The findings come from the Avoid programme, run by the Met Office in conjunction with other UK research institutions under government funding.
The latest results were presented at the UN climate summit in Copenhagen.
The Avoid programme aims to use the latest scientific understanding to make risk-based assessments of the impacts, global and regional, of rising greenhouse gas emissions.
Echoing the general conclusion of other analyses, including by the Intergovernmental Panel on Climate Change (IPCC), the Avoid researchers conclude that the earlier global emissions peak and begin to decline, then the gentler that decline can be.
"If you reduced everything to zero immediately you'd still get about 1.3C because of the greenhouse gases already in the atmosphere."
Policies to ensure a reasonable chance of remaining under 1.5C would involve "negative emissions" - sucking CO2 out of the air - she said.
And if emissions peak later than 2020, negative emissions - a form of geo-engineering - would be needed even to hit the G8's 2C target.
The concept of geo-engineering has its adherents but it is also fraught with economic, social and technical difficulties.
A report by the UK's Royal Society, released in September, concluded that although the approach might have a role, there were "major uncertainties regarding its effectiveness, costs and environmental impacts", and was no substitute for curbing emissions.
It was announced on Thursday that more than 1,700 scientists had signed a statement to defend climate science.
The response has been co-ordinated by the Met Office in the wake of the "ClimateGate" row.
The scientists became concerned after climate change "sceptics" claimed e-mails taken from servers at the University of East Anglia's Climatic Research Unit (CRU) suggested researchers had manipulated evidence.
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