US climate-change report adds to urgent call for mitigation and adaptation

Neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts are even close to sufficient to avoid substantial damages to the US economy, environment, and human health over the coming decades.

That’s the blunt message from the Fourth National Climate Assessment , the quadrennial, statutorily mandated report prepared by the 13-agency US Global Climate Research Program. The 1600-page report, released by the White House on 23 November, offers a dire projection of the impacts of climate change in the US. In the absence of a substantial global reduction in greenhouse-gas emissions and sustained regional adaptation measures, US GDP could decline 10% by the end of the century, the report says. Some sectors of the US economy could lose hundreds of billions of dollars annually.

The US assessment follows last month’s Special Report on Global Warming of 1.5 °C from the United Nations Intergovernmental Panel on Climate Change, which advocates mitigation on an unprecedented global scale. And a report from the United Nations Environment Programme released on 26 November warns that the emissions reductions pledged by the world’s nations in the 2016 Paris climate agreement must be quintupled to limit warming to 1.5 °C. The report notes that after three years of stagnation, global CO₂ emissions actually increased in 2017 .

The US report’s call for more aggressive actions on mitigation and adaptation is at odds with Trump administration policies, which have included taking steps to withdraw from the Paris accord and promoting greater use of fossil fuels, particularly coal. President Trump said that he does not believe the report from his own administration.

The assessment lays out how extreme weather events, deteriorating air quality, the spread of new diseases by insects and pests, and changes in food and water availability threaten the health of the US population. Lower-income and marginalized communities will be hit hardest. The continued rise of sea level could threaten $1 trillion of coastal real estate, the report says, with some inundation inevitable on the Atlantic and Gulf coasts through midcentury. Yet there’s still a lot that can be done. For example, the report says that more than half of projected damage to coastal property could be avoided through timely adaptation measures, such as shoreline protection and conservation of coastal ecosystems.

Similarly, the report notes that numerous adaptation strategies are available to cope with other adverse impacts of climate change. Farmers could plant crops that are genetically modified to enhance resilience to climate stress. Engineers and policymakers could improve designs and long-term planning to minimize damage caused by climate change to bridges, pipelines, transportation systems, electrical systems, and other infrastructure.

John Holdren, who served as science adviser to President Obama, says adaptation is as important as mitigation. “We have three choices: mitigation, adaptation, or suffering. We’re doing some of each. What’s up for grabs is the future mix. If we want to minimize future suffering, we’ve got to maximize mitigation and adaptation.”

Holdren calls the report “very well done, comprehensive, and heavily documented” and says it continues an established pattern wherein each new major climate change report becomes more dire. He adds that the report’s estimate of the potential harm to the economy is likely too conservative.

Gina McCarthy, the EPA administrator during most of Obama’s second term, praises the report as “a rallying cry and an important reminder and opportunity to stop denying the science, look at what’s happening in the real world today, and start making investments in adaptation and reducing fossil fuels that are fueling climate change.”

The report is the second half of a two-volume assessment. The first part , released last year, concluded that “it is extremely likely that human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century.”