What Is the Carbon Limit? That Depends Who You Ask

Scientists are offering widely varying estimates of how much carbon we can emit into the atmosphere without causing dangerous climate change. But establishing a so-called carbon budget is critical if we are to keep the planet a safe place to live in the coming century.

How much carbon can we safely emit into the atmosphere without the planet suffering dangerous climate change? It would be good to know. The world’s governments have agreed that “dangerous” should mean any warming above two degrees Celsius. And in recent reports, the Intergovernmental Panel on Climate Change (IPCC) has tried to translate that into a future carbon budget.

But too many different numbers are still floating around. We could have more than 500 billion tons of carbon that we could safely emit, or the real

The science is a work in progress, but we know how to make judgments on what it will take to be on the safe side.
figure might be close to 100 billion tons — it depends on whose estimates you decide to accept.

The carbon budget looks to be one of the most critical single metrics for keeping planet Earth a safe place to live through the coming century. So it would be a good idea to get to the bottom of the discrepancies, especially since the countdown to dangerous climate change may be shorter than the lifetime of a new coal-fired power plant.

Here is an attempt to cut through the statistical fog.

Some of the uncertainty is inevitable. We still don’t know the true sensitivity of global temperatures to the greenhouse gases that cause warming. Embarrassingly for climate researchers, there is a three-fold uncertainty range that, according to Susan Solomon, the former co-chair of an IPCC science-assessment working group who is now at MIT, “has not narrowed appreciably in 30 years of research.”

As fast as one uncertainty is resolved, another shows up. Just last month, for example, it emerged that global climate models have probably underestimated the amount of carbon dioxide that plants absorb by about a sixth. This, said Ying Sun of the University of Texas at Austin and colleagues, explained why CO2 wasn’t accumulating in the atmosphere

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Carbon budget

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Estimates of how much carbon can still be emitted into the atmosphere vary widely.
quite as fast as climate models predicted it should be.

But if the science is still work in progress, we do know enough to make some judgments about what it will take to be on the safe side — to reduce the risk of exceeding two degrees Celsius (3.6 degrees Fahrenheit) to an acceptable level.

The bottom line is that, despite short-term natural climatic variability, there is a fairly straightforward relationship between average global temperatures and the concentration in the atmosphere of the critical greenhouse gases — primarily carbon dioxide. Moreover, we know that about half of the CO2 we put into the atmosphere when we burn fossil fuels and trash natural ecosystems stays there. It has an atmospheric lifetime measured in centuries. So we can calculate a carbon budget for what it takes to keep warming below two degrees.

And we should. Solomon argued a couple of years ago that cumulative carbon emissions are the best way of assessing climate risk, since they avoid problems such as time lags that mess with other measures, such as atmospheric concentrations. That view was reflected when in its most recent assessment of the science of climate change, completed last September, the IPCC for the first time tried to calculate that carbon budget.

Ever since, climate scientists, think tanks and pundits have been interpreting those numbers. But the process has gotten confusing. Everybody seems to have a different budget.

The IPCC’s first analysis was included in its fifth scientific assessment of climate change, published in September 2013 and reiterated in the
According to the IPCC, we have just under 500 billion tons of our carbon budget left.
synthesis report released last Sunday. It suggested that a two-thirds chance of keeping warming below two degrees required the world to limit its total carbon emissions since 1860 to no more than a trillion tons of carbon. Of this grand all-time total, 515 billion tons had already been emitted by 2011. So, according to the IPCC, we have just under 500 billion tons of our budget left. Then we have to stop. Totally.

The synthesis report said that fossil-fuel power generation would have to be “phased out almost entirely by 2100” — unless the largely untried technology of capturing CO2 emissions and burying them out of harm’s way could be deployed on a massive scale. Without a drastic slowdown in emissions within the next decade, the phase-out date could happen much earlier, probably before 2050.

The arithmetic seemed straightforward enough. But carbon budget numbers since quoted by other sources do not all follow this IPCC bottom-line figure. They reveal a bewildering array of different estimates for our remaining budget. Among environmental groups, the World Resources Institute (WRI) sticks with the IPCC estimate that we have 485 billion tons left. But other environment groups quote other numbers. For instance, Greenpeace and WWF say 350 billion tons.

Scientists are even less coordinated. A big study in Nature Climate Change in September by Michael Raupach of the Australian National University in Canberra and others, quotes 381 billion tons. The International Institute for Applied Systems Analysis, a think tank based in Laxenberg, Austria, and the Global Carbon Project says we have 327 billion tons to go. While the International Geosphere-Biosphere Programme, an international research consortium based in Sweden, say 250 billion tons.

To confuse things further, another blue-chip study, published last December by Jim Hansen of the Earth Institute at Columbia University and others, argued that we could emit a further 350 billion tons and still keep below 1.5 degrees of warming.

So what is going on? The good news is that most of the discrepancies arise from different assumptions and policy scenarios, rather than outright disagreements about the science. The bad news is that means there is no single right answer.

For instance, some estimates of our future carbon budget have been based on the concept that we might be safe with a 50 percent chance of keeping below two degrees, rather than 66 percent. Some assume we must stop all emissions from deforestation immediately, so that all our remaining carbon budget can be used for industrial emissions, while others reckon we
The choice of two degrees as the threshold for ‘dangerous’ climate change is ultimately a political choice.
should keep part of the budget available for continued deforestation.

The most complex to disentangle is the dissenting claim of Hansen that we can emit 350 billion tons and still limit warming to 1.5 degrees. According to Hansen’s modeler and co-author, Pushker Kharecha of Goddard Institute for Space Studies in New York, this is based, among other things, on the assumption that we can both halt deforestation and plant sufficient new trees to soak up the 100 billion tons of carbon already emitted from past deforestation.

Some of the budget estimates also make an allowance for the effects of anthropogenic emissions of warming gases other than CO2, such as methane. They assume we will carry on making such emissions and that the carbon budget has to be adjusted downward to allow for that. The IPCC itself reckoned that allowing for these non-CO2 gases would cut the total budget from a trillion tons to 790 billion tons. That would leave just 275 billion tons of the budget remaining at the end of 2011.

This approach is controversial. On the face of it, if non-CO2 gases warm the atmosphere, they have to be included. But the difference is that most of them don’t hang around in the air for anything like as long as CO2. Methane’s half-life is around 10 years, for instance. And warming aerosols last for only a few days. So they don’t accumulate in the way that CO2 does, and can be cleaned up much more quickly with emissions reductions.

Solomon argues that “long-term temperature change remains primarily associated with total cumulative carbon emissions, owing to [their] much longer atmospheric residence time.” So it is a moot point whether these non-CO2 gases should be factored into a carbon budget devoted to emissions that will last for many centuries. Some argue that we should have separate targets for them.

The choice of two degrees as the threshold for “dangerous” climate change is subjective and ultimately a political choice. As Solomon puts it: “There is little by way of quantitative evidence that this represents a ‘safe’ policy target.” It is also recent. While the global commitment to prevent “dangerous” climate change was included in the UN climate change convention in Rio de Janeiro in 1992, the decision to interpret that as a
Long-term global carbon budgets have barely been mentioned in proposals for the Paris climate conference next year.
two-degree warming was only adopted at the Copenhagen climate conference in 2009.

It remains a subject of debate. David Victor of the University of California at San Diego — a long-standing critic of international approaches to tackling climate change — says we should ditch it because it is scientifically “wrong-headed” and politically unachievable.

Others say it would lead to unacceptably severe impacts, especially in the longer term, and we should aim for something tougher. They worry in particular that two degrees warming would be enough to set off natural positive feedbacks that would be difficult to halt. Hansen’s 2013 paper argued that “cumulative emissions of 1000 billion tons, sometimes associated with 2C global warming, would spur ‘slow’ feedbacks and eventual warming of 3-4C with disastrous consequences.” These “slow feedbacks,” he says, include greenhouse-gas releases from ecosystems as forests die and permafrost melts.

For that reason, Hansen thinks we should try and avoid a temperature rise of more than one degree. That would keep the world within the temperature boundaries we have experience in the past 10,000 years, he says. (It would also be broadly consistent with returning CO2 levels in the atmosphere to around 350 parts per million, as proposed by groups such as activist Bill McKibben’s 350.org.

Hansen calculates that keeping below one degree of warming will mean we only have 130 billion tons of carbon from fossil-fuel emissions to go. That is not much more than a decade’s worth at current emissions rates.

Such carbon budgets sound stringent. But they have the advantage of being measurable and predictable, and we can plan for how to achieve them. However, while the latest IPCC assessment raises the idea of a fixed carbon budget as a desirable and scientifically valid objective, climate negotiators so far have not picked up the gauntlet. Long-term global carbon budgets have barely been mentioned in proposals for the next major climate

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conference planned for Paris next year.

But the rest of us can at least think about what a fair and scientific approach to meeting a carbon budget might look like. The longest-standing — and most elegantly simple — proposal is the “contraction and convergence” formula from the UK-based Global Commons Institute. It envisions contracting global emissions to meet an agreed global carbon budget, while at the same time having national emissions converge toward strict per-capita entitlements, in which each nation’s annual emissions allowance would be dictated by its population. Those who have trouble meeting those targets would have the option of trading in the entitlements.

More recently, the WRI has given researchers and the public the chance to create their own formula for fair national limits. WRI’s CAIT Equity Explorer incorporates criteria such as historical emissions, levels of economic development, capacity to adapt to climate change, and potential to reduce emissions.

A fair-share formula that included historical emissions has been suggested in the past by the Brazilian government. It would create drastically fewer future emissions entitlements for countries that industrialized early, notably in Europe and North America. Some could end up with no rights to emit at all, having already exhausted their budget, and would be dependent on buying other countries’ quotas.

However it is done, says Raupach, such a formula would for the first time allow the global community to develop “a transparent methodology for translating global to national carbon quotas under a wide range of possible sharing principles.”

In other words, the horse trading that has characterized the negotiations over carbon emissions targets during the past two decades could be put to one side. Once an overall carbon budget and the basic rules for what is fair have been agreed, national targets would automatically flow. Any takers?