The aim to limit global warming to two degrees of pre-industrial levels is "crazy" and "a prescription for disaster", according to a long-time NASA climate scientist.
The paleo-climate record shows sea-levels were six to eight metres higher than current levels when global temperatures were less than two degrees warmer than they are now, Professor James Hansen, formerly head of NASA's Goddard Institute for Space Studies and now at Columbia University in New York, said.
"It's crazy to think that 2 degrees celsius is a safe limit," Professor Hansen told RN Breakfast on ABC Radio on Tuesday, adding that this would lock in several metres of sea-level rise by the middle of the century,
New satellite data over the past decade indicate that the ice sheets are disintegrating faster than had been modelled by climate scientists.
"The ice sheets are losing mass faster and faster, with a doubling time of about 10 years," Professor Hansen said. "If that continues, we would get sea-level rises of several metres by 40-50 years."
"The consequences are almost unthinkable. It would mean that all coastal cities would become dysfunctional," he told ABC Radio.
Even the goal of limiting warming by two degrees is looking like it is unlikely to be met. A study out this week by Lord Nicholas Stern and colleagues found that the commitment and likely pledges by nations to cut greenhouse gas emissions by 2030 fall about half short of the reductions needed to restrict the two-degree increase on pre-industrial levels.
Professor Hansen said fossil fuel prices had to be priced high enough to make them "honest". Nuclear energy will need to play a big role in creating "carbon-free electricity" because non-hydro renewable energy - which now accounts for just 3 per cent of total supplies - would not be able to be ramped up fast enough, he said.
"If we have carbon-free electricity, the problem is solved," Professor Hansen said.
Greens leader Christine Milne said Pacific Island nations are among those advocating 1.5 degrees as the maximum warming that the world should target.
"We need a complete overhaul of policies to make climate top of mind because right now we are living in a climate emergency, on track for 4 degrees of warming," Senator Milne said.
"The major parties are teaming up to cut the Renewable Energy Target - which will kill jobs and investment in large-scale solar - [that's] the last thing they should be doing right now," she said. "We need more ambition, not less, in all areas of Australian policy and that's sorely lacking in this parliament."
The Abbott government may secure agreement as soon as this week to reduce the renewable energy target from the current 41,000 gigawatt-hours a year by 2020 to 33,000 gWh.
Paris summit
Climate change issues are likely to gain more attention in the run-up to the Paris climate summit planned for France in December.
Global temperatures may also be in focus with the prospects of an El Nino in the Pacific making it likely that 2015 will be the hottest year on record - breaking a high set only in 2014.
The first three months of 2015 - and the past 12 months - were also the hottest in 136 years of data, the US National Oceanic and Atmospheric Administration said last month.