How much warming in the pipeline? Part II – it’s as tricky as ABC

Warming ‘in the pipeline’ is a term used to describe lags and inertia in the climate system. As explained in my previous post on this topic,  the planet is committed to further heating and sea level rise, irrespective of what choices we make now, or in the immediate future, to reduce carbon emissions. The global warming trend over the last 100 years (actually, from 1906 to 2005), of 0.56°C to 0.92°C, is not all that we would have expected. That is, there is a ‘missing’ quanta of  warming, which is being hidden by a number of poorly understood factors.

First, let’s consider 21st century carbon emissions mitigation scenarios. A range of possible future energy and economic development storylines have been developed by the Special Report on Emissions Scenarios (SRES), such as A1FI, A2, B1 etc. (based on global vs regional action, and different tech pathways). For details, see here. These scenarios were used as a basis for the IPCC 2007 AR4 projections of future climate change. Two important points with these SRES storylines: (i) there is no explicit preference given to any particular scenario, and (ii) they are not active mitigation scenarios (mitigation is a byproduct of economic, social and technology choices). As such, it can be confusing to work out what ‘is likely’, and indeed, whether any are particularly realistic. 

More usefully in terms of future predictions, a recent paper in PNAS by Van Vuuren and co-workers (including a friend of mine, Tom Wigley, who is an Adjunct Professor at the University of Adelaide), assessed the impact on climate change of some plausible real-world actions. Here is the abstract, with some bolding by me:

Estimates of 21st Century global-mean surface temperature increase have generally been based on scenarios that do not include climate policies. Newly developed multigas mitigation scenarios, based on a wide range of modeling approaches and socioeconomic assumptions, now allow the assessment of possible impacts of climate policies on projected warming ranges. This article assesses the atmospheric CO2 concentrations, radiative forcing, and temperature increase for these new scenarios using two reduced-complexity climate models. These scenarios result in temperature increase of 0.5-4.4°C over 1990 levels or 0.3-3.4°C less than the no-policy cases. The range results from differences in the assumed stringency of climate policy and uncertainty in our understanding of the climate system. Notably, an average minimum warming of ≈1.4°C (with a full range of 0.5-2.8°C) remains for even the most stringent stabilization scenarios analyzed here. This value is substantially above previously estimated committed warming based on climate system inertia alone. The results show that, although ambitious mitigation efforts can significantly reduce global warming, adaptation measures will be needed in addition to mitigation to reduce the impact of the residual warming.

Their conclusion? Even with a strong and concerted effort towards rapid carbon mitigation, we are committed to 0.5 to 2.8°C additional warming (on top of what has already been experienced), due to the combined effects of climate system, economic and technological inertia. 

This sobering view is supported by many climate scientists in this field; most prominently, by Prof V Ramanathan of University of California — San Diego. Ram is the scientific guru of the study of tropospheric aerosols (soot, dust, sulphates, nitrates and other chemicals that together constitute the low-lying ‘haze’ seen over polluted cities worldwide, and now, large parts of Asia). Aerosols can cool the climate by reflecting solar energy back out to space before it has a chance to be absorbed and re-emitted as infrared radition by the Earth’s surface, and also warm the climate by absorbing extra energy in the lower atmosphere (coming mostly from incompletely burnt carbon from coal-fired power stations and dung braziers). Collectively, it’s called the Atmospheric Brown Cloud (it used to be known as the Asian Brown Cloud, ’cause that is where most of it is). Also known as ABC.

Last year, Ramanathan and co-author Y. Feng published a remarkable paper in PNAS called “On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead“). Here, once again, is the abstract:

The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial 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. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4°C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols. The range of 1.4°C to 4.3°C in the committed warming overlaps and surpasses the currently perceived threshold range of 1°C to 3°C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan-Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that ≈25% (0.6°C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6°C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO2 mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4°C.

You can download the PDF of the full paper, for free, here. It’s an excellent piece that should, in the most part, be intelligible to the majority of interested readers.

The net climate-forcing effect of ABCs is much more poorly known than that of long-lived trace greenhouse gases, as explained here. Our best estimate is that in sum, ABCs cool the climate system — potentially offsetting more than half the warming we would have otherwise expected to date.  That is, the new industries and traditional stoves of Asia may have delayed the worst impacts of climate change. Here is a key point made by R&F (I removed the reference numbers for clarify of reading here — see the original paper for the links to the relevant peer-reviewed literature; GHG = greenhouse gases, CEWGA = committed equilibrium warming from greenhouse gases and aerosols, Wm2 = watts per metre squared, DAI = dangerous anthropogenic interference with the climate system):

First, we have to consider the effect of aerosols, which start off as urban haze or rural smoke and ultimately become transcontinental and transoceanic plumes o ABCs consisting of sulfate, nitrate, hundreds of organics, black carbon, soil dust, fly ash, and other aerosols. ABCs have masked GHG warming by enhancing the albedo (percent of incoming solar radiation re-flected back to space) of the planet. A recent review of available literature estimates the masking effect of ABCs to be
47% (1.4 Wm2) with a 90% confidence interval of 20-80%. The IPCC-AR4  value for the masking is 40%. Effectively, the forcing ‘‘felt” by the climate system is only 53%, i.e., 1.3°C, which is identical to CEW
GA, the committed warming adapted by earlier studies. About 8% of the committe warming (0.2°C) is compensated by increases in the surface albedo because of land-use changes;
20% (0.5°C) is delayed by the thermal inertia of the oceans and it is only the balance of
25%, i.e., 0.6°C, that should by now have manifested as observed warming. This algebraic exercise demonstrates that the observed surface warming of 0.76°C (since the latter half of 1800s) is not inconsistent with the committed warming of 2.4°C.

The fundamental deduction (subject to the assumption of IPCC climate sensitivity) is that if we get rid of the ABCs today the Earth could warm another 1.6° (which includes the delayed warming caused by ocean thermal inertia) unless we act now to reduce GHG concentrations. As shown by couple ocean atmosphere models used in IPCC,
50% of this warming can happen in few decades, and most of the balance will manifest during the course of this century. The situation with respect to sea-level rise is considerably more complex. Sea-level rise caused by thermal expansion (in the range of 10 to 30 cm per century) is likely to continue for centuries (even if the warming asymptotes to values close to CEW
G by 2100) because of the time required for mixing of the heating to deeper oceans. In addition, the range of CEW
G (1.4-4.3°C) raises another major DAI-related issue. As suggested by the IPCC the Greenland Ice Sheet can disappear completely if surface warming is maintained in excess of 1.9-4.6°C for millennia and raise sea level by 7 m or more.

Prof HJ Schellnhuber, Director of the Potsdam Institute, commented formally in PNAS on the R&F paper here: “Global warming: Stop worrying, start panicking?“.  His conclusion was that R&F’s assessment is technically right, but, as Mark Twain once commented on a Wagner opera, it’s not as bad as it sounds. That is, there is still a fair chance that we can ‘hold the 2°C line’, if strong mitigation of greenhouse gases is combined with the following three actions: (i) a slow, rather than instant, elimination of aerosol cooling, (ii) a directed effort to first remove warming aerosols like black carbon, and (iii) a concerted and sustained programme,  over this century, to draw-down excessive CO2 (geo- and bio-engineering) and simultaneously reduce non-CO2 forcings, such that the final equilibrium temperature rise will be lower than would otherwise be expected on the basis of current concentrations.

His bottom line? “This requires an industrial revolution for sustainability starting now“.