By Alister Doyle
BONN, Germany (Reuters) - Harmful impacts of global warming such as heat waves and sea level rise are mounting and show a need for a "radical transition" to a greener economy, a study presented at U.N. climate talks said on Tuesday.
Damage is growing even though average temperatures have risen only 0.85 degree Celsius (1.5 Fahrenheit) above pre-industrial times, less than half the 2C set as a maximum acceptable rise by almost 200 nations, it said.
"Negative impacts are not only something in the future – they are something now," said Zou Ji, a co-leader of the U.N. review of consultations about science policy for governments working on a U.N. climate deal in Paris in December.
All sides at the presentation of the report, on the sidelines of June 1-11 talks on the Paris accord, said government promises so far for curbs on greenhouse gas emissions were too weak to stay below the 2C goal.
"Limiting global warming to below 2C necessitates a radical transition ... not merely a fine tuning of current trends," according to the report based on talks between experts and governments.
Such a transition would mean deep cuts in greenhouse gases, shifting from fossil fuels such as coal and oil to renewable energies such as wind, hydro and solar power, it said.
The report also concluded that the 2C goal was too often wrongly viewed as an acceptable maximum, a "guardrail" up to which climate change would be manageable.
But impacts of climate change, such as damage to coral reefs or a melt of Greenland's ice that is raising sea levels, showed risks were already increasing.
"The guardrail concept in which up to 2C would be considered safe would be better seen as an upper limit, a defense line," said Andreas Fischlin, a co-leader of the report.
Thomas Stocker, a senior Swiss scientist from the U.N.'s Intergovernmental Panel on Climate Change, said governments faced tough choices in managing the risks of warming.
"The elephant in the room is what we can do to change the trend in emissions," he told delegates.
Many developing nations favor setting a ceiling of 1.5C above pre-industrial times, arguing that their economies are vulnerable to impacts such as storms, floods, droughts and sea level rise.
Collin Beck, representing the Solomon Islands, said scientists should do more to examine ways to set up a defense line against 1.5 degrees.
(Reporting by Alister Doyle; Editing by Tom Heneghan)