Comparing global warming potential of CO2 and black carbon has its limitations
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
|H = 20||H = 100||H = 20||H = 100|
(270 to 6200)
(100 to 1700)
(95 to 2400)
(5 to 340)
|1200 ± 720||345 ± 207||420 ± 190||56 ± 25|
|2900 ± 1500||830 ± 440|
(–60 to –320)
(–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
|Comparing warming impacts of short-lived pollutants
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)).
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 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)|
|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)|
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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