate scientists have become most confident in their attributions, according to the 2016 National Academies report. Meanwhile, the 2013 assessment report of the UN Intergovernmental Panel on Climate Change (IPCC) assigned “medium confidence” to the idea that climate change is increasing the frequency of extreme precipitation events, and “low confidence” to claims that climate change is causing more-intense droughts and tropical storms.

The climate scientists’ assessments aren’t necessarily all correct, of course. Spend a few hours reading through the attribution literature, though, and the impression one gets is of a good-faith effort to define what is known so far — and what isn’t — about the impact that increased concentrations of greenhouse gases are having on the climate and the weather.

Much public discussion of climate change and its impacts hasn’t been like that at all. The biggest difference has been the mostly bad-faith campaign to sow doubt that climate change is occurring, which currently seems to be headquartered in the White House. But there’s also a tendency among journalists (myself included), politicians, and other non-scientists who are concerned about climate change to attribute pretty much every untoward weather-related event to it.

Such attributions aren’t necessarily wrong — give it another few decades, sadly, and I imagine scientists will be able to link a lot more weather phenomena to greenhouse gas-related warming. But they often seem unhelpful, in that they can give a misleading impression of where the biggest risks lie and what we might be able to do about them.

Mississippi River ready for a new course

My personal bugbear on this front is the disappearance of much of Louisiana into the Gulf of Mexico, which is frequently depicted in the media as the result of climate change but is a much more complicated story than that. The state’s coastal marshes used to be replenished by sediment from Mississippi River flooding, and by the Mississippi’s penchant for choosing a new outlet to the Gulf every 1,000 years or so.

As John McPhee wrote in the classic New Yorker article “Atchafalaya” in 1987, the Mississippi has been ready to carve a new course since the 1950s, but the US Army Corps of Engineers has so far succeeded in preventing it from doing so.

Elizabeth Kolbert described in the New Yorker just this March how the corps and other government agencies are trying to reverse some of the resulting damage with controlled flooding, but thanks to the series of dams built along the famously muddy Missouri River in Montana and the Dakotas from the 1930s to and throughout the 1960s, the Mississippi carries a lot less sediment than it used to. Rising global sea levels are making this problem worse, but Louisiana would be shedding acres even if they weren’t rising at all.

The floods that have battered the US Midwest this year present another tricky case. The finding that global warming, by allowing the atmosphere to hold more water vapour, probably increases the likelihood of extreme rainfall is definitely relevant, and I wouldn’t be surprised if the 2020 “Explaining Extreme Events from a Climate Perspective” special issue includes an article on the Midwestern deluges of 2019. But other causes of the flooding include development along floodplains, the replacement of rainfall-absorbing native grasslands with less-permeable cornfields and impermeable parking lots, and the unintended consequences of dams, levees, and river channelisation.

The fact that many local officials in the Midwest avoid discussing climate change when talking about the floods, as the New York Times reported in May, is partly the result of climate-change denialism and the polarised political climate, which is bad. But it’s surely also that mayors prefer to focus on causes they can directly address, which they should.

Hurricane confusion

The extreme-weather phenomenon about which popular discourse may be most muddled is hurricanes — the western hemisphere name for tropical cyclones. As noted earlier, scientific opinion on their relationship to global warming is far from settled. The number of hurricanes making landfall in North America has trended slightly downward since 1900; hurricane damage is up mainly because more people have moved into harm’s way. This evidence is not inconsistent with theories that warming seas and air will result in fewer but more-intense tropical cyclones, but the infrequency of such events and the complex atmospheric dynamics involved make it hard for scientists to offer definitive pronouncements.

A paper published in May by the Bulletin of the American Meteorological Society proposed a more-forgiving “balance of evidence” standard to give a better sense of the risks ahead, and by that standard 10 of the 11 authors concluded that cyclones were probably becoming more intense, and eight that climate change was contributing to this. But only two of the 11 agreed that warming had contributed to an increase in hurricane storm surges along US coasts.

In other words, it’s complicated. Some aspects of climate change really aren’t — to cite the IPCC’s 2013 assessment report again, it is “virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas”. Taking climate change seriously requires differentiating between what’s “virtually certain” and the extreme weather events about which far less is understood.

• Fox is a Bloomberg Opinion columnist covering business. This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Bloomberg