The most important number in history is now the annual measure of carbon emissions. That number reveals humanity's steady billion-tonne by billion-tonne march to the edge of the carbon cliff, beyond which scientists warn lies a fateful fall to catastrophic climate change.
With the global total of climate-disrupting emissions likely to come in at around 52 gigatonnes (billion metric tonnes) this year
, we're already at the edge, according to new research
To have a good chance of staying below two degrees C of warming, global emissions should be between 41 and 47 gigatonnes (Gt) by 2020, said Joeri Rogelj, a climate scientist at Switzerland's Institute for Atmospheric and Climate Science in Zurich.
The study is the first to comprehensively quantify the costs and risks of emissions surpassing critical thresholds by 2020.
"Delay is by far the riskier option," Rogelj said, noting that failure to act now means those additional risks, costs and social disruption will land on the heads of the next generation.
"We're deciding that the next generation will have to pay significantly higher costs because we're not doing anything now."
In 1990, global emissions were 38.2 Gt, and in recent years, they have been growing at a rate of three percent per year. This growth is despite commitments by industrialised countries to reduce their emissions.
Despite the urgent need to reduce emissions, the fossil fuel industry received a record 523 billion dollars in public subsidies in 2011, 30 percent more than the previous year, according to the International Energy Agency.
"At 44Gt (in 2020) we can choose the most cost-effective reduction options. Above 55Gt, we need everything and they'd all better work," he said.
The authors of the study acknowledge these numbers might be too optimistic because current climate models cannot incorporate emissions from melting permafrost and other natural sources of greenhouse gases that might result from increasing temperatures.
Staying below two degrees is not a matter of science or technology. It will be determined by political and social decisions to take the necessary steps to shift to low-carbon living, said McCollum.