What Fraction of Global Warming is Due to the Radiative Forcing of Increased Atmospheric Concentrations of CO2?

This is a long weblog. The bottom line conclusions are written here to motivate reading the entire weblog.

CONCLUSIONS:

1. The primary focus on carbon dioxide inappropriately deemphasizes the first order importance of the other climate system heat system forcings (both cooling and warming forcings), as well as does not address the spatially complex, and incompletely understood, actual pattern of global climate system heat changes.

2. Attempts to significantly influence regional and local-scale climate based on controlling CO2 emissions alone is an inadequate policy for this purpose.

A starting point for the assessment of the relative fraction of global warming that is attributable to the radiative forcing of CO2 is the Summary Figure from the 2002 IPCC report (see). Clearly, according to their analysis, in comparing the change of radiative forcing since pre-industrial times until the present as estimated from the IPCC summary figure, the well-mixed greenhouse gases dominate the forcings which cause warming (about a 2.4 Watts per meter squared difference between these two time periods), of which about 1.4 Watts per meter squared is from CO2. Other warming forcings that they include, if the mean value plotted is used, are black carbon from burning fossil fuels (about 0.2 Watts per meter squared), tropospheric ozone (about 0.3 Watts per meter squared), and solar (about 0.25 Watts per meter squared).

Using these values about 58% of the radiative forcing of the well-mixed greenhouse gases results from CO2, and about 48% of the warming human-caused climate forcings result from the radiative forcing of CO2.

The following extracts from research studies reduce the relative contribution of the radiative forcing of CO2 as reported in the 2002 IPCC Report, as summarized above. These studies report the following,

“NASA scientists have found that a major form of global air pollution involved in summertime “smogâ€? has also played a significant role in warming the Arctic……According to this new research, ozone was responsible for one-third to half of the observed warming trend in the Arctic during winter and spring. Ozone is transported from the industrialized countries in the Northern Hemisphere to the Arctic quite efficiently during these seasons. â€?
(see).

â€?Even within the well-mixed greenhouse gas forcings, there are new complications. Drew Shindell and colleagues, as reported in Pollution Online found that, ’According to new calculations, the impacts of methane on climate warming may be double the standard amount attributed to the gas. The new interpretations reveal methane emissions may account for a third of the climate warming from well-mixed greenhouse gases between the 1750s and today. The IPCC report, which calculates methane’s affects once it exists in the atmosphere, states that methane increases in our atmosphere account for only about one sixth of the total effect of well-mixed greenhouse gases on warming. ’â€? (see).

Moreover, from the 2006 Nature paper “Methane emissions from terrestrial plants under aerobic conditions” by Keppler et al,

” If our measurements are typical for short-lived biomass and scaled on a global basis, we estimate a methane source strength of 62–236 Tg yr-1 for living plants and 1–7 Tg yr-1 for plant litter (1 Tg = 1012 g). We suggest that this newly identified source may have important implications for the global methane budget and may call for a reconsideration of the role of natural methane sources in past climate change.”

“A recent study by the CERES Science Team has added to the uncertainty associated with the contributions of climate forcings to global warming by finding that for the period 2000-2004, their assessment of the shortwave albedo decreased by 0.0015 which corresponds to an extra 0.5 Watts per meter squared of radiative imbalance according to their assessment. (see)

“Deposition of BC aerosols over snow-covered areas can result in changes to the surface albedo (Chylek et al. 1983). Further reductions in albedo occur due to the enhanced melting that accompanies the heating of absorbing soot particles in snow. Chylek et al. (1983) estimate this enhancement to be up to a factor of ten in the rate of melting. Recent model results indicate radiative forcings of +0.3 W m−2 in the Northern Hemisphere associated with albedo effects of soot on snow and ice (Hansen and Nazarenko 2004).â€? (see)

We can summarize these findings below:

i) “ozone was responsible for one-third to half of the observed warming trend in the Arctic during winter and spring”.

ii) “The new interpretations reveal methane emissions may account for a third of the climate warming from well-mixed greenhouse gases between the 1750s and today”.

iii) “for the period 2000-2004, their assessment of the shortwave albedo decreased by 0.0015 which corresponds to an extra 0.5 Watts per meter squared of radiative imbalance according to their assessment.”

iv) “Recent model results indicate radiative forcings of +0.3 W m−2 in the Northern Hemisphere associated with albedo effects of soot on snow and ice”

v) There are a variety of direct and indirect aerosol effects that cause global warming including the black carbon direct effect, the semidirect indirect effect, and the glaciation indirect effect, with the thermodynamic effect having an unknown influence (see).

If we use the IPCC estimate of the fraction of the radiative forcing change of the well-mixed greenhouse gases from the pre-industrial to the present (i.e. see), which is about 2.4 Watts per meter squared, we can use the estimates of the radiative forcing from the other human climate forcings that are listed above to compare with this value, and with the fraction of the well-mixed greenhouse gas forcing that is due to CO2.

It need to be emphasized that the IPCC figure of the radiative forcing of 1.4 Watts per meter squared due to CO2 is not the current radiative imbalance since, presumably, some of the imbalance earlier in the industrial period with respect to CO2 increases has been removed as the climate system warmed. Nonetheless, these values can be used to scale the relative contribution to global warming due to the radiative effect of CO2. Also, since the observed radiative imbalance based on the 2004 Willis et al assessment is significantly less than the change from preindustrial to the present, the effect of human climate forcings that cool the climate system are, of course, also occurring.

With respect to the finding listed above, methane has a value of 0.8 Watts per meter squared, the shortwave albedo change is 0.5 Watts per meter squared, and the albedo effect of soot is 0.3 Watts per meter squared (which, however, may not be independent of the “shortwave albedo change). Tropospheric ozone, the aerosol black carbon direct effect, the semidirect indirect effect, and the glaciation indirect effect also add Watts per meter squared.

By summing the 0.8 Watts per meter squared for methane and using the total of 2.4 Watts per meter squared of the well-mixed greenhouse gases from the IPCC Report, the radiative contribution of CO2 reduces to about 46% of this component of radiative forcing (1.1 Watts per meter squared). The 46% value, of course, assumes that none of the radiative forcing of CO2 emitted earlier in the industrial period has equilibrated, so that the 46% is actually a high number, but is used here to be conservative.

For all of the human-caused warming radiative forcings, which includes the 0.5 Watts per meter squared value for the shortwave albedo change, and estimating tropospheric ozone as 0.3 Watts per meter squared, the aerosol black carbon direct effect as 0.2 Watts per meter squared, the black carbon on snow and ice as 0.3 Watts per meter squared, the semidirect indirect effect as 0.1 Watt per meter squared, and the glaciation indirect effect as 0.1 Watt per meter squared (with the latter two forcings using a nominal value, since these forcings are very poorly known), the contribution due to CO2 will fall to about 28%.

This analysis also ignores solar influences on the heating in which a published paper concludes,

“We estimate that the sun contributed as much as 45–50% of the 1900–2000 global warming, and 25–35% of the 1980–2000 global warming. ‘ (see). Even the IPCC estimates that there has been a warming influence from the Sun in their radiative forcing summary figure of about 0.25 Watts per meter squared (see). Adding this 0.25 Watts per meter squared value reduces the percent contribution of CO2 to about 26.5%.

This calculation does not mean that there is not merit in reducing the human input of CO2 into the atmosphere, but it does mean that even in the context of global warming, it is only a fraction of the actual positive radiative forcings.

This specific weblog focuses on the specific subset of climate variability and change that is referred to as “global warming” However, the assessment of radiative forcing directly is not the most appropriate procedure to use to assess global climate system heat changes. As was discussed in 2003 in “Heat storage within the Earth system”, the ocean heat content change is the proper metric to monitor.

Moreover, the observed ocean heat content changes have been spatially complex as has been discussed on this weblog (see and see). As reported based on the paper in 2004 by Willis et al that has been discussed on Climate Science (e.g. see and see),

“Maps of yearly heat content anomaly show patterns of warming commensurate with ENSO variability in the tropics, but also show that a large part of the trend in global, oceanic heat content is caused by regional warming at midlatitudes in the Southern Hemisphere.â€?

This heating is

“…centered on 40S is spread more uniformly over the water column and warms steadily throughout the entire time series…â€?

The climate science that is presented in this weblog summarizes one of the reasons for the conclusion that,

‘Attempts to significantly influence regional and local-scale climate based on controlling CO2 emissions alone is an inadequate policy for this purpose. ” (see).

The primary focus on carbon dioxide inappropriately deemphasizes the first order importance of the other climate system heat system forcings (both cooling and warming forcings), as well as does not address the spatially complex, and incompletely understood actual pattern of global climate system heat changes.