Extreme Weather Can Feel ‘Normal’ After Just a Few Years, Study Finds

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While unusual weather patterns have always come and gone, in a changing climate extreme weather events will become more frequent. And this year has already had its fair share of extremes.

But if extreme temperatures arrived in your area, would you even notice?

Often, the answer is no, according to a study published Monday in the journal Proceedings of the National Academy of Sciences. In it, researchers analyzed more than two billion Twitter messages to see how people reacted to weather events.

In fact, the study suggests, people learn to accept extreme weather as normal in as little as two years.

That might lead people to underestimate the extent of global warming, given that it has already caused extreme temperature changes. If you were born after 1976, for example, the Earth has been warmer than the 20th century average every year of your life (though local temperatures vary).

Frances C. Moore, the lead author of the new study, wanted to find out how people contextualized extreme temperatures based on past weather experiences.

“You go anywhere in the world and people are going to be commenting on the weather,” said Dr. Moore, an assistant professor in the Department of Environmental Science and Policy at the University of California, Davis. “But the type of weather that people talk about, it’s going to differ from place to place. And the way in which it differs, it’s telling us something about what type of weather people are finding unusual.”

The research team analyzed messages posted to Twitter between 2014 and 2016 that included location data within the United States; they found 60 million tweets that mentioned the weather. They then compared the local temperatures at the time those tweets were sent to a baseline of the 1981-1990 average temperatures for those areas at the same time of year.

“Then what we’re looking at is: How does the volume of tweets about weather change, and in particular, how did they change as temperatures change?” Dr. Moore said.

She and her colleagues found that if people experienced extreme temperatures they were unaccustomed to — hot or cold — they tweeted a lot about it. But if the location in question had already experienced those sorts of temperatures in recent years, even if the weather was extreme compared to the baseline, people tended not to tweet about it. The extreme weather wasn’t remarkable anymore.

Generally, it took just two to eight years for Americans in a given location to adjust their mental baselines of what was normal — in other words, to stop recognizing that those extreme temperatures were in fact extreme.

“The definition of ‘normal weather’ shifts rapidly over time in a changing climate,” the authors wrote.

It’s Not Your Imagination. Summers Are Getting Hotter.

Summer temperatures have shifted toward more extreme heat over the past several decades.

Saif M. Mohammad, a senior research scientist at the National Research Council Canada, who was not part of the study, said it had some limitations. “We don’t know how representative tweets are to general public opinion,” he said. “Because a certain kind of people are tweeting, it is not necessarily representative of people at large.”

Still, Dr. Mohammad said the study was a good early attempt at trying to understand how people perceive climate change through extreme weather. “It is sort of sounding a warning bell saying that we keep seeing these anomalous weather events over many years and we can quickly get adjusted to that,” he said.

If this recalls the apocryphal tale of the frog that allows itself to be cooked to death if placed in a pot of water that is slowly boiled, it should be noted that frogs won’t actually allow themselves to die this way; they will jump out of the pot.

But if people stop registering extreme temperatures as extreme, it may limit the public’s willingness to take or support action on global warming, the researchers said. Social scientists refer to “windows of opportunity” when extreme events can act as triggers for social change.

“When people survive something that’s so threatening or at times even deadly, it really shapes how they think about what was before, and that often can spur action,” said Katja Brundiers, an assistant research professor in sustainability at Arizona State University, who was not involved in the study.

Other severe, repeated weather events that are exacerbated by climate change, like wildfires and hurricanes, could be more effective at spurring people to action, said Elisabeth Hamin Infield, a professor of regional planning at the University of Massachusetts, Amherst, who was also not part of the research.

Even if we’ve learned to normalize temperatures that are out of the ordinary, we might still feel the effects. Dr. Moore and her colleagues used a technique called sentiment analysis to analyze the moods of tweets from people in regions that were experiencing extreme temperatures, even if the people weren’t tweeting about the weather itself. (Sentiment analysis looks for words with positive and negative connotations.)

They found that people experiencing extreme temperatures still had more negative sentiment scores than average.

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