Heat wave: Your questions answered

Source: Gayathri Vaidyanathan, E&E reporter • Posted: Friday, July 29, 2016

The changing winds bear good news: The heat wave is on the wane.

For much of this week, temperatures across the United States have been blistering. In Washington, D.C., thermometers hit 100 degrees Fahrenheit on Monday, breaking records for July. The heat was caused by a strong ridge of high pressure that parked over most of the United States and formed a dome of stifling heat and humidity.

The weather pattern is dissipating and being replaced, from the Mississippi Valley through the Northeast, with rain, said Bob Oravec, lead forecaster at the National Oceanic and Atmospheric Administration’s National Weather Service.

“The worst of the heat wave is over, the weather pattern has definitely changed,” Oravec said.

Only a handful of regions will continue to swelter, including eastern North Carolina, southeastern Virginia, northeastern South Carolina and the desert Southwest, Oravec said.

Could humans have contributed to this heat wave through their carbon emissions? ClimateWire ponders this, and other questions:

1. Was this heat wave unusual?

Not really. This heat wave persisted for just over a week. Similar weather patterns have previously stayed in place for months, Oravec of NWS said. Typically speaking, the longer a heat wave persists, the more damage it does.

“It is summertime, so you get hot weather in summer,” Oravec said.

But this is just the weather report. With respect to the climate — the long-term behavior of weather — heat waves seem to be occurring more frequently, according to the National Climate Assessment.

Some studies have linked this trend to global warming. If humans were not warming the planet, the number of heat waves in the past decade may have been lower by a factor of five, one study found.

Heat waves globally are expected to become longer, more frequent and more intense by 2100, according to the Intergovernmental Panel on Climate Change.

2. Was the U.S. heat wave linked to climate change?

Probably, because all weather events are happening on a planet that has been fundamentally changed by humans’ carbon emissions.

Heat waves can be easily tied to climate change, according to the National Academies of Sciences, Engineering and Medicine (ClimateWire, March 14). Basic physics dictates that rising carbon dioxide levels in the atmosphere will drive a rise in temperatures, and this is captured well in climate models.

“Temperature is the simplest part of climate change,” Ted Shepherd, a climate scientist at the University of Reading, said in a March interview. “We know temperatures are warming for sure, and they are warming on continental scales as well as globally, so we expect there will be more warm extremes. It’d be almost incredible not to have an increase in warm extremes.”

Climate scientists every year assemble a massive report in the Bulletin of the American Meteorological Society, where they attribute a role for humans in some extreme events. Since 2012, they have studied around 20 heat waves, Stephanie Herring, an attribution scientist at NOAA’s National Centers for Environmental Information, said in an interview earlier this year.

For the vast majority of heat waves they looked at, “we have been able to find a fingerprint of human influence on either the intensity or frequency of the event,” she said.

In some cases, climate change increased the probability of the heat wave by more than a factor of 10.

3. So, how much of it was due to humans?

Figuring out exactly how much of a role humans played will take a few months to a year, provided it piques a scientist’s interest.

Some attempts are afoot to attribute extreme weather events, such as heat waves, while the memory of the event is still fresh in people’s minds (ClimateWire, June 14). But these are still preliminary, and scientists are selective about the events they study.

Separately, Dáithí Stone, an attribution scientist at the Lawrence Berkeley National Laboratory, every month tries to “forecast” the link between climate change and heat events. His efforts are rudimentary and have large uncertainties attached, Stone cautioned, but the broader findings are interesting.

“It tells us how much more — or less — likely something like a heat wave might be than it would have been without greenhouse gas emissions,” he said.

Stone released his July forecast last month. His models suggested that greenhouse gas emissions had increased the chances of an unusually hot month for some regions of the United States. It was less conclusive for other regions, he said.

“Over the several years that we have been running this, there is a clear increase in the chance of a hot July over all parts of the U.S., and probably at least a doubling of the chance [of a hot July],” he said.

4. Has the planet warmed overall?

The past June continued the inexorable upward march of temperatures as it was the 14th consecutive month to break records.

Global temperatures have risen by more than 1 degree Celsius above the late 19th-century average, according to NASA scientists (ClimateWire, July 20). 2016 will almost certainly set a record, due to global warming and an El Niño, a circulation pattern that elevated temperatures around the world and is just dying down.

Anomalous temperatures have been recorded around the world this summer. A heat wave set a record in India as temperatures of 123.8 F were recorded in Rajasthan in May.

On July 21, temperatures at Mitrabah, Kuwait, rose to 129.2 F, the highest recorded in the Eastern Hemisphere and Asia, according to the World Meteorological Organization.

China, too, is simmering as the mercury has risen to 102 F in some parts of the country, according to the China Meteorological Administration, which issued a three-day heat wave alert. To keep cool, zoo animals were sheltered in air conditioning and played with giant blocks of ice, according to media reports.