Climate scientists have welcomed a pact to battle global warming as a major political advance, but warned of a gaping hole – the lack of a detailed roadmap for cutting greenhouse gases that cause the problem.

The new accord, embraced by 195 nations, aims to cap warming to “well below” two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels, and to “pursue efforts” to limit the increase to 1.5C.

“This is an historic agreement,” said Steffen Kallbekken, director of the Centre for International Climate and Energy Policy.

“But this ambitious temperature goal is not matched by an equally ambitious mitigation goal,” he said, using the scientific term for the drawing-down of heat-trapping gases.

To have a two-thirds chance of limiting warming to two degrees, emissions would have to fall by 40-70 per cent by mid-century, according to the Intergovernmental Panel on Climate Change (IPCC), the UN’s climate science body.

And to reach the 1.5C target also embraced in the newborn pact, those mid-century cuts would have to be even deeper: 70 to 95 per cent.

Without these hard numbers – dropped from an earlier draft – the climate pact “does not send a clear signal about the level and timing of emissions cuts,” Kallbekken cautioned.

Need to reduce CO2 output

Many scientists highlighted the imbalance created by boosting the ambition of the temperature target on the one hand, while removing the yardsticks against which progress toward that goal could be measured, on the other.

“How are we going to reach our objective unless we set out in the right direction?” asked Professor Bill Collins at the University of Reading in southern England, pointing to the need to slash CO2 output by 70 per cent before by mid-century.

“Until governments accept this, we should restrain our optimism.”

Keveh Madani, a professor at Imperial College London, said international summits were better at setting aspirational goals than laying out a pathway for achieving them.

“What matters more is how to get to the target,” he noted.

But scientific reality is unyielding, said Miles Allen at Oxford University.

Stabilising greenhouse gases “in the second half of this century will require net carbon dioxide emissions to be reduced, in effect, to zero,” he said.

“It seems governments understand this, even if they couldn’t quite bring themselves to say so.”

Other scientists voiced concern about the fact that the new accord allows several years to pass before ramping up emissions reduction efforts.