What’s New in the Latest U.S. Climate Assessment

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WASHINGTON — Global warming is now affecting the United States more than ever, and the risks of future disasters — from flooding along the coasts to crop failures in the Midwest — could pose a profound threat to Americans’ well-being.

That’s the gist of Volume Two of the latest National Climate Assessment, a 1,656-page report issued on Friday that explores both the current and future impacts of climate change. The scientific report, which comes out every four years as mandated by Congress, was produced by 13 federal agencies and released by the Trump administration.

This year’s report contains many of the same findings cited in the previous National Climate Assessment, published in 2014. Temperatures are still going up, and the odds of dangers such as wildfires in the West continue to increase. But reflecting some of the impacts that have been felt across the country in the past four years, some of the report’s emphasis has changed.

More and more of the predicted impacts of global warming are now becoming a reality.

For instance, the 2014 assessment forecast that coastal cities would see more flooding in the coming years as sea levels rose. That’s no longer theoretical: Scientists have now documented a record number of “nuisance flooding” events during high tides in cities like Miami and Charleston, S.C.

“High tide flooding is now posing daily risks to businesses, neighborhoods, infrastructure, transportation, and ecosystems in the Southeast,” the report says.

As the oceans have warmed, disruptions in United States fisheries, long predicted, are now underway. In 2012, record ocean temperatures caused lobster catches in Maine to peak a month earlier than usual, and the distribution chain was unprepared.

The report suggests a different approach to assessing the effects of climate change, by considering how various impacts — on food supplies, water and electricity generation, for example — interact with each other.

“It is not possible to fully understand the implications of climate change on the United States without considering the interactions among sectors and their consequences,” the report says.

It gives several examples, including recent droughts in California and elsewhere that, in combination with population changes, affect demand for water and energy. The report also cites Superstorm Sandy, six years ago, which caused cascading impacts on interconnected systems in the New York area, some of which had not been anticipated. Flooding of subway and highway tunnels, for example, made it more difficult to repair the electrical system, which suffered widespread damage.

The United States military has long taken climate change seriously, both for its potential impacts on troops and infrastructure around the world and for its potential to cause political instability in other countries.

The report cites these international concerns, but goes far beyond the military. Climate change is already affecting American companies’ overseas operations and supply chains, it says, and as these impacts worsen it will take a toll on trade and the economy.

Global warming and natural disasters are also affecting development in less affluent countries. That, the report says, puts additional burdens on the United States for humanitarian assistance and disaster aid.

Since 2014, more detailed economic research has estimated that climate change could cause hundreds of billions of dollars in annual damage, as deadly heat waves, coastal flooding, and an increase in extreme weather take their toll. To limit that harm, communities will need to take steps to prepare beforehand.

The previous assessment warned that few states and cities were taking steps to adapt to the impacts of climate change. That’s slowly changing, the new report finds. More and more communities are taking measures such as preserving wetlands along the coasts to act as buffers against storms.

But outside of a few places in Louisiana and Alaska, few coastal communities are rethinking their development patterns in order to avoid the impacts from rising seas and severe weather that the report says are surely coming.

The report warns that the country is particularly unprepared for the upheavals that will come as rising sea levels swamp coastal cities: “The potential need for millions of people and billions of dollars of coastal infrastructure to be relocated in the future creates challenging legal, financial, and equity issues that have not yet been addressed.”

While much of the discussion of climate change looks at the role of carbon dioxide and other greenhouse gases in warming the planet, the report puts a renewed emphasis on the impacts of other atmospheric pollutants like ozone and smoke, which can cause respiratory problems and lead to premature death.

The report notes with “high confidence” that climate change will increase ozone levels, as rising temperatures and changes in atmospheric circulation affect local weather conditions. But the increases will not be uniform. By near the end of the century, the worst ozone levels will be found across a wide expanse of the Midwest and Northern Great Plains, while levels are expected to improve, at least somewhat, in parts of the Southeast.

The report reiterates what residents of the West have learned from hard experience: that warmer springs, longer dry seasons in the summer and other impacts are lengthening the fire season. The smoke from fires affects not only health, the report says, but visibility.

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