Carbon-cutting pledges from 146 nations are "far from enough" to stave off dangerous global warming, the UN has warned, three weeks before a crucial climate summit in Paris.
The voluntary efforts to curb greenhouse gases – if respected – would yield only a third of the cuts needed by 2030 to keep Earth from overheating, a UN Environment Programme report says.
Countries have made "an historic level of commitment" with their pledges, UNEP executive director Achim Steiner said in a statement.
"However, they are not sufficient to limit global temperature rise to the recommended level of 2degC this century," Steiner said.
Beyond that threshold, scientists say, lies a climate-addled world plagued by deadly drought, superstorms and mass migration.
The emissions reduction plans – called Intended Nationally Determined Contributions, or INDCs – would result in a temperature rise of 3degC or more by 2100, UNEP said.
Other scientific estimates have varied between 2.7degC and 3.5degC.
The annual "Emissions Gap" analysis tracks the difference between projected CO2 pollution, on the one hand, and the levels required to stay under the UN 2degC target, on the other.
This year's report is the first to take into account greenhouse gas reduction promises made before the November 30-December 11 summit, tasked with delivering the first-ever universal climate pact.
Without the INDCs, humanity is set to spew about 60 billion tonnes of carbon dioxide equivalent – a measure that groups different greenhouse gases including methane and nitrous oxide – into the atmosphere in 2030.
The INDCs would shave six billion tonnes off the top, leaving 54 billion tonnes.
But to keep the 2degC target on track, total emissions in 2030 should not exceed 42 billion tonnes, according to the UN's climate science panel.
That means the INDCs cover only a third of the 18 billion tonne gap between the present trajectory and where we need to be in 2030.
Moreover, the report notes, greenhouse gas output would still be rising in 2030.
Many scientists say it is crucial to bend the emissions curve – to make it peak, in other words – as soon as possible.
The longer we wait, they say, the harder and more expensive it will be to transition to a low-carbon global economy.
In 2014, greenhouse gas emissions from all sources totalled just under the equivalent of 53 billion tonnes of CO2.
That output will have to drop to near zero by 2075 at the latest to stay under 2degC, scientists say.