Does short-lived black carbon have long-term climate impact?

The understanding about black carbon has come a long way since the nebulous beginning in the seventies, when all the world understood was suspended particulate matter (SPM) – a local pollutant from incomplete combustion, indicted for pollution from fires and vehicles.

It is now understood that black carbon comes from all combustion processes, all dust generating activities, secondary particulates – nitrates and sulfate, and the condensation of gases into liquid droplets. Black carbon is largely a product of low temperature combustion of carbonaceous fuels, and incomplete combustion. The composition of black carbon varies by the type of fuel used, the combustion process, and emission control technologies or practices. Black carbon particles vary in size and can be much smaller than PM2.5 and as small as PM0.1. These last up to minutes, hours and one week or little more in the atmosphere depending on the combustion process and size.

Black carbon and warming: Black carbon can absorb heat and warm up the surrounding atmosphere. Scientists calculate the potential of a gas to cause global warming in terms of ‘radiative forcing’. Radiative forcing is the difference of sunlight absorbed by the Earth and energy radiated back to space in watts per square meter of the Earth's surface. More incoming energy is more warming.

More outgoing energy is negative forcing that cools. Black carbon has the shortest life – between 3-8 days. And there is uncertainty regarding its potential of causing climate change. The uncertainty in the emission metrics such as Global Warming Potential (GWP) and Global Temperature change Potential (GTP) of Black Carbon is wide reflecting the current challenges related to understanding and quantifying the various effects of black carbon on climate systems in different regions of the world.

However, the science of black carbon has improved over the years and so has the understanding on its impacts on climate. The latest IPCC report AR5 has taken note of the recent research and is more explicit in its discussion on black carbon than it was ever before. For instance, AR5 has doubled the estimate of warming (Global mean radiative forcing) of black carbon aerosol from fossil fuels and biofuels from its previous AR4 report.

There is also considerable uncertainty about comparing the global warming potential of long-lived CO2 with short-lived pollutants like black carbon. For instance, if GWP of CO2 over 100 years is 1, that of black carbon is estimated to be 900. But this comparison has limitations. For black carbon, a short time horizon like 20 years will capture all of its radiative forcing because it is short lived – few hours to few days. Their effect is gone as soon as they fall on ground.

But a very small part of CO2 forcing can be captured in such short time as most of its impact will show up in 100 years or more. But in that long time horizon the effect of black carbon will become very small. This explains why the 100-year GWP for black carbon is much lower than the 20-year GWP.

GWP and GTP from the literature for BC and OC for time horizons of 20 and 100 years For the reference gas CO2, RE and IRF from AR4 are used in the calculations. The GWP100 and GTP100 values can be scaled by 0.94 and 0.92, respectively, to account for updated values for the reference gas CO2. For 20 years the changes are negligible
	GWP		GTP
	H = 20	H = 100	H = 20	H = 100
BC total, globalc	3200 (270 to 6200)	900 (100 to 1700)	920 (95 to 2400)	130 (5 to 340)
BC (four regions)d	1200 ± 720	345 ± 207	420 ± 190	56 ± 25
BC globala	1600	460	470	64
BC aerosol–radiation interaction + albedo, globalb	2900 ± 1500	830 ± 440
OC globala	–240	–69	–71	–10
OC globalb	–160 (–60 to –320)	–46 (–18 to –19)
OC (4 regions)d	–160 ± 68	–46 ± 20	–55 ± 16	–7.3±2.1
Note: a. Fuglestvedt et al. (2010). b. Bond et al. (2011). Uncertainties for OC are asymmetric and are presented as ranges. c. Bond et al. (2013). Metric values are given for total effect. d. Collins et al. (2013). The four regions are East Asia, EU + North Africa, North America and South Asia (as also given in Fry et al., 2012). Only aerosolradiation interaction is included. Source: Working Group I: Contribution to Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2013, Climate Change 2013: The physical Science Basis, UNEP and WHO, Cambridge University Press

Also GWP assumes that the emission being compared is evenly spread across the globe as is the case with CO2. But black carbon is short-lived and its radiative forcing is regionally concentrated. So this assumption does not hold. Black carbon travels short distances and creates hot spots and varies according to local conditions. Black carbon emissions weighted by the GWP do not necessarily represent a CO2-equivalent value. Scientists are now discussing if an alternative method can be found for comparison (see Box: Comparing warming impacts of shortlived pollutants).

On the other hand, there is difference in opinion among the scientific community on the relative importance to be attached to the short-lived climate pollutants as a climate mitigation strategy. For instance, a group of scientists in the 2014 Proceedings of the National Academy of Sciences (PNAS) have taken the view on the potential impact of black carbon on long term warming. It states that reducing the emissions of short-lived climate pollutants can reduce the rate of warming in the short term but will only have a limited effect on long-term warming, which is mainly driven by CO2 emissions. Early action on local pollutants will influence near-term temperatures and bring small benefits for limiting maximum warming relative to comparable reductions taking place later. They caution against overestimating the effect of reducing short-lived forcers in long term climate stabilisation.

Comparing warming impacts of short-lived pollutants   Under the Kyoto Protocol, action to reduce greenhouse gases is evaluated using the 100-year global warming potential matrix. This considers the effect of CO2 over 100 years and is given a GWP value of 1. The GWP of all other pollutants are compared in relation to CO2. Scientists use a weighting factor that indicates the ratio of the total radiative forcing (the change in net energy radiated in and out of the atmosphere) of a greenhouse emission to that of carbon dioxide and over a specific time horizon. The temperature effects of GHGs are generally proportional to their radiative forcings as measured high in the atmosphere. But this is not as simple for black carbon. Radiative forcing has to be ‘normalized’ in a complex way to translate into a true measure of the temperature effect on the globe. Comparing the radiative forcing figures of black carbon to CO2 is therefore difficult. The comparison requires the choice of a time horizon for the atmospheric lifetime of the pollutant – how long it stays in the atmosphere. This varies widely for all pollutants (see Table: Global Warming Potentials (GWP)). Global Warming Potentials (GWP) Pollutants GWP 20 years GWP 100 years Carbon dioxide 1 1 Carbon monoxide 18.6 5 Sulphur dioxide -268 -71 Oxide of Nitrogen -560 -149 Fossil methane 85 30 Nitrous oxide 264 265 Black carbon 3200 900 Organic carbon -160 - 46 Sources: AR5 WGI Alternative metric The IPCC acknowledged the limitations of the GWP method to assess short-lived forcers and called for a new metric for short-lived emissions in its 2007 report. The other method gaining ground is global temperature change potential. It is the ratio of temperature change from a pulse emission of a climate species to a pulse emission of carbon dioxide. Long-lived and short-lived pollutants that are equivalent in terms of GTP-weighted emissions will produce an equivalent global mean temperature response for a chosen year. This captures effect of one pulse of emissions vs another in a given year. However, policy makers will still need to choose a time period over which the metric will be calculated. This is still an evolving concept. Global Temperature Change Potentials (GTP) for black carbon and other pollutants Pollutant GTP 20 GTP 100 Black carbon 470 64 Methane 57 4 Nitrous oxide 303 265 Organic Carbon -71 -10 Sulphur dioxide -41 -5.7 Carbon dioxide 1 1 Source: Fuglestvedt, J., K. Shine, T. Berntsen, et al. (2009) Transport impacts on Atmosphere and Climate: Metrics. Atmos Environ In press/ International Council on Clean Transportation

 

Global and annual mean RF (W m–2) due to aerosol–radiation interaction between 1750 and 2011 of seven aerosol components for AR5 Values and uncertainties from SAR, TAR, AR4 and AR5 are provided when available. Note that for SAR, TAR and AR4 the end year is somewhat different than for AR5 with 1993, 1998 and 2005, respectively
	Global Mean Radiative Forcing (W m–2)
	SAR	TAR	AR4	AR5
Sulphate aerosol	–0.40 (–0.80 to –0.20)	–0.40 (–0.80 to –0.20)	–0.40 (–0.60 to –0.20)	–0.40 (–0.60 to –0.20)
Black carbon aerosol from fossil fuel and biofuel	+0.10 (+0.03 to +0.30)	+0.20 (+0.10 to +0.40)	+0.20 (+0.05 to +0.35)	+0.40 (+0.05 to +0.80)
Primary organic aerosol from fossil fuel and biofuel	Not estimated	–0.10 (–0.30 to –0.03)	–0.05 (0.00 to –0.10)	–0.09 (–0.16 to –0.03)
Biomass burning Secondary organic aerosol	–0.20 (–0.60 to –0.07)	–0.20 (–0.60 to –0.07)	+0.03(–0.09 to +0.15)	–0.0 (–0.20 to +0.20)
Secondary organic aerosol	Not estimated	Not estimated	Not estimated	–0.03 (–0.27 to +0.20)
Nitrate	Not estimated	Not estimated	–0.10 (–0.20 to 0.00)	–0.11 (–0.30 to –0.03)
Dust	Not estimated	–0.60 to +0.40	–0.10 (–0.30 to +0.10)	–0.10 (–0.30 to +0.10)
Total	Not estimated	Not estimated	–0.50 (–0.90 to –0.10)	–0.35 (–0.85 to +0.15)
Source: Working Group I: Contribution to Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2013, Climate Change 2013: The physical Science Basis, UNEP and WHO, Cambridge University Press

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