It seems the news has no shortage of extreme weather events: wildfires raged across Greece and the northwestern United States, flooding washed out the northeastern US, and intense heatwaves blanketed Japan and the United Kingdom. The island of Puerto Rico is facing down another hurricane season while many areas only recently regained power after Hurricane Maria devastated the island in 2017.
If it feels like we are hearing about extreme weather events more and more frequently now, it’s because we are. Large fires are now five times more common and fire season lasts three months longer than 40 years ago. The most intense rainstorms have increased by as much as 70% in the last 50 years and the city of Houston, Texas has seen three five-hundred-year floods—that is, floods so intense they are only expected to happen once every 500 years—in the last 3 years.
What does science say about the link between climate change and this increase in extreme weather? Can we attribute a single event, like a particular heat wave or wildfire or flood, to climate change?
Linking Climate Change and Extreme Weather
Global temperatures are rising. This fact is indisputable. Temperatures are also rising at the same time that extreme weather events are increasing across the globe. But as any scientist’s favorite warning tells us: correlation does not equal causation. Just because it seems to rain every time I forget my umbrella doesn’t mean I’m actually making it rain with my poor memory function. We’ve also seen a significant decrease in the number of seafaring pirates in the last several decades as temperatures have continued to rise, but that does not mean we all need to reconsider piracy as a profession in our efforts to find a viable solution to climate change.
So to understand whether or not climate change and extreme weather events are linked, we have to look at the physics behind their possible connections and not just the fact that they are occurring at the same time. It turns out, the strengths of those connections vary with different kinds of extreme weather.
Heat Waves
The extreme weather events with perhaps the most obvious link to rising global temperatures are heat waves or prolonged periods of higher than average temperatures. Around one third of the world’s population experience life-threateningly high temperatures at least 20 days out of the year, and that fraction is expected to increase to one half to three quarters of the population by 2100. As we become more and more used to the heat, it may be easy to forget that our circumstances weren’t always like this. As a friend said to me just last week, “People keep saying we’re in for another heat wave, but isn’t this just the weather now?”
Floods and Storms
Water expands when it warms so increased global temperatures lead to higher sea levels as the ocean waters expand. (Note that sea levels also rise due to melting Arctic ice.) The resulting higher sea levels, as well as the energy added by raising the temperature of the water, lead to an increased risk of storm surges that can cause coastal flooding.
According to National Oceanic and Atmospheric Administration, the average global surface temperature of our oceans is higher by 1 degree Fahrenheit since 1880. That’s only one degree over almost 140 years. However, comparing current ocean surface temperatures to temperatures from 1971 to 2000, half of that rise in global average ocean surface temperature happened in just the past 10-15 years.
Not only do the oceans warm under the influence of global warming, but the atmosphere does as well. A warmer atmosphere is better at holding moisture which provides any given storm with more fuel and thus can lead to heavier rainfall and flooding. This increased moisture can also cause more precipitation in the form of heavier snowfall when temperatures are colder.
Fires
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 Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein 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 everydayeinstein@quickanddirtytips.com. Image courtesy of shutterstock.
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