The National Academy of Sciences wants to put the United States on a low-carbon diet.
That is the underlying message of a hotly anticipated trio of reports requested by Congress and released today. In them, the academy describes an "urgent need" for the nation to trim its greenhouse gas emissions.
The reports say available evidence "makes a compelling case" that climate change is happening now, is largely driven by human activities and threatens the well-being of people today and in future generations.
"There is good evidence that not all climate changes will be smooth and gradual and thus easy to adapt to," said Pamela Matson, dean of Stanford University's School of Earth Sciences and chairwoman of the NAS panel on climate change science.
"There are still some uncertainties, and there always will be in understanding a complex system like Earth's climate," the reports say. "Nevertheless, there is a strong, credible body of evidence, based on multiple lines of research, documenting that climate is changing and that these changes are in large part caused by human activities."
Together, the three panel reports comprise more than 800 pages of analysis on the underlying science of climate change and measures to limit and adapt to it. NAS will release two more reports under the "America's Climate Choices" banner: a committee report on improving climate-related decisionmaking, and a final report that draws together and builds on the recommendations from all four panels.
The new reports recommend cutting emissions swiftly by setting a price on carbon. That could take the form of a carbon tax or a cap-and-trade plan.
The ongoing climate debate in the United States has focused on the latter option, and the NAS authors say they "see no strong reason" to argue cap and trade should be abandoned in favor of a carbon tax. Cap and trade, they say, has several advantages: It is more in line with the idea of setting an overall "budget" or target for U.S. greenhouse gas emissions and it may make it easier to monitor progress toward such goals.
While the report shies away from recommending an overall budget target for emissions, it says recent studies advise limiting emissions between 2012 and 2050 to either 170 gigatons of CO2 equivalent (80 percent below 1990 levels) or 200 gigatons (50 percent below 1990 levels). Those levels are in line with proposals from the Obama administration and Capitol Hill.
Achieving those goals won't be easy, the NAS authors say. By their calculations, continuing the current rate of emissions from the nation's energy sector alone would wipe out the U.S. greenhouse gas emissions budget "well before" 2050.
But no matter the approach, the nation's greenhouse gas output must begin to decline soon, in large part because CO2 lingers in the atmosphere for decades to centuries. Thus, the NAS analyses say, action to cut emissions won't produce instant results.
"It is mainly future generations that will have to deal with the consequences (both positive and negative) of decisions made today," the authors conclude.
Blaming human activities
Some effects are already apparent, however. Earth's average surface temperature rose 1.4 degrees Fahrenheit over the past 100 years, with most of the warming occurring over the past three decades, concludes the NAS panel report on the state of climate science.
Natural climate cycles alone cannot account for that change, the report says, pinning much of the blame on human activities that burn fossil fuels and to a lesser extent, agriculture, forest clearing and other industrial processes.
But overall warming is not the only apparent effect of climate change. The NAS report includes a laundry list of impacts, including increases in the frequency of intense rainfall, decreases in snow cover and ice, more frequent and intense heat waves, rising sea levels, and widespread ocean acidification.
Projections of future climate change range from a warming of 2 to 11.5 degrees Fahrenheit during this century. The numbers vary widely in part because they are based on different scenarios of energy use over that time, the academy's analysis notes.
"While it is clear that the Earth's future climate will be unlike the climate that ecosystems and human societies have become accustomed to during the last 10,000 years, the exact magnitude of future climate change and the nature of its impacts will always remain somewhat uncertain," it concludes.
While proposals to fight climate change by geoengineering are gaining steam as a potential stopgap in case emissions cuts fail to stave off dangerous climate change, the science academy's analyses say there is limited research on the feasibility or risk of implementing one of the most-discussed approaches.
Solar radiation management techniques -- which seek to limit warming by increasing the amount of sunlight reflected into space by increasing cloud cover or placing reflective particles or mirrors into the upper atmosphere -- "all involve considerable risk and potential for unintended ... side effects," the analyses conclude.
Meanwhile, even with aggressive policies to limit the magnitude of climate change, the NAS reports find some shifts will be unavoidable and will require society to adapt.
Because policymakers have only recently begun thinking about adaptation, there is little information available on the strengths and weaknesses of different approaches, the analyses say.
They recommend that the federal government strengthen climate research carried out by the U.S. Global Change Research Program and work with state, local and tribal governments, as well as businesses and environmental groups, to develop and implement a national strategy for adapting to climate change.
Click here to read the report entitled "Advancing the Science of Climate Change."
Click here to read the report entitled "Limiting the Magnitude of Climate Change."
Click here to read the report entitled "Adapting to the Impacts of Climate Change."