We have three years to stop the worst effects of climate change through an unprecedented and coordinated global effort, prominent experts are now warning.

Ecosystems have been collapsing, ice sheets have been disappearing, and coral reefs have been dying from the heat.

But global emissions remained stable over the past three years while economies grew, showing it is possible to mitigate the worst effects of climate change, a letter published in Nature journal on Wednesday said.

The authors said we have until 2020 to stop unrelenting heatwaves, droughts and sea-level rise.

More than 60 people, including former UN climate executive Christiana Figueres and Hans Joachim Schellnhuber of the Intergovernmental Panel on Climate Change, signed the letter.

The authors listed six goals for 2020, which could be adopted at the G20 meeting in Hamburg next month.

Renewables should make up at least 30 per cent of the world’s electricity supply, electric vehicles should be 15 per cent of new car sales, and 3 per cent of buildings should be upgraded to be emission free each year.

The authors also said heavy-duty vehicles and aeroplanes should be 20 per cent more efficient by 2020. Emissions from deforestation should be reduced to zero, and heavy industries should be working towards halving emissions by 2050. The global financial sector should be spending at least $1 trillion a year for climate action by 2020, the letter said.

“Lowering emissions globally is a monumental task, but research tells us that it is necessary, desirable and achievable,” the authors said.

Professor David Karoly, of the School of Earth Sciences at the University of Melbourne and a member of the Climate Change Authority, said the letter was “a stark reminder that Australia’s current climate policies are inadequate”.

“Australia’s greenhouse gas emissions have grown by 1 per cent per year since 2014 and are projected to continue growing to 2020 and beyond in the government’s latest emission projections,” Dr Karoly told The New Daily.

“This is in contrast to the plateauing of global emissions and fall in emissions in most developed countries.”

He said Australia’s emissions reduction target from the Paris Agreement was not high enough.

“Australia’s greenhouse gas emissions need to fall to zero by 2040 if Australia is to do its fair share of global emission reductions,” Dr Karoly said.

In 2015, the Australian government committed to reducing emissions to 26-28 per cent below 2005 levels by 2030.

Climate Council CEO Amanda McKenzie said Australia was falling behind global progress on climate action.

“Australia is falling behind compared to the rest of the world, all at a time when there’s no room for exceptions in order to limit global warming,” Ms McKenzie told The New Daily.

“The urgency highlighted by the former UN climate chief Christiana Figueres and the other signatories to this letter is true. This is the critical decade.”

She said there was “no more time to waste”.

Governments pledged to stop global warming at between 1.5 and 2 degrees Celsius in the 2015 Paris Agreement. But the target won’t be met if current emissions continue, the article warned.

“Should emissions continue to rise beyond 2020, or even remain level, the temperature goals set in Paris become almost unattainable,” the signatories said.