The Climate Change Committee (CCC) wrote in its December report that the world could expect about a 2 degree rise in temperature by 2100 if global emissions fell by about 50% by mid-century. It said that the risk of more than a 4 degrees rise is less than 1% if the world achieved this reduction. Because the UK has per capita emissions much higher than the global average, the Committee recommended that the country should cut its emissions by about 80%. This would eventually leave the UK’s emissions per head as about the same as the rest of the world.

The CCC report is thorough, robust and clear. But is its recommendation sufficiently prudent? In four main respects, the Committee has chosen a more optimistic conclusion than I believe is warranted. The implication is that its emissions reductions targets are not severe enough.

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A grossly simplified model of climate change The amount of global warming we can eventually expect depends on three key variables:

Emissions of greenhouse gases.
The percentage of these gases absorbed by ‘sinks’ such as the oceans, forests, and soils and thus extracted from the air.
The sensitivity of temperature to the total amount of greenhouse gas in the atmosphere.

Put another way, average global temperatures depend on the stock of greenhouse gases and the degree to which these gases heat up the atmosphere. Let’s look at the assumptions used by the CCC for each of these three variables:

1) Emissions of greenhouse gases The CCC’s method was essentially to calculate numbers for 2) and for 3) and then to ask itself ‘What are maximum emission that can be allowed if the global temperature rise is kept to about 2 degrees?’ It found that if world emissions peaked in 2016 and then fell by 3% a year until 2050 the median expectation of temperature rise in 210 is about 2 degrees. This is essentially an arithmetic calculation that springs from the Committee’s assumptions about CO2 absorption and the sensitivity of temperature to levels of greenhouse gases.

2) The percentage of these gases absorbed by ‘sinks’ such as the oceans, forests, and soils and thus extracted from the air Scientists have a general rule that in each year total absorption of carbon dioxide is slightly over 50% of the amount emitted. In other words, for every 2 tonnes of CO2 emitted every year, the total stock of carbon dioxide in the atmosphere rises about 1 tonne.

The relationship between the CO2 emitted and the amount absorbed is a complex one. Scientists say that if a single large ‘pulse’ of carbon dioxide entered the atmosphere the equivalent of 15-20% of this pulse would be typically absorbed within a year. Smaller and smaller amounts would be taken out in future years but almost a quarter would remain after several hundred years. Standard figures for how much carbon dioxide remains in the atmosphere (usually called the ‘airborne fraction’ or AF) are used in the runs of the immensely complex computer models employed by climate scientists. There can also be very substantial year-to-year variations in the AF because of such things as severe drought, which may result in low net CO2 absorption because of the dieback of vegetation containing carbon.

The predicted amount absorbed each year is a function of both the level and of the rate of change of emissions (sometimes called the trajectory). An increase in the rate of growth of emissions will typically produce a temporary increase in the amount of CO2 remaining in the atmosphere. This has happened in recent years and scientists have, as expected, noted a slow rise in the average AF. 2008 interrupted this trend because of the generally good growing conditions for vegetation, which increased the absorption of carbon dioxide.

Net CO2 added to the atmosphere as percentage of yearly emissions

The CCC’s projections show the AF falling fast from an expected peak in about 2011. By 2035, the atmosphere will add greenhouse gas emissions equivalent to only about 30% of the CO2 emitted each year. This change, in line with predictions by the standard models, means that for every tonne emitted in 2035 the net impact on atmospheric concentrations will be little more than half what it is today.

Net CO2 added to the atmosphere as percentage of yearly emissions (with CCC forecasts to 2035)

So where is the issue with the Committee’s projections? The problem is that the world’s ecosystems show increasing signs of not absorbing as much CO2 as they did in the past. The report acknowledges this in its text but does not appear to have included substantial weakening of the biosphere’s capacity to absorb carbon dioxide in its detailed forecasts. This raises the possibility that much more carbon will remain in the atmosphere than the Committee suggests. We cannot know with certainty that the forecasts from the CCC are wrong but we can suggest both that the ‘known unknowns’ and the ‘unknown unknowns’ are likely to reduce unexpectedly the total re-absorption of greenhouse gases.

A prudential approach might have employed more pessimistic assumptions about carbon uptake, with absorption declining more strongly over time. This recommendation is in line with the conclusions of increasing numbers of recent scientific papers.

3) The sensitivity of temperature to the total amount of greenhouse gas in the atmosphere There is still considerable uncertainty about how much temperatures will rise in response to increased levels of climate changing gases. The uncertainty arises both from the imprecise understanding of exactly how much infra-red radiation (heat) is absorbed by carbon dioxide and other molecules and, second, from a lack of knowledge of how much the reflectivity of the earth is likely to change. Ice largely reflects visible light, which CO2 does not absorb as heat. But if ice melts, the surface underneath is darker (whether on sea or land). This reduces the reflection of light and increases the absorption of light energy by the surface. The absorption of energy results in higher levels of infra-red output eventually transmitted out to space, some of which is trapped by CO2.

The increase in temperature resulting from a doubling of greenhouse gas levels is known as the ‘climate sensitivity’. The Committee says that it uses a figure of 3 degrees Celsius in its work. Although this is not far from standard estimates, it is probably somewhat too low. The Fourth Report of the IPCC (2007) gives a figure of 3.26 degrees with a standard deviation estimate of 0.69 degrees. It may have been more appropriate to use this higher figure. In addition, the IPCC comments (p. 60 of Working Group III report) that:

Non-linearities in the feedbacks (including e.g. ice cover and carbon cycle) may cause time dependence of the effective climate sensitivity, as well as leading to larger uncertainties for greater warming levels.

The implication of this remark is that the distribution of probability of the future value of climate sensitivity is wider than we currently expect – there are more likely to be unpredicted extreme outcomes. The Committee might therefore have appropriately chosen a wider range of potential outcomes, as well as a higher median figure, for climate sensitivity.

Other issues a) The median and the mean temperature rise The Committee produced extremely detailed technical appendices. One spreadsheet looked at the distribution of expected temperature rise resulting from the various emissions trajectories modelled by the CCC. The Committee favours one scenario as meeting its requirements that the average figure for expected temperature rise is about 2 degrees. This scenario requires global emissions to peak in 2016 and then fall at 3% a year.

This outcome is said by the Committee to result in a distribution of possible temperature rises that is centred around 2 degrees. The chance of the actual figure being more than 4 degrees is less than 1%. The 2 degree figure has substantial totemic significance: most scientists and policy-makers see the world being able to cope with this rise, but with the risks of catastrophe rising sharply above this level.

The illustrative graphic below illustrates one problem with the CCC’s assessment. Its own assessment of the distribution of possible outcomes actually peak at 2.2 degrees. Secondly, the distribution has a larger area to the right of the median than it has to the left. The ‘central model estimate’ of temperature rise is 2.2 degrees, but this is not the average increase we can expect. There is a larger chance of outcomes to the right of the 2.2 degrees than there is to the left. The actual average figure (which is the statistical mean of this distribution) is about 2.3 degrees. It seems a small change but in climate science tenths of one degree are important.

In this case I think it would have been prudent to state that the mean temperature rise expected under the Committee’s projections is 2.3 degrees. The report should have stressed that this figure is substantially different from the 2 degree level which the CCC indicates is an important line to hold.

Schematic representation of the distribution of temperature rises by 2100 under the CCC’s 2016 3% p.a. reduction scenario

b) Overshooting The CCC’s proposed global emissions trajectory and its assumptions about AF show greenhouse gas concentrations rising to about 500 ppm and then slowly declining. If concentrations stayed at 500 ppm, the mean expected increase in temperature would be higher than the Committee forecasts. (As it stands, even the Committee’s own figures see temperatures peaking under the ‘3% p.a. from 2016’ scenario at over 2.6 degrees above pre-industrial levels sometime well after 2100.)

The CCC is recommending what the climate science trade calls ‘overshooting’ or letting greenhouse gas concentrations rise to above a certain level and then reducing them to what might be called a ‘safer’ number. Underlying this recommendation is, in effect, a belief that we can rely on lags in the climate system. The climate will, as it were, not ‘notice’ the temporary higher levels of climate changing gases, largely because the oceans will absorb the extra heat for at least a few decades.

‘Overshooting’ has some supporters. But the CCC’s predecessor, the 2006 Stern Review, took a dim view of this tactic:

An overshooting path to any stabilisation level would lead to greater impacts. [...] Overshooting is potentially high risk. (p. 228 of the printed edition)

The CCC acknowledges that overshooting has potential perils. But it goes on to say that not overshooting would increase the required reduction rate from 3% from 2016 to almost 7% a year. Understandably, it sees this sharper cut as more difficult to achieve. Nevertheless, a more prudent approach would have pushed the Committee towards recommending that the UK government adopt a tighter emissions reduction regime than it actually specifies in its report.

Why do we want a more prudent approach? Loosely speaking, this article notes four areas in which Committee showed limited risk aversion in its recommendations. It may be right to have done this. However, as climate science progresses it seems to be increasingly telling us that the probability distributions of expected ‘climate sensitivity’ and of annual rates of CO2 absorption by sinks are widening. In other words, our assessment of the risks from climate change should be rising. We should be incorporating more risk aversion, not less, into our policies.

The CCC itself admits that the climate science is worryingly pessimistic. At one stage, the report acknowledges one recent paper that suggests that the world is already locked into a temperature rise of 2.4 degrees because atmospheric aerosol pollution is masking the underlying existing impact of greenhouse gases. Having mentioned this particular scientific work, it then seems to have largely ignored the implications.

This could be a completely unfair criticism of the CCC’s approach. The Committee may, quite rationally, have said to itself that the political elite can only be dragged slowly into accepting the need for rapid decarbonisation of the economy. A more pessimistic conclusion from the CCC’s December report might have made policy-makers despair and thus actually reduced the likely rate of change. But the rest of us Jeremiahs need to keep the pressure up for commitment to even faster rates of progress.