WASHINGTON, Sept. 3, 2010 - Every hurricane season, the Union of Concerned Scientists (UCS) receives questions about the science linking hurricanes to climate change. In short, global warming is making the ocean warmer, which in turn makes it more likely for hurricanes, when they do form, to become stronger and more destructive. Meanwhile, the link between global warming and the number of hurricanes that form each year remains unclear.
When examining the link between climate change and extreme weather, it is important to bear in mind that each storm is just one data point in Earth’s climate history. Statistically, an individual hurricane is like a single at-bat for a baseball player, while conclusions about how climate change affects hurricanes is like calculating a baseball player’s batting average. Scientists draw meaningful conclusions from analyzing many storms over many decades.
Besides contributing to hurricane intensity, climate change is also causing sea levels to rise. As the ocean warms and expands and as land-based glaciers melt into the ocean, sea level rises and storms reach further inland before dissipating. Higher sea levels give coastal storms a higher starting point when they make landfall, especially if storms make landfall during high tides. Unless communities improve coastal planning, hurricanes will cause more property damage over time due to the proliferation of public and private development along coastal areas.
Two links between climate change and hurricanes are less clear, however. Scientists aren’t sure how climate change will impact wind shear -- the difference in wind speed and direction in a given location. Low wind shear makes storms more likely to grow into hurricanes while high wind shear can put the brakes on storms and prevent them from becoming hurricanes. Additionally, natural ocean cycles can enhance or mask the impact of climate change over a period perhaps as long as 20 or 30 years. Scientists expect that global warming will overwhelm those natural cycles in the long-term, especially if atmospheric carbon dioxide levels from burning fossil fuels and destroying tropical forests keep rising.
Often, people don’t understand that scientists have varying levels of certainty when it comes to different consequences of climate change. Climate contrarians try to take advantage of this knowledge gap when they conflate scientists’ relatively low certainty about how climate change affects hurricanes with their relatively high certainty regarding other, serious consequences of climate change, including sea level rise, long-term drought, reduced crop yields and species extinctions.
Much of the scientific uncertainty regarding climate change and hurricanes has to do with a lack of data. The most accurate hurricane data come from satellites, which have been observing weather for a little more than 30 years. And while pre-satellite data from ships and land-based stations can help scientists understand when hurricanes hit and roughly how powerful they were, those data are not specific enough to allow scientists to draw precise conclusions. Scientists have a less precise, but longer record of hurricane data from the geologic record. Because hurricanes are so powerful, scientists can still see evidence of their impact on coastal lands going back thousands of years. By contrast, the global temperature record comes from reliable thermometers and goes back to the 1880s, so scientists’ conclusions about global temperatures are quite certain. Similarly, scientists have accurate precipitation records going back decades and can therefore draw robust conclusions about how climate change is affecting precipitation.
Uncertainty is also related to scale. It is easier for scientists to make certain conclusions about global phenomena, such as the increase in temperature over the past several decades or changes in precipitation over large areas of land. However, it is harder to determine climate change’s impact on smaller-scale events. When studying hurricanes, scientists use specialized hurricane models that run within global climate models to understand the factors that affect hurricane frequency and intensity. Overall, scientists can have greater confidence in their conclusions regarding hurricanes than they can other types of smaller-scale extreme weather. For instance, climate change’s impact on tornadoes remains unclear.
