As Houston struggled in the aftermath of the record-breaking rainfall from Hurricane Harvey, and no fewer than three hurricanes simultaneously moved across the Atlantic basin, a reader asked if I could explain the role of climate change in the recent extreme weather events.
I want to connect the dots with arguments that aren’t really controversial.
First, is the climate changing? Almost everyone would agree that this is the case. What some would dispute is whether human activity is a significant contributor. I accept that it is, but I won’t attempt to make that case here. If you don’t accept that human activity is impacting the climate, I won’t be able to convince you in a short article here.
But let’s agree that the climate is changing. There are multiple lines of evidence that indicate that the earth is warming. One piece of evidence is that the surface temperature of the oceans is climbing. NOAA (National Oceanic and Atmospheric Administration) has determined that since 1880, the average global surface temperature of the ocean has increased by about 1 degree Fahrenheit (°F). (See their methodology here). Relative to the average temperature from 1971 to 2000, the surface temperature over the past five years has risen about 0.5°F.
These measurements aren’t controversial, as they have been confirmed by many different studies. But the temperature change varies depending on where it is measured. For example off the coast of Greenland, the surface temperature has actually declined by about 1°F. But according to data from NOAA and IPCC (Intergovernmental Panel on Climate Change), across much of the Atlantic and Pacific Oceans, the surface temperature has risen 1.5°F-2.0°F since 1900.
Again, you might dispute the reason, but there is really no disputing the measurements (other than perhaps the magnitude of the increase).
There are some effects we can expect from a warmer ocean surface. I will discuss two, that are based on physics. Warmer water will lead to greater evaporation, which should show up as higher humidity in the atmosphere. That has been observed, although as with the ocean surface temperature, some areas have seen humidity decline.
The net effect is that more water vapor in the air means more rainfall on average. This helps explain why Houston, for example, could experience three “500-year” flood events in three years. Since there’s more water in the atmosphere, the probability of these heavy rainfall events has increased. In other words, they aren’t really “500-year” floods anymore. Normal has shifted. Some areas are going to experience more rain and some less, but on average precipitation is on the rise.
The other impact of warmer ocean surfaces is stronger hurricanes. Hurricanes (as well as typhoons and cyclones elsewhere in the world) are massive heat engines that convert warm water into strong winds. Hurricanes are fueled by warm seawater, which is why you see them dissipate when they move over land, or into cooler ocean waters.
Thus, it would be expected that we would see stronger storms as a result of warmer ocean surfaces. That was certainly the case with Hurricane Irma, which set a record with top winds of 185 miles per hour (mph) for 37 hours.
While some are quick to blame climate change anytime there is an extreme weather event, these events have been taking place throughout history. It is wrong to blame any particular event on climate change, but the physics of why we can expect some of these events to become more extreme is well understood — even if some still reject that human activity is driving it.