Radiative Forcing, Radiative Feedbacks and Radiative Imbalance – The 2013 WG1 IPCC Report Failed to Properly Report on this Issue

Guest essay by Roger A. Pielke Sr.

Main Points

1. The difference in ocean heat content at two different time periods provides the global average radiative imbalance over that time [within the uncertainty of the ocean heat measurements]

2. This global average radiative imbalance is equal to the sum of the global average radiative forcings and the global average radiative feedbacks.

3. The global average radiative forcing change since 1750 is presented in the 2013 IPCC WG1 Figure SPM.5 as 2.29 [1.13 -3.33] Watts per meter squared.

4. The global average radiative imbalance is given in the 2013 IPCC report as 0.59 Watts per meter squared for 1971-2010 while for 1993-2010 it is 0.71 Watts per meter squared.

5. Thus, assuming that a large fraction of the global average radiative forcing change since 1750 is still occurring, the global average radiative feedbacks are significantly less than the global average forcings; i.e. a negative feedback.

6. Such a negative feedback is expected (since the surface temperature, and thus the loss of long wave radiation to space would increase).

7. However, the water vapor and cloud radiative feedback must also be part of the feedback. This water vapor feedback is a key claim in terms of amplifying warming due to the addition of CO2 and other human inputs of greenhouse gases. The IPCC claims that the net cloud radiative feedback is also positive.

8. The IPCC failed to report on the global average radiative feedbacks of water vapor and clouds in terms in Watts per meter squared, and how they fit into the magnitude of the diagnosed global average radiative imbalance.

9. The reason is likely that they would to avoid discussing that in recent years; at least, there has been no significant addition of water vapor into the atmosphere. Indeed, this water vapor feedback, along with any other feedbacks must be ALL accommodated within the magnitude of the global average radiative imbalance that is diagnosed from the ocean heating data!

It certainly appears that, even using the 2013 IPCC WG1 assessment estimates, that the vapor amplification of global warming is not, as least yet, occurring.

I explain and elaborate on these issues below.

Introduction

As I wrote above, the 2013 WG1 IPCC assessment of the magnitude of the radiative forcings on the climate system persists in missing discussing a key fundamental issue, namely the estimated magnitudes of

· the global annual average radiative imbalance,

· the global annual average radiative forcing

· the global annual average radiative feedbacks

and how these quantities are related to each other.

Section 1 The Fundamental Budget Equation

The relationship between the annual global average radiative forcings, radiative feedbacks and radiative imbalance can be expressed by this budget equation

Radiative Imbalance = Radiative Forcing + Radiative Feedbacks

where the units are in Joules per time period [and can be expressed as Watts per area].

The fundamental difference with this approach and that presented in papers such as Stephens et al (2012) – see http://bobtisdale.files.wordpress.com/2013/10/05-figure-1-from-stephens-et-al-2013.png

is that instead of computing the radiative imbalance as a residual as a result of large positive and negative values in the radiative flux budget with its large uncertainty as shown by Stephens et al, this metric is a robust constraint on the analysis of the radiative fluxes.

As Bob Tisdale reports, the Stephens et al value of the global average radiative imbalance [which Stephens et al calls the “surface imbalance”] is 0.70 Watts per meter squared, but with the large uncertainty of 17 Watts per meter squared!

The Stephens et al paper is

Stephens et al, 2012: An update on Earth’s energy balance in light of the latest global observations. Nature Geoscience 5, 691–696 (2012) doi:10.1038/ngeo158 http://www.nature.com/ngeo/journal/v5/n10/abs/ngeo1580.html [as an aside, that paper, unfortunately, makes the typical IPCC type mistake in stating that the

“Climate change is governed by changes to the global energy balance.”

Changes in the climate system on any time scale is much more than just any changes in the global energy budget as we discuss, for example, in

Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell,  W. Rossow,  J. Schaake, J. Smith, S. Sorooshian,  and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union. http://pielkeclimatesci.files.wordpress.com/2009/12/r-354.pdf

Section 2 The Radiative Imbalance

With respect to the Radiative Imbalance, as I proposed in my paper

Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331- 335.

http://pielkeclimatesci.files.wordpress.com/2009/10/r-247.pdf

the radiative imbalance can be estimated based on the changes in the ocean heat content. As written in

Levitus, S., et al. (2012), World ocean heat content and thermosteric sea level change (0-2000), 1955-2010, Geophys. Res. Lett.,doi:10.1029/2012GL051106

“The world ocean accounts for approximately 90% of the warming of the earth system that has occurred since 1955”

Jim Hansen had provided his value of the heating rate in a communication to me in 2005 http://pielkeclimatesci.files.wordpress.com/2009/09/1116592hansen.pdf]

as

The Willis et al. measured heat storage of 0.62 W/m2 refers to the decadal mean for the upper 750 m of the ocean. Our simulated 1993-2003 heat storage rate was 0.6 W/m2 in the upper 750 m of the ocean. The decadal mean planetary energy imbalance, 0.75 W/m2 , includes heat storage in the deeper ocean and energy used to melt ice and warm the air and land. 0.85 W/m2 is the imbalance at the end of the decade.”

More recent information, with respect to the Radiative Imbalance is reported in

Levitus, S., et al. (2012), World ocean heat content and thermosteric sea level change (0-2000), 1955-2010, Geophys. Res. Lett.,doi:10.1029/2012GL051106

“The heat content of the world ocean for the 0-2000 m layer increased by 24.0×1022 J corresponding to a rate of 0.39 Wm-2 (per unit area of the world ocean)…. This warming rate corresponds to a rate of 0.27 Wm-2 per unit area of earth’s surface.”

The IPCC WG1 Chapter 3 [http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter03.pdf]

writes

“It is virtually certain that Earth has gained substantial energy from 1971–2010 — the estimated increase in energy inventory between 1971 and 2010 is 274 [196 to 351] ZJ (1 ZJ = 1021 J), with a rate of 213 TW from a linear fit to the annual values over that time period (Box 3.1, Figure 1). Ocean warming dominates the total energy change inventory, accounting for roughly 93% on average from 1971–2010. Melting ice (including Arctic sea ice, ice sheets, and glaciers) accounts for 3% of the total, and warming of the continents 3%. Warming of the atmosphere makes up the remaining 1%. The 1971–2010 estimated rate of oceanic energy gain is 199 TW from a linear fit to data over that time period, implying a mean heat flux of 0.55 W m–2 across the global ocean surface area. Earth’s net estimated energy increase from 1993–2010 is 163 [127 to 201] ZJ with a trend estimate of 275 TW. The ocean portion of the trend for 1993–2010 is 257 TW, equivalent to a mean heat flux into the ocean of 0.71 W m–2.”

Using the 93% dominance of the ocean in this heating, then from the 2013 IPCC report

· 1971-2010 the total earth surface heating rate is 0.59 Watts per meter squared

· 1993-2010 it is 0.71 Watts per meter squared.

Of course, there is the question as to whether the Levitus et al 2012 calculation below 700 meters before 2005 is even robust. The 2013 IPCC WG1 Chapter 3 report writes [highlight added] – http://www.climatechange2013.org/images/uploads/WGIAR5_WGI-12Doc2b_FinalDraft_Chapter03.pdf

“Below 700 m data coverage is too sparse to produce annual global ocean heat content estimates prior to about 2005, but from 2005–2010 and 0–1500 m the global ocean is warming (von Schuckmann and Le Traon, 2011). Five-year running mean estimates yield a 700–2000 m global ocean heat content trend from 1957 to 2009 (Figure 3.2b) that is about 30% of that for 0–2000 m over the length of the record (Levitus et al., 2012). Ocean heat uptake from 700–2000 m continues unabated since 2003 (Figure 3.2b); as a result, ocean heat content from 0–2000 m shows less slowing after 2003 than does 0–700 m heat content (Levitus et al., 2012). “

Remarkably, the IPCC report persists in making claims regarding deeper ocean heating before 2005. But that is a subject for another time.

Section 3 The Radiative Forcing

Using even the largest value [the 0.85 W/me value for the Radiative Imbalance from Jim Hansen], however, it is still significantly less than the total anthropogenic change in radiative forcing since 1750 reported by the IPCC.

In Figure SPM.5 in the 2013 IPCC WG, they report that the total anthropogenic change in radiative forcing since 1750 is

2.29 [1.13 -3.33] Watts per meter squared.

They write that

“The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750.”

and

“The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m−2 (see Figure SPM.5), and it has increased more rapidly since 1970 than during prior decades.”

Unfortunately, the IPCC did not provide an estimate of the CURRENT “total anthropogenic RF”.

Some of this forcing would have been accommodated with warming of the climate system since 1750. When I served on the NRC (2005) assessment [http://www.nap.edu/openbook/0309095069/html/], one of my colleagues on the Committee (V. Ramanthan), when I asked him this question, he said that perhaps 20% of the CO2 radiative forcing was already equilibrated to. In any case, the CURRENT forcing must be somewhat less, but not probably by more than 20% or so.

Regardless, unless the IPCC estimates of the Radiative Forcing are too positive, this means that the

Radiative Imbalance < Radiative Forcing.

4. Radiative Feedbacks

However, while the warming of the climate system is a negative radiative feedback, and thus we should expect this part to be a negative feedback [since a surface temperature results in an increase of the outgoing long wave radiation to space], added water vapor, if it is there, would be a positive radiative feedback.

In my book

Cotton, W.R. and R.A. Pielke Sr., 2007: Human impacts on weather and climate, Cambridge University Press, 330 pp

in Section 8.2.8 we reported on an analysis of the water vapor feedback by Norm Woods using column assessments for three selected vertical soundings. Norm showed that the positive significant radiative forcing from even modest (e.g. 5% increase is atmospheric water vapor) is significant. [See also http://pielkeclimatesci.wordpress.com/2006/05/05/co2h2o/].

Norm Woods’s further analysis can be read on this posts

http://pielkeclimatesci.wordpress.com/2007/08/24/further-analysis-of-radiatve-forcing-by-norm-woods/

Among the conclusions for the representative soundings Norm used are

“with the tropical sounding ….adding 5% more water vapor, results in a 3.88 Watts per meter squared increase in the downwelling longwave flux. In contrast, due to the much lower atmospheric concentrations of water vapor in the subarctic winter sounding, the change from a zero concentration to its current value results in an increase of 116.46 Watts per meter squared, while adding 5% to the current value results in a 0.70 Watts per meter squared increase.”

and

“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.”

However, there appears to be no long trend in atmospheric water vapor! This can be seen in the latest analysis we have;

Vonder Haar, T. H., J. L. Bytheway, and J. M. Forsythe (2012), Weather and climate analyses using improved global water vapor observations, Geophys. Res. Lett., 39, L15802, doi:10.1029/2012GL052094. [http://onlinelibrary.wiley.com/doi/10.1029/2012GL052094/abstract]

Although they write in the paper

“at this time, we can neither prove nor disprove a robust trend in the global water vapor data.”

just the difficulty in showing a positive trend suggests a very muted water vapor feedback at most.

The figure from their paper with respect to this analysis is shown below

The 2013 IPCC WG1 SPM report states with respect to the radiative feedbacks that

“The net feedback from the combined effect of changes in water vapour, and differences between atmospheric and surface warming is extremely likely positive and therefore amplifies changes in climate. The net radiative feedback due to all cloud types combined is likely positive. Uncertainty in the sign and magnitude of the cloud feedback is due primarily to continuing uncertainty in the impact of warming on low clouds.” [http://www.climatechange2013.org/images/uploads/WGIAR5SPM_Approved27Sep2013.pdf]

They also write, in contrast to what is seen in the Vonderhaar et al 2012 paper,

“Anthropogenic influences have contributed to observed increases in atmospheric moisture content in the atmosphere (medium confidence)”

This report also write that

“The rate and magnitude of global climate change is determined by radiative forcing, climate feedbacks and the storage of energy by the climate system.”

Of course the report also fails to distinguish “global climate change” [which is much more than just the global average radiative forcings and feedbacks; a mistake also made in Stephens et al 2012].

The IPCC WG1 report discuss the reduced heating and Radiative Forcing in recent years as follows

“The observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols)…”

Nowhere in this discussion, except implicitly in the mention of internal variability, is the role of the radiative feedbacks including the role of water vapor and clouds presented.

5. The IPCC Failure

The IPCC report has failed to report on the implications of the real world radiative imbalance being significantly smaller than the radiative forcing. This means not only that the net radiative feedbacks must be negative, but they failed to document the magnitude in Watts per meter squared of the contributions to positive feedbacks from surface warming, and from atmospheric water vapor and clouds.

These must be smaller than what the IPCC models are producing.

One clear conclusion from their failure is that the climate system has larger variations in the Radiative Imbalance, Forcing and Feedbacks than is predicted by the model and accepted in the 2013 IPCC assessment report. Judy Curry David Douglass, Roy Spencer, Bob Tisdale, Anastasios Tsonis, Marcia Wyatt and others have been pioneers in advocating this perspective, and the failure in the SPM of the 2013 IPCC WG1 report to discuss this issue is a major failing of the assessment.