What does science say about the link between climate change and this increase in extreme weather? Can we attribute a single extreme weather event, like a particular heat wave or wildfire or flood, to climate change?
Higher temperatures lead to more evaporation of water leaving soil dryer and thus better fodder for spreading fires. Snow also melts earlier each year which also results in dryer conditions.
Tornados and Hurricanes
Evidence of direct links between higher global temperatures and an increase in the intensities of tornados and hurricanes is less clear than with these other weather phenomena. While scientists agree that warmer, moister air hovering over the oceans will provide more fuel for a given hurricane, whether or not their frequency increases is still an area of ongoing research. Tornados involve factors like variations in vertical versus horizontal winds that complicate their connection to higher temperatures alone.
As summed up by the International Panel on Climate Change, a group of leading climate scientists from around the world, climate change is leading to changes in the “frequency, intensity, spatial extent, duration, and timing of extreme weather and climate events”.
Can a Single Extreme Weather Event be Attributed to Climate Change?
Ten years ago the answer to this question was a solid "no." Such extreme events like heat waves and major storms have happened throughout history so climate scientists were hesitant to blame climate change, and especially human-induced climate change, for any single event. But the science of identifying singular extreme weather events as results of human-caused climate change is changing fast. This new field of study even has a name: event attribution science.
The National Academies of Science, Engineering and Medicine recently published a 186-page report on the latest findings of event attribution science. But in case you don’t have a day to spare on the full report, they summarized their findings in a helpful infographic.
In order to link a single extreme weather event to human-caused global warming, scientists will need: a) an event they can simulate in models; b) a record of observed related weather trends that is long and frequent enough to show trends; and c) “physical processes that are well understood.”
Computer models of climate events are like having multiple earths where scientists can run different versions of the same experiment. Each individual input to the model—air temperature, ocean temperature, human activity, etc—is its own knob that can be dialed up or down in the model. So scientists can look at individual events like Hurricane Harvey and set all other conditions the same but remove human greenhouse gas emissions to see if the effects of the storm are as devastating as they were in reality.
So why are scientists suddenly better at building and using these models? The main factor is computing power. As computing power increases, scientists can add more knobs to account for more specific conditions. And as our knowledge of one particular knob increases, that helps inform how the other knobs are designed. Better computing power also means better resolution or detail. For now, models are better at simulating heat waves because they occur over large areas but worse at recreating tornados because they are more localized.
The NAS infographic gives the confidence levels of each of the three requirements for different kinds of extreme weather events. Extreme cold and extreme heat events have the most confidence across the board—meaning they have the most developed models, the most detailed observational records, and the best understood physical processes—while wildfires have the lowest confidence levels.
For example, the international World Weather Attribution project found links to human-induced climate change in four of the five extreme weather events they modeled from 2016, including coral bleaching in the Pacific Ocean, spring rain storms across Europe, summer flooding in Louisiana, and extreme Arctic warming, but not cold air outbreaks at the end of the year across the United States.
As for Hurricane Harvey in Houston? The models suggest that human-caused climate change increased the rainfall during the storm to levels that were three times more likely and 15% more intense than without the human activity.
More than 80% of people living in the United States report having experienced an extreme weather event in their lifetimes and the severity of those experiences are only set to increase. Our extreme weather preparedness plans already must not only include efforts to mitigate the occurrence of extreme weather but also adapt to changes in climate that are now inevitable.
Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at email@example.com. Image courtesy of shutterstock.