Addressing world leaders at the climate summit in Sharm El-Sheikh, Egypt, he said “we are in the fight of our lives – and we are losing”, with greenhouse gases still growing and temperatures still rising.
And while the world’s attention is gripped by war in Ukraine, prompting an energy, food and cost of living crisis, and other conflicts, Mr Guterres said: “Climate change is on a different timeline and a different scale.
“It is the defining issue of our age. It is the central challenge of our century. It is unacceptable, outrageous and self-defeating to put it on the backburner.”
He warned: “Today’s crises cannot be an excuse for backsliding or greenwashing.”
The latest round of talks comes amid high geopolitical tensions sparked by Vladimir Putin’s invasion of Ukraine, worsening extreme weather, and calls for rich nations to provide finance for the loss and damage hitting poor countries as a result of the climate crisis.
Leaders including Egyptian president Abdel Fattah El-Sisi called for an end to the war between Russia and Ukraine as they spoke to the opening ceremony of the summit.
Attending a side event at Cop27, Boris Johnson warned the fight against climate change had become a “collateral victim” of the invasion of Ukraine, with countries questioning the goal of cutting emissions at a time of soaring energy prices.
And he warned against the risk that “some people will go weak and wobbly” on commitments to cut emissions to zero overall – known as net zero – to tackle the crisis.
His comments come ahead of Rishi Sunak’s speech to the conference, which the prime minister is attending following what opponents called a “screeching U-turn”, having initially planned to stay at home to work on domestic financial issues.
Mr Sunak will use his speech to the conference to call for a “global mission for clean growth”.
He will say it is essential that countries stick to commitments made at the Cop26 summit hosted by the UK in Glasgow, if it is to limit warming to 1.5C above pre-industrial levels and avoid the worst impacts of climate change.
At the same time, he will argue that the transition away from fossil fuels has the potential to drive growth and deliver jobs in the new green industries of the future, while cutting off funding for Russia’s war in Ukraine.
But as he passed on the baton to the Egyptians, Mr Sunak faced criticism at home over the government’s decision to issue more licences for oil and gas exploration in the North Sea and its continued opposition to new onshore wind.
The UK is also under pressure over its climate finance commitments, amid questions over the delivery of promised funding contributions to help poorer countries with the crisis.
Speaking to broadcasters in Egypt, Mr Sunak said the UK remains committed to its £11.6bn contribution to international climate finance over five years.
His attendance at the gathering in the Red Sea resort, alongside leaders such as US president Joe Biden and French president Emmanuel Macron, marks his first outing on the international stage since becoming prime minister last month.
In a packed day at the conference, the prime minister was holding a series of bilateral meetings, including with Italy’s new far-right prime minister Giorgia Meloni and European Commission president Ursula von der Leyen amid ongoing tensions over post-Brexit trading arrangements.
After wrangling between delegations, the issue of funding for loss and damage from climate change, such as destruction of crops, buildings and infrastructure in poorer countries, is now an item on the official agenda for the talks – a move supported by the UK.
Scottish first minister Nicola Sturgeon, who is also at the summit, said there is an obligation on richer countries that have largely caused climate change to help those suffering from its impacts.
Funding for loss and damage is a key demand for some of the world’s most vulnerable countries, who are on the front line of climate impact and yet have done the least to cause the crisis – though developed countries have historically been reluctant to discuss it.
In a statement as the summit kicked off, the Alliance of Small Island States called for a new loss and damage response fund that is operational by 2024, as well as for emissions to peak and decline immediately, and reform of the financial system away from fossil fuels.
In his speech to world leaders, Mr Guterres called for action to double support to help poorer countries adapt to climate change, but also for acknowledgement of a “harsh truth” that it was not possible to adapt to the growing number of catastrophic events around the world.
“The deadly impacts of climate change are here and now. Loss and damage can no longer be swept under the rug. It is a moral imperative,” he said, calling for concrete results on the issue at Cop27.
Mr Guterres also called for a pact between developed and developing countries, in which all nations make an extra effort to reduce emissions, and richer countries provide financial assistance to help emerging economies speed their own renewable transition, and to end dependence on fossil fuels.
He said the United States and China had a particular responsibility to join efforts to make the pact a reality, warning humanity had a choice to “cooperate or perish”.