French scientists unveiling new estimates for global warming said on Thursday the 2 C (3.6 F) goal enshrined by the United Nations was "the most optimistic" scenario left for greenhouse-gas emissions.

The estimates, compiled by five scientific institutes, will be handed to the UN's Intergovernmental Panel on Climate Change (IPCC) for consideration in its next big overview on global warming and its impacts.

The report -- the fifth in the series -- will be published in three volumes, in September 2013, March 2014 and April 2014.

The French team said that by 2100, warming over pre-industrial times would range from two degrees Celsius (3.6 degrees Fahrenheit) to 5.0 C (9.0 F).

The most pessimistic scenarios foresee warming of 3.5-5.0 C (6.3-9.0 F), the scientists said in a press release.

Achieving 2C, "the most optimistic scenario," is possible but "only by applying climate policies to reduce greenhouse gases," they said.

In its Fourth Assessment Report published 2007, the IPCC said Earth had already warmed in the 20th century by 0.74 C (1.33 F).

It predicted additional warming in the 21st century of 1.1-6.4 C (1.98-11.52 F), of which the likeliest range was 1.8-4.0 C (3.24-7.2 F).

The French estimates are derived from two different computer models that crunch data for four scenarios based on atmospheric levels of carbon dioxide (CO2), the main greenhouse gas.

The work differs from previous calculations as it takes into account the reflectivity of clouds and uptake of CO2 by the oceans and other factors that can skew the equation, the authors said.

Meeting in Cancun, Mexico in December 2010, countries under the UN Framework Convention on Climate Change (UNFCCC) set 2 C (3.6 F) above pre-industrial times as the maximum limit for warming.

They vowed to consider lowering it to 1.5 C (2.7 F) if scientific evidence warranted this.

Small island states and other poor nations badly exposed to climate change are lobbying for the 1.5 C (2.7 F) limit.