The evidence is now crystal clear that climate change is real, caused by humans, happening faster than predicted, and poses a tremendous threat to the United States and the rest of the world, according to the Climate Science Special Report released Friday.
This 470-page U.S. government report evaluated the latest scientific evidence and concludes that storms, including hurricanes, have become more powerful; heavy rainfall is more common in some parts of the U.S.; and heat waves, wildfires, and droughts are more intense and happening more frequently.
These conclusions were made with an unprecedented level of scientific certainty, utterly refuting statements made by senior Trump administration officials about the causes and effects of our changing climate. (See more on the Trump administration’s approach to climate.)
The White House downplayed the new report on Friday. “The climate has changed and is always changing,” White House spokesperson Raj Shah said in a statement.
In contrast, the report pins today’s climate change squarely on human activities and says there is “no convincing alternative explanation” supported by science to explain the warming seen in the last century. (See a "Running List of How Trump Is Changing the Environment.")
“This period is now the warmest in the history of modern civilization,” says Katharine Hayhoe of Texas Tech University, one of the report’s lead authors.
“We’ve known for decades that human activities are warming the planet and that this is potentially very dangerous,” Hayhoe says. Burning fossil fuels and deforestation are two of the main human activities causing global warming—by putting billions of tons of carbon dioxide, or CO2, into the atmosphere every year. (See "Global Warming 101" below for more.)
“Almost every year we’re learning that the impacts of climate change are coming faster and are worse than expected,” she says.
The Climate Science Special Report, or CSSR, is part of the Fourth National Climate Assessment, which will be completed in 2018. The CSSR is Congressionally mandated and designed to be the authoritative assessment of the science of climate change, with a focus on the United States.
"The CSSR is extremely thorough and well-written," says Kevin Trenberth, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. It uses the most current scientific evidence to reveal the human fingerprints on the drivers of extreme weather events, such as heavier rainfall and increases in heatwaves and wildfires, Trenberth adds. Trenberth was an official National Academy of Science reviewer of a draft version of the CSSR.
In looking at the latest science on the melting of the Greenland and Antarctic ice sheets, the report concludes that sea levels could rise globally by as much as eight feet by the year 2100. Sea level rise does not happen evenly, and the coasts of the U.S. Northeast and the western Gulf of Mexico are likely to experience much higher levels than the global average. A 10-foot sea level rise would put the New York City subway system and parts of the city permanently underwater, another study found. (See “Sea Level Rise Will Flood Hundreds of Cities in the Near Future.”)
The CSSR employs a “balance of evidence” approach and inherently is scientifically conservative in its conclusions, says Hayhoe.
One example of this cautious approach: Some scientists believe that the Atlantic Ocean’s “conveyor belt” current, which carries warm water from the Caribbean northward toward the British Isles and northern Europe, is already slowing down due to the influx of cold, fresh water from the melting of the ice sheet in Greenland.
The report concludes, however, that there isn’t enough evidence yet of such a slowdown, largely because there hasn’t been enough research. Should this current slow, it would have “dramatic climate feedbacks as the ocean absorbs less heat and CO2 from the atmosphere … [and] affect the climates of North America and Europe.”
The report documents a dozen other potential “dramatic climate feedbacks” including the complete loss of Arctic ice in the summer, the thawing of Earth’s permafrost zone, and the loss of the Greenland ice sheet. These tipping points, or more accurately "tipping elements," are major aspects of the climate system that could ‘tip’ into a new state with far-ranging consequences for the climate, says Hayhoe.
Climate models that provide a window into the future largely do not include these tipping elements. Moreover, these are unlikely to occur in isolation and may act more like dominoes, one triggering another. In fact, the CSSR notes that “compound extreme events” such as hot and dry conditions can trigger heat waves, drought, and wildfires, resulting in economic and social impacts that can be greater than the sum of the parts.
“My biggest worry is cascading consequences of these events,” Hayhoe says.
Trenberth agrees, saying things usually happen on top of each other and often with unexpected results. This is why “economists have greatly underestimated the impacts of climate change,” he says.
Global Warming 101
Earth's atmosphere is made up of CO2 and other greenhouse gases, as well as oxygen and nitrogen. Without this atmosphere, conditions on Earth would be more like those on the moon, which is scorching hot (+100° C) during the day and bitterly cold (-150° C) at night. More than 150 years ago, scientists proved that CO2 traps heat from the sun and that burning fossil fuels such as oil, gas, and coal emits CO2.
Measurements show that there is now 44 percent more CO2 in the atmosphere than 150 years ago, before massive use of fossil fuels. That extra CO2 is like putting another warm blanket on at night, even though you are already nice and warm.