An island in the Indian Ocean, vital to the US military, disappears as the sea level rises. Rivers critical to India and Pakistan shrink, increasing military tensions in South Asia. Drought, famine and disease forces population shifts and political turmoil in the Middle East.

US defense and intelligence agencies, viewing these and other potential impacts of global warming, have concluded if they materialize it would become ever more likely global alliances will shift, the need to respond to massive relief efforts will increase and American forces will become entangled in more regional military conflicts.

It is a bleak picture of national security that backers of a climate bill in Congress hope will draw in reluctant Republicans who have denounced the bill as an energy tax and jobs killer because it would shift the country away from fossil fuels by limiting carbon dioxide emissions from power plants and industrial facilities.

At the current increasing rate of global carbon dioxide pollution, average world temperatures at the end of this century will likely be about 7 degrees (4 Celsius) higher than at the end of the 20th century, and seas would be expected to rise by as much as 2 feet (0.61 meters), according to a consensus of scientists on the Intergovernmental Panel on Climate Change.

The security implications of global warming were center stage Wednesday at a Senate Environment and Public Works Committee hearing, one of a series of sessions in advance of voting on the climate bill, possibly as early as next week.

"Our economic, energy and climate change challenges are all inextricably linked," retired Vice Adm. Dennis McGinn told the committee. "If we don't address these challenges in a bold way and timely way, fragile governments have great potential to become failed states ... a virile breeding ground for extremism."

"The US military will be called to respond to these threats," added McGinn, a member of the CNA Military Advisory Board, an influential think tank on military and security issues.

The security implications of climate change have been an issue of growing concern in the defense and intelligence communities.

Dennis Blair, the Obama administration's national intelligence director, has told Congress that global warming will have broad security implications over the next two decades. Also, the Central Intelligence Agency has created a new group of experts to study the security fallout of increased droughts, population shifts, sea level rise and other likely impacts of severe climate change, and the Pentagon has embarked on a detailed study on the military's vulnerabilities from a warmer world.

"US vulnerabilities to climate change are linked to the fate of other nations," says Kathleen Hicks, a deputy undersecretary for defense. She told the Senate panel that senior defense officials believe climate change will make US security challenges more difficult and complex.

While the debate over climate legislation has been sharply split along partisan lines, the alarm over impacts on national security has come from both Democrats and Republicans in the defense and intelligence communities.

A recent report by the American Security Project, an advisory group of high-powered Republicans and Democrats, called global warming "not simply about saving polar bears or preserving beautiful mountain glaciers ... (but) a threat to our security." The group has on its board Republicans such as former Sen. Warren Rudman as well as Democrats including Sen. John Kerry of Massachusetts, the chief author of the Senate climate bill.

Across the globe there exist conflicts and security challenges including ethnic conflicts and emerging radicalism and often "these are also the parts of the world where we will see the most severe consequences from climate change," Bernard Finel, a co-author of the American Security Project report, said in an interview. " The intelligence community, CIA, (military) commanders, they're all looking at these issues."

Former Republican Sen. John Warner, a longtime chairman of the Armed Services Committee and a close ally of the military, has been touring the country to talk about climate change and national security.

"We are talking about energy insecurity, water and food shortages, and climate-driven social instability," says Warner. "We ignore these threats at the peril of our national security and at great risk to those in uniform."

Among the flash points:

- Himalayan glaciers are likely to recede, producing fresh water shortages in India, Pakistan, Bangladesh and parts of China.

- Receding Arctic ice could trigger a territorial conflict involving Russia, the United States, Canada and others.

- Sea level rise in Bangladesh, and drought in other parts of the world could unleash a flood of cross-border "climate refugees" and violence.

- The Indian Ocean island of Diego Garcia, an atoll only a few feet (meter) above sea level, likely would disappear, taking away a critical US military staging area.

Still these concerns are not unanimous.

At Wednesday's hearing, retired Army Major General Robert Scales, who said he had "deep reservations" about the science of climate change, worried that if fossil fuels were curtailed it would reduce the availability of diesel and jet fuel "that might reduce our ability to go to war."

On the prospects of global political and military instability from climate change, Scales said, "such unlikely events would cause enormous suffering and social dislocation. But the history record strongly suggests that such devastating humanitarian disasters rarely if ever result in large-scale wars."