Nation

Study links extreme weather to global warming

A car was buried under snow in Hyde Park in February. Last winter was Boston’s snowiest on record.
Sean Proctor/Globe Staff
A car was buried under snow in Hyde Park in February. Last winter was Boston’s snowiest on record.

NEW YORK — The moderate global warming that has already occurred as a result of human emissions is responsible for about 75 percent of daily heat extremes and about 18 percent of precipitation extremes, scientists reported Monday.

Especially hot days of a sort that occurred only once every 30 years or so before the Industrial Revolution are now occurring every six or seven years, the scientists found.

Such scorching days could become near-annual events by late this century even if emissions are brought under control, the researchers said, and they could occur several times a year if they are not.

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The new research builds on those predictions of frequency and attempts to answer the question: To what degree are the changes attributable to human-caused climate change?

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“People can argue that we had these kinds of extremes well before human influence on the climate — we had them centuries ago,” said Erich M. Fischer, lead author of a study published Monday by the journal Nature Climate Change. “And that’s correct. But the odds have changed, and we get more of them.”

On Tuesday, Pope Francis will step into the climate-change debate by convening scientists, government officials, and religious leaders for a Vatican meeting on the subject. Vatican officials will host a conference titled “Protect the Earth, Dignify Humanity,” as Francis prepares an encyclical on humans’ relationship with nature.

The study by Fischer and his colleague, Reto Knutti of the Swiss Federal Institute of Technology in Zurich, is not the first to attribute large-scale changes in extreme weather to human influence on the climate. But it is among the first to forecast, at a global scale, how those extremes might change with continued global warming.

The question is important because while a gradual increase in average temperatures can have profound ecological consequences, it is weather extremes that have the greatest effect on human society. A 1995 heat wave in Chicago killed hundreds of people, and a 2003 heat wave in Europe killed an estimated 70,000.

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Scientists believe that both were made more likely by the human emissions that are warming the planet, and heat on that scale will become commonplace if emissions are allowed to continue unabated. For now, though, such heat extremes — Chicago temperatures were near or above 100 degrees for four days running that July — are still rare, which makes them difficult to study in a statistical sense.

For their paper, Fischer and Knutti focused on more common heat and precipitation extremes, the sort occurring about every three years. To perform a global study, they defined extremes not on an absolute scale but in terms of what local weather is like; after all, a hot day in North Dakota might be pretty routine in Texas.

These weather extremes still have important consequences, even if they are not as severe as those of great heat waves. Such hot days can, for example, cut farm yields sharply and drive up food prices.

The researchers found that 75 percent of the daily heat extremes in the present-day climate were a consequence of the planetary warming that has occurred since the 19th century, a global average rise of about 1.5 degrees Fahrenheit.

They found a smaller effect on precipitation, but it was still significant, with about 18 percent of the extremes in today’s climate attributable to global warming. That is a planetary average, Fischer said, with the effect being small in some regions and much larger in others.

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If efforts to control emissions are unsuccessful, and temperatures by the end of the century rise as much as some scientists fear, both heat and precipitation extremes will skyrocket, with human-caused global warming likely to be responsible for half the precipitation extremes and perhaps 90 percent of the heat extremes in that climate, the researchers found.

The Vatican summit, which has angered some conservatives, will focus on the links between poverty, economic development, and climate change.

The change being seen today in precipitation patterns was predicted decades ago, based on the elementary fact that warmer air can hold more moisture.

Other researchers have found that the increase is leading to heavier rainstorms across large parts of the United States, with the biggest effect occurring in the Northeast. At the same time, warmer temperatures are worsening the effects of droughts when they do occur, as in California over the last few years.

Since his first homily in 2013, the pope has preached about the need to protect the Earth and all of creation as part of a broad message on the environment.

Now, as Francis prepares to deliver what is likely to be a highly influential encyclical this summer on environmental degradation and the effects of human-caused climate change on the poor, he is alarming some conservatives in the United States who are loath to see the Catholic Church reposition itself as a mighty voice in a cause they do not believe in.

The Tuesday meeting at the Vatican is part of efforts to build momentum for a campaign by Francis to urge world leaders to enact a sweeping UN climate change accord in Paris in December. The accord would for the first time commit every nation to enact tough new laws to cut the emissions that cause global warming.

The Vatican summit will focus on the links between poverty, economic development, and climate change, with speeches and panel discussions by climate scientists and religious leaders, and economists such as Jeffrey Sachs of Columbia University. UN Secretary General Ban Ki-moon, who is leading efforts to forge the Paris accord, will deliver the opening address.

In the United States, the encyclical will be accompanied by a 12-week campaign, now being prepared by a committee of Catholic bishops, to raise the issue of climate change and environmental stewardship in sermons, homilies, news media interviews, and letters to newspaper editors, said Dan Misleh, executive director of the Catholic Climate Covenant in Washington.

The effort is angering a number of American conservatives, among them members of the Heartland Institute, a libertarian group partly funded by the Charles G. Koch Foundation, run by the billionaire industrialist Koch brothers, who oppose climate policy.