America's thirst for oil is a gathering threat to its national security – and the risk will grow further as the world's population touches 7 billion, a military adviser to the Pentagon told the Senate today.

In a second day of debate on energy, Democratic senators today pivoted from the economy to national security to try to make the case for a climate change bill.

The threat to Americans' security ranged from the here and now – with troops in Afghanistan and Iraq tied down by their reliance on gas-guzzling equipment – to years into the future when extreme temperatures and rising sea levels could lead to a widespread social breakdown.

"We have never before on this planet had close to 7 billion people which we will have in 2011. We have never had the unprecedented level of per capita energy use multiplied by that 7 billion people," Dennis McGinn, a member of the Military Advisory Board, composed of senior retired admirals and generals, told the Senate. "We have a whole host of indicators, warnings and trends that tells us climate change is bad for national security."

He said the country would face risks on multiple fronts. "America's current energy posture constitutes a serious and urgent threat to national security – militarily, diplomatically and economically."

The Pentagon is already beginning to focus more acutely on the threat posed by climate change.

Military research labs are exploring new energy-saving devices, and other ways of conserving fuel in the battlefield. The conflicts in Afghanistan and Iraq have made planners acutely conscious that fuel dependence is putting US forces at risk. The US marines corps recently ordered an energy audit of its operations in Afghanistan, in a bid to reduce enormous fuel costs.

"We are tied down by fuel. Fuel is a real day-today concern for our forces in the field who are tethered to that fossil fuel tail," said Kathleen Hicks, the deputy undersecretary of defence for strategy.

The US military is beginning to focus more intensely on the threat posed by climate change.

Hicks also told the Senate that global warming was emerging as a dangerous "accelerant" – fuelling conflicts and speeding the breakdown of fragile states.

It also created opportunities for extremist groups such as al-Qaida.

Progress on the climate change bill is seen as essential a to a successful outcome at the international meeting on carbon reduction in Copenhagen in December.

The White House has also stepped up its efforts to shepherd the bill through the Senate. This week's hearings, the formal start of the legislative process, were carefully coordinated with the White House.

Obama yesterday toured a solar facility in Florida and announced the award of some $3.5bn (£2.1bn) in grants to modernise America's electrical grid. Today, the White House sponsored a public forum on energy.

The White House and Democratic leaders are also trying to rally support around the climate change bill in the Senate – in part by bowing to Republican demands for a larger place for nuclear power.

Barbara Boxer, the co-author of the proposed legislation, said today she would press ahead to get her environment and public works committee for early approval of the draft – despite opposition from Republicans and even a powerful Democrat.

"I think there will be good news out of the committee this week, so stay tuned," she said.