Hurricanes Harvey and now Irma became monster storms while swirling over two separate stretches of unusually warm ocean water, a feature that has reignited debate on climate change and the degree it is adding to the intensity of hurricanes.
Scientists all agree that global warming is not the cause of hurricanes, a fact made obvious by the long history of tropical cyclones. But there is scientific consensus that a warming planet will produce bigger and more destructive hurricanes, with many scientists arguing that those impacts are already occurring.
Peter J. Webster, an atmospheric scientist at Georgia Institute of Technology, said it’s clear that Harvey intensified amid some abnormally warm waters in the Gulf of Mexico, and that Irma formed during a season when the Atlantic was also warmer than average.
“I stand by what I said in 2005 — warmer sea temperatures will lead to stronger hurricanes,” said Webster, who 12 years ago published a hotly debated study reporting a rise in Category 4 and Category 5 hurricanes since 1970.
Webster cautioned, however, that sea temperatures are just one factor in spawning hurricanes. “We have two things going on,” he said. “Natural variability and warmer sea temperature.”
As of Wednesday afternoon, forecasters were calling Irma the strongest hurricane ever recorded in the Atlantic, with winds up to 185 miles per hour. Depending on its track, it could strike Florida by weekend, possibly landing as a Category 5 storm.
It has long been known that warmer ocean waters can serve as “fuel” for hurricanes, including those in the Atlantic and Caribbean. The Atlantic this year has been unusually warm, but there is scientific debate on the reasons why.
Temperatures in the Atlantic Ocean are affected by a natural phenomenon called the “Atlantic multidecadal oscillation,” which results partly from a change in ocean currents. From the 1970s to the early 1990s, Atlantic temperatures were relatively cool because of this oscillation. Since then, the Atlantic has been generally warmer, coinciding with scientific concern over rising greenhouse gases and elevated global temperatures.
To analyze what is occurring, scientists at the National Center for Atmospheric Research (NCAR) have developed climate models based on temperatures recorded over the last century. They also factor in various environmental conditions, ranging from the sun’s energy output to the impacts of volcanic eruptions.
Those models show the Atlantic has warmed beyond the impact of natural oscillations, said Kevin E. Trenberth, who heads the climate analysis section at NCAR in Boulder, Colo.
Trenberth says there is also strong evidence that global warming contributed to the intensification of Hurricane Harvey.
Harvey was spawned from a tropical wave that developed to the east of the Lesser Antilles. It reached tropical storm status on Aug. 17, limped into the Gulf of Mexico and rapidly intensified on Aug. 24 as it took aim at Texas.
During this period, surface temperatures in the Gulf were 2.7 to 7.2 degrees Fahrenheit above average, with “record levels” of heat deep into the water column and dense air moisture above, said Trenberth. “The conditions were ripe” for the hurricane to intensify, he said, and later unleash record rainfall.
Trenberth’s observations contrast with that of Scott Pruitt, President Donald Trump’s Environmental Protection Agency administrator. Interviewed by Breitbart News last week, Pruitt said it was “opportunistic” and “misplaced” to tie Hurricane Harvey to climate change.
Hurricane Irma also formed in a general region where Atlantic waters were abnormally warm, about 2 degrees above average, said Trenberth. But meteorologists say that other factors were at play as Irma quickly built into a Category 5 storm.
Joe Cione, a hurricane researcher with the National Oceanic and Atmospheric Administration, said that his analysis shows that Irma intensified in a stretch of Atlantic water that was relatively cool to the surrounding warmer waters. That occurred on Sept. 4 and 5, when Irma strengthened into a Category 4 and then a Category 5 storm.
Cione and fellow NOAA researcher Neal Dorst say that other factors — such as low vertical wind shear — were crucial in supercharging the storm. “Irma’s explosive strengthening was as much a matter of the proper atmospheric elements coming together as the ocean warmth,” said Dorst.
While there is general consensus that global climate change will cause more extreme hurricanes and other weather events, scientific organizations differ on whether it is already occurring.
In May, the Intergovernmental Panel on Climate Change concluded “there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea-surface temperatures.” That statement is similar to what Georgia Tech’s Webster and other researchers concluded in 2005.
By contrast, NOAA says on its website: “It is premature to conclude that human activities — and particularly greenhouse gas emissions that cause global warming — have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations.”
There is also scientific debate on why Atlantic hurricanes diminished in the year after 2005, when Hurricanes Katrina and Wilma killed nearly 1,900 people and caused roughly $137 billion in damage. Climate change skeptics seized on this lull to argue that predictions of more-intense hurricanes were bogus. “We were criticized heavily,” said Webster.
He and other scientists say a strong El Nino warming in the Pacific is the best explanation for the lull. When El Ninos form during the spring and summer, the jet stream dips, creating wind shear that tears apart small storms before they can organize in the Atlantic.
Webster said the continental United States can expect similar swings in the future, even with a general rise in ocean temperatures.
“I wouldn’t put everything in the global warming basket,” he said. “But it certainly has an impact.”
Stuart Leavenworth: 202-383-6070, @sleavenworth