Response to Andy Revkin’s Science Question of August 26, 2005

“Is most of the observed warming over the last 50 years likely to have been due to the increase in greenhouse gas concentrations”

On Global Warming:

There are natural explanations for global warming of which a change in the output of solar energy is a candidate. However, none of the published work has convinced me that this can explain much of the observed global warming over the last several decades. Volcanic emissions are another natural global forcing, and it is well known that they produce cooling, such as after the eruption of Mount Pintatubo, where in August of 1991 it was estimated as -4 Watts per meter squared. There have not been eruptions of that magnitude since, such that the absence of such major eruptions might permit greater absorbed solar radiation in the climate system than otherwise would occur. However, this absence of eruptions resulting in any positive radiative imbalance for a period of time well after a major volcanic emission has also not been shown to occur. This leaves anthropogenic emissions as a source for global warming.

There are multiple first-order anthropogenic contributors to global warming in addition to the well-mixed greenhouse gases. As identified by the IPCC (see their summary figure which is reproduced as ES-2 in Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties ) the significant human-caused radiative forcings that produce warming that they recognized were the well-mixed greenhouse gases, tropospheric ozone and mineral dust, with a large uncertainty in the later forcing. This figure has been used to show that well-mixed greenhouse gases dominant the global warming signal. The figure does correctly state that most of the radiative forcings that they list have “a very low level of scientific understanding”, although this qualification is usually lost when references are made to the relative importance of the well-mixed greenhouse gases.

However, recent work has complicated the IPCC conclusion. First, the figure from the IPCC is not the current radiative imbalance from each of the forcings, but a difference since pre-industrial times. There is not a current well-mixed greenhouse gas forcing of 2.4 Watts per meter squared as given in that Figure. As I discussed in 2003 (see Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335 ) only part of the radiative effect of the increase in the well-mixed greenhouse gases remains as part of the radiative imbalance from earlier years presumably has been adjusted for as the climate system has warmed over the past century.

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.”

While this does not specifically address your question, it indicates that even the well mixed greenhouse gas contributions to the radiative imbalance are not as well understood as indicated in the IPCC figure.

With respect to other major global warming forcings, the National Research Council 2005 report has broken the indirect radiative forcing identified in Figure ES-2 into 6 categories of which two have global warming radiative forcing effects: the semi-indirect effect and the glaciation indirect effect, and one has an unknown global warming effect. The magnitude of their effects is uncertain.

Even the direct radiative effect from black carbon (BC) has been shown to be more complicated than summarized in the IPCC figure. For example, as reported in the National Research Council report,
“A portion of the direct solar beam is absorbed by the aerosol, and this atmospheric absorption leads to further reduction in solar radiation reaching the surface. As shown later, this shielding of the surface by BC is the dominant absorption term for anthropogenic aerosols with as little as 10 percent of BC. This absorption leads to a positive radiative forcing of the atmosphere and a negative radiative forcing of the surface.”
Also, from this report, “Black carbon emissions may have increased by a factor of two to four during the last 50 years (Novakov et al. 2003).” At the surface, the deposition of black carbon can also be a major global warming effect. As reported in the National Research Council report,

“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).”

There are also complex, poorly understood temporal changes in the albedo of the climate system which has a significant effect on global warming. 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.

Each of these global warming climate forcings make the percent attribution of well-mixed greenhouse to global warming a much more challenging problem than is implied by the IPCC figure reproduced in Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties .

This is the basis for my conclusion that the observed global warming involves the integral effect of a diversity of positive and negative radiative climate forcings, and is not dominated by the well-mixed greenhouse gases. Whenever anyone makes the statement that the predominance of global warming is from the well-mixed greenhouse gases, or CO2 by itself, oversimplifies a much more complicated explanation for global warming. Indeed, it is the integrated effect of all of the climate forcings which determine whether we have anthropogenic global warming or cooling. It would be only fortuitous if these forcings balanced to zero. Thus, addressing the policy issues associated with global warming by the policy community is much more difficult due to the number of climate forcings which affect the heat budget of the climate system.

On Climate Change

To add even more complication, global warming is only one component of climate change. As discussed in our July 28th blog (What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?), it is the alteration of atmospheric and ocean circulations as a result of the diversity of climate forcings which have a larger impact on the climate that we experience, than can be described by the metric of the total climate system heat change. The climate forcing of land-use/land-cover change is just one example of such a climate forcing. As shown in a a variety of papers (e.g. see Chase et al. 2000 and Chase et al. 2001 ), there are large regional changes in weather patterns due to landscape change as simulated in the models with implications on whether a region warms or cools, and becomes wetter or drier over time. This occurs despite little global average heat changes associated with land-use/land-cover change, since areas of cooling balance with areas with warming. We can see the importance of atmospheric circulation changes in hurricane tracks. Whether the USA is pummeled by landfalling hurricanes such as Katrina or recurves offshore depends on the regional tropospheric wind field not a global average metric.

Thus we limit the communication to policymakers if we use climate change as a synonym for global warming. Global warming is just one aspect of a much more complicated environmental issue.

Constructive science comments on this blog are welcome.

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