Exchange Of Viewpoints By Issac Held and Roger A. Pielke Sr.

On June 1 2011 I posted

A Different Interpretation Of Issac Held’s View Of Forced Climate Change

I also then alerted Issac of this post on his weblog under his title

Summer is warmer than winter.

Issac and I then had an informative exchange of viewpooints which I have reproduced below from the comments on his weblog. Issac Held is an internationally well-respected climate scientist, so his perspective on this subject is important to know and is influential. I also appreciate his willingness to engage in constructive dialog.

Roger A. Pielke Sr. says:

Hi Issac – I have posted on my weblog regarding your “Summer is warmer than winter” post. I look forward to a constructive discussion on our disagreement on this science issue.

My post is accessible at

Isaac Held says:

Roger: I prefer to call my anticipation that next summer will be warmer than the past winter a “prediction” (a very high confidence one, admittedly) and you don’t, and we could discuss this semantic point. But I think the substantive issue that we disagree on is simply the magnitude of the forced response to an increase in CO2 as compared to internal variability.

If this signal/noise ratio is too small, than predicting climate anomalies a century from now will be analogous to the prediction of departures from the climatological seasonal cycle (for example, the phase of ENSO) many seasons in advance — and extremely difficult if not impossible due to the underlying chaos in the system. If this signal/noise ratio is large enough, as I am confident it is (I hope to explain my own personal basis for this confidence as these posts evolve) then predicting climate anomalies a century from now, given plausible CO2 trajectories, becomes possible. Almost all of the complexity of the atmosphere, including that inherent in the hydrological cycle and clouds that you mention, comes into play on the seasonal scale without making the seasonal cycle hopelessly complex. I find this encouraging.

Roger A. Pielke Sr.says:

Issac – Thank you for your reply.

In my view, there is a need to develop a way to test our two perspectives. My conclusion has been that, for the climate forcing of added CO2 and other greenhouse gases to be considered a forced boundary problem, the multi-decadal global model predictions

i) must first skillfully predict the current spatial and temporal statistics of the occurrence of ENSO events, the NAO, PDO etc with the “~current” atmospheric concentrations of these gases [say 1979 to the present]


ii) these models must also skillfully predict the changes in the statistics of these atmospheric/ocean features due to the added atmospheric concentrations of the greenhouse gases.

It is these climate circulations that have the most important influence on societally important events such as drought, floods, tropical cyclone tracks etc.

The challenge is that while requirement i) is testable today [and evidence on this would be welcome], but requirement ii) cannot be achieved until several decades from now, unless one can show the effect of added greenhouse gases for the period, for example, form 1979 to the present.

I suggest 1979 to the present as the global data quality and coverage became more homogeneous starting in 1979.

How do you see testing these requirements? What evidence do you have the requirements i) and ii) have been achieved?

Isaac Held says:

Roger — I think it is important to recognize that warming superposed on unchanged variability (ENSO, PDO, cyclone frequency) can be very significant. Think of heat waves — add 3C to summertime temperatures in Chicago and keep the pdf of temperatures fixed — that has consequences. You don’t need to make the case that some model adequately simulates the statistics of heat waves in Chicago. If you can make the case that you have such a model, then you can address the question of how the shape of the pdf might change — that’s great — but even without it, the implications of the conservative assumption of no change in pdf can still be very significant.

I would also emphasize that it is important not to lump all phenomena of relevance together and condemn modeling efforts because they cannot do everything. The ability of models to speak to different issues varies a lot across phenomena. I have included a couple of posts on the Atlantic hurricane issue. Post #2 describes the quality of some simulations of hurricane frequency in the post-1980 period. Post #10 then addresses the use of this model to study attribution of current trends and future projections (I think I will have to return to this, since several emails have indicated that I managed to confuse some readers). It think this is a very good case study for discussing how to evaluate a model/theory for use in projections. It’s a process, requiring an understanding of which model deficiencies future projections are most sensitive to, and whether past variability samples the space of possibilities adequately or if new physics comes into play as the climate warms.

Roger A. Pielke Sr.says:

Issac – We agree on this point – “…. that warming superposed on unchanged variability (ENSO, PDO, cyclone frequency) can be very significant.”. Indeed, we and others have shown that land use change (such as urbanization such as experienced in Chicago and elsewhere) can result in hotter weather even with unchanged large scale variability. Added CO2, while adding to the warming, may actually be a secondary influence as contrasted with this local land use change in this example.

Since land use change has occurred over such large areas of the globe, its affect on temperatures can ameliorate or amplify what occurs as a result of other climate forcings. The attribution of temperature changes as a boundary problem is much more complex than relating to just the radiative forcing of added CO2, and other greenhouse gases.

The issue that I have concluded is more fundamental, however, is the extent that added greenhouse gases, aerosols, and land use change alter regional and hemispheric circulation features. The question is the extent that these circulation featues are altered from what they would be in the absence of these human forcings, as I wrote in my earlier comment.

In 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.,

for example, we found that the spatially variable forcing from the diabatic atmospheric heating from aerosols is large enough to alter regional pressure gradients in some regions. We wrote in our paper that

“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….”

Our paper, however, is only a start at answering this question, as we did not compare the relative role of this human forcing from aerosols with natural variability. To answer the question as to whether or not added greenhouse gases can be treated as a boundary problem cannot be adequately answered until both the role of other human climate forcings and of natural variability are quantitatively evaluated by using real world data in conjunction with the model predictions on multi-decadal time scales.

Tomorrow on my weblog, I am posting on the new paper

Fildes, R. and N. Kourentzes, 2011: Validation and forecasting accuracy in models of climate change. Journal of Forecasting. doi 10.1016/j.ijforecast.2011.03.008

which addresses the extent the IPCC type models have been adequately evaluted as tools to answer the questions such as you and I are discussing. Your comments on that paper in a post on your weblog would be very valuable.

Isaac Held says:

I don’t disagree with the argument that forcing with more horizontal structure is more efficient at generating changes in the mean atmospheric circulation than more uniform forcing of the same overall magnitude. I think it is very generally recognized that, for the same global mean forcing, aerosols perturb the mean precipitation field more than do the well-mixed greenhouse gases (WMGGs). So if, up to the present, anthropogenic aerosols and WMGGs have had comparable effects on regional precipitation, say, the WMGG effect will undoubtedly grow and will be essentially irreversible on the time scale of several centuries, in the absence of geoengineering, while the aerosol effect will likely be bounded by its current magnitude, and the WMGGs will dominate. The ozone hole vs WMGG effects on Southern Hemisphere circulation are analogous in this regard. I gather you feel that WMGG effects are so small that they will not dominate over other forcing agents for most phenomena of interest even as concentrations continue to grow over the next century. This is where we part ways if I understand your position correctly.

Roger A. Pielke Sr. says:

Issac – In terms of very long term effects, the aerosol influence continues even if the emissions were stopped (which is not likely to occur, unfortunately). Nitrogen emissions, as one example, do relatively quickly deposit on the Earth’s surface, but their effect on biogeochemistry of plants and other parts of the biosphere will persist long after the atmospheric concentrations would fall back to more natural levels. With respect to human land management (advertant and inadvertant), land use change will effect climate indefinitely.

I agree with you that the effect of added carbon dioxide will also persist indefinitely and it is a significant human climate forcing. I am actually more concerned about its biogeochemical effects than its radiative forcing, but both are important.

Where I see us disagreeing is that you conclude CO2 and a few other greenhouse gases will dominate climate change in the coming decades. However, as we summarize 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.

other human climate forcings are of at least as much importance on the time scale of decades. This broader perspective, including the recognition that natural climate variability is larger than simulated by the global models, makes the climate predicition problem much more difficult than suggested in your analogy to the annual cycle. We are too conservative if the focus is just on CO2 and a few other greenhouse gases as the dominate first order climate forcings.

If I am correct, than the proposals to geoengineer away the radiative effect of added greenhouse gases is fraught with risk, as we would be just adding further to the complexity of how humans are altering the climate system.

Isaac Held says:

Some climate changes will be dominated by greenhouse gas increases on the time scale of several decades and others will not be due to the higher noise level. Table 11.1 in Ch. 11 of the WG1/AR4 provides estimates of the time it takes 20yr mean seasonal temperature and precipitation changes averaged over various sub-continental regions to emerge from internal variability, according to the CMIP3 ensemble of models (ie, given both their responses and their internal variability). I lean on the models a lot, plus I was one of the Lead Authors for this chapter, so this can be taken as a more explicit statement of my starting point for discussing this issue, at least for this subset of variables. The time required for these 20yr means to emerge from the model’s own noise varies a lot. Temperature responses emerge in most regions after 20 years — 20 year mean precipitation signals emerge in some regions in 30 years, some not for 100 years if then. Evidence that the model responses, or model internal variability levels, are biased one way or the other can then be brought to bear to criticize these numbers (and on a more minor point, one can question how the choice of regions and seasons affects the results) — but it helps to have a clear idea of the model results that one is then arguing should be modified.

Roger A. Pielke Sr. says:

Issac – Thank you for clearly clarifying our area of disagreement. In my view, the use of the CMIP3 ensemble of models (or other global climate models on this time scale) does not permit the testing of your hypothesis on the dominance of the added greenhouse gas relative to natural variability over multi-decadal time periods.

While I agree that models are very valuable tools, they are, however, just hypotheses. They must be tested against real world data. This means, for example, they need to show skill at predicting the statistics of large scale circulation features, and the change in these statistics due to human climate forcing.

As shown, however, in papers such as

Stephens, G. L., T. L’Ecuyer, R. Forbes, A. Gettlemen, J.‐C. Golaz, A. Bodas‐Salcedo, K. Suzuki, P. Gabriel, and J. Haynes (2010), Dreary state of precipitation in global models, J. Geophys. Res., 115, D24211, doi:10.1029/2010JD014532

the models still have a way to go to be accepted as robust predictive tools, in the similar manner as we accept numerical weather prediction model results.

The recent paper

Wyatt, Marcia Glaze , Sergey Kravtsov, and Anastasios A. Tsonis, 2011: Atlantic Multidecadal Oscillation and Northern Hemisphere’s climate variability Climate Dynamics: DOI: 10.1007/s00382-011-1071-8.

illustrates the type of variability that the global climate models must skillfully simulate.

The CMIP3 ensemble of models need to show that they can recreate this observed variability [in fully coupled ocean-atmosphere-land form] given initial conditions for the ocean, atmosphere, and land. Than the next step is to show they can skillfully predict changes in this variability due to added CO2 (and other human climate forcings) over the time period for which we have real world observations.

This is, in my view, a necessary condition to accept global climate predictions of regional climate in the coming decades.

Isaac Held says:

Model quality is one of the things that I hope to discuss, carefully and critically, on this blog

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