Like a household that has been living beyond its means, the world has been expending more greenhouse gas emitting energy than it can afford.
With the costs of profligate CO2 emissions increasingly apparent and less than 90 days until the start of the United Nations Climate Change Conference in Copenhagen that will determine the successor to the Kyoto Protocol, early September has brought a number of new proposals for achieving carbon reductions.
Among these is a report from the German Advisory Council on Climate Change (WBGU) outlining what the council – an independent scientific advisory board – calls a "budget approach" to reducing CO2 emissions that in effect proposes putting the world on an emissions diet.
The budget approach, explains Stefan Rahmstorf, professor of ocean physics at Potsdam University and WBGU member, is based on the premise that there's a limited amount – or fixed budget – of CO2 that can be released worldwide between now and 2050 if we're to avoid raising global temperatures beyond a point that would cause irreversible climate change.
"The fundamental idea behind the budget approach," says Rahmstorf, is that it ties reduction targets to total – or cumulative – rather than annual CO2 emissions.
Working within this 40-year time frame and against a maximum global mean temperature rise of 2ºC above pre-industrial levels – the threshold set out in the 2007 IPCC report and adopted as a goal by the European Union – the WBGU proposes distributing the resulting finite "budget" of CO2 emissions equally worldwide on a per capita basis. How each country would stay within budget would depend on current per capita emissions and the mechanisms chosen for achieving required reductions.
"This approach makes sense from a natural science perspective," says Rahmstorf, "because of the long lifetime of CO2."
Given that about 50% of any given year's CO2 emissions can last for decades – if not longer – basing reductions on annual emissions does not account for the extended global warming impacts of what's released in a single year. Basing reductions targets on total cumulative emissions offers a way of responding to the global warming effects that are already in the pipeline.
The "budget approach" also makes sense from an ethical point of view, says the WBGU, as it allocates emissions equally and calculates national emissions budgets according to current population numbers.
"Countries with high population growth will have to make do with less CO2 per capita," says Rahmstorf, "just as they have to make do with less land per capita. It is the same as with any other finite resource. The climate system," he adds, "cares only about total emissions."
Working within this framework, legally binding national CO2 budgets would be calculated for all countries. To stay within budget, high CO2 emitters, like the U.S. and other industrialized countries, would have to make what the WBGU calls "extensive emissions reduction commitments." These would be accompanied by technology and financial commitments designed to facilitate emissions trading and transitions away from fossil fuel energy sources. The WBGU also recommends establishing "a world climate bank" to oversee emissions trading and what it calls "decarbonization roadmaps."
A global CO2 budget that is allocated equally per capita provides flexibility for developing countries, says Rahmstorf. It gives low-emission countries like India room to grow, he explains, while also providing "an immediate incentive to be careful about their emissions, both because they are worth something – they can be sold – and because any excess emissions now will mean that their budget is more limited later on."
"A key signal we need now throughout the entire world, is that wasting a ton of CO2 is in fact wasting real money," says Rahmstorf. "I think for a country like India this scheme offers real benefits in terms of finance and technology transfer which may make them want to join a global agreement and is an excellent way for them to get onto the green growth path."
If binding CO2 budgets and other commitments are honored – including an interim 2020 reductions target – the "budget approach" compels countries with the highest current emissions to reduce them swiftly while allowing low-emitters to do so more gradually.
Yet as Pushker Kharecha of NASA's Goddard Institute for Space Studies points out, one of the big "ifs" of the WBGU's "budget approach" is its reliance on a global temperature rise ceiling of 2ºC.
This number may be too generous an allowance, says Kharecha, who explains that it's based on climate models that in his view do not properly account for long term climate impacts – most notably ice sheet loss – prompted by historical greenhouse gas emissions.
Whether the German government adopts the "budget approach" as official policy or if any of these WBGU proposals influence outcomes at Copenhagen remains to be seen. But as news comes in from the field of global climate conditions and Arctic sea ice cover continues its downward trend, keeping the world to a rigorous carbon budget seems increasingly imperative.