The following is a guest post from Terry Tamminen and Stewart J. Hudson. Tamminen is the Cullman Senior Climate Policy Fellow at the New America Foundation. His latest book is Lives Per Gallon: The True Cost of Our Oil Addiction. Hudson is president of the Emily Hall Tremaine Foundation, and co-chair of the U.S. Climate and Energy Funders Group.

Preparations for President Bush’s Sept. 27-28 summit of world leaders on climate change are underway and will determine how the president sets the tone for this historic meeting. He can restore American leadership by calling for mandatory reductions in greenhouse-gas emissions, or he can shoot for the lowest common denominator as a means of sticking with the status quo.

“Science has deepened our understanding of climate change and opened new possibilities for confronting it,” the president said recently. In keeping with this new perspective, there are three steps he could take now to make this summit a success.

First, based on what the hard science tells us is needed, the president could commit the United States to measurable actions that avoid catastrophic changes in the Earth’s climate. This requires making a national commitment to numerical targets that would at a minimum reduce greenhouse-gas emissions to 1990 levels by 2020, and reduce these emissions 80 percent below 1990 levels by 2050.

Second, the president could acknowledge that voluntary reductions have not proven sufficient to address this problem. As Republican governors in California, Florida, Connecticut, and elsewhere have realized, mandatory reduction measures are a necessary addition to voluntary programs and can be good for the economy as well as the environment. Climate experts tell us that such measures are imperative. The president should now endorse mandatory reductions.

Third, the president could highlight and support state and local climate initiatives. From California to Connecticut, and Missoula to Miami, mandatory strategies are now being crafted and implemented by Republicans and Democrats alike. The president can empower these efforts by highlighting them at the upcoming summit. He can add incentives for state and local initiatives in federal policy. And he can start down this path by granting a waiver allowing California and other states to implement clean cars legislation, a move that would communicate his desire to lead on this issue.

Why would an administration not known for leadership on climate and energy issues assume this new stance on climate policy? To begin, the president might take a cue from one of his most powerful constituencies — American evangelicals. Given the moral dimensions of this issue, some of the newer leaders in the evangelical movement have spoken out on the need for action. As some have described it, climate change is really a huge, unprecedented experiment that humans are performing on Earth’s ecosystems and all creatures, great and small. Moreover, Americans comprise only 5 percent of the world’s population but use 20 percent of the world’s energy, and are responsible for at least 25 percent of the global climate problem. For many religious leaders, the lack of individual or collective action on climate change is contrary to their spiritual beliefs about creation and what we owe future generations at home and abroad.

Polls show that a vast majority of Americans are also concerned that steep energy prices and our dependence on foreign sources have weakened our national security and standing in the world. Diversifying U.S. energy sources, and reducing our use through high-priority investments in energy efficiency, will enhance our security and provide valuable political capital for the U.S. in world affairs.

While this updated approach to climate change would run contrary to the findings of Vice President Dick Cheney’s 2001 energy task force, it is in line with what science has told us since then. It also fits President Bush’s penchant for making deeply personal decisions on moral issues of global and historical significance.

Finally, there is the issue of political legacy. Presidents often hunger for a moment when they can leave a positive mark on history. This is such a moment, and President Bush can create a positive historical legacy for his administration by assuming a leadership position on climate and energy. Calling for mandatory measures would be a smart, courageous, and enduring way of doing so.