as has happened at times earlier in the planet’s history, it could pose formidable challenges for adaptation measures. In the worst case, warming may trigger tipping points—thresholds for irreversible changes in the way Earth’s climate operates and how human and ecological systems respond.

Given this scenario, it is likely going to be a Herculean task to limit climate change to 2°C of warming from preindustrial levels as desired by many governments. The 1997 Kyoto Protocol was an important initial step toward attempting to manage greenhouse gas emissions at the international level. At the national level, nearly 80 percent of U.S. states have adopted or are preparing climate action plans, some of which include mitigation measures such as cap and trade programs. However, many policy decisions on mitigation and adaptation are being made without the scientific support that could help shape better outcomes. Robust and effective responses to climate change demand a vastly improved body of scientific knowledge, including observations and better understanding and predictions of the changing climate system, the human drivers of climate change, the response of the climate system to these drivers, and the response of society to climate changes.

The research, observations, and modeling needed to develop the knowledge foundation for understanding and responding to climate change at the federal level is the responsibility of the U.S. Climate Change Science Program (CCSP). At the request of Dr. James Mahoney, then director of the CCSP, the National Research Council established a committee to carry out two tasks over a 3-year period. The report on the committee’s first task, Evaluating Progress of the U.S. Climate Change Science Program: Methods and Preliminary Results, was published in 2007 (NRC, 2007c). The second task—future priorities for the program—is the subject of this report: