For the Army, it is the size of its training estate that poses the greatest challenge when it comes to tackling climate change and achieving net-zero.
Nationally, the Ministry of Defence (MOD) operates 230 thousand hectares of land – about twice the area of Greater London and 2% of the total UK landmass.
A vast proportion of it is used by the Army for training.
Major General Bobby Walton-Knight, head of plans and strategy at the Defence Infrastructure Organisation (DIO), the body that manages the MOD's land, said the Army will increasingly train on simulators to reduce its environmental impact.
He also stressed that much of the MOD's land is actually protected by the fact it is used by the military and has, therefore, not been developed.
"You can see the momentum there now is to try and do this," he said.
"This is no longer a niche sport that goes on in quiet corridors, this is stuff that everyone is involved in."
At Millbrook Training Ground in Bedfordshire, they are testing hybrid versions of the Jackal and Foxhound combat vehicles which the Army used extensively in Afghanistan.
Watch: Climate Warriors – RAF in a race to drastically transform its carbon footprint.
The Sheffield-based firm behind the conversion removed the vehicle's gearbox and fuel tanks and shoe-horned in electric generators.
The units have so much power they have had to turn it down so the chassis of the vehicles can cope.
Lieutenant General Richard Nugee wrote the MOD's strategy on climate change and sustainability and he's cautious about how far the Army can go with electric vehicles but does believe they can have a real tactical benefit too.
"By having an electric or hybrid vehicle you can steer around the battlefield without being seen, heard, or your emissions being seen," he said.
The Army is also 'greening' its vast built estate.
At Westdown Camp on Salisbury Plain they are demolishing the old 1930s accommodation and replacing them with carbon neutral 'NetCap' buildings.
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The modular design of the transit buildings mean they can be built quickly and relatively cheaply.
Powered by solar energy, they use heat pumps and thermal sensors to maximise their heat efficiency, as well as plans for the future to recycle water from the showers.
The Army is also planting 2,000,000 trees at 25 sites nationally, exploring geothermal energy, and building 80 new solar farms.
The first solar array opened at the Defence School of Transport at Leconfield last year.
The 4,000 solar panels will cut the base's carbon emissions by 700 tonnes and save £250,000 a year.
"We want to turn [the defence training estate] from a carbon emitter to a carbon sequester," said Major General David Southall, Director of Army Basing and Infrastructure.
"It's absolutely non-discretionary. It's not just an energy supply problem nor is it just an environmental issue, it's a national stability issue."