Such climate-induced crises could topple governments, feed terrorist
movements or destabilize entire regions, say the analysts, experts at the
Pentagon and intelligence agencies who for the first time are taking a
serious look at the national security implications of climate change.

Recent war games and intelligence studies conclude that over the next 20
to 30 years, vulnerable regions, particularly sub-Saharan Africa, the
Middle East and South and Southeast Asia, will face the prospect of food
shortages, water crises and catastrophic flooding driven by climate change
that could demand an American humanitarian relief or military response.

An exercise last December at the National Defense University, an
educational institute that is overseen by the military, explored the
potential impact of a destructive flood in Bangladesh that sent hundreds
of thousands of refugees streaming into neighboring India, touching off
religious conflict, the spread of contagious diseases and vast damage to
infrastructure. "It gets real complicated real quickly," said Amanda J.
Dory, the deputy assistant secretary of defense for strategy, who is
working with a Pentagon group assigned to incorporate climate change into
national security strategy planning.

Much of the public and political debate on global warming has focused on
finding substitutes for fossil fuels, reducing emissions that contribute
to greenhouse gases and furthering negotiations toward an international
climate treaty — not potential security challenges.

But a growing number of policy makers say that the world's rising
temperatures, surging seas and melting glaciers are a direct threat to the
national interest.

If the United States does not lead the world in reducing fossil-fuel
consumption and thus emissions of global warming gases, proponents of this
view say, a series of global environmental, social, political and possibly
military crises loom that the nation will urgently have to address.

This argument could prove a fulcrum for debate in the Senate next month
when it takes up climate and energy legislation passed in June by the
House.

Lawmakers leading the debate before Congress are only now beginning to
make the national security argument for approving the legislation.

Senator John Kerry, the Massachusetts Democrat who is the chairman of the
Foreign Relations Committee and a leading advocate for the climate
legislation, said he hoped to sway Senate skeptics by pressing that issue
to pass a meaningful bill.

Mr. Kerry said he did not know whether he would succeed but had spoken
with 30 undecided senators on the matter.

He did not identify those senators, but the list of undecided includes
many from coal and manufacturing states and from the South and Southeast,
which will face the sharpest energy price increases from any carbon
emissions control program.

"I've been making this argument for a number of years," Mr. Kerry said,
"but it has not been a focus because a lot of people had not connected the
dots." He said he had urged President Obama to make the case, too.

Mr. Kerry said the continuing conflict in southern Sudan, which has killed
and displaced tens of thousands of people, is a result of drought and
expansion of deserts in the north. "That is going to be repeated many
times over and on a much larger scale," he said.

The Department of Defense's assessment of the security issue came about
after prodding by Congress to include climate issues in its strategic
plans — specifically, in 2008 budget authorizations by Hillary Rodham
Clinton and John W. Warner, then senators. The department's climate
modeling is based on sophisticated Navy and Air Force weather programs and
other government climate research programs at NASA and the National
Oceanic and Atmospheric Administration.

The Pentagon and the State Department have studied issues arising from
dependence on foreign sources of energy for years but are only now
considering the effects of global warming in their long-term planning
documents. The Pentagon will include a climate section in the Quadrennial
Defense Review, due in February; the State Department will address the
issue in its new Quadrennial Diplomacy and Development Review.

"The sense that climate change poses security and geopolitical challenges
is central to the thinking of the State Department and the climate
office," said Peter Ogden, chief of staff to Todd Stern, the State
Department's top climate negotiator.

Although military and intelligence planners have been aware of the
challenge posed by climate changes for some years, the Obama
administration has made it a central policy focus.

A changing climate presents a range of challenges for the military. Many
of its critical installations are vulnerable to rising seas and storm
surges. In Florida, Homestead Air Force Base was essentially destroyed by
Hurricane Andrew in 1992, and Hurricane Ivan badly damaged Naval Air
Station Pensacola in 2004. Military planners are studying ways to protect
the major naval stations in Norfolk, Va., and San Diego from
climate-induced rising seas and severe storms.

Another vulnerable installation is Diego Garcia, an atoll in the Indian
Ocean that serves as a logistics hub for American and British forces in
the Middle East and sits a few feet above sea level.

Arctic melting also presents new problems for the military. The shrinking
of the ice cap, which is proceeding faster than anticipated only a few
years ago, opens a shipping channel that must be defended and undersea
resources that are already the focus of international competition.

Ms. Dory, who has held senior Pentagon posts since the Clinton
administration, said she had seen a "sea change" in the military's
thinking about climate change in the past year. "These issues now have to
be included and wrestled with" in drafting national security strategy, she
said.

The National Intelligence Council, which produces government-wide
intelligence analyses, finished the first assessment of the national
security implications of climate change just last year.

It concluded that climate change by itself would have significant
geopolitical impacts around the world and would contribute to a host of
problems, including poverty, environmental degradation and the weakening
of national governments.

The assessment warned that the storms, droughts and food shortages that
might result from a warming planet in coming decades would create numerous
relief emergencies.

"The demands of these potential humanitarian responses may significantly
tax U.S. military transportation and support force structures, resulting
in a strained readiness posture and decreased strategic depth for combat
operations," the report said.

The intelligence community is preparing a series of reports on the impacts
of climate change on individual countries like China and India, a study of
alternative fuels and a look at how major power relations could be
strained by a changing climate.

"We will pay for this one way or another," Gen. Anthony C. Zinni, a
retired Marine and the former head of the Central Command, wrote recently
in a report he prepared as a member of a military advisory board on energy
and climate at CNA, a private group that does research for the Navy. "We
will pay to reduce greenhouse gas emissions today, and we'll have to take
an economic hit of some kind.

"Or we will pay the price later in military terms," he warned. "And that
will involve human lives."