Human activities added 1,430 gigatons of carbon to the atmosphere from 1870 to 2013. That's 45 percent of the total carbon budget the world has to maintain a rise in global temperatures below 2 degrees Celsius. At this rate of emissions, the world will hit its carbon quota in the next three decades.

The numbers were reported by the Global Carbon Project, an international consortium of scientists who track the total accumulation of carbon annually, in a series of articles in the journals Nature Climate ChangeNature Geoscience and Earth System Science Data Discussions.

The reports are meant to guide policymakers to a new way of thinking about emissions ahead of the New York climate summit this week. Policymakers are trying to hash out a new global agreement by 2015 following the expiration of the Kyoto Protocol last year. Some consider Kyoto a failure as the world's emissions trajectory remains poised to increase global temperatures by 3.2 to 5.4 C in 2100.

Some scientists are suggesting that policymakers approach emissions reductions in a different way. In recent research, scientists have identified that the world can altogether emit 3,200 gigatons of carbon dioxide and still keep temperature rise below 2 C. Of that, we have already emitted 1,430 gigatons from 1870 to today.

This leaves roughly 1,770 gigatons of carbon dioxide, which should be split up among the nations, according to scientists at the Global Carbon Project.

"We are nowhere near the commitments necessary to stay below 2°C of climate change, a level that will be already challenging to manage for most countries around the world, even for rich nations," Corinne Le Quéré, director of the Tyndall Centre at the University of East Anglia, said in a statement. "Politicians meeting in New York need to think very carefully about their diminishing choices exposed by climate science."

The approach is diametrically opposite to the one policymakers currently favor, in which nations choose to make emissions reductions below a base line. For example, the United States' mitigation target is 17 percent below 2005 levels by 2020.

Is a strict budget limit possible?
But this focus on modern emissions is myopic, David Frame, director of the New Zealand Climate Change Research Institute, and his colleagues argue in a commentary in Nature Geoscience.

They say it places an unfair emphasis on emissions in the short term, up to 2020 or 2030, but the world may drop the ball beyond that. For instance, nations may adopt a host of energy efficiency measures or tackle short-lived climate pollutants to get to their Kyoto goals. But these initiatives cannot exist alone; they have to be combined with carbon dioxide reductions in order to make a dent in long-term climate change.

Frame and his colleagues suggest that the U.N. Framework Convention on Climate Change assign fixed limits on how much each nation is allowed to emit. This would initially stress Western nations, which have contributed most of the world's emissions so far. But as developing nations like China and India grow, they, too, would have to cut their emissions.

But policymakers have said the carbon budget is a nonstarter in climate negotiations because there is no global actor—the UNFCCC lacks that mandate—who could assign the quotas.

"I don't think it's possible," Christiana Figueres, the executive director of the UNFCCC, told the LondonGuardian in an interview. "Politically, it would be very difficult. I don't know who would hold the pen" in setting out allocations of future budgets.

2 competing painful solutions
The largest emitters in 2013 were China, the United States, the European Union and India, according to the Global Carbon Project. A large part of China's emissions were from industries that supplied services to the developed world. China has overall contributed 11 percent to the world's cumulative carbon budget.

Emissions in the United States grew by 2.9 percent in 2013 due to an increase in coal use. The nation has so far contributed 26 percent of the cumulative carbon budget.

Emissions in India grew by 5.1 percent, but the nation has contributed just 3 percent to the world's cumulative emissions.

The scientific basis for carbon budgets is this: As CO2 concentrations have accumulated in the atmosphere since the industrial era, they have led to a predictable linear increase in global temperatures. The latest report by the Intergovernmental Panel on Climate Change graphed this relationship for the first time, showing a direct link between cumulative emissions and temperature.

In a study published in Nature Climate Change, scientists have suggested three possible methods to partition the carbon budget.

The first method takes the world's current emissions into account. North America and Europe would get 30 percent of the total allocations, while China would get 22 percent, India 7 percent and Africa just 4 percent. This scenario would restrict developing nations from accessing energy and development opportunities.

'Very difficult choices' ahead
A second method is more equitable, based on the size of the population. Under this scenario, North America and Europe would get 11 percent of the carbon quota. China would get 17 percent, India would get 25 percent and Africa would get 18 percent. But this scenario would place an extremely high burden on developed nations to mitigate their emissions, the study states.

Instead, a blend of the two scenarios might be a good option. It would give North America and Europe 21 percent of the world's carbon quota, while China would get 22 percent, India 16 percent, and Africa 11 percent.

Assigning carbon quotas has two implications for nations: North America, Europe and China would need to retire or improve some of their existing infrastructure to keep within their quotas under the equity scenario. And a significant portion of the carbon in the form of fossil fuel reserves would have to be left untapped by nations. These are difficult choices.

"If this were a bank statement, it would say our credit is running out," said Dave Reay, professor of carbon management at the University of Edinburgh, who is not affiliated with the Global Carbon Project.

"We've already burned through two-thirds of our global carbon allowance, and avoiding dangerous climate change now requires some very difficult choices. Not least of these is how a shrinking global carbon allowance can be shared equitably between more than 7 billion people and where the differences between rich and poor are so immense."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500