Common Science

Harvey, the tropical storm that made landfall as a Category 4 hurricane on Friday night, is one of the worst to hit the U.S. in over a decade. It has sustained 130 mph winds, traveled 3,100 miles and is expected to dump over 50 inches of rain throughout its reign, according to The Weather Channel. 

Climate scientists are hesitant (as they should be, this is science) to diagnose any storm as being directly related to climate change. With that being said, they’re fairly certain 200 years of fossil fuel burning has raised temperatures enough to make matters worse. 

“In general, the way to think about it is: climate change has changed the environment that everything is happening in,” senior scientist at the National Center for Atmospheric Research Kevin Trenberth told National Geographic. “When you add in the climate’s natural variability and then the right conditions come along, you can get a storm which is stronger than you might otherwise have expected.” 

According to NASA’s Third National Climate Assessment Report, long-term effects of global climate change include changes in precipitation patterns, a rise in sea level and stronger, more intense hurricanes. Since the early 1980s, the frequency of Category 4 and 5 hurricanes has increased and hurricane-associated rainfall is expected to increase as the climate continues to warm. 

Hurricanes tend to lose strength as they get closer to land because they lose access to energy from the hot, wet ocean air. Harvey’s winds, however, intensified by 45 mph in the 24 hours before it hit land, according to the National Hurricane Center. 

The warm ocean waters provide energy for the hurricane, but as they grow, they also pick up seawater, which churns the ocean and pushes warm water deep below the surface. According to Trenberth, the storm is then weakened by the new, cold water being close to the atmosphere. 

Harvey did this too — it churned up the ocean water — but the problem was that ocean water 200 meters below the surface was STILL warm. Instead of weakening the storm, the churning intensified it. 

“Hurricanes are powered by the evaporation of sea water,” MIT atmospheric sciences professor Kerry Emanuel told National Geographic. “Water evaporates faster from hot surface than a cold surface.” 

Because the storm pulls up moisture it already dumped on land that’s once again evaporating, the storm is basically feeding itself as a self-sustaining system. 

According to Emanuel’s research earlier this year on changes in hurricane speeds, a storm that increases by 60 knots in the 24 hours before landfall was likely to occur once per century in the 1900s. Later this century, Emanuel expects them to come every 5-10 years.  

“[The ocean heat] is the main fuel for the storm,” Trenberth said. “Although these storms occur naturally, the storm is apt to be more intense, maybe a bit bigger, longer-lasting, and with much heavier rainfalls.” 

The increased precipitation has been the most damaging aspect of Harvey, and it’s also what’s worrying climate scientists the most: when the earth heats up, rainfall increases in regions along the equator. Heat from the ocean allows the storm to intake more water vapor, and because the atmosphere is warmer, its capacity to hold moisture increases. 

“The big story is the precipitation — it’s a no-brainer,” senior staff scientist at the DOE’s Lawrence Berkley National Laboratory Michael Wehner told National Geographic. “Pretty much everything we’ve done so far has suggested that in this situation, precipitation will be increased. Lots of simulations are being done. In every one of them, it rains more.” 

According to National Geographic, every scientist contacted was in agreement that the increased volume of rain was almost certainly driven by increased temperatures from human carbon dioxide emissions. 

“The human contribution can be up to 30 percent or so of the total rainfall coming out of the storm,” Trenberth said. “It may have been a strong storm, and it may have caused a lot of problems anyway, but human-caused climate change amplifies the damage considerably.” 

A draft of the U.S. governmental review of climate science, due later this year, says there is “medium confidence” that human activities have contributed to the upward trend of hurricane activity since the 1970s. 

So while we can’t say that climate change directly “caused” Hurricane Harvey, we can say that it played a part in exacerbating certain characteristics of the storm and intensified its rainfall and wind speeds. This is exactly the kind of weird weather climate scientists have been warning us about. 

Rachael Snodgrass is the opinion editor. Follow her on Twitter @RachaelReneeee.