Quick measures to cut emissions would give a far better chance of keeping global warming within an agreed UN limit of 2o Celsius above pre-industrial times to avert more floods, heatwaves, droughts and rising sea levels.
"If you delay action by 10, 20 years you significantly reduce the chances of meeting the 2o target," said Keywan Riahi, one of the authors of the report at the International Institute for Applied Systems Analysis in Austria.
"It was generally known that costs increase when you delay action. It was not clear how quickly they change," he told Reuters on Wednesday (2 January) of the findings in the science journal Nature Climate Change based on 500 computer-generated scenarios.
It said the timing of cuts in greenhouse gases was more important than other uncertainties - about things like how the climate system works, future energy demand, carbon prices or new energy technologies.
The study indicated that an immediate global price of €15 a tonne on emissions of carbon dioxide (CO2), the main greenhouse gas, would give a roughly 60% chance of limiting warming to below 2o.
Wait until 2020 and the carbon price would have to be around €75 a tonne to retain that 60% chance, Riahi told Reuters of the study made with other experts in Switzerland, New Zealand, Australia and Germany.
And a delay of action until 2030 might put the 2o limit - which some of the more pessimistic scientists say is already unattainable - completely out of reach, whatever the carbon price.
"The window for effective action on climate change is closing quickly," wrote Steve Hatfield-Dodds of the Commonwealth Scientific and Industrial Research Organisation in Australia in a separate commentary in Nature.
Governments agreed to the 2o limit in 2010, viewing it as a threshold to avert dangerous climate change. Temperatures have already risen by 0.8o Celsius since wide use of fossil fuels began 200 years ago.
The report also showed that greener policies, such as more efficient public transport or better-insulated buildings, would raise the chances of meeting the 2o goal.
And fighting climate change would be easier with certain new technologies, such as capturing and burying carbon emissions from power plants and factories. In some scenarios, the 2o goal could not be met unless carbon capture was adopted.