GLASGOW -- The world's armies must keep pace with global efforts to tackle climate change and cut their huge carbon footprints according to clearly defined benchmarks, NATO Secretary-General Jens Stoltenberg said on Tuesday.

Little reliable data exists on emissions created by military activities but there is no doubt they are big: a 2019 report conluded the U.S. military would be the world's 47th largest emitter of greenhouse gases if it were a country.

"There is no way to reach net zero without also including emissions from the military," Stoltenberg said in an interview at the COP26 climate conference, referring to the ambitions of many nations to hit net-zero emissions by mid-century.

Stoltenberg, a former UN special envoy on climate change, said work had already started within the alliance on a methodology to measure armed forces' emissions and that his aspiration was that this should be completed by end-2022.

"That is aim but of course I am dependent on agreement among 30 allies," he told Reuters.

Stoltenberg acknowledged the heavy existing footprint of the world's armies but said there were already efforts to address it: for example, the U.S. military using solar panels in its installations, the British army exploring alternative fuels and Spain planting trees on military land to capture carbon.

"There is an energy revolution taking place out there in civil society ... and this is about keeping up the pace, being part of that transformation," he said.

Stoltenberg began pushing for joint NATO moves on climate change some time ago but NATO diplomats said efforts to focus on the issue were stymied during Donald Trump's U.S. presidency.

He likened the current energy challenge facing militaries to the pre-World War I decision of Winston Churchill to replace coal with oil as the main fuel for the British navy - a move Stoltenberg said had both environmental and strategic benefits.

"We need to find a way to reconcile the need for green and environment-friendly armed forces with strong and effective armed forces," he said.

Editing by Gareth Jones