GENEVA - Countries must take "unprecedented" action to slash carbon emissions to zero by 2050 and limit dangerous global warming, a key report has warned.
Impacts of climate change, from droughts to rising seas, will be less extreme if temperature rises are curbed at 1.5C above pre-industrial levels than if they climb to 2C, the UN-backed study said.
Limiting warming to 1.5C is possible but will require fast and far-reaching changes to power generation, industry, transport, buildings and potential shifts in lifestyle such as eating less meat.
It will also require action to take excess carbon emissions out of the atmosphere, the Intergovernmental Panel on Climate Change report said.
The review of thousands of scientific papers sets out the impacts of temperature rises of 1.5C compared to 2C, and what is needed to curb temperatures at that level.
Impacts ranging from increased droughts and water scarcity to extreme weather, spread of diseases such as malaria, economic damage, and harm to yields of maize, rice and wheat will be less severe at 1.5C than 2C.
Sea level rises would be 10cm lower with a 1.5C temperature rise compared to 2C by 2100, while there would be worse impacts on coral reefs and the Arctic at higher temperatures.
The world has seen 1C of warming so far, with consequences such as more extreme weather already being felt, and there is more to come as temperatures continue to rise, the report said.
Prof Jim Skea, from Imperial College London, one of the experts leading on the assessment, said the report was "unambiguous" on the difference in impacts between 1.5C and 2C of warming.
He said: "The changes that would be needed to keep global warming to 1.5C are really unprecedented in terms of their scale. We can't find any historical analogies for it.
"There are some areas we are making progress quickly enough that they are compatible with 1.5C, the example of renewables is one, where we've seen costs falling and deployment across the world.
"We need to extend this kind of progress on renewables to other areas."
Promises made by countries to cut their emissions up to 2030 will not limit global warming to 1.5C even if action is massively scaled up after the end of the next decade, the report warns.
Carbon dioxide emissions need to fall about 45% from 2010 levels by 2030 and to "net zero" – so no more is being put into the atmosphere than is being removed by measures such as planting trees – by 2050, while other greenhouse gases will also need to decline steeply.
It will require a huge ramp-up in renewables so they generate 70-85% of electricity supplies by 2050, while coal power's share of the mix tumbles to almost nothing.
There will also need to be emissions cuts in industry, transport and buildings as well as the restoring of forests and potential changes to lifestyle.
Responding to the report, Prof Corinne Le Quere, from the University of East Anglia, said: "For the UK, this means a rapid switch to renewable energy and electric cars, insulating our homes, planting trees, where possible walking or cycling and eating well – more plants and less meat – and developing an industry to capture carbon and store it underground.
"It also means adapting to the growing impacts of climate change that are felt here, particularly to the increasing flood risks from heavy rainfall and from sea level rise along our coasts."
Taking steps to curb temperature rises to 1.5C can help with other aims such as improving health through lower air pollution and more sustainable diets, and alleviating poverty in the developing world.
The report stresses the need for measures to take carbon out of the atmosphere, such as planting forests or using land for crops to burn for energy and capturing the carbon and storing it underground, known as bioenergy and carbon capture and storage (Beccs).
But it contains a warning about "overshooting" the 1.5C limit and trying to remove excess carbon dioxide from the atmosphere to get temperatures back down, due to the problems of relying on largely unproved technologies such as Beccs on a large scale.
Claire Perry, Minister for Energy and Clean Growth said: "This report should act as a rallying cry for governments around the world to innovate, invest, and raise ambition to avert catastrophic climate change.
"The UK has already shown carbon abatement and prosperity can go hand in hand and we lead the world in clean growth – slashing emissions by more than 40% since 1990 while growing our economy ahead of the G7.
"There is now no excuse and real action is needed."