Global warming is making some extreme weather events worse.
As Earth’s climate has warmed, a new pattern of more frequent and more intense weather events has unfolded around the world. Scientists identify these extreme weather events based on the historical record of weather in a particular region. They consider extreme weather events to be those that produce unusually high or low levels of rain or snow, temperature, wind, or other effects. Typically, these events are considered extreme if they are unlike 90% or 95% of similar weather events that happened before in that same area.
Global warming can contribute to the intensity of heat waves by increasing the chances of very hot days and nights. Warming air also boosts evaporation, which can worsen drought. More drought creates dry fields and forests that are prone to catching fire, and increasing temperatures mean a longer wildfire season. Global warming also increases water vapor in the atmosphere, which can lead to more frequent heavy rain and snowstorms.
A warmer and more moist atmosphere over the oceans makes it likely that the strongest hurricanes will be more intense, produce more rainfall, and possibly be larger. In addition, global warming causes sea level to rise, which increases the amount of seawater, along with more rainfall, that is pushed on to shore during coastal storms. That seawater, along with more rainfall, can result in destructive flooding. While global warming is likely making hurricanes more intense, scientists don’t know yet if global warming is increasing the number of hurricanes each year. The effect of global warming on the frequency, intensity, size, and speed of hurricanes remains a subject of scientific research.
Many factors contribute to any individual extreme weather event.
Extreme weather events are influenced by many factors in addition to global warming. Daily and seasonal weather patterns and natural climate patterns such as El Niño or La Niña affect when and where extreme weather events take place.
For example, many studies have linked an increase in wildfire activity to global warming. In addition, the risk of a fire could depend on past forest management, natural climate variability, human activities, and other factors, in addition to human-caused climate change. Determining how much climate change contributes to extreme weather events such as wildfires continues to be studied.
New scientific approaches make it possible to determine how global warming affected individual extreme weather events.
Even a decade ago, it was hard to link a specific weather event, such as a heat wave or an intense rainstorm, with climate changes happening on a global scale. However, climate scientists are getting better at making these kinds of connections, called extreme event attribution. These studies can’t say whether global warming caused a specific event—but they can look at whether the warming climate made an event more severe or more likely to happen. Scientists use computer models to simulate weather conditions with and without global warming and other contributing factors. By comparing different scenarios, they can identify how global warming has affected observed extreme weather events.
For example, scientists completed extreme event attribution studies after Hurricane Harvey soaked Texas in 2017 with record-breaking rains of more than 60 inches in some places. They concluded that global warming worsened the flooding and made a Harvey-sized storm at least three times more likely.
Understanding global warming’s impacts on extreme weather is important because it can help inform choices about managing risks. For example, if a community knows that increased rainfall from global warming has turned what was previously a “500-year flood” into a “100-year flood” (or more accurately: a flood that had a 1-in-500 chance of happening each year into a 1-in-100 chance of happening each year), it may make different choices about how to manage land, what and where people can build, or whether to build a floodwall.
- Climate at the National Academies
- U.S. Global Change Research Program – Fourth National Climate Assessment (2018)
- American Meteorological Society – Explaining Extreme Events in 2017 from a Climate Perspective
- National Academies of Sciences, Engineering, and Medicine – Attribution of Extreme Weather Events in the Context of Climate Change (2016)
- American Meteorological Society – Explaining Extreme Events in 2016 from a Climate Perspective