Is “Global Dimming” Really Global?

The answer according to a new paper is NO.

This paper provides further evidence as to why we need to focus on local and regional scales if we are to better understand climate science. The new paper is

Alpert, P., P. Kishcha, Y. J. Kaufman and R. Schwarzbard, “Global Dimming or Local Dimming? — Effect of Urbanization on Sunlight Availability” Geophysical Research Letters,32, L17802, doi:10.1029/2005GL023320. 2005.

The abstract of the paper is

From the 1950s to the 1980s, a significant decrease of surface solar radiation has been observed at different locations throughout the world. Here we show that this phenomenon, widely termed global dimming, is dominated by the large urban sites. The global-scale analysis of year-to-year variations of solar radiation fluxes shows a decline of 0.41 W/m2/yr for highly populated sites compared to only 0.16 W/m2/yr for sparsely populated sites (<0.1 million). Since most of the globe has sparse population, this suggests that solar dimming is of local or regional nature. The dimming is sharpest for the sites at 10°N to 40°N with great industrial activity. In the equatorial regions even the opposite trend to dimming is observed for sparsely populated sites.

This paper illustrates the importance of spatially heterogeneous diabatic heating, as was discussed, for example, on our weblog of July 28th entitled “What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?â€?, and recommended in the 2005 National Research Council report “Radiative forcing of climate change: Expanding the concept and addressing uncertainties”.

The answer is that this is another well-documented example of cherry picking.

I have posted below my submission to Nature and the subsequent exchange of communications with Nature. David Parker requested that I not post his reply, which I will respect. I invite him, however, to respond to the issues that are raised in my Comment and in the Geophysical Research Letter article.

The article:

Pielke Sr., R.A., and T. Matsui, 2005: Should light wind and windy nights have the same temperature trends at individual levels even if the boundary layer averaged heat content change is the same? Geophys. Res. Letts., 32, No. 21, L21813,10.1029/2005GL024407.

resulted from my inability to publish my comment on his paper.

Comment on Parker (2004; “Large-scale warming is not urbanâ€? Nature. Vol. 432 18 Nov 2004).

By Roger A. Pielke Sr. June 7, 2005 Nature

Parker (2004) summarized a very interesting study in which he segmented observed surface temperature data into “calmâ€? (defined as the lower tercile of daily averaged wind speeds) and “windyâ€? (defined as the upper tercile of daily averaged wind speeds) in order to assess whether the reported large-scale global averaged temperature increases are attributable to urban warming. This note, however, questions, whether trends of surface air temperature should even be expected to have the same trends for these different sets of days.

The Parker article focused on nighttime, minimum temperatures. During this time of the day, it is well understood that temperature change with height in the lowest tens of meters can be quite large, particularly in light wind, clear sky conditions (e.g., Stull, 1988; Oke, 1987). For relatively windy nighttime conditions, surface similarity theory has been developed which can be used to describe the vertical temperature lapse rate in the lowest few tens of meters. Pielke (1984; Figure 7-4), for example, illustrates the vertical lapse rate as a function of the intensity of mechanical generation and convective suppression of turbulence. For strong winds, the vertical lapse rate becomes adiabatic (0.1°C per 10 meters). For light wind conditions, longwave radiative flux divergence becomes dominant and the lapse rates can become quite large (e.g., 10°C per 10 m or even larger).

Thus, if the upper tercile of wind conditions, as defined in the Parker article, is well represented by an adiabatic lapse rate, a temperature trend at 1 m, for instance, would be essentially the same as at 2 m. However, when the large lapse rates typical of lighter wind conditions occur, a trend at 1 m would, in general, be expected to be significantly different at 2 m.

The relative warming of windy nights in the winter in the extratropical Northern Hemisphere that he reports could also, at least partially, be a result of changes in the lapse rate due to trends in surface windiness rather than due to changes in onshore warm advection from the oceans.

If the urban and rural sites have a different aerodynamic roughness, this presents an additional complication by producing greater mechanically-forced turbulent mixing over the rougher surfaces (presumably the urban sites) than for the rural sites. Even with the same trend of temperature when averaged over the lower few tens of meters above the surface, there would be different trends at specific levels within that layer when the aerodynamic roughness effect is included.

This influence of vertical temperature stratification on the temperature trends raises the issue as what is actually meant by the term “surface temperature trend.â€? Along with the issues of surface temperature as contrasted with surface moist enthalpy (Pielke et al. 2004) and microclimate station exposure changes (Davey and Pielke 2005), the reported regionally- and globally-averaged surface temperatures trends have unresolved uncertainties.

The Parker (2004) conclusions, therefore, need further analysis and interpretation before they can be used to conclude whether or not there is an influence of urban warming on the large-scale temperature trends. More broadly, these issues regarding surface temperature trends need investigation to determine whether these effects increase or decrease long-term spatially representative globally-averaged and regionally-averaged surface temperature trends, as well as a clearer definition of what is meant by “surface temperature.â€?

Davey, C.A., and R.A. Pielke Sr., 2005: Microclimate exposures of surface-based weather stations – implications for the assessment of long-term temperature trends. Bull. Amer. Meteor. Soc., 86, No. 4, 497–504.

Oke, T.R., 1987: Boundary layer climates. 2nd Edition. Routledge, London, and John Wiley & Sons, New York, 435 pp.

Parker, D.E., 2004: Large-scale warming is not urban. Nature, November 18, 2004.

Pielke, R.A. Sr., C. Davey, and J. Morgan, 2004: Assessing “global warming” with surface heat content. Eos, 85, No. 21, 210-211.

Stull, R., 1988: An introduction to boundary layer meteorology. Kluwer Academic Pub., 666 pp.

Date: Tue, 21 Jun 2005 13:26:06 UT
From: r.cotter@nature.com
To: pielke@atmos.colostate.edu
Subject: Nature – 2005-06-06864 out to Nature Author

Dear Professor Pielke

Thank you for submitting your Communications Arising entitled “Comment on Parker (2004; “Large-scale warming is not urban” Nature. Vol. 432 18 Nov 2004)” to Nature. This message is to tell you that we are sending your paper out to the Nature authors for their response according to our policy (see our guidelines to authors of Communications Arising on http://nature.com/nature/authors/gta/index.html#a8). Once we have received their reply, we shall decide whether or not to send the exchange out for review.

This decision is reached after discussion with the appropriate specialist editors and depends on a number of factors, including the likely impact of the criticism, the topicality of the discussion, and its interest to the non-specialist readers of this section of the journal (many debates are referred to the specialist literature at this point or the issues are addressed in the form of a published clarification from the criticised authors).

If we have several comments under consideration on one of our published papers, this may introduce delays while we coordinate these steps in the editorial process, so please allow four weeks before sending us any status enquiries.

Please note that consideration of your comment by Nature is contigent on there being no discussion of its content with the media in advance of publication (see http://www.nature.com/nature/authors/policy).

Yours sincerely

Rosalind Cotter
Editor, Brief Communications

Date: Tue, 5 Jul 2005 14:03:24 UT
From: r.cotter@nature.com
To: pielke@atmos.colostate.edu
Subject: Decision on Nature Manuscript 2005-06-06864

5th July 2005

Dear Professor Pielke

Thank you for your comment on the Brief Communication by David Parker, which I am afraid we must decline to publish. As is our policy on these occasions, we showed your comment to the earlier authors, and their response is enclosed. We also sent the exchange to 2 referees, whose comments are attached.

In the light of this advice and of the competition for our space, we have regretfully decided that publication of this debate is not justified as it would not add to our understanding or otherwise clarify this issue in the minds of our readers.

Thank you again, however, for writing to us.

Yours sincerely

Rosalind Cotter
Editor, Brief Communications

Nature Author #1(Remarks to the Author):

Reply by D E Parker to R A Pielke, Sr. s comment on Parker (2004; large-scale warming is not urban Nature. Vol 432 18 Nov 2004).

Prof. Pielke provides no physical hypothesis as to why, in the absence of urban warming, trends of near-surface air temperature on calm nights, when similarity theory is inapplicable and longwave radiative loss dominates, should differ from those on windy nights, when lapse-rate trends are close to adiabatic. In the absence of a known mechanism, Occams razor demands a null hypothesis of equal trends. Parker (2004) found that the temperature trends on calm nights are insensitive to the definition of calm as winds in the lightest decile rather than the lightest terce, so the trends appear to be insensitive to the turbulence-structure of the boundary layer.

Trends in surface windiness will not affect near-surface lapse rates on windy (upper tercile) nights in winter in the extratropical Northern Hemisphere; these lapse-rates will remain close to adiabatic. The temperature trend on windy nights is expected to be least influenced by local effects and microclimate station exposure changes, as horizontal and especially vertical mixing is then strongest. As the global trend of night-time temperature in windy conditions equals that for the whole sample used by Parker (2004), and the global trend for mean temperature based on this sample is similar to the global trend of the Jones and Moberg (2003) dataset, it must be concluded that the Jones and Moberg (2003) global trends are not much biased by urban and other small-scale influences.

Nonetheless, Prof. Pielke is right in stressing the need to analyze enthalpy (i.e. total sensible and latent heat content) for a full understanding of climatic variations and changes.

References:

Jones, P. D., and A. Moberg, 2003: Hemispheric and large-scale surface air temperature variations: an extensive revision and an update to 2001. J. Climate, 16, 206-223.

Parker, D. E., 2004: Large-scale warming is not urban. Nature, 432, 290.

Referee #2(Remarks to the Author):

Pielke has failed to adequately assess whether there are any trends in windiness in the Parker data set. Parker stratified by wind conditions, both at rural and urban sites, so any trends in windiness (even if this were possible in a stratified data set) would occur both at rural and urban sites. To suggest that there would be different turbulent mixing at rural and urban sites would then require differences in trends in temperature to be found, which is exactly what Parker found not to be the case. The logic presented in Pielke’s comment is circular and incorrect.

I cannot recommend publication.

Referee #3(Remarks to the Author):

The key result of the Parker paper that made it worthy of publication in Science is the demonstration that daily minimum temperatures windy nights exhibit an upward trend no less than the trend observed on calm nights. Hence it is unlikely that the observed upward trend in daily minimum temperature during the past few decades could be simply an artifact of the “heat island” effect, as alleged by some global warming skeptics. That the upward trend on windy nights proved to be even larger than the trend observed on calm nights is interesting, but I don’t regard it as central to the paper or of particular interest to Nature’s interdisciplinary reading audience. Parker’s article would have been of no less interest if this material had not been included,

The points raised in this letter address the second, in my view less important result. They have no bearing on the primary result. Hence, I think they would be more appropriately recast as a short article published in a more specialized technical journal. This change of venue would also give the author space to articulate his argument more clearly. As it’s written now I have difficulty getting his point.

Date: Tue, 5 Jul 2005 09:05:08 -0600 (MDT)
From: Roger Pielke
To: r.cotter@nature.com
Subject: Re: Decision on Nature Manuscript 2005-06-06864

Dear Dr. Cotter

I am disappointed regarding your choice of reviewers who did not address
the substance of my comment. For the trends to be the same on light and
windy nights violates our understanding of the physics of the surface
boundary layer.

As a former Chief Editor of both the American Meteorological Society’s
Monthly Weather Review and the Journal of Atmospheric Science, a
communication such as mine on another paper would never have been
rejected. The community who read the original paper would have been
provided the opportunity to read and, therefore, discuss the issue raised
in my comment.

Unfortunately, the rejection of such a comment perpetuates the impression
that Nature is not permitting a balanced debate on the climate issue.

Sincerely

Roger A. Pielke Sr.

Date: Wed, 20 Jul 2005 15:10:33 +0100
From: “Cotter, Rosalind”
To: “‘pielke@atmos.colostate.edu'”
Subject: FW: Decision on Nature Manuscript 2005-06-06864

Dear Professor Pielke
Thank you for your message. Although we appreciate the points you raise, I
am afraid that in view of the severe competition for our space and in the
light of the referees’ recommendations (who are respected experts in the
field), we are unable to offer to reconsider your Communication Arising from
the short paper by Parker.
Thank you, however, for writing to us.
Yours sincerely
Rosalind Cotter
Editor, Brief Communications

Is there a Warm Bias in the Nightime Land Surface Temperature Record?

The answer is YES.

The paper Pielke Sr., R.A., and T. Matsui, 2005: Should light wind and windy nights have the same temperature trends at individual levels even if the boundary layer averaged heat content change is the same? Geophys. Res. Letts., 32, No. 21, L21813, 10.1029/2005GL024407 has appeared.

The consequences of this paper go well beyond just questioning the conclusions of the 2004 D.E. Parker paper in Nature entitled Large-scale warming is not urban. In fact, the requirement, based on our physical understanding of the vertical profile of air temperatures in the lowest levels of the atmosphere, is that the averaging of air temperature trends at night over land will introduce a warm temperature bias in comparison to any boundary layer averaged long-term temperature increase. This occurs because, while the stronger wind nights will have similar temperature trends as a function of height, on lighter wind nights a long-term boundary layer temperature increase would produce an amplified temperature increase in the lowest levels.

This paper provides further evidence of why we need to move away from using surface temperatures as a metric to monitor global climate heat changes.

Coral as a Component of the Climate System.

I have been fortunate to become familiar with a lay-accessible summary of the World’s coral and want to share this excellent source of information with our weblog readers. As discussed frequently on the weblog, biological processes are as much a part of the climate system as the physical components such as temperature and precipitation. This perspective is summarized in the 2005 National Research Council report where marine biota is specifically identified as part of the ocean component of climate (http://www.nap.edu/books/0309095069/html/12.html). The e-book information can be accessed from The Pomacanthus Snorkelling Guide to the World’s Coral Reefs where the summary of the guide discusses how different types of reefs and how they were formed, how to recognize different types of corals and reef fishes, worldwide variations in abundance and types of reefs, corals, fishes, and variations in reef threats and snorkeling conditions, and regional accounts of reefs of the west Atlantic (and Caribbean), Indian, and Pacific Ocean. In studying climate science, we need to move beyond the traditional perspective of focusing on atmospheric and physical oceanographic processes.

New Procedure for Climate Model Parameterizations

A new procedure to parameterize climate processes within climate models has been developed. It is described in a paper that is in press with the National Weather Digest entitled “A New Paradigm for Parameterizations in Numerical Weather Prediction and other Atmospheric Models” with authors Roger A. Pielke Sr., Toshihisa Matsui, Giovanni Leoncini, Timothy Nobis, Udaysankar S. Nair,Er Lu, Joe Eastman, Sujay Kumar, Christa Peters-Lidard, Yudong Tian and Robert L. Walko. The abstract states,

“The use of look-up-tables (LUTs) to represent parameterizations within numerical weather prediction and other atmospheric models is presented. We discuss several approaches as to how the use of LUTs can be optimized in order to retain the physical representation of the parameterization, yet be much more computationally efficient than the parent parameterization from which they are derived.”

As we elaborate on in our paper,

“….the LUT-approach, utilizes data access and retrieval procedures, and methods to reduce the dimensionality of the original parameterization to create this method of model improvement. A major advantage of the LUT-approach, for example, includes the ability to create more realizations in the creation of ensemble forecasts. ”

This approach is an alternate procedure to the so-called “superparameterizations”. With that approach, as we discuss in our paper,

“…… a cloud-resolving model (is embedded) within a larger-scale model in order to improve the accuracy of simulating cloud interactions with the larger-scale model. This has been called a “superparameterizationâ€?. Superparameterization refers to using a 2-D or 3-D cloud-resolving model to simulate a process in place of a very simplified parameterization that has been commonly used in weather and climate models in order to keep the computational cost low. Superparameterization-embedded Multi-Modeling Frameworks (MMF) are recently under development at several institutions, and there are plans to create global cloud libraries, which includes detailed mass and energy output from cloud resolving models. With the LUT-based approach the superparameterization approach could be used much more efficiently since the simulations (e.g., the 3-D cloud model) are integrated offline and the results are archived in a database for future retrieval. ”

The LUT-approach will allow more computationally efficient climate model simulations than are possible using the currently applied parameterization procedure.

Atmospheric Chemistry Within the Climate System

A recent seminar by Qinbin Li of NASA held at Colorado State University highlighted an important interaction between atmospheric chemistry, atmospheric transport, and weather. The talk was titled

“Trapping of deep convective pollution by upper level anticyclones: Implications for global climate”, and the abstract was

“The GEOS-Chem global 3D chemical transport model is used to analyze observations of CO and upper tropospheric clouds from the EOS Microwave Limb Sounder (MLS). MLS observations during 25 August-6 September 2004 reveal elevated CO concentrations and dense high-altitude clouds in the upper troposphere over the Tibetan plateau and southwest China, collocating with the upper-level Tibetan anticyclone. The observed dense high-altitude clouds are accompanied with relatively little precipitation, as indicated by the CMAP precipitation rate. A large fraction of these dense high clouds have relatively small particle sizes and are thus distinguished from convective clouds which have larger particles. Model simulations indicate the transport of boundary layer pollution by Asian summer monsoon convection and orographic lifting to the upper troposphere over South Asia, where the simulated distributions of CO closely resemble the MLS observations. Model results also show elevated aerosols and ozone in the anticyclone region. Analysis of simulated CO and aerosols indicate that the upper-level Tibetan anticyclone effectively ‘traps’ anthropogenic emissions lifted from northeast India and southwest China. These aerosols may be responsible for the formation of some of the dense high-altitude clouds. In a separate study, we find that in summer the semi-permanent upper-level anticyclone over the southern United States traps the convective outflow and allows it to age in the upper troposphere over the United States for several days. Rapid ozone production takes place in this outflow driven in part by anthropogenic and lightning NOx, and in part by HOx radicals produced from convectively lifted CH2O that originates from biogenic isoprene. This mechanism could explain ozonesonde observations of elevated ozone in the upper troposphere over the southeastern United States.”

One of the papers by Li et al on this important climate study is in Geophysical Research Letters in 2005 entitled “Convective outflow of South Asian pollution: A global CTM simulation compared with EOS MLS observations”. The abstract from the paper states

“A global 3-D chemical transport model is used to analyze observations of carbon monoxide (CO) and upper tropospheric clouds from the EOS Microwave Limb Sounder (MLS). MLS observations during 25 August–6 September 2004 reveal elevated CO and dense high clouds in the upper troposphere over the Tibetan plateau and southwest China, collocating with the upper level Tibetan anticyclone. Model simulations indicate the transport of boundary layer pollution by Asian summer monsoon (ASM) convection and orographic lifting to the upper troposphere over South Asia, where simulated distributions of CO resemble MLS observations. Model results also show elevated aerosols in the anticyclone region. Analysis of model simulated CO and aerosols indicate that the Tibetan anticyclone could ‘trap’ anthropogenic emissions lifted from northeast India and southwest China. These aerosols may be responsible for the formation of some of the dense high clouds.”

The paper concludes with the statement,

“Changes in cloud properties could have profound impact on the global cloud system, hydrological cycle, and climate.”

This work provides additional information to support the findings in the 2005 National Research Council report entitled “Radiative forcing of climate change: Expanding the concept and addressing uncertainties.” Climate, as influenced by human activities and by natural processes, must be considered an integrated physical, chemical and biological system. Predictions of future climate are made much more difficult as a result of the complexity exemplified in the research by Qinbin Li and colleagues.

An Overview Book on Why Land-Surface Processes Are So Important in Climate Science

We have cited in the weblog the book

Kabat, P., Claussen, M., Dirmeyer, P.A., J.H.C. Gash, L. Bravo de Guenni, M. Meybeck, R.A. Pielke Sr., C.J. Vorosmarty, R.W.A. Hutjes, and S. Lutkemeier, Editors, 2004: Vegetation, water, humans and the climate: A new perspective on an interactive system. Springer, Berlin, Global Change – The IGBP Series, 566 pp.

The book’s abstract states,

“This volume focuses particularly on the interactions between the terrestrial biosphere and atmosphere via the hydrological cycle, and their interactions with anthropogenic activities. Measurements from integrated field experiments are complemented by modelling studies simulating flows and transport in river catchments, coupled land-cover and climate, and Earth System processes. The impact of humans on river basins through land use, pollution and river engineering is discussed, and the book ends with a discussion of environmental vulnerability and methodologies of assessing the risks associated with global change.”

This book has contributions from a wide range of authors and editors, and provides additional scientific support for the 2005 National Research Council report “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.

The new IPCC drafts that are circulating ignore the significance of both reports. See, for example, our weblog of October 4, 2005 entitled “Overlooked Issues in Prior IPCC Reports and the Current IPCC Report Process: Is There a Change From the Past?” Unbiased readers of the IPCC report will clearly conclude it is an advocacy document if the major findings of these scientific summaries are not included in the IPCC report.

More on the Limitations of Radiative Forcing as an Indicator of Climate Change

As discussed on this weblog (e.g., see the August 1, 2005 posting Is There a Human Effect on the Climate System? ) and documented in the 2005 National Research Council Report , we need to move beyond global radiative forcing as the primary metric to define climate change. A new paper provides additional support for this view.

The paper in Climate Dynamics by Stuber et al. entitled “Why radiative forcing might fail as a predictor of climate change” (subscription required). Their paper states,

“A series of climate model simulations involving ozone changes of different spatial structure reveals that the climate sensitivity parameter λ is highly variable: for an ozone increase in the northern hemisphere lower stratosphere, it is more than twice as large as for a homogeneous CO2 perturbation. A global ozone perturbation in the upper troposphere, however, causes a significantly smaller surface temperature response than CO2. The variability of the climate sensitivity parameter is shown to be mostly due to the varying strength of the stratospheric water vapour feedback.”

The climate sensitivity parameter λ is defined by the equation ΔTsurf = λ times RF where RF is the radiative forcing. The term ΔTsurf is the global mean surface temperature and RF is the global mean radiative forcing. This paper further shows the inadequacies of using the concept of a climate sensitivity, as defined by λ, as well as the overly simplistic concept of a global averaged surface temperature and a global mean radiative forcing (e.g., see the weblog of September 25, 2005 Is Global Warming Spatially Complex? and July 28th What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?). The climate system is much more complex and spatially heterogeneous than represented by this simple relationship. The IPCC and other climate assessments need to move beyond a narrow perspective of the climate system.

More Evidence On The Spatial Complexity Of Climate Forcings

A new paper provides more evidence on the complexity of climate forcings. The paper, entitled “Radiative effect of surface albedo change from biomass burning” by Myhre et al. has the following abstract,

“The radiative impact of burn scars from biomass is investigated. Changes in surface albedo derived from satellite observations over the African continent are used as a first order indication of this impact. Because the direct radiative effect of aerosols from biomass burning is dependent on the underlying surface albedo, we investigate the interaction of the direct radiative effect due to biomass burning aerosols with the change in surface reflection due to the burn scars. The radiative effect of reduced surface albedo from burn scars is estimated to be close to 0.1 W m⁻² over a region covering the African continent.”

This paper is important, since not only does it show that biomass burning alters the surface albedo, the vertical profile of radiative heating and temperature are changed as a result. The paper also quantifies the magnitude of this effect for Africa where biomass burning is extensive.

Figure 2 of their paper shows significant spatial variations in the annual mean radiative effect which further illustrates that climate forcings often have large spatial heterogeneity that result in heterogeneous spatial trends in weather patterns, as has been discussed on this weblog (see, for example, the weblog of July 28, 2005 What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures? ).

Reponse to a November 4, 2005 post on RealClimate

RealClimate posted a comment today on Chaos and Climate.

My response, which I also submitted to RealClimate, is as follows:

James and William- your post, unfortunately, perpetuates the use of climate to refer to long term weather statistics. You state that

“The chaotic nature of atmospheric solutions of the Navier-Stokes equations for fluid flow has great impact on weather forecasting (which we discuss first), but the evidence suggests that it has much less importance for climate prediction.”

This is incorrect.

First, the more appropriate scientific definition of climate is that it is a system involving the oceans, land, atmosphere and continental ice sheets with interfacial fluxes between these components, as we concluded in the 2005 National Research Council report . Observations show chaotic behavior of the climate system on all time scales, including sudden regime transitions, as we documented in Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38.

That the model simulations that you discuss in your weblog do not simulate rapid climate transitions such as we document in our paper illustrates that the models do not skillfully create chaotic behavior over long time periods as clearly occurs in the real world.

That climate is an integrated system and is sensitive to initial conditions is overviewed in Pielke, R.A., 1998: Climate prediction as an initial value problem. Bull. Amer. Meteor. Soc., 79, 2743-2746. Even within the atmospheric portion of the climate system, and applying a simple nonlinear model, based on the work of Lorenz, a chaotic response can be generated which is not evident in the model results you refer to (see Pielke, R.A. and X. Zeng, 1994: Long-term variability of climate. J. Atmos. Sci., 51, 155-159). We show in this study that even short-periodic natural variations of climate forcing can lead to significant long-term variability in the climate system.

We need to move the discussion to studying climate as a complex, nonlinear system which displays chaotic behavior if we are going to provide scientifically robust understanding to policymakers. Readers of your weblog are invited to read my postings at http://www.climatesci.org if they would like to read a different perspective on climate science.