Overseas aid funding may need to be shifted away from humans to the natural environment to protect the planet and reduce poverty, Rory Stewart has said.
The international development secretary told MPs hard decisions would be needed if the UK was serious about mitigating the impact of climate change globally.
“We may have to target our money directly on the trees,” he said. “We may actually have to plant trees.”
He also warned a no-deal Brexit could, at worse, cut aid funds by up to £400m.
Discussing his department’s priorities in front of the Commons international development committee, Mr Stewart warned MPs it might be the last time he spoke to them.
Mr Stewart, who caused a stir during his recent Conservative leadership bid, has said he will not serve under Boris Johnson if he becomes Tory leader due to disagreements over Brexit.
With Mr Johnson considered favourite to succeed Mrs May, Mr Stewart said he may only have a month left in the role.
In that time, he said his main goal was to try and double the amount of money his department spent on climate and environment programmes.
In the longer-term, he said the department may need to reconsider how it allocates its resources – the bulk of which are spent on bilateral programmes in developing countries.
Dealing with the climate emergency was key to tackling global poverty, he said, because without it, the number of people below the poverty line would increase by up to 100 million.
“It has been very tempting for this department, in the short term, to think it can’t spend money on climate change because there are also these poor people out there,” he said.
“But the harsh reality is that if we don’t start tackling climate and the environment, we are going to start going backwards rather than forwards.”
Using a Venn diagram to illustrate his argument, he said the overlapping, shared area between environmental and poverty initiatives was “not always a very good place to be”.
As an example, he said while it might seem sensible to provide rural communities dependent on forestry with alternative sources of employment nearby, such diversification may end up having a marginal economic benefit while taking people away from the land.
In contrast, tree planting could provide sustainable income sources for communities while benefiting eco-systems, increasing carbon capture and providing defence against flooding.
“That is quite a shift in the way we think. Because we have tended to assume that best way to deal with humans is to target your money on humans.
“But sometimes, in the long run, the best way is to target your money on non-humans.”
During his leadership campaign, Mr Stewart – who worked as a diplomat in Iraq and Afghanistan before entering politics – claimed to have planted 5,000 trees himself.
With successive governments committing to spend 0.7% of UK national income on aid spending, the aid department has seen a huge increase in its budgets in the past decade.
Mr Stewart said there would be “problems” if, for whatever reason, the economy grew at a slower rate than was currently projected.
In the worst-case scenario of a no-deal Brexit, he said there could be a £300-£400m annual shortfall in the Overseas Development Assistance budget.
Asked how the gaps would be plugged, he suggested the UK could look to reduce the amount of money it gave to multilateral partners such as the World Bank.
This, he suggested, would be an “easier conversation” for the UK to have than cutting funding for programmes in individual countries.
Mr Stewart has also been setting out the UK’s progress towards hitting its Sustainable Development Goals, a series of international targets for eradicating poverty and hunger, reducing inequality and securing gender equality.
He said the UK had made “significant strides” by having a faster rate of decarbonisation than any other advanced economy but there was “more to do”.