Thousands of scientists from 153 countries have come together to warn the world that "untold suffering" is inevitable if we do not make significant changes to the way we live.
The 11,258 scientists signed a letter which says researchers have a "moral obligation" to warn humanity "clearly and unequivocally that planet Earth is facing a climate emergency".
The declaration, published in the journal Bioscience, is based on analysis of more than 40 years of data covering a range of measures from energy use to deforestation and carbon emissions.
Scientists from the University of Sydney, Australia, Oregon State University and Tufts University in the US and the University of Cape Town in South Africa are joined in the warning by 11,000 signatories from 153 countries including the UK.
William Ripple, the ecology professor at Oregon State University who spearheaded the letter, said: "Despite 40 years of major global negotiations, we have continued to conduct business as usual and have failed to address this crisis.
"The climate crisis has arrived and is accelerating faster than most scientists expected.
"It is more severe than anticipated, threatening natural ecosystems and the fate of humanity."
Although researchers say there are some positive indicators, like declining birth rates and increasing renewable energies, most suggest humans are not doing enough to stop the world becoming a "hothouse Earth" beyond mankind's control.
Air travel, rising meat consumption, faster deforestation and increasing carbon dioxide emissions only show humanity moving in the wrong direction, they said.
Scientists say they want the public to “understand the magnitude of this crisis, track progress, and realign priorities for alleviating climate change”.
"To secure a sustainable future, we must change how we live," they said.
The letter outlines six crucial objectives humans need to carry out - replacing fossil fuels, cutting pollutants like methane and soot, restoring and protecting ecosystems, eating less meat, and converting the economy into being carbon-free.
“Global surface temperature, ocean heat content, extreme weather and its costs, sea level, ocean acidity and land area are all rising,” Professor Ripple said.
“Ice has been rapidly disappearing, evidenced by declining trends in minimum summer Arctic sea ice, Greenland and Antarctic ice sheets, and glacier thickness worldwide."
Despite the urgency, the scientists say they are encouraged by the recent surge of concern around the world.
"Schoolchildren are striking. Ecocide lawsuits are proceeding in the courts. Grassroots citizen movements are demanding change, and many countries, states and provinces, cities, and businesses are responding.
"We believe that the prospects will be greatest if decision-makers and all of humanity promptly respond to this warning and declaration of a climate emergency and act to sustain life on planet Earth, our only home."
