Why We Need Estimates Of The Current Global Average Radiative Forcing

Climate Science asked questions to Real Climate regarding Figure SPM.2 in the 2007 Statement for Policymakers in the weblog

http://www.climatesci.org/2007/12/18/question-the-weblog-real-climate/

http://www.climatesci.org/2007/12/19/follow-up-to-question-to-real-climate/

The questions are straightforward:

1. What are the radiative forcings that are plotted in Figure SPM.2? The figure label states they are “Global average radiative forcing (RF) estimates and ranges in 2005…”

2. What are the global average radiative forcings currently, if the caption in Figure SPM.2 is incorrect?

An evaluation of the current global radiative imbalance requires knowledge of the current radiative forcings and feedbacks, not the difference since 1750.

As shown in the Climate Science weblog

http://www.climatesci.org/2007/11/30/climate-metric-reality-check-1-the-sum-of-climate-forcings-and-feedbacks-is-less-than-the-2007-ipcc-best-estimate-of-human-climate-forcings/

the highest near-current estimate for the radiative imbalance (by Jim Hansen) is +0.85 Watts per meter squared (other estimates are smaller than this; e.g. see and see). To determine whether the globally averaged radiative feedbacks are positive or negative, the value of the current global radiative forcing is needed.

If this global radiative forcing is greater than +0.85 Watts per meter squared, the radiative feedbacks when averaged globally would be negative. If this forcing is less than +0.85 Watts per meter squared, the radiative feedbacks when averaged globally would be positive.

From the responses, it is clear that the data that is actually plotted in Figure SPM.2 of the 2007 IPCC report is the

“The difference in the global average radiative forcing (RF) estimates and ranges between 2005 and pre-industrial conditions defined at 1750…”

Using a footnote to correct the figure caption, rather than just correctly labeling the figure caption to start with, is misleading to a policymaker who looks at just the Figure.

This is an important issue. As James Annan stated in a reply on the weblog Stoat

âI think RP is really asking about the current radiative imbalance: while I do not think it is wrong or misleading to talk about total forcing (with a 1750 baseline) as the IPCC do, the other question is also interesting as it relates directly to warming âin the pipelineâ. Of course the answer is we do not know for sure, since it directly depends on the climate sensitivity (and even the effective climate sensitivity of the current climate state, which may be slightly different again). But a rough ballpark estimate would be that a little more than half of the total forcing (IPCC terminology) remains as a current imbalance (the âcommitmentâ? runs in the AR4 show the future warming due to this imbalance). Of course splitting this up further into the contribution of each component would then become rather arbitraryâ?.

Thus, while he writes that this is a “rough ballpark estimate”, his insight that

“…. a little more than half of the total forcing (IPCC terminology) remains as a current balance”,

is the type of answer that is being requested.

Gavin Schmidt on Real Climate also added constructively to this when he responded that

“I donât think it can be done robustly. A straight-forward apportioning based on the fractional contribution to the original forcing neglects the differing transient behaviour. For instance if one forcing agent rose quickly and stabilised, while another increased later, then the impact of each on the current imbalance should be weighted towards the latter. So thatâs no good. Maybe you could do it by examining the single forcing transient runs we did for our recent paper (table 1) and looking at the year 2000-2003 (say) imbalances in Ann/Net TOA radiation. Youâd need to check that the individual components do in fact add up to something close to the combined effect (not obviously true). However, different models might give quite different results, and you can only do this for forcings weâve run. Other groups didnât do as many single forcing experiments and so you might not be able to find another set of numbers to compare with. Attribution requires models however, and so I donât see how you could do it any other way.”

The reason that this issue is so important is that

“if one forcing agent rose quickly and stabilised, while another increased later…”

as Gavin wrote, than the fractional contribution to the current radiative imbalance is weighted towards the more recent forcings. Since CO2 has been rising since 1750, at least part of the radiative forcing of CO2 has equilibrated. Thus the claim that CO2 is 50% (or about 30% as estimated on Climate Science based on the 2007 IPCC figure SPM.2; see) is an overstatement of its actual current radiative forcing.

There are two major conclusions from this analysis:

1. The fractional contribution of carbon dioxide to the current global radiative heating is overstated in Figure SPM.2 of the 2007 IPCC report.

2. The estimates that the current global average radiative forcing and the current radiative imbalance are close in value provide further support that the global radiative feedbacks, including water vapor, are averaging out close to zero. As written by Issac Held and Brian Soden in a 2000 paper and discussed on Climate Science (see) âIf this feedback is, in fact, substantially weaker than predicted in current models, sensitivities in the upper half of this range would be much less likely, a conclusion that would clearly have important policy implicationsâ?.

I look forward to further weblogs (and, even more important, peer reviewed papers) refuting or supporting these two conclusions.

However, until and unless these two conclusions are disproved, it should be recognized that the 2007 IPCC report overstated the magnitude of global warming due to CO2, as well as failed to identify a serious shortcoming in the multi-decadal global climate models to accurately predict global average radiative feedbacks and forcings.

Comments Off on Why We Need Estimates Of The Current Global Average Radiative Forcing

Filed under Climate Change Forcings & Feedbacks