Last-ditch efforts to hold climate change to the most ambitious target set by governments will likely require using every available technique rather than picking and choosing the most attractive ones, climate scientists said on Monday.
Dramatically reducing the use of coal, planting huge swaths of land with carbon-absorbing forest or powering most transport with electricity are no longer sufficient to bring about the swift transition needed, they said, with warming expected to pass the 1.5 degree Celsius (2.7 degrees Fahrenheit) mark in as little as 12 years.
"We can make choices about how much of each option to choose, but the idea you can leave anything out is impossible," said Jim Skea, who jointly led a major scientific report analyzing the feasibility of holding global average temperature rise to 1.5 degrees C above pre-industrial levels, the most ambitious goal of the 2015 Paris Agreement.
The Intergovernmental Panel on Climate Change (IPCC) report, requested by governments, was issued ahead of a U.N. conference in December in Poland that will consider how to increase country ambitions to cut emissions and manage climate risks better.
Current government commitments to curb climate change under the Paris pact, even if fully met, would still leave the world on track for about 3 degrees of warming, scientists said.
To have a chance of meeting the 1.5 degrees goal, climate-changing emissions would have to plunge 45 percent by 2030 compared to 2010 levels, the report said.
As that would be an "unprecedented" rate of decline, it is more likely the world will overshoot the target, then try to return to it by sucking carbon from the air, scientists said.
Such "carbon removal" might happen by developing better technology to take out carbon dioxide from the atmosphere - now an extremely expensive process - or by planting many more forests that could be harvested and burned for energy, with emissions pumped into underground storage.
"We have not identified any pathways that get to 1.5 degrees Celsius without some kind of carbon dioxide removal," Skea said.
But turning over much more land for energy production "could have implications for food security, ecosystems and biodiversity," the British scientist warned, as competition for land grows.
All on board
Swiftly reducing emissions - even with carbon removal - will also require unprecedented levels of international cooperation, a particular challenge as some national governments, like that in the United States, look increasingly inward.
Making the needed emissions changes "is within the scope of what humans can achieve", said Hans-Otto Portner, a German climate scientist and IPCC report co-chair.
But success "depends on political leadership," he added.
Henri Waisman, a senior researcher at Paris-based think tank IDDRI and one of 91 report authors, said the report's aim was to set out the types of transformation required as clearly as possible to inform discussions at U.N. climate talks and beyond.
Delaying action on climate change "is something that is explicitly contradicted in the report," he told the Thomson Reuters Foundation.
If governments fail to ramp up their ambition to reduce heat-trapping emissions over the next two years, they will have consciously abandoned the 1.5 degree goal, he added.
Action in cities - which consume more than two-thirds of energy globally and account for about three-quarters of carbon emissions - are pivotal to meeting the target, said report author William Solecki, a professor at Hunter College-City University of New York.
That's particularly true because most population growth in coming years "is going to be in urban areas - a lot of it particularly in small and medium-sized cities ... in the global south," he said.
Those cities will need more support to develop cleanly, prevent disasters and adapt to climate shifts, he added.
The scientists said the report was intended to guide more than just governments, however, and that action by everyone - including individuals and businesses - would be required to hold the line on climate change.
"There's a lot we can do individually or within our communities," said Debora Ley, a report author who works on adaptation and renewable energy in Latin America.
Personal changes might include everything from eating less meat to using energy-efficient appliances and reducing air travel, said Patricia Pinho, a Brazilian climate scientist and report author.
Individuals and civic groups have a big role to play in pushing governments to tackle climate threats, and are stepping up pressure as recognition of the danger grows, she said.
"We have to live our lives in a way that makes a difference. "Our life on this planet, our kids are at risk," she said.