Choices the world makes about whether to cut man-made carbon dioxide emissions will determine the severity of climate change over the next thousand years -- or longer, according to a new report by the country's leading scientific advisory body, the National Academy of Sciences.
That's because the greenhouse gas lingers in the atmosphere for hundreds or even thousands of years.
"The bottom line is that because CO2 is so long-lived in our atmosphere, it could effectively lock the Earth and future generations into warming -- not just for decades or centuries, but literally for thousands of years," said committee chairwoman Susan Solomon, an atmospheric scientist at the National Oceanic and Atmospheric Administration.
The report was sponsored by the Energy Foundation -- a coalition of major philanthropic foundations with an interest in energy policy -- and U.S. EPA.
It breaks down how each additional degree of warming would affect the Earth, cataloging impacts on forests, fresh water supplies, fisheries, Arctic sea ice, sea level and agriculture.
The picture it paints is stark:
- Every 1 degree Celsius of warming -- roughly 1.8 degrees Fahrenheit -- would reduce rainfall in the American Southwest, the Mediterranean and southern Africa by 5 to 10 percent.
- Stream flow in some river basins -- including the Arkansas and the Rio Grande -- would drop by 5 to 10 percent.
- Yields of some crops, including U.S. and African corn and Indian wheat, would fall 5 to 15 percent.
- And 1 to 2 degrees Celsius of warming is enough to double or even quadruple the area burned by wildfires in the American West.
Sea ice and forests disappear
Determining what would happen if Earth warms beyond 2 degrees Celsius is difficult, the NAS committee said, because there would be little forested land left to fuel flames -- meaning parts of the West that are now tree-covered would transform into entirely new ecosystems.
Focusing on effects of incremental rises in temperature, rather than different levels of CO2 in the atmosphere, revealed commonalities in the projections of climate models that appeared to disagree, the scientists who wrote the report said.
One example is models that attempt to project how warming will affect sea ice cover in the Arctic.
"When you look at the various models, you may see quite a range in what they have to say about how fast sea ice looks like it's going to disappear," Solomon said. "But when you actually look at it as a function of temperature, it's really kind of amazing how consistent they all become."
That's because sea ice, like many of the other parts of the environment discussed in the report, is most sensitive to temperature change, she said.
While the report focuses on how climate change could affect life on Earth into the far future, it also takes stock of the present.
Welcome to the 'Anthropocene'
Humans have already changed climate to the point that Earth has entered a new geologic epoch, which the analysis dubs the "Anthropocene."
And even if the primary human activities that produce greenhouse emissions -- cutting down forests and burning fossil fuels -- were to stop immediately, a certain amount of additional warming is already baked into the climate system.
The level of carbon dioxide in the atmosphere has jumped from roughly 280 parts per million in preindustrial times to 390 ppm today, raising Earth's temperature roughly 0.9 degree Celsius along the way.
The seas, slow to warm, have absorbed a lot of extra heat trapped by that CO2, along with about 80 percent of CO2 emissions. But over time, that will change. As oceans start pumping some of that accumulated heat and CO2 back into the atmosphere, the CO2 emitted up until now will cause another 1 degree Celsius of warming.
Building a backlog of more heat
"It's like we're paying for things with credit and running up a deficit and we won't have the full consequences of that for some time," said David Lobell, a Stanford University scientist who studies climate impacts on agricultural crops.
Ultimately, whether the Anthropocene will amount to a blip in Earth's history or a major climate shift lasting "many thousands" of years depends on the choices society makes about whether -- and how much -- to curtail greenhouse gas emissions, the new report says.
That requires making value judgments about the level of risk we're willing to endure -- a question that goes beyond scientific projections, said experts who worked on the new report.
"When I think about things I can do in my own life where I can go back and reverse them, I treat them differently from things that are irreversible," Solomon said. "If I knew that every piece of cheesecake would give me a pound of weight gain that would never be reversed, I would eat a lot less cheesecake."
The new report doesn't make the call on what level of warming is tolerable, the scientists who worked on the analysis said, but it is designed in part to help lawmakers understand the likely impact of different policy choices.
It does note, however, that "emissions reductions larger than about 80 percent, relative to whatever peak global emissions rate may be reached, are required to approximately stabilize carbon dioxide concentrations."
Click here to view the NAS report.