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?
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.
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.