the united states is witnessing a massive, dangerous heat wave, as a huge system of high pressure covers the central part of the country. it’s a big enough deal that yesterday president obama even tweeted about it, including a map showing the maximum heat index in some parts of the midwest and southeast reaching 110 or 115 degrees on saturday. here in washington, temperatures could break 100 degrees friday or over the weekend.   this will, inevitably, lead to much talk of climate change in the coming days. so it’s important to separate the scientific wheat from the chaff and figure out what science can, and can’t, reliably say about the link between an event like this and a warming planet — especially in a year that, on a global scale, has shattered past temperature records for six out of the last six months. and the gist is that when it comes to extreme heat waves in general — heat waves that appear out of the norm in some way, for instance in their intensity, frequency, or duration — while scientists never say individual events are “caused” by climate change, they are getting less and less circumspect about making some connection. “as predictable as the sunrise, some will say heat waves always happened,” said marshall shepherd, director of the atmospheric sciences program at the university of georgia and former president of the american meteorological society. “yep, so did home runs in baseball, but the steroid era brought more and longer home runs. a new national academies study suggests that ‘heat waves’ may be one of the primary climate change markers like home runs were in baseball.” in other words, when a planet warms, the odds shift in favor of more intense or long lasting heat waves. that’s just plain logic. indeed, the national academy of sciences report in question notes that, “confidence in attribution findings of anthropogenic influence is greatest for those extreme events that are related to an aspect of temperature, such as the observed long-term warming of the region or global climate, where there is little doubt that human activities have caused an observed change.” “in particular, for extreme heat and cold events, changes in long-term mean conditions provide a basis for expecting that there should also be changes in extreme conditions.” and we’ve definitely already had changes in not only “long-term mean conditions,” but in heat waves themselves. the u.s. national climate assessment found that u.s. heat waves have already “become more frequent and intense,” that the u.s. is shattering high temperature records far more frequently than it is shattering low temperature records (just as you’d expect), and that it is seeing correspondingly fewer cold spells. as for future projections, meanwhile, the assessment added that “climate models project that the same summertime temperatures that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064 in the u.s. if global emissions of heat-trapping gases continue to grow.” however, pointing all of this out is not the same as making a specific attribution for this specific heat event — rather, it is saying that attribution can be made for this class or type of event. beyond that, specific attribution requires active research, and an attention to the actual temperatures, duration and other aspects of the weather phenomenon. that’s because, as shepherd notes, there are always heat waves, even in a stable climate. typically, in such an attribution study, scientists will use sets of climate models — one set including the factors that drive human global warming and the other including purely “natural” factors — and see if an event like the one in question is more likely to occur in the first set of models. researchers are getting better and better at performing these kinds of studies fast, in near real time. so don’t be at all surprised if we see such a study for the current heat wave event — just as we saw for, most recently, the devastating bleaching of the great barrier reef, which was tied to an extreme marine heatwave off australia’s northeastern coast. but in the meantime, some scientists and experts aren’t holding back. with every heat wave, probably the number one question is, is it climate change, or is it not? well the answer is, it’s both,” said texas tech university climate scientist katharine hayhoe on a press call thursday. “we get heat waves naturally, but climate change is amping them up, it’s giving them that extra energy, to make them even more serious, and have even greater impacts.” hayhoe didn’t attempt a specific attribution of the current heat wave, any more than shepherd did. rather, she’s once again articulating the general connection. “we are not used to having heat waves that are extreme as the ones we see today,” she said. it’s important to underscore, as hayhoe did, that this event poses severe risks to health — particularly for children and the elderly — and also to crops across the u.s. heartland. she pointed to an extreme 2003 heat wave that affected paris and europe, and which has indeed been connected to climate change through statistical attribution analysis. that event killed hundreds of people in paris and london, and a recent study attributed at least some of those deaths, themselves, to climate change. so these links are real, if not always simple to characterize. now, it’s all about watching this current event carefully, taking preparations, and seeing what scientists have to say once they run their analyses.
what science can tell us about the links between global warming and massive heat waves