The British military accounts for half of all UK Government emissions. Now it is exploring ways it can reduce its carbon footprint, with the aim of being net zero by 2050.
According to Ministry of Defence (MOD) figures it generates 2.5m tonnes of CO2, and each year its aircraft burn 461 million litres of aviation fuel.
In 2021 the MOD published a strategy on Climate Change and Sustainability, written by Lieutenant General Richard Nugee, a former Chief of Defence People, and the military's first climate champion.
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"Climate change is happening to us whether we like it or not," he said.
"If our equipment doesn't work because it's too hot for it to work, if our equipment doesn't work because the surface sea temperature has risen to a level that it doesn't cool our engines anymore, or if our training can't happen because Salisbury Plain is on fire... then we need to think about how we deal with that."
The Royal Air Force has set a goal to be net-zero by 2040. The Royal Navy is aiming to reduce its emissions by up to 40% in the same period by introducing hybrid engines.
The Army is planting two million trees across its estate and building 80 new solar farms.
Watch: Climate warriors – the military's fight against global warming.
Doctor Richard Milburn lectures on environmental security at King's College London and is also an Army reservist.
He believes the services need to channel the "spirit of Bletchley Park" and encourage disruptive thinking.
"It (the MOD) needs to reach out to regulars and reserves, and at all ranks. It needs to almost go back to Churchill's corkscrew thinkers of World War Two.
"Putting them in rooms with engineers and scientists and saying 'what about?'"
Campaign groups claim UK defence actually produces 11 million tonnes of CO2, not three, if you count the supply chain and operations.
"It's excellent to hear these policies and initiatives," said Linsey Cottrell from the Conflict and Environment Observatory.
"But we need to make sure what's being said is being delivered. There's a lot of talk about 'green washing' in the commercial sector, and that can also happen in the military."
General Nugee would like the UK military to set the benchmark on sustainability, but he also believes going green must bring more than just capability gains. It also has to work for the military's balance sheet.
"When it gets into the finance boardroom, the moral high ground doesn't work," he said.
"We need a really solid business argument for doing this, in my view. If you have a moral argument on top of that, fantastic – there is such a strong moral argument – but it's not enough."