Confused about the new IPCC's carbon budget? So am I.

by David Spratt

When the IPCC's new report on the physical basis of climate change was released in late September, media attention focused on a conclusion from the Summary for Policymakers that the world had emitted just over half of the allowable emissions if global warming is to be kept to 2 degrees Celsius (2°C) of warming.

Unfortunately, because many people think if you have a budget you should spend every last dollar, the "carbon budget" message could be interpreted as saying there is plenty of budget left to spend. The respected climate researcher Ken Caldeira told Climate Progress that the carbon budget concept is dangerous for two reasons:

There are no such things as an “allowable carbon dioxide (CO2) emissions.” There are only “damaging CO2 emissions” or “dangerous CO2 emissions.” Every CO2 emission causes additional damage and creates additional risk. Causing additional damage and creating additional risk with our CO2 emissions should not be allowed.
If you look at how our politicians operate, if you tell them you have a budget of XYZ, they will spend XYZ. Politicians will reason: “If we’re not over budget, what’s to stop us to spending? Let the guys down the road deal with it when the budget has been exceeded.” The CO2 emissions budget framing is a recipe for delaying concrete action now.

And the idea that 2°C of warming is safe is not a sustainable proposition. Prof. Kevin Anderson says that "the impacts associated with 2°C have been revised upwards, sufficiently so that 2°C now more appropriately represents the threshold between ‘dangerous’ and ‘extremely dangerous’ climate change".

Typical of the media's coverage of IPCC 2013 was The Guardian's headline: "IPCC: 30 years to climate calamity if we carry on blowing the carbon budget. Global 2C warming threshold will be breached within 30 years, leading scientists report, with humans unequivocally to blame" and its reporting that:

The scientists found that to hold warming to 2°C, total emissions cannot exceed 1,000 gigatons of carbon. Yet by 2011, more than half of that total "allowance" – 531 gigatons – had already been emitted.

But hold on, I thought the current level of greenhouse gases was enough in the long run to produce 2°C of warming? This is what researchers such as Ramanthan and Feng found back in 2008:

The observed increase in the concentration of greenhouse gases (GHGs) since the pre-industrial era has most likely committed the world to a warming of 2.4°C (1.4°C to 4.3°C) above the preindustrial surface temperatures.

It takes a while for greenhouse gases to produce their full warming effect because 90% of the additional energy goes into heating the oceans and another 7% into melting ice sheets. This is called thermal inertia, and it means that after an increase in the atmospheric greenhouse gas level, about one-third is realised as a temperature increase during the first decade, getting to two-thirds of the warming potential takes 50 years, and most of the rest is realised within a century. So for emissions back in the 1960s, we have felt about two-thirds of the warming; for emissions around 2000, we have felt only one-third of the heating effect so far.

When, after a burst of greenhouse gas emissions, all these processes have worked through the climate system, the resultant effect on temperature is known as equilibrium climate sensitivity (ECS).  This defines the amount of warming for a doubling of greenhouse gas levels, which the new IPCC report finds to be in the range of 1.5–4.5°C, with a median of around 3°C.   From this, we can calculate for the present level of all greenhouse gases of around 478 parts per million carbon dioxide equivalent (ppm CO2e), the equilibrium temperature increase will be 2.3°C if ECS is taken as 3°C.

So how can this be reconciled with the IPCC "headline" story that there are plenty of emissions left in the carbon budget for 2°C of warming?

The new Climate Council's Prof. Will Steffen says that:

This budget may, in fact, be rather generous. Accounting for non-CO2 greenhouse gases, including the possible release of methane from melting permafrost and ocean sediments, or increasing the probability of meeting the 2°C target all imply a substantially lower carbon budget.

Here's some pointers as to why.


1. OTHER GREENHOUSE GASES  The budget is for CO2 emissions only, and does not include other greenhouse gases. When these "non-CO2 forcings" are included, the IPCC's Summary for Policy Makers says the total allowable emissions is 800 gigatonnes of carbon (GtC) for a 66% chance of not exceeding 2°C. Take away the 530 GtC already omitted, and the budget remaining is now 270 GtC.  That a lot less than the "half of 1000 GtC" line that led the news.

2. RISK  And what if didn't want a one-in-three risk of exceeding 2°C?  That would be very prudent given the escalating impacts above 2°C. The IPCC report doesn't seem to give the answer.  But an earlier report by Anderson and Bows, quoting work by Meinshausen, said that:

to provide a 93 per cent mid-value probability of not exceeding 2°C, the concentration would need to be stabilized at, or below, 350 ppmv CO2e, i.e. below current levels.

In other words, if you want a very low risk of not exceeding 2°C, there is probably no budget left.   Let's hope this figure can be clarified.

3. ARCTIC SEA ICE  The IPCC's carbon budget relies on Coupled Model Intercomparison Project Phase 5 (CMIP5) computer modelling results. In another part of the report, results are given for the ~2°C warming scenario (known as RCP2.6) of a 43% reduction in September Arctic sea-ice extent by end of 21st century (compared to a 1985–2005 reference period).  This is so at odds with the reality on the ground as to be not credible.  In just 30 years and with warming of less than 1°C, the sea-ice extent has dropped by half, and the sea-ice volume by more than three-quarters.  Switched-on Arctic researchers suggest that the Arctic will be sea-ice free in summer within the next decade or so, as discussed here and here and here.

Changes in September sea-ice conditions from
IPCC for four scenarios using CMIP5, and
observations (green line)

In fact the IPCC projection for September sea-ice extent by century's end (deep blue line on figure at right) for ~2°C of warming is greater than the actual conditions now (green line) with less than 1°C of warming.  Losing the sea-ice earlier than the IPCC projects will change the planet's surface reflectivity (albedo) and drive further warming. This, again, would reduce the carbon budget for 2°C this century, but in appears this has not been accounted for fully using the CMIP5 Arctic sea-ice results.

4. CARBON STORES The Summary for Policymakers offers this qualification:

A lower warming target, or a higher likelihood of remaining below a specific warming target, will require lower cumulative CO2 emissions. Accounting for warming effects of increases in non-CO2 greenhouse gases, reductions in aerosols, or the release of greenhouse gases from permafrost will also lower the cumulative CO2 emissions for a specific warming target.

In December 2012, the UNEP reported that "the IPCC Fifth Assessment Report, due for release in stages between September 2013 and October 2014, will not include the potential effects of the permafrost carbon feedback on global climate."  Yet even for the ~2°C warming pathway, permafrost release of greenhouse gases is pertinent. As I reported recently in Is climate change already dangerous?:

A 2012 UNEP report on Policy implications of warming permafrost says the recent observations “indicate that large-scale thawing of permafrost may have already started.”  In February 2013, scientists using radiometric dating techniques on Russian cave formations to measure historic melting rates warned that a +1.5ºC global rise in temperature compared to pre-industrial was enough to start a general permafrost melt.  Vaks, Gutareva et al. found that “global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.” Vaks says that: “1.5ºC appears to be something of a tipping point”.

And in April 2011, the paper "Amount and timing of permafrost carbon release in response to climate warming" concluded:

The thaw and release of carbon currently frozen in permafrost will increase atmospheric CO2 concentrations and amplify surface warming to initiate a positive permafrost carbon feedback (PCF) on climate…. [Our] estimate may be low because it does not account for amplified surface warming due to the PCF itself….
We predict that the PCF will change the arctic from a carbon sink to a source after the mid-2020s and is strong enough to cancel 42-88% of the total global land sink. The thaw and decay of permafrost carbon is irreversible and accounting for the PCF will require larger reductions in fossil fuel emissions to reach a target atmospheric CO2 concentration.

For the other three IPCC warming scenarios, permafrost must be a key component. Climate Progress reported in 2012 that:

Back in 2005, before the IPCC’s Fourth Assessment, a major study led by NCAR climate researcher David Lawrence, found that virtually the entire top 11 feet of permafrost around the globe could disappear by the end of this century. Using the first “fully interactive climate system model” applied to study permafrost, the researchers found that if we tried to stabilize CO2 concentrations in the air at 550 ppm, permafrost would plummet from over 4 million square miles today to 1.5 million.
That matters because the permafrost permamelt contains a staggering “1.5 trillion tons of frozen carbon, about twice as much carbon as contained in the atmosphere, much of which would be released as methane.  Methane is 25 times as potent a heat-trapping gas as CO2 over a 100 year time horizon, but 72 to 100 times as potent over 20 years!

All of which suggests to me that for a high probability of not exceeding 2C of warming, and including the likely impacts of a period of sea-ice-free summer conditions in the Arctic sooner rather than later, and significant release of CO2 and methane from Arctic permafrost stores, then the available carbon budget is probably zero, or less.

But that real-world question is not one to which I could find an answer in the IPCC report.

NOT DISCUSSED: At a broader level, the IPCC physical basis report seems weak on many Arctic-related issues.  As far as I can see:

• The model predictions of sea-ice extent loss are so discordant with recent summer extent observations as to be not credible.
• The loss of three-quarters of Arctic sea-ice volume (a more robust indicator than extent) since the 1980s seems irreconcilable with the IPCC model predictions for September sea-ice extent by 2100.
• Given the deep concern by some scientists over significant methane hydrate releases from shallow sea-floor deposits and especially from the East Siberian Arctic shelf, the IPPC's conclusion that "large (hydrate) CH4 release to the atmosphere during this century is unlikely" seems premature. 
• The impacts of Arctic-derived carbon cycle positive feedbacks such as permafrost loss on future temperature projections and on allowable carbon budgets are not given.
• No mention is made of the cascading additive effect of multiple Arctic positive feedbacks on the rate of global warming, nor an assessment of Arctic amplification sensitivity.