(The author is a Reuters columnist. The opinions expressed are his own)
By Gerard Wynn
LONDON, Oct 1 (Reuters) - A U.N. panel of climate scientists has for the first time backed a limit on cumulative global carbon emissions, in a so-called budget approach that is useful but with certain limitations.
Carbon budgets estimate the maximum greenhouse gases that countries can emit cumulatively for the world to stay below a certain amount of global warming, and so are related to targets that set an emissions limit in any particular year.
The Intergovernmental Panel on Climate Change (IPCC), a United Nations body, published on Friday estimates for different “budgets” that would limit global warming.
Governments have never agreed a global emissions target at annual climate conferences dating back to the signing of the U.N. convention on climate change in 1992.
It is therefore very unlikely they would adopt a budget.
Instead, lobby groups and countries can use the IPCC estimate to test the ambition of new commitments in a future climate deal.
Another limitation concerns communicating such budgets to wider audiences, including voters.
Such communication is difficult because of the various time frames, assumptions and uncertainties in how they are calculated.
At multilateral talks, governments have never agreed an aggregate, global carbon emissions target, probably because - depending on the ambition - it could put the brakes on growth for emerging economies such as top carbon emitter China.
The second-biggest emitter, the United States, is also doubtful about international emissions targets, preferring national measures for curbing greenhouse gases.
Nevertheless, governments have repeatedly backed a target not to exceed 2 degrees Celsius warming, “according to science”, for example under the 2009 Copenhagen Accord.
Therefore, the IPCC’s estimate of a carbon budget that could meet the 2 degrees target provides a useful benchmark for weighing the ambition of national carbon cuts committed to in future deals.
The next major climate conference is in Paris in 2015, when governments are supposed to agree a carbon-cutting deal for implementation from 2020.
Explaining carbon budgets is complicated by their uncertainty, and by assumptions that make comparisons difficult.
Carbon budgets have a long history, with many studies to which the IPCC has now added.
One of the first explicit mentions was in 1997 by Bill Hare, then climate policy director for Greenpeace, in his publication “Fossil Fuels and Climate Protection: the Carbon Logic”.
But the idea of calculating the impact of cumulative carbon emissions goes back much further.
For example, two authors from Switzerland’s University of Bern (U. Siegenthaler and H. Oeschger) calculated how much emitted carbon dioxide would stay in the atmosphere, writing in the journal Science in 1978.
They concluded that to limit growth in atmospheric CO2 to no more than 50 percent above pre-industrial levels, most fossil fuels could not be burned.
“The value of 50 percent for the tolerable increase may be too high or too low; if it is correct, we may burn in total over the next centuries not much more than 10 percent of the known fossil fuel reserves,” they said.
As it turns out, they were right about a 50 percent increase - scientists now say this is around the upper limit on CO2 levels, to avoid more than 2 degrees Celsius warming.
The latest IPCC publication is a good example of how important it is to note the assumptions in budget estimates, when comparing studies.
On Friday, much media coverage focused on a carbon emissions budget of 1 trillion tonnes, for a two-thirds chance of staying within 2 degrees warming.
While a nice, round number, that was not the best figure as it ignored the warming impact of other greenhouse gases besides carbon dioxide; to account for these, a lower estimate was more relevant.
If estimates are compared using the same assumptions they are rather similar, suggesting the underlying science is reasonably consistent.
For example, studies roughly agree on the amount of carbon emissions this century that will give a 50 percent chance of staying below 2 degrees, including the effect of all greenhouse gases.
In 1997, Hare calculated a budget from burning fossil fuels of about 410 billion tonnes of carbon this century; he did not refer to probability or chance, but a “50 percent uncertainty”.
Two much more formal studies published in the journal Nature in 2009 by large teams of researchers showed how warming was correlated with rising cumulative emissions, and for the first time calculated the probability of staying under 2 degrees warming.
They estimated a carbon budget for this century of about 430 billion tonnes of carbon from burning fossil fuels and deforestation.
And the IPCC on Friday estimated a remaining carbon budget of no more than 417 billion tonnes of carbon (from 2000), for a 50 percent chance of staying under 2 degrees.
At present fossil fuel emissions rates of 9.4 billion tonnes of carbon a year, such budgets would be used up in about four decades.
The estimates show the monumental effort needed to curb warming: the budgets are far lower than fossil fuel reserves, implying most cannot be burned, as Siegenthaler and Oeschger concluded in 1978.
The International Energy Agency last year estimated the amount of carbon emissions locked up in current, proven fossil fuel reserves - including unconventional gas and oil - at 10,500 billion tonnes.
Regarding the future outlook, the IPCC showed last week that only the most ambitious of its four emissions scenarios would stay within this particular carbon budget.
That scenario required global emissions to stop rising this decade, fall rapidly after 2020 and reach near zero by 2070 - something that seems irreconcilable with China’s rapid growth.
See chart (page 115): link.reuters.com/qaj53v
Another complexity for the carbon budget approach is uncertainty, which accounts for the probability ranges published with the numbers.
There are two main sources of uncertainty.
First, regarding the carbon cycle, it is unclear exactly how much carbon dioxide emitted will stay in the air.
Until now, the world’s oceans have absorbed about a quarter of all manmade emissions.
As these continue to rise, oceans and soils may take up less carbon, meaning atmospheric levels will increase faster.
And second, it is unclear exactly how much warming a given level of atmospheric CO2 will cause, given the complexity of the climate system.
The IPCC on Friday published a wider range of temperature rises it expects from a doubling of atmospheric CO2, at 1.5-4.5 degrees Celsius, compared with 2.0-4.5 degrees as published six years ago.
Additionally, it is unclear what human harm will result from 2 degrees warming, from higher sea levels to floods, heatwaves and crop failures.
Environmentalists argue that uncertainty cannot be used to justify inaction, citing a “precautionary principle”.
One advantage of a carbon budget is that it at least adds some specific numbers, where high agreement between estimates makes the case for action more convincing despite the uncertainty. (Reporting by Gerard Wynn; Editing by Dale Hudson)