Experts are more certain than ever that human activity is changing the global climate, even though they don't fully understand every detail of the climate system, according to a new report released Wednesday by two of the world's leading scientific bodies.
The document from the U.S. National Academy of Sciences and the United Kingdom's Royal Society aims to move the climate change debate beyond humans' role in global warming to a discussion of how to limit the impacts on society.
"Climate change is happening. We see it in temperature, we see it in the melting ice, and we see it in sea-level rise," Inez Fung, an atmospheric scientist at the University of California at Berkeley and a co-lead author of the report, told NBC News. The changes are due to rising atmospheric concentrations of carbon dioxide with a chemical signature from the burning of fossil fuels, she added.
The report, "Climate Change: Evidence and Causes," is written in simple language and filled with pictures and graphs to illustrate why scientists are certain human activity is causing the climate to change.
Clear evidence and uncertainty
Concentrations of carbon dioxide, the document notes, increased by 40 percent between 1880 and 2012 and are now higher than at any time in at least 800,000 years. As a result, global temperatures are 1.4 degrees Fahrenheit warmer than in 1900, Arctic sea ice is shrinking, sea levels are 8 inches higher, ocean acidity is on the rise, and the geographical ranges of many plants and animals are shifting.
"The evidence is clear," reads the report. "However, due to the nature of science, not every single detail is ever totally settled or completely certain. Nor has every pertinent question yet been answered."
Key areas of uncertainty highlighted in the report include the cause of the recent slowdown in warming, known as the hiatus; estimates of how much warming can be expected in the future; the connections between climate change and extreme weather events such as the frequency and intensity of hurricanes, droughts, and floods; and the role of clouds.
The bulk of the document is a 20-part series of answers to questions that range from the basic "Is the climate warming?" – Answer: Yes – to the thornier "Does the recent slowdown of warming mean that climate change is no longer happening?" – Answer: No, blame the slowdown on interactions between the ocean and atmosphere, quiet solar activity and an increase in sunlight reflecting particles.
'Ho-hum' but necessary
Outside experts asked to comment on the report noted that it lacks new information, but neatly packages mainstream climate science for a general audience. "Ultimately, [it is] rather ho-hum, and pretty redundant to everything else that is out there," Roger Pielke Jr., a climate policy analyst and professor of environmental studies at the University of Colorado at Boulder, told NBC News in an email.
Ho-hummery aside, the National Academy of Sciences and Royal Society were compelled to make a statement, according to Michael Mann, a climate scientist at Pennsylvania State University in University Park. "Sadly, in today's political environment, where climate change denial is pervasive at our highest levels of government, it seems that the message is not being heard," he told NBC News via email.
In fact, from Mann's perspective, the report is too conservative. For example, he said, it fails to acknowledge that climate models often underestimate the rate of change. "This is clearly true with respect to Arctic sea ice, where the precipitous decline seen in the observations over the past several decades is way beyond where the models generally conclude we should be," he said.
Which option do we choose?
To move the debate forward on how to respond to climate change, the document describes available options, ranging from doing nothing and accepting the "losses, damage and suffering that arise," to a change in energy production and usage to limit greenhouse gas emissions or "geoengineering" of the climate to counteract some of the changes.
Which option — or portfolio of options — society ultimately chooses is up for debate. What's important for the scientific academies is that the debate occurs. Even as the scientific process evolves and raises new questions about climate change, the evidence is clear that human activity is forcing the climate to change, according to the report.
"Uncertainty," Fung said, "does not challenge my certainty about the fact the planet will warm."