As climate change makes hurricanes more intense, can US live in denial anymore?

After Hurricane Harvey ripped through Texas on August 25, Irma is now battering Caribbean island nations with wind speed of 185 miles per hour. Even as one of the most powerful hurricanes on record inches closer to Florida, two other tropical storms are brewing in mid-Atlantic and southwestern Gulf of Mexico and have grown to hurricane strength. Four hurricanes in two weeks! While formation of storms and hurricanes in quick succession is not uncommon, especially during late summer and early autumn (August-October), some climate scientists believe that climate change has somehow made this year worse.

Rising ocean temperature decides how many tropical storms and hurricanes will develop each year, according to geography professor Jim Elsner. "We're seeing fewer hurricanes, but the ones we do see are more intense," said Elsner based on the study he and his ex-student Namyoung Kang did.  

According to the US Environmental Protection Agency, the average surface temperature across the contiguous 48 states has risen at an average rate of 0.14°F per decade since 1901. Average temperatures have increased more swiftly since 1979 (0.29 to 0.46°F per decade). In fact, eight of the top 10 warmest years on record were experienced since 1998. Also, 2012 and 2015 were the two warmest years on record.

This is Brickell in downtown Miami. Totally underwater. #HurrcaneIrma @wsvn w pic.twitter.com/Fc0GsBP1T5

— Brian Entin (@BrianEntin) September 10, 2017

Claiming that yearly temperatures are a good indicator of how the storm season is going to be, both Elsner and Kang pointed to the trend of stronger but fewer tropical cyclones during warmer year and weaker but more tropical cyclones during a colder year. According to their findings, storm speeds over the past 30 years have increased on average by 1.3 metres per second (3 miles per hour). Moreover, there were 6.1 fewer storms on an average. Hurricane Irma is the latest example. “Irma now had winds of 185 mph for 33 hrs—no other tropical cyclone around the globe has been this strong for so long in satellite era since 1966,” tweeted Philip Klotzbach, meteorologist at Colorado State University.

Hurricanes source their energy from warming oceans. They are formed when water temperatures are 79 degrees Fahrenheit or higher. As the warm water evaporates, the storm gains the energy to become a hurricane. Higher temperatures mean higher level of energy, and finally, more destructive wind. As the link between burning of fossil fuels and warming of our oceans has already been established, it is safe to assume that climate change exacerbates the impact of hurricanes.

Climate change link can’t be clearer

Climate change deniers try to debunk the claims made by climate scientists by pointing out that Hurricane Harvey was the first ‘major hurricane’ that the US has seen since 2005. NASA had an answer to this idea of "hurricane drought" that climate change sceptics have been propagating. In a 2015 study, two NASA scientists concluded that the fact that the US did not experience major storms between 2005 and 10 was merely "a matter of luck".

The argument that no "major hurricanes" made landfall in the US between 2005 and August 2017 does not hold water as we cannot forget how Category 4 Hurricane Ike affected Texas in September 2008, causing extensive damage. Hurricane Sandy, which happened in 2012, turned out to be the deadliest and the most destructive hurricane of that year.

But 2017 has been different. It is experiencing an “above normal hurricane season". While the 30-year average is 12 named storms, six hurricanes and three major hurricanes, the NOAA had predicted 14 to 19 named storms and two to five major hurricanes. While the link between climate change and hurricanes is complex, most experts agree on the probability that hurricane seasons like 2017 will be repeated more often.

The US president Donald Trump seems to be convinced that Hurricane Irma is the largest ever recorded in the Atlantic” and he tweets about the “horrors and devastations” caused by Hurricane Harvey. However, the president continues to refute all claims pertaining to global warming.  This August, he disbanded an important advisory panel aimed at fighting climate change. Few days later, the country’s fourth largest city was seen several feet under water. He has to realise that no country is immune to the effects of extreme climate events.

What made the rain in Hurricane Harvey so extreme?

Fifty inches of rain. Nine trillion gallons of water. The Gulf Coast of Texas, and especially the Houston metropolitan area, has been inundated by rain produced by Hurricane Harvey. And as of this writing, the rain continues along a broad swath of the Gulf Coast, with a flood threat extending all the way east through New Orleans to the Florida Panhandle.

Even for one of the wettest and most flood-prone parts of the United States, the rainfall totals and flooding are breaking records. So, what has made Harvey such a prodigious rain producer?

A 'train' of rainstorms

The amount of rain that falls at a given location can be boiled down to a surprisingly simple equation: The total precipitation equals the average rainfall rate, multiplied by the rainfall duration. In other words, the most rain falls where it rains the hardest for the longest.

Tropical cyclones in general are very efficient rain producers, because they draw large quantities of water vapor into the atmosphere from a warm ocean. That moist air rises and the water vapor condenses, and a large fraction of that water falls as rain. Tropical cyclones can also last a long time; if their motion slows, then a particular region can experience that heavy rainfall for multiple days.

Even compared to other tropical cyclones, the rain from Harvey has been very hard, and gone for a very long time. On Saturday evening (August 26) into Sunday morning (August 27), an intense band of storms developed to the east of Harvey’s center, and lined itself up right over Houston. This is a process known as “echo training,” in which it appears that the individual thunderstorm cells are like train cars that repeatedly pass over the same spot and bring with them heavy precipitation.

This precipitation band was producing up to six inches of rain per hour – an extremely high rate – and it remained over the Houston metro area for several hours, with a couple more that followed immediately after. One location just southeast of downtown Houston recorded 13.84 inches in just three hours. These rains from Saturday night into Sunday morning initiated the massive flooding in the Houston metro area.

Relentless rainfall

Then, after this initial intense burst, there has been no respite. Usually, when a tropical cyclone turns poleward from the tropics toward the United States, it will interact with one or more midlatitude weather systems that will send the storm on its way after a day or two. But this August, the jet stream has been positioned well to the north of Texas, so none of these disturbances has approached, and Harvey’s center of circulation has barely moved since it made landfall. As a result, across the Texas (and now Louisiana) coast, there have been periods with intense rainfall (in more of the rainbands described above), along with lighter, but still substantial, accumulations.

This combination of unusually high rain rates and long duration has resulted in a very large area with 30 to 45 inches of rain in a few days.

Those of us who study extreme rainfall and flooding, and those who live in and around Houston, know this area is vulnerable to both very heavy rainfall and destructive and deadly floods. The previous standard-bearer for extreme rainfall in the region was Tropical Storm Allison in June 2001, which produced just over 40" of rain around Houston. But the excessive accumulations were fairly localized. Major floods again occurred on Memorial Day in 2015, and on April 18-19, 2016.

In the April 2016 event, an intense line of overnight storms produced up to 15" of rain in a few hours, similar to the “training” rainbands in Harvey. But with Harvey, the area covered by the heavy rainfall has been vastly larger, and the rain has persisted for days. For comparison, in just one day (ending Sunday morning, August 27, 2017) the area covered by rainfall from Harvey exceeding 16" is several times larger than the entire April 2016 flood event, and at least two more days of similar accumulations have followed.

Tornado risks

To make matters even worse, there were also numerous tornadoes reported as the rainbands came on shore. It’s fairly common to have tornadoes occur in association with landfalling hurricanes, but what struck me in this case was that tornado warnings were being issued in the same places that had just received massive amounts of rain.

My research group has studied the challenges associated with multi-hazard situations, and specifically when the threats of tornadoes and flash flooding occur in the same place at the same time, as the protective responses to those hazards can be at odds with each other. For people to be under both a tornado and a flash-flood warning at the same time is surprisingly common – these overlapping warnings occur around 400 times per year on average.

But this situation was taken to a new extreme during Harvey, when tornado warnings were being issued at the same time that emergency officials were sending messages for people to go to their roofs for safety (rather than risk getting caught in the attic). The heartbreaking (but also heroic) video footage of water rescues speaks to the immense human impact of this multifaceted storm.

Spot-on forecast

One final remarkable aspect of Hurricane Harvey’s rainfall is how accurate numerical weather prediction models – and the human forecasters who use them to make official forecasts – were at highlighting the incredible precipitation accumulations.

Medium-range forecast models at least a week in advance were showing Harvey stalling out along the Texas coast and producing extreme rainfall. As the event neared, essentially every numerical model was showing accumulations over 25 inches. Often, when meteorologists see models making predictions of events that would be unprecedented, we are rather skeptical of that guidance, because there are no points of reference to compare to. But in this case, the models were in close agreement about the potential for a truly major event, and forecasters saw the gravity of the situation.

The NOAA Weather Prediction Center, which makes official rainfall forecasts (and rarely includes extreme amounts), on Friday afternoon (August 25) predicted a broad swath of over 20 inches, with isolated areas up to 40". Never before had the Weather Prediction Center issued a “high risk” of excessive rainfall three days in advance, as usually the uncertainties with forecasting precipitation don’t allow enough confidence to do so. In fact, their protocol didn’t even allow for such an alert so far in advance! But for Harvey’s rainfall, they did this on consecutive days, and with high accuracy, because of the expected extremity of the event. (The primary errors in the rainfall forecasts in advance of the storm is that they placed the maxima a bit southwest of Houston, instead of centered over Houston.)

One of the fascinating aspects of studying extreme rainfall and flash floods is the wide variety of storm systems that can produce heavy rain, and trying to figure out how the ingredients came together in each of those diverse situations to inform and improve future forecasting. For Hurricane Harvey, researchers and forecasters will be analyzing the ingredients that led to this record-setting flood for many years to come.

Russ Schumacher, Associate Professor of Atmospheric Science, Colorado State University

This article was originally published on The Conversation. Read the original article.

Climate change has affected every part of US: government report

People in the US are already feeling the effects of climate change as the average temperature in the country has increased rapidly and drastically since 1980, according to a federal climate change report prepared by scientists from 13 federal agencies. The report, which is awaiting Trump administration’s approval, claims that the recent decades have been the warmest in the past 1,500 years.

Attributing the climate change to human activities, especially greenhouse gas emissions, the scientists observed that even if greenhouse gas emission is completely stopped today, the world would still feel at least an additional 0.30°C of warming over this century as compared with today. The report concludes that a small difference in global temperatures can make a huge difference in the climate. In fact, the difference between 1.5°C and 2°C rise in global temperatures could mean longer heat waves and more intense rainstorms.

The Environment Protection Agency is one of 13 agencies that must approve the report by Aug. 18. The agency’s administrator, Scott Pruitt, does not believe that carbon dioxide is a primary contributor to global warming. “This is the first case in which an analysis of climate change of this scope has come up in the Trump administration, and scientists will be watching very carefully to see how they handle it,” Michael Oppenheimer, a professor of geoscience and international affairs at Princeton University, told New York Times.

Warmer days and rise in annual average temperature

The authors of the report attributed 2003 European heat wave and the record heat in Australia in 2013 to a man-made factor, and added that more than half of the global mean temperature increase since 1951 can be linked to human influence.

After examining every part of the US, the report found that all of it was touched by climate change. The number and severity of cool nights have decreased since the 1960s and the frequency and severity of warm days have increased. Similarly, extreme cold waves have become less common and heat waves more common since the 1980s.

Much to the fear of climate change believers, the report noted that the average annual temperature in the US will increase, making recent record-setting years “relatively common” in the future. Depending on the level of future emissions, the annual temperature may increase between 2.8 and 4.8°C before the end of this century.

More alarming facts were revealed when it comes to precipitation. The average annual rainfall across the US has increased by about 4 per cent since the beginning of the 20th century, and while parts of the West, Southwest and Southeast are drying up, the Southern Plains and the Midwest are getting wetter. The scientists also observed that the accelerated rate of Arctic warming will have a significant consequence for the United States pointing out how accelerating land and sea ice melting is causing sea level to rise and threatening country’s coastal communities. According to them, restricting the global mean temperature increase to 2°C will require significant reductions in global CO2 levels.

Despite mounting evidence in support of anthropogenic climate change, President Trump continues to stay in denial mode. He not only walked out of the Paris Agreement but also called it detrimental for the US economy. His government is still pursuing a policy of expanding access to fossil fuels is part and turning the US into an oil, natural gas and coal exporting powerhouse. It also wants to continue to export fracking technology to other countries.

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