Speaking at the first UN Security Council meeting on climate and security, Sir David said that the world had “left the stable and secure climatic period that gave birth to our civilisation”.
“Today, there are threats to security of a new and unprecedented kind,” he told leaders.
“These threats do not divide us. They are threats which should unite us no matter from which part of the world we come, for they face us all.
“If we continue on our current path, we will face the collapse of everything that gives us our security: food production, access to fresh water, habitable ambient temperature and ocean food chains. And if the natural world can no longer support the most basic of our needs, then much of the rest of civilisation will quickly break down.
“Please, make no mistake. Climate change is the biggest threat to security that modern humans have ever faced.”
His words were echoed by UN secretary-general António Guterres, who described climate change as a “crisis multiplier”.
“We need to protect the people and communities that are being hit already by climate disruption,” said the UN chief.
“We must step up preparations for the escalating implications of the climate crisis for international peace and security.”
Boris Johnson, who chaired the UN Security Council meeting, also said that it was “absolutely clear that climate change is a threat to our collective security”.
“Whether you like it or not, it is a matter of when, not if, your country and your people will have to deal with the security impacts of climate change,” he said.
“So let’s do what this council was created to do and let’s show the kind of global leadership that is needed to protect the peace, the security and the stability of our nations, of our regions and of our world.”
(His comments came just hours after he told an annual meeting of the National Farmers’ Union that he hoped this year would see the UK selling more “Welsh lamb” and “Aberdeen Angus beef” around the world.)
French president Emmanuel Macron and the US special envoy for climate John Kerry also spoke on the need to take greater action to address the security risks posed by the climate crisis.
Mr Kerry said that the climate crisis was “indisputably” an issue for the Security Council, calling it “one of the most complex and compelling security issues that we’ve ever faced”.
“When farmers can no longer make a living because the weather is so extreme and unpredictable, they become increasingly desperate. When people already impoverished lose water and heat drives them from their homes, the embers of conflict burn brighter and faster,” he added.
However, a small number of countries, including Russia, questioned whether the climate crisis was an issue that should be discussed by the UN Security Council.
Russia’s representative to the UN Vasily Nebenzya said: “Climate change is one of the contemporary global changes that the world is facing … We agree that climate change and environmental issues exacerbate conflicts, but are they really the root cause of these conflicts? There are serious doubts about this.”
The issue of how climate change could be contributing to human conflict is still an active area of research among scientists.
The most recent global assessment from the Intergovernmental Panel on Climate Change (IPCC), the world’s authority on climate science, said there was some evidence to suggest that the factors that increase the risk of violent conflict within states are sensitive to climate change, but there was still more to be learned about the possible links.
A study published in 2019 which drew on the knowledge of a range of experts concluded that “climate has affected organised armed conflict within countries”. However, the study authors added that “other drivers, such as low socioeconomic development and low capabilities of the state, are judged to be substantially more influential, and the mechanisms of climate-conflict linkages remain a key uncertainty”.
At the council meeting, representatives from the least-developed countries welcomed more discussion of the security risks posed by the climate crisis. However, they also urged developed countries to meet their existing commitments on providing financial assistance to those most vulnerable to climate impacts.
Under the Paris Agreement – the global climate deal aimed at curbing global temperature rise – high-income countries pledged to provide poorer nations with $100bn (£73bn) a year by 2020 to help them both tackle and adapt to the climate crisis.
But the latest figures show that high-income countries mobilised just $78.9bn (£57bn) in climate finance in 2018.
Gaston Alphonso Browne, prime minister of Antigua and Barbuda, said that the security threats posed by the climate crisis were a “current reality” for small island developing states.
He told the council session: “For [small island developing states], our peace and security can be decimated on multiple fronts, sometimes at a moment’s notice – whether it be through sea level rise devouring our coastal and low-lying communities and territories … or even more severe and tropical cyclones rendering our states uninhabitable.
“There is considerable need for support from developed countries, through grant and concessional financing, capacity building and technology transfer.”