Greenhouse-gas emissions are increasing the frequency of heatwaves

EXTREME HEAT is ruinous to productivity, particularly if you are a criminal. Several American police forces posted messages to their social-media accounts last weekend declaring a moratorium on crime. “It is just too hot to be outside committing crimes,” wrote the Park Forest Police Department in Illinois, on its Facebook page. In some cases, it seems to have worked. “We have had zero customers stay the night at our ‘hotel’, so we appreciate all of the criminals adhering to the heat advisory,” tweeted the Malden Police in Massachusetts on Sunday.

The messages came as scorching temperatures swept across America, placing more than 100m people under excessive-heat warnings. Temperatures hovered either side of 40°C on the east coast. On July 18th Mitch Petrus, a well-known retired player of American football, died of heatstroke after working outdoors all day. At least five other deaths have been reported.

Europeans have also been sweating, for the second time this summer. A month ago, warm air from the Sahara contributed to making it the continent’s hottest June on record. At the top of Mont Blanc, western Europe’s highest mountain, instruments recorded 7°C (the normal June temperature would be below freezing). At Gallargues-le-Montueux near Nîmes, in France, temperatures peaked at 45.9°C. The previous record anywhere in that country was 1.8°C lower. Linked to these temperatures, in Alaska (see article) and Portugal (pictured), forest fires are raging.

If your hunch is that this kind of extreme weather is more common today than it was once-upon-a-time, you are correct. When, in 2003, tens of thousands of people in Europe died prematurely as a result of a two-week heatwave, it was deemed to be a once-in-1,000-years event. Twelve years later, a study led by Nikolaos Christidis of the Hadley Centre, the climate-research division of Britain’s Met Office, found that heatwaves of this severity had become once-in-100-years events, and would be commonplace by the 2040s.

The question on many people’s minds is whether these changes, and specific events like this week’s temperatures in America and Europe, are caused by greenhouse gases accumulating in the atmosphere. For years, the semi-official line was that no single weather event could be blamed on climate change, only trends. That began to change in 2004, with the publication of the first “attribution” study. This focused on the European heatwave of 2003, when average summer temperatures broke through a threshold until then unbreached in 150 years of records. By comparing simulations of a world with and without greenhouse-gas emissions, Peter Stott at the Met Office and his colleagues found that climate change had made the record-breaking heatwave at least twice as likely as it would otherwise have been.

Since then, research of this sort, intended to study how climate change is already promoting extreme weather, has grown rapidly. A recent, extended drought in California has been linked to greenhouse-gas emissions, as was the extreme heat southern Europe experienced during the summer of 2017. That event was made at least ten times more likely by climate change according to work published later that year by World Weather Attribution, a collaboration between experts in these sorts of analyses.

Shortening odds

Attribution work does not concern itself only with heat. Floods, storms and cold spells also carry a climatic fingerprint. When Hurricane Harvey hit America in August 2017, it stalled over Texas, delivering huge quantities of rain, which caused heavy flooding and more than 80 deaths. On that occasion, World Weather Attribution found that climate change was responsible for intensifying precipitation levels by between 8% and 19%. Since 2012, the Bulletin of the American Meteorological Society has published an annual compendium of attribution studies. Roughly 70% of events scrutinised show some influence from climate change.

One challenge has been to do the analyses faster. Findings connected with the heatwave of 2003 took a year to appear, by which time public interest had mostly moved on. The goal today is to offer a verdict on the influence of climate change on particular meteorological events more or less as they are happening. Here, the Met Office has been leading the way, with its Dutch, French and German counterparts close behind. But many other places do not have the capacity to carry out the onerous computer-modelling required. As a result, a European Union project planned to start before November will seek to provide contemporaneous weather-attribution analyses for the continent.

An inadvertent early test of how this could work took place last month, when many of Europe’s attribution scientists gathered at a statistical-climatology meeting in Toulouse, just as the June heatwave hit. Within days they published their conclusions. Accumulating greenhouse gases in the atmosphere had made the event at least five times more likely than would otherwise have been the case.

Such statements help show that the danger posed by climate change is clear and present, not just something for future generations to worry about. Heatwaves, for example, sometimes kill by the thousand—and can cause more casualties than other meteorological extremes, such as floods and hurricanes. But attribution also provides useful guidance to policymakers.

For instance, information about how much more likely an event is today than it was 50 or 100 years ago can assist decisions about building and adapting infrastructure. If what were thought of as once-in-a-millennium heatwaves now come once a century and will soon become so frequent as to be normal, then public-health systems need to be designed to cope with an influx of people suffering from heat stress. Likewise, if big floods are more frequent, water-handling systems need to be expanded and flood defences raised. Insurance and reinsurance companies are paying particular attention, because these calculations help them reassess risk levels.

Conversely, some people blamed climate change for a drought in south-eastern Brazil in 2014 and 2015, in which water levels in reservoirs around São Paulo and Rio de Janeiro fell to between 3% and 5% of capacity. But a study published in 2015 by Friederike Otto of the Environmental Change Institute at Oxford University found no sign that greenhouse-gas emissions had raised the risk of drought. Dr Otto concluded instead that a quadrupling of São Paulo’s population since 1960 had put pressure on scarce water supplies.

Attribution science is also playing a role in courtrooms and human-rights hearings. A study published in 2015 showed that climate change contributed to the high wind speeds of supertyphoon Haiyan, which blew through the Philippines in 2013, killing more than 6,000 people. Those stronger winds created a much bigger storm surge. The matter was raised during hearings held by the Philippine Commission on Human Rights last year, which sought to explore the question of whether fossil-fuel companies could be held responsible.

Clear, present and lethal?

Others have sought to pin companies down more specifically. In one widely reported lawsuit, Saúl Luciano Lliuya, a Peruvian farmer, is suing RWE, a German energy firm, for contributing to the melting of a mountain glacier that threatens to sweep away his village. Mr Luciano Lliuya’s counsel, Roda Verheyen, has said that the case “was mostly made possible by the advancement of…attribution science”. Lindene Patton, a lawyer with the Earth and Water Law Group, a firm specialising in environmental law, has written that “the science of event attribution may become a driver of litigation, as it shifts understanding of what weather is expected and, relevantly for law, foreseeable.”

To a layman, however good attribution science has become, trying to use it to link an event in the Peruvian Andes to a particular firm in Germany looks a bit of a stretch. But whether or not Mr Luciano Lliuya wins his case, the fact it is even being heard is a straw in the wind—and a sign that global warming can change metaphorical weather patterns as well as real ones.