Mike Mann: Harvey Not Caused, but Made Worse, by Climate Change September 1, 2017

Michael Mann in the Guardian:

What can we say about the role of climate change in the unprecedented disaster that is unfolding in Houston with Hurricane Harvey? There are certain climate change-related factors that we can, with great confidence, say worsened the flooding. Sea level rise attributable to climate change – some of which is due to coastal subsidence caused by human disturbance such as oil drilling – is more than half a foot (15cm) over the past few decades (see here for a decent discussion). That means the storm surge was half a foot higher than it would have been just decades ago, meaning far more flooding and destruction. In addition to that, sea surface temperatures in the region have risen about 0.5C (close to 1F) over the past few decades from roughly 30C (86F) to 30.5C (87F), which contributed to the very warm sea surface temperatures (30.5-31C, or 87-88F). There is a simple thermodynamic relationship known as the Clausius-Clapeyron equation that tells us there is a roughly 3% increase in average atmospheric moisture content for each 0.5C of warming. Sea surface temperatures in the area where Harvey intensified were 0.5-1C warmer than current-day average temperatures, which translates to 1-1.5C warmer than “average” temperatures a few decades ago. That means 3-5% more moisture in the atmosphere. That large amount of moisture creates the potential for much greater rainfalls and greater flooding. The combination of coastal flooding and heavy rainfall is responsible for the devastating flooding that Houston is experiencing. Not only are the surface waters of the Gulf of Mexico unusually warm right now, but there is a deep layer of warm water that Harvey was able to feed upon when it intensified at near record pace as it neared the coast. Human-caused warming is penetrating down into the ocean. It’s creating deeper layers of warm water in the Gulf and elsewhere. Harvey was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage and a larger storm surge. (As an example of how this works, we have shown that climate change has led to a dramatic increase in storm surge risk in New York City, making devastating events like Hurricane Sandy more likely.) Finally, the more tenuous but potentially relevant climate factors: part of what has made Harvey such a devastating storm is the way it has stalled near the coast. It continues to pummel Houston and surrounding regions with a seemingly endless deluge, which will likely top out at nearly 4ft (1.22m) of rainfall over a days-long period before it is done.

The stalling is due to very weak prevailing winds, which are failing to steer the storm off to sea, allowing it to spin around and wobble back and forth. This pattern, in turn, is associated with a greatly expanded subtropical high pressure system over much of the US at the moment, with the jet stream pushed well to the north. This pattern of subtropical expansion is predicted in model simulations of human-caused climate change. More tenuous, but possibly relevant still, is the fact that very persistent, nearly “stationary” summer weather patterns of this sort, where weather anomalies (both high-pressure dry hot regions and low-pressure stormy/rainy regions) stay locked in place for many days at a time, appears to be favoured by human-caused climate change. We recently published a paper in the academic journal Scientific Reports on this phenomenon. In conclusion, while we cannot say climate change “caused” Hurricane Harvey (that is an ill-posed question), we can say is that it exacerbated several characteristics of the storm in a way that greatly increased the risk of damage and loss of life. Climate change worsened the impact of Hurricane Harvey.

Politifact:

Keep in mind: None of the experts we talked to said climate change caused Hurricane Harvey. Instead, some posited that climate change exacerbated the effects of the storm. The degrees of exacerbation vary, though, sometimes significantly. A draft report on climate science conducted by 13 federal agencies as part of the National Climate Assessment said models showed the number of very intense storms have been rising as a result of a warmer world. But the trend has yet to rise above normal variation. The report also said that scientists are better able to attribute weather events to climate change than they used to be, but linking individual events to climate change is more complicated. The scientists we spoke to about Hurricane Harvey expressed a similar challenge. Below, we’ll outline some general concepts of climate change and then address whether they influenced Hurricane Harvey. Precipitation In a warmer world, there’s more water vapor that storms can sweep up and dump on us. “While it is not yet possible to determine exactly how much of the rainfall associated with Harvey was due to climate change versus how much would have occurred naturally, nearly every scientific study agrees that, as the world warms, on average the amount of rainfall associated with hurricanes will increase,” said Katharine Hayhoe, a climate scientist at Texas Tech University in Lubbock.

Just astounding the amount and extent of rainfall that #Harvey2017 produced in Southeast Texas. From a NOAA/NWS research partner @UWCIMSS pic.twitter.com/o4NFetyMHH — NWS Brownsville (@NWSBrownsville) September 1, 2017

Heavy downpours are on the rise in the United States, according to a 2014 report by 13 federal agencies. But we found differing estimates for how climate change might have affected rainfall from Harvey. According to Chris Landsea, science and operations officer at the National Hurricane Center, the amount of increased precipitation in Harvey is not significant. Landsea expects a 10 percent surge in rainfall by the end of the century due to climate change, which he predicts would only have only increased rainfall by an inch or two in this case. Kevin Trenberth, a scientist at the National Center for Atmospheric Research, said he could justify a 5 to 15 percent increase in rainfall during Harvey from climate change effects, which then increase with natural variability. “So the storm is a bit more intense, bigger and longer lasting than it otherwise would be,” Trenberth wrote to PolitiFact. Storm surge As Earth’s temperature warms, land-based ice melts and ocean water expands, causing sea levels to rise. This in turn increases the risks that the sea will rise with the atmospheric pressures of a storm, causing more waves and flooding. If sea level rise continues at the current pattern, Landsea expects every single hurricane to have a 2-foot higher storm surge by the end of the century. Trenberth attributed half a foot of the flooding in this case to climate change. In addition to rising sea levels, the storm surge in Harvey was also tied to erosion, subsidence, saltwater intrusion and the impact of past hurricanes. Storm intensity Warm oceans may also increase the intensity of hurricanes. Tropical storms use the ocean to fuel their growth like a battery, so warmer temperatures make storms stronger and intensify them more quickly. Over 90 percent of the excess heat in the atmosphere caused by climate change is going to the ocean. However, it’s not so simple to connect climate change to the intensity of Harvey, or any other hurricane for that matter. There are other atmospheric factors at play, such as changes in air temperature at the top of the hurricane, that can offset warmer ocean temperatures. Climate science researchers want to answer this question, but they haven’t reached a verdict just yet. What some researchers have shown is that the frequency of hurricanes is decreasing, while the intensity increases. That would mean fewer storms, but the storms would be more intense with greater potential for destruction. Some scientists attribute abnormally high Gulf of Mexico temperatures to the storm’s rapid intensification before it hit the ground, but they also can’t wholly link the temperatures to climate change. The Gulf has been ripe enough for hurricanes for three decades, but it still doesn’t produce them every year. Stagnation Almost every scientist we spoke with agreed that the most remarkable feature of Harvey is how long it lingered atop Houston, which was crucial in creating the staggering rain totals. But that is also the part of the storm with the least conclusive evidence linking it to climate change. Several studies found that hurricane-steering winds have weakened and shifted northward in recent decades over North America, increasing the likelihood of slow-movers like Harvey in the future. A recent paper by Penn State professor Michael Mann showed how blocking patterns in general become more common in a warmer world. “These studies require painstaking, detailed analyses that take years to complete so there is nothing we as scientists can say right now about the extent to which human-induced change was involved in determining Harvey’s path, or even whether it was at all,” Hayhoe said. Kerry Emanuel, a professor of atmospheric science at Massachusetts Institute of Technology, found that steering currents had weakened significantly since 2010, but “the suddenness of the decline weighs against an explanation in terms of anthropogenic climate change.” Preparation Scientists may disagree on the degree to which anthropogenic (or human-caused) climate change intensified Harvey, but almost all concurred that Houston’s lack of preparation for it magnified its ramifications. Urbanization turned prairies and forests into concrete, reducing the land’s capacity to absorb rainfall, and lax zoning codes gave way to development more prone to cave to the flooding. Paired with an explosion in the city’s population, the damages snowballed. Many of the scientists we spoke with nonetheless stressed that in addition to infrastructure, lawmakers must focus on addressing climate change. “We’re doing a huge disservice if we put off the reality that we are changing the climate and the sea levels if we wait until we do fancy analyses (on storms like Harvey) years later. It’s time to face up to the fact that some of these are going to become the new norm,” said Harold Wanless, chair of the geological sciences department at the University of Miami.