In the United States, the report finds that every part of the country has been touched by warming, from droughts in the Southeast to flooding in the Midwest to a worrying rise in air and ground temperatures in Alaska, and conditions will continue to worsen.
“This assessment concludes, based on extensive evidence, 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. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence,” the report states.
“Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse (heat-trapping) gases emitted globally and on the remaining uncertainty in the sensitivity of Earth’s climate to those emissions,” it adds.
Credit Ralph Freso/Getty Images
The climate science report is part of a congressionally mandated review conducted every four years known as the National Climate Assessment. The product of hundreds of experts and scientists within the government and academia and peer reviewed by the National Academy of Sciences, it is considered the United States’ most definitive statement on climate change science.
But the Environmental Protection Agency has wiped references to climate change from its website and barred its scientists from presenting scientific reports on the subject, and its administrator, Scott Pruitt, does not believe carbon dioxide is a primary contributor to warming. Rick Perry, the energy secretary, asserted Wednesday that “the science is out” on whether humans cause climate change.
Jim Bridenstine, the Oklahoma congressman whom President Trump nominated to lead NASA, came under fire this week from multiple lawmakers demanding to know his position on climate change. “Carbon dioxide is in fact a greenhouse gas,” Mr. Bridenstine allowed, but declined to say if he accepted it as the primary cause of climate change. He did pledge that scientists would not be punished or reassigned for working on or speaking about global warming.
Yet none of those agencies nor the White House moved to stop the report’s publication, and the White House Office of Science and Technology Policy signed off on the final release, despite fear among some scientists involved in the research that the Trump administration would block it or seek to water it down. That, researchers said, now creates an unusual situation in which government policies are in direct opposition to the science it is producing.
Christopher Field, director of the Stanford Woods Institute for the Environment at Stanford University, called that “tragic.”
We know. Global warming is daunting. So here’s a place to start: 17 often-asked questions with some straightforward answers.
“This profoundly affects our ability to be leaders in developing new technologies and understanding how to build successful communities and businesses in the 21st century,” Mr. Field said. “Choosing to be dumb about our relationship with the natural world is choosing to be behind the eight ball.”
Critics of climate change science attacked the report as the product of holdovers from former President Obama’s administration, and also chastised the Trump administration for allowing it to be published. Others said the science may be valid but those findings should not affect laws to address the rise of emissions.
“I really don’t think that determines policy at all,” said Marlo Lewis Jr., a senior fellow at the Competitive Enterprise Institute. Mr. Lewis said he does not deny that the majority of warming is caused by man-made emissions. But, he said, “The thing is, I’m also going to affirm that there are risks of climate policy as well as climate change. To me the real issue is, where do the risks lie? Suppressing your economy is never a good solution.”
The report finds with very high confidence that the average annual temperature over the contiguous United States has increased by 1.2 degrees Fahrenheit (0.7 degrees Celsius) since 1986, relative to the previous century. It is projected to rise, scientists said with an equally high degree of confidence, about 2.5 degrees Fahrenheit, or 1.4 degrees Celsius by midcentury. That will mean hotter days and nights, particularly in urban and densely populated areas.
The report finds with high confidence that if greenhouse gas concentrations were stabilized at their current level, the world will still see at least an additional 1.1 degree Fahrenheit (0.6 degree Celsius) of warming over this century.
“This new report simply confirms what we already knew. Human-caused climate change isn’t just a theory, it’s reality,” said Michael E. Mann, a distinguished professor of atmospheric science and the director of the Earth System Science Center at Penn State University. “Whether we’re talking about unprecedented heat waves, increasingly destructive hurricanes, epic drought and inundation of our coastal cities, the impacts of climate change are no longer subtle. They are upon us. That’s the consensus of our best scientists, as laid bare by this latest report.”