Research endorsed by 11,000 scientists in 153 countries "clearly and unequivocally" declares a climate emergency and gives a stark warning of "untold suffering" unless there is urgent action.

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

The paper describes "profoundly troubling" signs from activities including sustained increases in human populations, the amount of meat consumed per person, the number of air passengers carried and global tree cover loss, as well as carbon emissions and fossil fuel consumption.

The declaration is based on analysis of more than 40 years of publicly available data covering a range of measures from energy use to deforestation and carbon emissions.

Since the first World Climate Conference held in Geneva in 1979, multiple global bodies have agreed that huge shifts in the way we live are needed, but greenhouse gas emissions continue to rise.

"Despite 40 years of major global negotiations, we have continued to conduct business as usual and have failed to address this crisis," said William Ripple, professor of ecology at Oregon State University, who spearheaded the letter.

"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," the paper states.

In another study, published in the Proceedings of the National Academy of Sciences, researchers have found that sea level rise is set to challenge human civilization for centuries to come.

It says that even if governments meet their commitments from the landmark Paris Climate Agreement signed in 2015, the first 15-year period of the deal will still result in enough emissions that would cause sea levels to increase by around 20 centimeters by the year 2300.

"Even with the Paris pledges there will be a large amount of sea level rise," said Peter Clark, an Oregon State University climate scientist and co-author of the study.

"Sea level rise is going to be an ongoing problem for centuries to come, we will have to keep on adapting over and over again. It's going to be a whole new expensive lifestyle, costing trillions of dollars.

"Sea level has a very long memory, so even if we start cooling temperatures the seas will continue to rise. It's a bit like trying to turn the Titanic around, rather than a speedboat."

Meanwhile, the Extinction Rebellion climate activist group this week won a High Court challenge against a police ban on protests throughout London.

The police used section 14 of the Public Order Act to try and prevent demonstrations involving more than two people. Extinction Rebellion's lawyers argued that the police's actions were beyond the powers of the act.

Speaking outside the High Court, Tobias Garnett, a human rights lawyer working with the climate group, said he was "delighted" at the result.

Garnett added: "It vindicates our belief that the police's blanket ban was an unprecedented and now unlawful infringement on our right to protest."