More than 11,000 scientists have "clearly and unequivocally" declared a climate emergency that could bring "untold suffering" unless there are significant transformations in the way humans live.

"Scientists have a moral obligation to clearly warn humanity of any great existential threat," the signatories, who hail from 153 countries, said in a paper published in BioScience magazine on Tuesday.

"To secure a sustainable future, we must change how we live ... [This] entails major transformations in the ways our global society functions and interacts with natural ecosystems."

The signatories suggest six steps that would lessen the worst effects of climate change: replacing fossil fuels with low-carbon renewables; reducing the emissions of pollutants such as methane; protecting the Earth's ecosystems; eating mostly plant-based foods and fewer animal products; creating a carbon-free economy and stabilising the human population.

In their statement, the alliance of scientists, led by William Ripple and Christopher Wolf of Oregon State University in the United States, said the climate crisis is "accelerating faster" than most researchers expected.

"Despite 40 years of global climate negotiations, with few exceptions, we have generally conducted business as usual and have largely failed to address this predicament," they wrote.

"Especially worrisome are potential irreversible climate tipping points and nature's reinforcing feedbacks (atmospheric, marine, and terrestrial) that could lead to a catastrophic 'hothouse Earth', well beyond the control of humans," they said.

The scientists say they are "encouraged by a recent surge of concern" over the climate crisis, demonstrated by the student-led Fridays for Future movement and other grassroots campaigns.

"As the Alliance of World Scientists, we stand ready to assist decision-makers in a just transition to a sustainable and equitable future," the paper concludes, adding that humanity should "act to sustain life on planet Earth, our only home".

The letter's publication comes one day after US President Donald Trump begun the process to quit Washington's participation in the landmark Paris climate accord, which seeks to fight climate change by mutual reductions in climate emissions.

Washington presented its withdrawal letter to the United Nations on Monday, the first possible date under the accord negotiated by Trump's predecessor Barack Obama, making the world's largest economy the sole outlier from the agreement.

Chinese leader Xi Jinping and French President Emmanuel Macron declared on Wednesday that the Paris climate pact is "irreversible", after the US formally withdrew from the accord.

Key powers expressed regret and concern after Trump went ahead with the pullout despite mounting evidence of the reality and effect of climate change.

In a joint statement released after Xi and Macron held talks in Beijing, the two leaders reaffirmed "their firm support for the Paris accord which they consider as an irreversible process and a compass for strong action on climate".

Without directly naming the US, Macron said he "deplores the choices made by others" as he sat next to Xi following the talks.

(Al Jazeera)

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