With two ongoing wars, the continuing specter of terrorism, nuclear programs in Iran and North Korea, and the challenges of the Middle East, the national security landscape is imposing. But one essential national security imperative has gone unaddressed too long, its risks unmet: the combined challenge of energy security and global climate change.

We can no longer afford to ignore the consequences of inaction or pass the buck to future generations. It is our responsibility to act now.

It's time for the United States to put in place a coherent national energy policy that limits the severe economic and national security risks of our over-dependence on oil. The American economy today is hostage to volatile oil prices. Four out of the last five U.S. recessions were preceded by an oil price spike. A geopolitical conflict that disrupts the global supply of oil could quickly escalate into a military confrontation and would leave us with few options to alleviate the impact on the U.S. economy. This strategic vulnerability draws us into intractable conflicts in the most volatile regions of the world.

Expanding our energy sources would have cascading benefits to our security: reducing our vulnerability to energy supply disruptions and price fluctuations; limiting the flow of funds to nations that support terrorism; diminishing the prospect of dangerous resource competition and minimizing the dramatic impact of climate change.

This last effect may be the most critical to our national security. Left unaddressed, climate change will cause political instability and humanitarian disaster with dire implications for American interests and global security. From rising sea levels, droughts, floods and storms, climate change threatens to trigger mass migration, uprooting as many as 700 million people by the end of the century -- an exodus of epic and explosive proportions. Increased sea levels could threaten the stability of forty nations worldwide. This is a prescription for war -- both within and between nations.

The threat is not some far-off nightmare: we already are seeing the destabilizing effects of climate change in regions critical to American interests. Droughts in the Middle East, a direct result of increased global temperatures, have put a severe strain on the Sea of Galilee and the Jordan River, limiting the availability of water in Israel and the West Bank. In South Asia, India and Pakistan face an escalating conflict over the Indus River, which runs through the hotly disputed territories of Jammu and Kashmir. In each case, competition over vital resources will continue to exacerbate tense political disputes, further destabilizing already troubled regions.

Nations undermined by desertification, crop failure, rising sea levels and resultant refugee crisis will be left to confront severe humanitarian, economic and political challenges. And when disaster strikes around the globe, states turn to America for help. With a military strained to its limits and a home front in desperate need of investment and renewal, we cannot afford a series of costly interventions that can be mitigated by responsible measures now.

Our military and intelligence agencies are painfully aware of the implications that our dependence on fossil fuels, and their contribution to global climate change, have on our security. For the first time, this year's Quadrennial Defense Review expressly considered the impact of climate change as a factor in destabilizing weak governments. The CIA has created a climate change center to analyze and respond to this threat. Our national security institutions are already mobilizing to combat this challenge. It is time for our political institutions to do the same.

Poll after poll has found strong support from voters for policies that invest in clean energy and reduce harmful carbon emissions. One such bipartisan effort, being advanced by Sens, John Kerry, Lindsey Graham and Joe Lieberman, sets us on a path to enhanced energy independence and economic growth. It's a sensible approach to a serious national security challenge.

Comprehensive climate and energy legislation will help us move toward a safer and stronger global position: enhancing our national security by cutting our dependence on foreign oil and reducing the harmful effects of climate change. By passing a meaningful clean energy bill, the Senate can take an important step in safeguarding the United States' interests at home and abroad.

is chairman of Albright Stonebridge Group, and was national security adviser from 1997 to 2001.