Heat waves are striking more people, disease-carrying mosquitoes are spreading and weather disasters are becoming more common, the authors note in the report published Monday by the British medical journal the Lancet. Climate change is a "threat multiplier," they write, and its blows hit hardest in the most vulnerable communities, where people are suffering from poverty, water scarcity, inadequate housing or other crises.
"We've been quite shocked and surprised by some of the results," said Nick Watts, a fellow at University College London's Institute for Global Health and executive director of the Lancet Countdown, a project aimed at examining the links between climate change and public health.
The effort involved 63 researchers from two dozen institutions worldwide, including climate scientists as well as ecologists, geographers, economists, engineers, mathematicians, political scientists and experts who study food, transportation and energy.
It is the latest in several Lancet reports to focus on climate change. In 2009, a Lancet commission described climate change as "the biggest global health threat of the 21st century." In 2015, a second commission recognized that the innovations required to match this threat represented "the greatest global health opportunity of the 21st century."
The Countdown, as its ticking-clock title suggests, outlines the way humans are adapting - or not - to a rapidly evolving climate. It was announced last year during the United Nations Climate Change Conference in Morocco. The project, a synthesis of scientific literature and media reports, tracks 40 indicators of human health, including migration, nutrition and air pollution.
Given the profound health dangers posed by a warming climate, the study's authors focused on a key question: How well is the world responding?
"The answer is, most of our indicators are headed in the wrong direction," Watts said. "Broadly, the world has not responded to climate change, and that lack of response has put lives at risk . . . The impacts we're experiencing today are already pretty bad. The things we're talking about in the future are potentially catastrophic."
Hotter global temperatures are exacting a human toll. Although the increase since 2000 may seem slight - about 0.75 degrees Fahrenheit - the planet is not a uniform oven. Local spikes can be dramatic and dangerous. Heat waves, defined as extreme temperatures that persist for at least three days, are on the rise.
Between 2000 and 2016, the number of people exposed to heat waves climbed by 125 million vulnerable adults, according to the report. During 2015, the worst year on record, 175 million people suffered through sweltering temperatures.
The number of potentially infectious bites from the Aedes aegypti mosquito, which spreads viruses such as dengue fever and Zika, is up 9 percent over 1950s levels.
And in recent years, the ranks of climate change migrants have grown. Just in the United States, more than 3,500 Alaskans have fled coastal erosion and permafrost melts. Twenty-five homes have been abandoned on Louisiana's Isle de Jean Charles, which is being overtaken by the Gulf of Mexico. In 2016, its former residents became the first to receive federal funds for a climate change retreat.
"If governments and the global health community do not learn from the past experiences of HIV/AIDS and the recent outbreaks of Ebola and Zika viruses," the authors warn in the paper, "another slow response will result in an irreversible and unacceptable cost to human health."
But Monday's report also finds "glimmers of hope," Watts said. For example, many countries are moving away from coal-fired power plants, which are a source both of carbon emissions that fuel global warming and pollution that can cause immediate health problems in nearby communities.
After President Donald Trump's announcement in June that the United States would pull out of the Paris climate accord, the global response to climate change has been heartening, the authors write, "affirming clear political will and ambition to reach the treaty's targets." Nicaragua, previously a holdout because it said the treaty was not stringent enough, is set to join the Paris agreement, leaving only Syria and the United States opposed.
Watts and his co-authors note the ways people are trying to cope with the effects of climate change - spending less time outdoors, for example - even as they warn that the world cannot rely on adaptation alone.
"If anybody says we can adapt our way out of this, the answer is, of course you can't," he said. "Some of the changes we're talking about are so enormous, you can't adapt your way out."
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