Climate Change Causes Extreme Weather—But Not All of It
Climate science is confusing. For decades, scientists have said that more CO2 means higher temperatures, longer dry spells, worse storms. But ask them whether global warming caused a Midwest heatwave, the California drought, or a New York hurricane, and they’ll explain ad nauseum how hard it is to untangle whether any single weather event is due to natural variation or climate change.
Hard, but not impossible. A new NOAA report released November 5 looked at how (or how not) climate change affected 28 different global events in 2014. These aren’t just nods of acknowledgement: Each study attempts to quantify how much climate change affected the duration, geographic extent, and severity of the weather event in question. For some events—like heat waves—the science is established enough that scientists can establish causal connections almost immediately. At the other end are things like tropical cyclones, rare and complex enough to elude current methods.
Yesterday, 50 years ago, Lyndon B. Johnson received a warning about the effects of climate change—the first of many to be issued to US presidents. Since that day, the discipline of climate study has been focused on large-scale effects of greenhouse gases: observing trends, establishing effects, making predictions. Those predictions—higher temperatures, stronger storms, yada yada yada—are familiar now. But mostly missing was any formalized way to look at climate change the other way; to identify a particularly nasty weather event and ask whether it was caused by climate change.
Event attribution began with a 2003 Nature paper that explored whether deductive tools from epidemiology could be applied to things like heat waves. “In epidemiology they look at things like, to what extend does cigarette smoking increase the chance of getting lung cancer,” says Heidi Cullen, chief scientist at Climate Central (Disclosure, this author interned for Climate Central briefly in 2013), and co-author of one of the papers in the new NOAA report. In 2004 came the first real example of an event attribution study (also published in Nature). “They found that climate change likely doubled or quadrupled the European heat wave of 2003,” says Cullen.
The general experiment is pretty simple to understand. Set up two models: one of the world without fossil fuels, the other one chuggin’ oil and burnin’ gas. Then run the scenario thousands, millions of times in each model, and eventually get a statistical likelihood for the heatwave happening in the world without humans versus the world as humans live today.
In fact, it’s so simple that some people in the field think these heatwave-climate change connections could soon be as rote as your local meteorological forecast. That makes sense. Warmer temperatures are what atmospheric greenhouse gases do best.
But drawing the connection between warmer temperatures and more complex weather events like drought, flooding, hurricanes—or even the expansion of the Antarctic ice sheet—is way more difficult. “For events like that, it’s not clear that we have an established agreement in the scientific community,” says Stephanie Herring, a NOAA climate scientist and editor of the report. The problem is complexity. Or maybe the problems are complexities. Anyway, it’s confusing.
The key metric in these models is a thing called fraction of attributable risk. That’s a measurement of how much stronger, higher, swirlier the thing being tested for—rain, temperature, wind—was in the emissions-heavy model. “For example, in a world without greenhouse gases a heat wave might come in at 5 degrees above normal,” says Herring. “With climate change, you add an extra 1.5 degrees.”
Flooding is only slightly more difficult to suss out than heatwaves, because scientists have a pretty good idea of the links between heat and moisture in the atmosphere. But the models don’t always agree on how that plays out. With a drought, the complexity increases because variables like soil moisture, snow pack, and deforestation come into play. In the study, authors determined that several of the droughts they looked at had no discernible links to climate change.
Tropical cyclones are the trickiest to study. Yes, warmer ocean temperatures mean cyclones have more available energy, but there are other complex things at play. Like how, if at all, does temperature relate to storm formation. Do wonky, change-afflicted weather patterns bring storms to virgin tracts of ocean? Also, despite their news value, cyclones are pretty rare. The satellite record only goes back to the 1970s. Prior to that, the sample size is limited to storms humans or their weather stations sat through first hand. “Because the hurricane data record is so short, there’s a lot of uncertainty connecting any individual storm to climate change,” says Cullen.
Considering that stronger storms are the marquee extreme weather event predicted by climate scientists, that uncertainty is a little frustrating. But scientists believe that eventuallyfiguring out all those complicated connections is the eventual goal.
Cullen is leading a project at Climate Central called World Weather Attribution to make these weather/climate connections faster and better. Ultimately she hopes to have this information readily available to people like politicians, leaders, and city planners so they can build a world that’s prepared for the worst climate change can dish out—and keep everyone else informed.