Response To A Question On Skeptical Science On The Fraction Of Positive Radiative Forcing From CO2

Skeptical Science has heeded my request for a more civil debate on the climate issues in their post

Pielke Sr. and SkS Disagreements and Open Questions

This is reaching out to build a bridge to discuss these issues is welcomed.

Today, I will respond to their question:

How did I estimate ~28% as the fraction of the current global annual average human-forced positive radiative forcing?

However, I want to emphasize that this is actually a very difficult question to answer.  The 2007 IPCC report did not provide their estimate of the current global average radiative forcing but, as they wrote

“In this report, radiative forcing values are for 2005 relative to pre-industrial conditions defined at 1750 and are expressed in watts per square metre.”

I presented the reasoning for my back-of-the envelope estimate of the fraction of positive radiative heating from human-added CO2 in my post on April 27 2006

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

[the text is slightly garbled as I have since 2006 switched the program that processes my weblog].

I summarized this view in my powerpoint talk

Pielke, R.A. Sr., 2006: Regional and Global Climate Forcings. Presented at the Conference on the Earth’s Radiative Energy Budget Related to SORCE, San Juan Islands, Washington, September 20-22, 2006.

i) Ozone was responsible for one-third to one-half of the observed warming trend in the Arctic during winter and spring [Drew Shindell]
 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. [Drew Shindell and colleagues; Keppler et al.]
 iii) For the period 2000-2004, a CERES Science Team assessment of the shortwave albedo found a decrease by 0.0015 which corresponds to an extra 0.5 W m−2 of radiative imbalance according to their assessment. [CIRES Science Team]
 iv) 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]
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 (NRC 2005).

(these findings are summarized at http://climatesci.atmos.colostate.edu/2006/04/27/what-fraction-of-globalwarming-is-due-to-the-radiative-

In Watts per meter squared [a back-of-the-envelope estimate]

Methane +0.8

Short-wave albedo change +0.5

Tropospheric ozone +0.3

Aerosol black carbon +0.2

Black carbon on snow and ice +0.3

Semi-direct aerosol effect +0.1

Glaciation effect +0.1

Solar influences +0.25

The CO2 contribution to the radiative warming decreases to 26.5% [human + natural] using the IPCC framework given in Slide 9

I also have other related posts; e.g. see

Comment On An Error In A Figure Caption In The Statement For Policymakers In The WG1 2007 IPCC Report

News Release On Soot Effects On Climate In The Himalayas – Its Larger Than the Forcing From The Human Input Of CO2

There is no doubt that the estimate of the fraction of positive radiative heating needs a lot of work. What we really need is not the difference from pre-industrial, but the radiative forcing in 2011. Some of the radiative forcing from the radiative forcings would have been accomodated by warming during this time period [at the meetings which culminated in NRC (2005), V. Ramanthan estimated to me when I asked him that ~20% of the radiative forcing from CO2 would have been compensated by the warming of the climate system in the intervening years. This would reduce the 2007 IPCC value by 20%.

The way this question should be addressed (albeit still incompletely), in my view, is to do the following:

1.  Use a column radiative transfer model (for all wavelengths – i.e. short and long wave) on a vertical profile of temperature, humidity and clouds at a sufficient number of locations (grid points) around the world (using all global reanalysis grid points) during a year (with hourly time intervals) to determine the baseline current radiative forcing. If resouces permit, do more than one year. Calculate the global average radiative forcing by integrating over the year at each grid point. While radiative feedbacks, of course, are implicit in the vertical profiles, the radiative transfer model provides the instantaneous forcing at that time.

2.  Use the column radiative transfer model with these same soundings but  [sensitivity test #1] change the CO2 level back to pre-industrial,  [sensitivity test #2] change the aerosol load back to preindustrial,  [sensitivity test #3]  change the land cover back to natural, etc and express the values in Watts per meter squared.

3. For each of these sensitivity tests, sum up the differences in radiative forcings to obtain the global annual average in Watts per meter squared.

Several years ago we did a very preliminary analysis of this type of approach for CO2 and for water vapor completed by Norm Woods, who worked with Graeme Stephens at Colorado State University at the time. This approach is a very preliminary evaluation of the more complete method I propose above.

I  discussed the relative role of the radiative forcings of added CO2 and water vapor  from that study in

Relative Roles of CO2 and Water Vapor in Radiative Forcing

Further Analysis Of Radiative Forcing By Norm Woods

Among our conclusions in these two posts are:

1. The effect of even small increases in water vapor content of the atmosphere in the tropics has a much larger effect on the downwelling fluxes, than does a significant increase of the CO2 concentrations. Thus, the monitoring of multi-decadal water vapor trends in the tropics should be a high priority. While the increase in CO2 concentrations, and resulting increase in downwelling longwave flux can result in surface ocean warming, and thus increase evaporation into the atmosphere, it is the atmospheric water vapor signal that should be monitored for long term trends, as it is the dominant greenhouse gas that has the greater climate response.

2. The fractional contribution of the effect of added CO2, relative to a 5% increase of water vapor in the subarctic winter is significantly larger than in the tropical sounding. This is because the subarctic sounding is quite dry. An increase in absolute terms of water vapor similar to a 5% increase in the tropical sounding would, however, dominate the increase of downwelling longwave fluxes. This again indicates that the assessment of long term water vapor atmospheric concentrations needs to be a climate science priority.

In August 2007, I made the following request to Gavin Schmidt

Request To Scientists Recommended By Gavin Schmidt To Assess The Relative Role of Human Climate Forcings In Altering Weather and Other Aspects of Climate

As I wrote in this post

“The fundamental hypothesis is that the more spatially heterogeneous human climate forcings, such as from land use/land cover change and aerosols, have as much, or more, of an effect in altering regional weather (and other aspects of climate), than does the more spatially homogenous effect of the radiative effect of added CO2 and other well-mixed greenhouse gases. This issue, which was reported in the 2005 NRC Report, was not explored by the 2007 IPCC Report in the format such as we present in the Matsui and Pielke paper, where it is the gradient of radiative forcing that needs to be quantified, in addition to the top-of-the-atmosphere radiative forcing.”

My question back to Skeptical Science, is to present your perspective on this issue. How would you propose including the assessment of the effect of each of the human climate forcings in terms of their effects on regional atmospheric and ocean circulations (i.e. by altering the pressure gradients through diabatic heating due to the radiative heating/cooling from CO2 and the other greenhouse gases, aerosols and land use/land cover change?

I recommend as a starting point for discussion on this issue, our paper

Matsui, T., and R.A. Pielke Sr., 2006: Measurement-based estimation of the spatial gradient of aerosol radiative forcing. Geophys. Res. Letts., 33, L11813, doi:10.1029/2006GL025974.

The abstract reads [highlight added]

This paper diagnoses the spatial mean and the spatial gradient of the aerosol radiative forcing in comparison with those of well-mixed green-house gases (GHG). Unlike GHG, aerosols have much greater spatial heterogeneity in their radiative forcing. The heterogeneous diabatic heating can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate. For this, we diagnose the Normalized Gradient of Radiative Forcing (NGoRF), as a fraction of the present global heterogeneous insolation attributed to human activity. Although the GHG has a larger forcing (+1.7 Wm-2) as measured than those of aerosol direct (-1.59 Wm-2) and possible indirect effect (-1.38 Wm-2) in terms of a spatially averaged top-of-atmosphere value, the aerosol direct and indirect effects have far greater NGoRF values (~0.18) than that of GHG (~0.003).

I will be posting two specific questions to Skeptical Science  this coming week in order to continue this constructive dialog.

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