“Observations throughout the world make it clear that climate change is occurring, and rigorous scientific research concludes that the greenhouse gases emitted by human activities are the primary driver,” the letter states. “This conclusion is based on multiple independent lines of evidence and the vast body of peer-reviewed science.”
The effort to draft the letter was spearheaded by the American Association for the Advancement of Science (AAAS), whose leader, Rush Holt — a former member of Congress, vigorously promoted its message.
“Climate change is real and happening now, and the United States urgently needs to reduce greenhouse gas emissions,” said Holt, who was the U.S. representative for New Jersey’s 12th congressional district from 1999 to 2015. “We must not delay, ignore the evidence, or be fearful of the challenge.”
The 31 organizations that signed the letter include the American Meteorological Society, the American Chemical Society, the American Public Health Association, the American Geophysical Union, and University Corporation for Atmospheric Research.
“What virtually every major science organization is warning us about is worrisome,” said Marshall Shepherd, a past-president of the American Meteorological Society and professor of atmospheric science at the University of Georgia. “There is no prize for being right as scientists. In fact, if my understanding of the science is correct, my kids lose. So I pray daily that we are wrong.”
Since 2009, the planet has experienced its two warmest years on record, 2015 and 2014, while atmospheric levels of carbon dioxide have risen relentlessly, passing 400 parts per million.
“For the United States, climate change impacts include greater threats of extreme weather events, sea level rise, and increased risk of regional water scarcity, heat waves, wildfires, and the disturbance of biological systems,” the updated 2016 letter says. “The severity of climate change impacts is increasing and is expected to increase substantially in the coming decades.”
The letter signatories said their statements are supported by key assessments from major scientific bodies, such as the U.S. government’s Global Change Research Program, the U.S. National Academies, and the Intergovernmental Panel on Climate Change.
Of prominent U.S. scientific organizations, only the American Physical Society (APS) abstained from participating in both the 2009 and 2016 letter efforts.
“The American Physical Society did not sign the  letter because it was presented as a fait accompli, and there are significant differences between the letter and the APS Statement on Earth’s Changing Climate,” it said in a statement. “The APS statement went through a two-year vetting process involving multiple committees, the society’s 53,000-plus membership and the board of directors.”
Though the APS statement about climate change is more nuanced than the AAAS letter, stating — for example — “scientific challenges remain in our abilities to observe, interpret, and project climate change,” it in no way disputes the scientific consensus on climate change or the risks it poses.