Scientists, environmentalists and politicians reacted with concern yesterday as a UN climate panel warned temperatures could rise by as much as 4.8 degrees Celsius this century due to man's voracious energy consumption.
"Yet another wake-up call," was how US Secretary of State John Kerry described the report by the Nobel-winning Intergovernmental Panel on Climate Change (IPCC), which also underlined the peril of heatwaves, drought and floods and warned sea levels could rise by as much as 82 centimetres.
"Those who deny the science or choose excuses over action are playing with fire," Kerry said.
The strongest scientific consensus yet that human activities drove warming since the 1950s, the report said heat-trapping greenhouse gas emissions must be urgently curbed to limit further damage to the climate system.
UN climate chief Christiana Figueres said the document's release in Stockholm was "an alarm clock moment for the world".
"To steer humanity out of the high danger zone, governments must step up immediate climate action" to meet the UN target of 2.0 C from pre-Industrial Revolution levels, she said. This is the ceiling at which many experts believe the worst climate fallout can be skirted.
Based on computer models of different emissions trajectories, the report's most optimistic scenario projects average warming of 1.0 C by 2100 over 2000 levels - ranging from 0.3 to 1.7 C.
Previous research said global carbon dioxide emissions, constantly scaling new highs, must peak around 2020 and then decline sharply for the lowest warming scenario to be possible. The world emits about 50 billion tonnes of greenhouse gas every year.
The IPCC's worst-case scenario projects average warming this century of 3.7 C - ranging from 2.6 C to 4.8 C.
"Without very strong cuts in emissions of greenhouse gases, we face huge risks from global warming of more than 2.0 C by the end of this century," said British climate economist Nicholas Stern of the report.
Green groups said such levels threatened the livelihoods of fishermen and farmers, would leave millions hungry and exposed to extreme weather events, and risked engulfing entire communities in coastal areas or low-lying small island states.