UPDATE September 26 2008: I have requested that Dr. Ramanathan write an unedited guest reply to the Climate Science weblog below, but so far have not had a response. If he accepts my invitation, Climate Science will promptly publish on this website.
There is a new paper by a eminent and distinguished climate scientist, Dr. Ramanathan in the Proceedings of the National Academy of Sciences. [Thanks to David Douglass for alerting us to this new paper!]. The paper is
Ramanathan, V. and Y. Feng, 2008: On avoiding dangerous anthropogenic interference with the climate system: Formidable challenges ahead, PNAS, 105, 14245-14250, Sept 23, 2008.
with the abstract
“The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial era has most likely committed the world to a warming of 2.4°C (1.4°C to 4.3°C) above the preindustrial surface temperatures. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4°C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols.The range of 1.4°C to 4.3°C in the committed warming overlaps and surpasses the currently perceived threshold range of 1°C to 3°C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan–Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that ≈25% (0.6°C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6°C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO2mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4°C.”
I agree with Dr. Ramanthan that the addition of carbon dioxide into the atmosphere is a significant positive (warming) radiative forcing. Until the last few years, at least one of the global models (the GISS model projections) accurately simulated the long term upper ocean heat content; see
Unfortunately, the Ramanthan and Feng PNAS paper does not use the more appropriate metric of ocean heat content changes as a diagnostic of global warming, but perpetuates a very significant misunderstanding of the Earth’s radiative fluxes by using the global average surface temperature anomaly.
When I served with Dr. Ramanthan on the 2005 National Research Council committee that produced the book
National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp
we discussed the issue of “heat in the pipeline” and “unrealized heat” and I assumed we had come to an agreement on this. The term “committed warming” is not used in the 2005 NRC report.
As has been discussed on Climate Science; see
there is no “committed” heat in the climate system.
They are confusing the concept of “committed”heat with that of a non-equilibrium radiative imbalance.
What they are assuming is that the difference in the IPCC multi-decadal global model simulations from the radiatively unforced runs (i.e. no human-caused radiative forcing) and the radiatively forced runs is the “committed” heat and that this will continue into the future.
However, the IPCC models have not been shown to accurately predict the difference in incoming and outgoing radiative fluxes at the tropopause or surface in comparison with observed values of these fluxes.
This is why the ocean heat content changes are so useful as they provide a mechanism to diagnose this imbalance.
Dr. Ramanathan even agreed with this viewpoint in the 2005 National Research Council report which he co-authored. It is written on page 98 of that report
“The ocean is the largest heat reservoir in the climate system (Levitus et al., 2000, 2001). Thus, the change in ocean heat storage with time can be used to calculate the net radiative imbalance of the Earth (Ellis et al., 1978; Piexoto and Oort, 1992). In essence, the ocean heat content provides a metric for the integral in time of the TOA radiative forcing. Furthermore, it offers a valuable constraint on the performance of climate models (Barnett et al., 2001).”
In lieu of the more robust metric of global warming that is given by upper ocean heat content changes, the “global average surface temperature” concept (e.g. see Equation 1-1 on page 19 in NRC 2005) is used by Ramanathan and Feng despite their recognition of its limitations.
The Ramanthan and Feng paper is, therefore, misleading. There is no “unrealized heat” or “heat in the pipeline”. What they must show is that the radiative imbalances are persisting in the observations.
They misapply a concept that, while appropriate for a pot of water on the stove with a burner turned on to heat the pot, is too simplistic to apply to the climate system for the following reasons:
1. The simple pot analog that Ramathan uses implicitly assumes a static climate system in which the radiative forcing is nearly constant (the “unforced” condition in the multi-decadal global climate model simulations). This is clearly not true even on the annual time scale in which significant (over 30 Watts per meter squared) global average radiative imbalance occurs each year as a result of the different distance of the Earth from the Sun during the year; e.g. see
Ellis et al. 1978: The annual variation in the global heat balance of the Earth. J. Climate. 83, 1958-1962.
Ramanthan and Feng are basing their assumption on a long term nearly static radiative imbalance on models, not observations. This is a circular argument as the models are hypotheses only.
2. The use of the term global average surface temperature anomaly is misleading as, unlike a pot of water, the surface temperature anomaly is i) spatially varying (e.g. see), ii) its effect on the radiative imbalance is proportional to temperature to its fourth power (T**4), e.g. see, and iii) the surface is often not thermodynamically coupled to the rest of the climate system including the troposphere (e.g. see Section 2 in
Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229.
3. The recent studies
Spencer, R.W., and W.D. Braswell, 2008: Abstract- Feedback vs. Chaotic Radiative Forcing: “Smoking Gun” Evidence for an Insensitive Climate System? Download Powerpoint of presentation
Douglass, D.H., and J.R. Christy, 2008: Limits on CO2 Climate Forcing from Recent Temperature Data of Earth. Energy and Environment, accepted. [although not a standard journal, their analysis still needs to be responded to with respect to the Ramanathan and Feng paper]
as well as our analysis; see slide 27 in
have raised issues on the magnitude of the feedbacks, particularly atmospheric water vapor increases, but also snow and sea ice albedo decreases, which, according to the IPCC global models, are expected to amplify the positive radiative forcing of the well-mixed greenhouse gases, including carbon dioxide.
The dominant radiative feedback is supposed to be an increase in atmospheric water vapor, but, at most, this increase is muted; e.g. see
These feedbacks do not exist in a pot of water.
4. The statement by Ramanthan and Feng that “ IPCC models suggest that ≈25% (0.6°C) of the committed warming has been realized as of now” illustrates that they assume that the climate system has an equilibrium radiative balance, when in reality it does not. Moreover, they are using the IPCC models to evaluate how out-of-radiative-balance the climate system is, yet those models fail to adequately simulate the radiative feedbacks, nor even have all of the first order climate forcings. In the 2005 National Research Council report (on which Dr. Ramanathan was a co-author), it is written on page 100 that
“To date, all model projections of future climate have included a subset of climate forcings, typically greenhouse gas emission scenarios, solar variability, and more recently, aerosol emissions. As the diverse types of radiative and nonradiative climate forcings are recognized (e.g., aerosol indirect effects, changes in land cover), skillful projections of future global and regional climate will need to take them into account, an increasingly challenging task (Pielke Jr., 2001). Addressing this challenge may require a greater focus on assessing key societal and environmental vulnerabilities (Sarewitz et al., 2000).”
The recommendation that concludes the Ramanathan and Feng paper illustrates the inappropriate and societally negative consequences of their paper. They write at the end of their paper
“Decisionmakers have to consider the tradeoffs between air pollution abatement and GHGs mitigation steps but they urgently need predictive tools for making such trade-offs rationally, informed fully of the consequences of policy actions, e.g., future climate changes caused by switching of fuel types, including switching to ethanol, bio diesel and other bio fuels; reducing SO2 emission without warming-offsetting emission reductions in black carbon, NOx, and CO2.”
This “tradeoff” is an seriously misleading recommendation. There are no tradeoffs with respect to air pollution abatement! Health benefits of reducing air pollution should be a worldwide goal irrespective of how it alters the global average radiative forcing.
Thus, the Ramanthan and Feng paper is misleading in both the science of the climate system and in its policy consequences. The science issues can be summarized as:
- Their model of climate change (including global warming) using a single temperature global averaged anomaly is inadequate to properly diagnose global warming,
- the actual radiative feedbacks, based on observations, are more muted than simulated by the IPCC models, and
- the climate system is not in radiative balance at any time.
My challenge to the authors of the PNAS report is the following. Tell us what accumulation in Joules you expect, based on the IPCC models, for the last five year, and for each of the next ten years. How much more would the accumulation of Joules be without the negative radiative forcing of the aerosols? What is the spatial pattern of the changes in upper ocean heat content in Joules with time?
This then provides the benchmark with which we can compare to observations of ocean heat content changes in order to track what the authors claim is “committed” heating.