“This event is unprecedented & all impacts are unknown & beyond anything experienced,” the National Weather Service (NWS) tweeted Sunday along with a projected precipitation map for Hurricane Harvey.
Although many meteorologists have said they had never seen such a storm — or such a tweet from the NWS — before, the fact is that Harvey is exactly the kind of off-the-charts hurricane we can expect to see more often because of climate change.
“Climate change worsened the unprecedented disaster unfolding in Houston from Hurricane Harvey,” as climatologist Michael Mann said in an email to ThinkProgress. “And unrestrained climate change means we will see many more Harveys in the future.”
Like a baseball player on steroids, our climate system is breaking records at an unnatural pace. Global warming is juicing storms — a key reason Harvey is the second 1-in-500-year superstorm in 16 years (and fourth 100-year rainstorm since spring 2015). And like a baseball player on steroids, it’s the wrong question to ask whether any given home run is “caused” by steroids.
Every stage of Harvey — the rapid intensification that makes for a forecasting nightmare, the brutal storm surge, and the unprecedented rainfall — were worsened by global warming. In fact, there’s been so much rain, the National Weather Service had to update their color maps to cover it all.
Hurricanes “extract heat energy from the ocean to convert it to the power of wind, and the warmer the ocean is, the stronger a hurricane can get if all other conditions that it needs to exist are present,” meteorologist and former hurricane hunter Jeff Masters explained last month on Living on Earth. “So, scientists are confident that as we continue to heat up the oceans, we’re going to see more of these high-end perfect storms.”
Let’s look at some of the latest climate science. The first stage of a hurricane is formation and intensification. Harvey spun up from a tropical depression to a Category 4 superstorm in two days as it crossed Gulf of Mexico waters that were 2.7 – 7.2°F warmer than “normal” (the 1961-1990 baseline).
The latest research says global warming is driving this trend. “Storms are intensifying at a much more rapid pace than they used to 25 years back,” explained the author of a 2012 study. “They are getting stronger more quickly and also [to a] higher category. The intensity as well as the rate of intensity is increasing.”
This warming-driven trend toward more rapid intensification is very worrisome. “The vast majority (79 percent) of major storms” are rapid intensification storms,” and “the most intense storms” are those that undergo rapid intensification according to a 2016 study. And rapid intensification makes it much harder to predict and plan for superstorms.
One 2013 paper found that “since 1975 there has been a substantial and observable regional and global increase in the proportion of Category 4–5 hurricanes of 25–30 percent per °C of anthropogenic global warming.” Another 2013 paper concluded that “dramatic changes in the frequency distribution of lifetime maximum intensity (LMI) have occurred in the North Atlantic,” and the stronger hurricanes “have become more intense.” A 2015 study on the impact of sea-surface temperatures on the intensity of hurricanes in the North Atlantic found “intensification increases by 16 percent for every 1°C increase in mean SST.”
While we aren’t seeing more total hurricanes, we are seeing more of the Category 4 or 5 super-hurricanes, the ones that historically have done the most damage and destroyed entire coastal cities. We’re also seeing a sharp rise in the most damaging storm surges, whereby even a Category 1 hurricane (such as Sandy) can cause unprecedented damage.
On our current path of unrestricted carbon pollution, NOAA researchers have determined that parts of the East Coast would see Sandy-level storm surges every year by mid-century.
After landfall, the winds die down, the storm surge recedes and it is rainfall that does the most damage. As climatologist Katharine Hayhoe tweeted last week, “As the world warms, evaporation speeds up. So on avg there’s more water vapour for a storm to sweep up & dump now, compared to 70 years ago.”
In big slow-moving storms like Harvey, the water vapor swept in can be even higher. As climatologist Kevin Trenberth told the Atlantic, “the human contribution can be up to 30 percent or so up to the total rainfall coming out of the storm. 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.”
This is the phase of Harvey we’re now in. And while it’s already been the worst storm Houston has ever seen, there is still more rain to come.