The Journal of Geophysical Research has published the paper
Murphy, D. M., S. Solomon, R. W., Portmann, K. H. Rosenlof, P. M. Forster, and T. Wong (2009), An observationally based energy balance for the Earth since 1950, J. Geophys. Res., 114, D17107, doi:10.1029/2009JD012105.
Their abstract reads
“We examine the Earth’s energy balance since 1950, identifying results that can be obtained without using global climate models. Important terms that can be constrained using only measurements and radiative transfer models are ocean heat content, radiative forcing by long-lived trace gases, and radiative forcing from volcanic eruptions. We explicitly consider the emission of energy by a warming Earth by using correlations between surface temperature and satellite radiant flux data and show that this term is already quite significant. About 20% of the integrated positive forcing by greenhouse gases and solar radiation since 1950 has been radiated to space. Only about 10% of the positive forcing (about 1/3 of the net forcing) has gone into heating the Earth, almost all into the oceans. About 20% of the positive forcing has been balanced by volcanic aerosols, and the remaining 50% is mainly attributable to tropospheric aerosols. After accounting for the measured terms, the residual forcing between 1970 and 2000 due to direct and indirect forcing by aerosols as well as semidirect forcing from greenhouse gases and any unknown mechanism can be estimated as 1.1 ± 0.4 W m2 (1s). This is consistent with the Intergovernmental Panel on Climate Change’s best estimates but rules out very large negative forcings from aerosol indirect effects. Further, the data imply an increase from the 1950s to the 1980s followed by constant or slightly declining aerosol forcing into the 1990s, consistent with estimates of trends in global sulfate emissions. An apparent increase in residual forcing in the late 1990s is discussed.”
On the topic of this paper, I recommend Bob Tisdale’s excellent weblog on an update on the Levitus et al ocean heat data. In terms of missing forcings and feedbacks in the Murphy et al paper, I recommend readers read Roy Spencer’s informative discussion of this issue at his weblog.
On the Murphy et al article, I e-mailed to Josh Wills and asked him a set of questions as well as made comments. These are written below with my text in bold face and his in italics. In the first e-mail we wrote to each other
What is your view on this new analysis? Have you been able to update your assessment? I plan to post on this next week, but would value your feedback and comments first.
Well, at first I was pretty skeptical about those results because the cooling in the Atlantic brings back bad memories of Argo floats that were biased cold there. In addition, I think we have to be very careful about any results that span the transition from the XBT network to Argo because I know that there are still systematic errors in the XBT dataset. Although we have all made attempts to reduce these errors, I don’t think anyone can claim to have eliminated all of them. So, we need to be cautious here.
That said, I also see a sort of transition in the North Atlantic from a period of rapid warming from the mid-1990s through about 2004, followed by a slight cooling during 2005 and 2006 and it has pretty much been level since then. This seems to agree well with the average over the altimeter data for the North Atlantic. So, perhaps the recent cooling of the North Atlantic is real.
The following set of e-mails are extracted and rewritten below:
1. The assumption that
“For the purposes of this paper we estimate that from 1950 to 2003 the increase in the heat content of the ocean deeper than 700 m was 40 +/- 15% of the increase from 0 to 700 m”
is rather cavalier, and this arbitrariness is compounded by their claim that
“For a given year, the deep ocean heat content is scaled to the heat content above 700 m averaged over the preceding 10 years.”
If the deeper ocean is actually such a large store of heat, this presumably is a sink that is not readily (or quickly) available to the atmospheric part of the climate system including the surface land temperatures.
Well there are regions such as the North Atlantic where convection occurs to depths of 2000 m or so most every year. Over longer time scales, these waters are advected away and become detached from the atmospheric connection, but I think that using the 10 year average rate of warming in the surface waters and scaling it down is a very reasonable assumption in lieu of actual data there.
Well, the rate of deep heating is really not known, and some assumption must be made. The long term trends in the deep ocean from Levitus suggest that the 700 m to 3000 m heat content is a small fraction of the 0 to 700 m rate. So I think this is not unreasonable.
2. They also claim that their data is corrected adequately
“There are several recent calculations of observed ocean heat contents from the surface to 700 m depth [Domingues et al., 2008; Ishii and Kimoto, 2009; Levitus et al., 2009]. In each study, temperature profiles were converted to estimates of the ocean heat content. Each study also corrects expendable bathy-thermograph (XBT) measurements using fall rate or empirical corrections. These corrections make the heat content estimates more accurate than previous estimates using similar data [Wijffels et al., 2008].”
This claim ignores your concerns on the remaining uncertainties in the data.
They also ignore the puzzling sudden jump in the Levitus et al data just prior to 2003, which, to my knowledge is not supported by other data analyses. It certainly cannot be due to one of the longer term climate forcings, even if it were true.
Well, I think that their comment is correct. These estimates ARE more accurate than previous estimates because they at least attempt to address the known errors. I think there is more work to do and that the biases in XBT data can be further reduced, but for the time being, those are the best estimates. I have seen plots overlaying all of the recent estimates and although the Levitus et al jump in 2003 seems a bit large to me, it is not WAY outside the scatter of all the estimates of recent ocean heat content change.
3. Their acceptance of the surface temperature trend as quantitatively robust is flawed, where they write
“First, we use lamda to predict net outgoing radiation from observed temperature changes and ordinary regression against temperature matches this use”
This ignores the major unresolved issues that we have found with this surface temperature metric: e.g.,
There clearly is a warm bias in the surface temperature data as we have shown in Klotzbach et al.
4. Rather than cite and discuss peer-reviewed papers that conflict with their paper; e.g., see their text
they use the popular media to make their point.
The available peer reviewed papers includes most recently
Douglass, D.H. and R. Knox, 2009: Ocean heat content and Earth’s radiation imbalance. Physics letters A.
5. Their Figure 6 looks like an example of knowing the answer (e.g. the ocean heat storage change) and then making the different forcings to fit. Their units are not even correct; 10** 21 Joules on their axes is not a forcing but what they claim to be the accumulation of heat over time. An appropriate figure for them to show would be the forcing of each term as the global average watts per meter squared. Converting Figure 6 to this units would be informative.
6. There is another problem with their Figure 6. In the right hand figure, they mix what they label as forcings with feedbacks. The outgoing radiation is a feedback.
I am not really familiar enough with the surface temperature data or the attribution of various forcings to feel comfortable commenting on your blog. However, I think their use of the ocean heat content data was reasonable. I also think that it makes sense to look at things in terms of their time-integrated values in order to make comparisons with ocean heat content. This is the part of the reason that ocean heat content estimates are so valuable–because the ocean accumulates the net radiative imbalance over time.
As always, you are more than welcome to use my emailed comments on your blog.
I want to thank Josh for engaging in a constructive discussion even though we disagree on the merits of the Murphy et al paper. As another issue with the Murphy et al. paper that I did not present in my e-mails, note that Figure 6 has exponentially increasing positive and negative radiative forcings. This behavoir conflicts for what we know about the radiative forcings of CO2 and other well-mixed greenhouse gas forcings. In the real world, they are logarithmic with increasing atmospheric concentration (due to the approach to saturation of the absorption lines in the long wave radiative spectrum) . They actually behave as Murphy et al. presented in Figure 1. Apparently, the authors ignored this in order to create Figure 6.
With respect to Figure 6, as was shown in
Ellis et al. 1978: The annual variation in the global heat balance of the Earth. J. Climate. 83, 1958-1962
Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335
Pielke Sr., R.A., 2008: A broader view of the role of humans in the climate system. Physics Today, 61, Vol. 11, 54-55
it is the heat storage change that is really global warming. Their plot of the ocean heating concludes that there was a change of heat of 200 * 10**21 Joules over a period of 54 years. This corresponds to 0.37 x 10**22 Joules per year which yields a radiative imbalance (i.e. global warming rate averaged over the 54 years) of 0.23 Watts per meter squared. The authors also (conveniently) end their analysis in 2004. Since 2003, as presented in the figure in my Physics Today article from the data analysis provided by Josh Willis, the global warming rate (their ocean heating term) up to at least the end of 2008 was essentially zero.