NEW ORLEANS, La. — For the first time, scientists have definitively linked human-caused climate change to extreme weather events.
Bouts of extreme weather occur every year. But a handful of events in 2016 had some help. These included a deadly heat wave that swept across Asia. It was one of several events that could not have been due solely to the natural ups and downs of climate.
That’s the finding of three new studies. They were part of a special December 13, 2017 issue of the Bulletin of the American Meteorological Society. (It’s also known as BAMS.)
Jeff Rosenfeld is editor in chief of BAMS. These new findings are a game changer — or should at least be a conversation changer, he says. Indeed, he maintains, “We can no longer be shy about talking about the connection between human causes of climate change and weather.”
Rosenfeld spoke at a news conference timed to the studies’ release. It was held at the fall meeting, here, of the American Geophysical Union (AGU).
For the past six years, BAMS has published a December issue containing research on extreme weather events the previous year. That research has sought to separate the role of human-caused climate change from natural variability. Both can contribute to types of extreme weather, such as heat waves, record rains or blistering cold. The goal from the start has been to find ways to improve the science of such “attribution,” said Stephanie Herring. She was lead editor of the latest BAMS issue and works at the National Centers for Environmental Information in Boulder, Colo. They're part of the National Oceanic and Atmospheric Administration.
To date, BAMS has published 137 attribution studies. Herring says that this year is the first time any of them concluded a weather event was so extreme that it was outside the bounds of natural variability. And this year, researchers identified three such events.
Attributing extreme weather
The Asian heat wave was one. Another was the record global heat. In fact, 2016 was the hottest year ever recorded.
The third event was the growth and persistence of a large swath of high ocean temperatures. Located in the Bering Sea off the coast of Alaska, it was nicknamed “the blob.” Those unusually warm waters lingered for about 18 months. They have been linked to mass die-offs of birds and sharply reduced codfish populations in the Gulf of Alaska. They also altered weather in such a way that California ended up with a severe drought.
The studies in the new issue looked at more events than just these three. Most of the 24 others found it highly likely that there was some human influence on those events. Yet those reports stopped short of saying these events could not be due solely to natural weather variability. In other words, there was some chance those events might have happened even without human-driven climate change.
El Niño (NEEN-yo) is a periodic warming of Pacific Ocean waters. And one study found that an already strong El Niño in 2016 probably was enhanced by factors associated with human activities (such as atmospheric warming due to fossil-fuel use). That El Niño contributed to drought and famine conditions in southern Africa.
Another study looked at the source of a heightened risk for coral bleaching along the Great Barrier Reef off the northeast coast of Australia. Here, the main factor driving the increase was greenhouse gases. They were shown to have driven warming of sea-surface temperatures in the nearby Coral Sea.
But not all of the studies linked 2016’s extreme events to human activity. There were record-breaking rains in southeastern Australia between July and September, for example. That had been due to natural variability, one study showed.
With hurricanes, wildfires and drought, 2017 is chock-full of extreme-event candidates for next year’s crop of BAMS studies. Already, three analyses have looked at Hurricane Harvey’s extreme rains. Two of those studies were presented at this year’s AGU meeting. The storm dropped about 1.3 meters (4.3 feet) of water on Houston, Texas, and surrounding areas in August. The three studies were discussed at a separate news conference on December 13. Human influence likely upped the hurricane’s total rainfall, they concluded. By how much? It was from at least 15 percent to 19 percent.
“I think [the BAMS studies] speak to the profound nature of the impacts we’re now seeing,” says Michael Mann. He was not involved in any of the studies, but is a climate scientist. He works at Pennsylvania State University in University Park.
Mann says he’s concerned that many researchers are too focused on quantifying how much people are responsible for certain events. Of more interest, he says, might be measuring how much human influence affects various planetary processes. One example, he notes, is the established link between rising temperatures and increased moisture in the atmosphere. This is another factor implicated in Hurricane Harvey’s extreme rains.
There’s also another possible issue with attribution science, he notes. The computer programs now used to model climate simply may not be capable of capturing some of the subtle changes in the climate and in the oceans. This is a particular danger when it comes to studies that claim to find no link to human activities.
Andrew King made the same point at the news conference. He’s a climate scientist at the University of Melbourne in Australia. He authored the paper on Australia’s severe rainfall.
“When we find no clear signal for climate change, there might not have been a human influence on the event.” Or, he notes, it might be that “the particular factors of the event that were investigated were not influenced by climate change.” But, he adds, “It’s also possible that the given tools we have today can’t find this climate-change signal.”
Rosenfeld noted that people tend to talk about the long odds of an extreme weather event happening. By that, he means that human events are unlikely to affect extreme weather — yet not impossible. In fact, some studies are now beginning to argue that climate change was a necessary condition for some extreme-weather events.
That means discussions about long odds no longer apply, he concludes. “These are new weather extremes made possible by a new climate.”
atmosphere The envelope of gases surrounding Earth or another planet.
attribution science A field of research, largely used in climate studies. It seeks to test whether — and by how much — climate change may be responsible for certain extreme weather events, such as droughts, extreme flooding, hurricanes, excessive heat or odd storm trajectories.
birds Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).
drought An extended period of abnormally low rainfall; a shortage of water resulting from this.
El Niño Extended periods when the surface water around the equator in the eastern and central Pacific warms. Scientists declare the arrival of an El Niño when that water warms by at least 0.4 degree Celsius (0.72 degree Fahrenheit) above average for five or more months in a row. El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have linked the arrival of El Niños to unusual weather events — including ice storms, droughts and mudslides.
factor Something that plays a role in a particular condition or event; a contributor.
famine A condition where many people go hungry because there is too little food. Droughts, flooding and other weather disasters often contribute to widespread crop failures causing famine.
Great Barrier Reef Some 2,300 kilometers (1,430 miles) long, this natural coral habitat is the largest living structure on Earth. In coastal waters off of northeastern Australia, It’s big enough to see from space. It’s home to some 3,000 coral reefs, 600 islands, and hundreds of types of 600 types corals, more jellyfish, mollusks, worms and fish. It’s also patrolled by more than 30 species of whales and dolphins.
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
herring A class of small schooling fish. There are three species. They are important as food for humans and whales.
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
information (as opposed to data) Facts provided or trends learned about something or someone, often as a result of studying data.
link A connection between two people or things.
moisture Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.
National Oceanic and Atmospheric Administration (or NOAA) A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
population (in biology) A group of individuals from the same species that lives in the same area.
profound The quality of being great in magnitude or very intense. Or it can describe a person who is insightful and/or has great knowledge.
reef A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.
risk The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
subtle Some feature that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be visible but subtle — small and hard to distinguish from nearby healthy tissues.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: S.C. Herring et al, eds. Explaining extreme events from a climate perspective. Bulletin of the American Meteorological Society. December 2017.
Meeting: K. Emanuel. How unusual were Hurricane Harvey’s rains? American Geophysical Union fall meeting, New Orleans, December 13, 2017.
Meeting: M.F. Wehner et al. Estimating the human influence on hurricanes Harvey, Irma and Maria. American Geophysical Union fall meeting, New Orleans, December 12, 2017.
Meeting: G. J. van Oldenborgh et al. Attribution of extreme rainfall from Hurricane Harvey, August 2017. American Geophysical Union fall meeting, New Orleans, December 12, 2017.
Journal: K. Emanuel. Assessing the present and future probability of Hurricane Harvey’s rainfall. Proceedings of the National Academy of Sciences. Vol. 114, November 28, 2017, p. 12681. doi:10.1073/pnas.1716222114.
Journal: G.J. van Oldenborgh et al. Attribution of extreme rainfall from Hurricane Harvey, August 2017. Environmental Research Letters. Published online December 13, 2017. doi: 10.1088/1748-9326/aa9ef2.
Journal: M.D. Risser and M.F. Wehner. Attributable human-induced changes in the likelihood and magnitude of the observed extreme precipitation during Hurricane Harvey. Geophysical Research Letters. Published online December 12, 2017. doi: 10.1002/2017GL075888.