Announcement Of A Meeting On Climate Models Intercomparison: Dynamics and Physical Processes

There is going to be an important session at the upcoming European Geosciences Union General Assembly, to be held in Vienna in the period 15-20 April 2007. A session is being organized by Valerio Lucarini, Aad Van Ulden and Masahide Kimoto to provide a context for discussing strategies for auditing climate models in terms of their dynamics and of their physical processes.

The session is CL40 “Climate Models Intercomparison: Dynamics and Physical Processes”.
Abstracts are due by January 15 2007.

The Climate Models Intercomparison: Dynamics and Physical Processes session announcement reads

“Convener: Lucarini, V. Co-Convener: Van Ulden, A.; Kimoto, M.

The goal of this session is to provide a space for discussing and detailing new approaches for auditing climate models (global and regional) in order to assess their degree of self-consistency and realism. Today, the intercomparison of climate models is greatly favoured by the collection on single servers of standardized outputs of several models running under well-defined scenarios. Contributions should focus on the dynamical properties of climate models as well as on their representation of some key physical processes, thus going beyond the typical time-mean diagnostics. Contributions focusing on nonlinearity of the processes as well as on nonlinear methodologies for data analysis are also welcome.

This session should provide a much needed discussion of multi-decadal global climate simulations.

A New Paper On The Role Of Agriculture Within The Climate System

One of the several excellent papers in the special issue of Global and Planetary Change on land use/land cover change and impact on climate is

Navin Ramankutty, Christine Delire and Peter Snyder, 2006: “Feedbacks between agriculture and climate: An illustration of the potential unintended consequences of human land use activities”. Global and Planetary Change – Land-use/land-cover change and its impact on climate, 79-93

The abstract reads,

“Agriculture has significantly transformed the face of the planet. In particular, croplands have replaced natural vegetation over large areas of the global land surface, covering around 18 million km2 of the land surface today. To grow crops, humans have taken advantage of the resource provided by climate — optimum temperature and precipitation. However, the clearing of land for establishing croplands might have resulted in an inadvertent change in the climate. This feedback might, in turn, have altered the suitability of land for growing crops. In this sensitivity study, we used a combination of land cover data sets, numerical models, and cropland suitability analysis, to estimate the degree to which the replacement of natural vegetation by croplands might have altered the land suitability for cultivation. We found that the global changes in cropland suitability are likely to have been fairly small, however large regional changes in cropland suitability might have occurred. Our theoretical study showed that major changes in suitability occurred in Canada, Eastern Europe, the Former Soviet Union, northern India, and China. Although the magnitude, sign, and spatial patterns of change indicated by this study may be an artifact of our particular model and experimental design, our study is illustrative of the potential inadvertent consequences of human activities on the land. Moreover, it offers a methodology for evaluating how climate changes due to human activities on the land may alter the multiple services offered by ecosystems to human beings.”

The paper concludes that,

“This study also provides a different perspective for evaluating climate change. Traditionally, climate and climate change has been viewed froma purely biophysical perspective, i.e., variables such as temperature, precipitation, humidity, wind speeds, etc., and many derived diagnostic thereof (Houghton et al., 2001). However, evaluating climate and its change, as relevant to human societies, needs to be critically tied to the concept of the “ecosystem servicesâ€? (Daily et al., 1997). In this study, we offer the concept of “land suitability for cultivationâ€? as one such measure of climate, as it is relevant to the global food production system. We have used change in this suitability index, rather than climatic variables such as temperature and precipitation, to measure how human modifications of the landscape, mediated through climate– vegetation interactions, may potentially affect human food production capacities.”

This paper illustrates further why landscape change and vegetation dynamics must be included in any assessment of the role of human- and natural caused climate variability and change.

Scitizen – New Website For Communicating Science News To The Public

A new communication to communicate science news to the public is available. It is called “Scitizen” and as their website writes,

“Scitizen is an open science news source by scientists and journalists, for the general public.”

This contribution is a valuable new addition as a science information resource.

I am pleased that Climate Science has been recognized and invited to contribute. The first publication from Climate Science was posted on December 1 2006 and is entitled “A Broader Assessment Of Climate Change Science Is Needed”.

Best Holiday Wishes to The Readers of Climate Science!

Today, I want to thank everyone who has taken time to read the weblogs on Climate Science, as well as those who have posted comments. This has been a successful year in communicating the breadth of issues in climate change science.

Best Wishes for the Holidays and for the New Year!

Roger A. Pielke Sr.

Further Comments Demonstrating that Climate Prediction Is An Initial Value Problem

There is a paper

F. Giorgi, 2005 : Climate Change Prediction: Climatic Change (2005) 73: 239–265
DOI: 10.1007/s10584-005-6857-4

which provides information by a well respected climate scientist which can be used to show why climate prediction is an initial value problem.

He starts the article by defining the two types of prediction that Ian Rutherford also commented on in his testimony as discussed on Climate Science (see). The two types listed in the Giorgi article are

“In the late 1960s and mid 1970s the chaotic nature of the climate system was first recognized. Lorenz (1969, 1975) defined two types of predictability problems:

1) Predictability of the first kind, which is essentially the prediction of the evolution of the atmosphere, or more generally the climate system, given some knowledge of its initial state. Predictability of the first kind is therefore primarily an initial value problem, and numerical weather prediction is a typical example of it.

2) Predictability of the second kind, in which the objective is to predict the evolution of the statistical properties of the climate system in response to changes in external forcings. Predictability of the second kind is thus essentially a boundary value problem.”

In the text of this paper, Dr. Giorgi writes,

“….because of the long time scales involved in ocean, cryosphere and biosphere processes a first kind predictability component also arises. The slower components of the climate system (e.g. the ocean and biosphere) affect the statistics of climate variables (e.g. precipitation) and since they may feel the influence of their initial state at multi decadal time scales, it is possible that climate changes also depend on the initial state of the climate system (e.g. Collins, 2002; Pielke, 1998). For example, the evolution of the THC in response to GHG forcing can depend on the THC initial state, and this evolution will in general affect the full climate system. As a result, the climate change prediction problem has components of both first and second kind which are deeply intertwined.

This concept is illustrated in Figure 2, which shows two hypothetical future climate evolutions as simulated by a climate model. In each simulation the GHG concentration increases in the same way but starting from different times of the Control run, and thus different initial ocean, sea ice and land surface conditions. As illustrated, the two climate evolutions can potentially differ both in their mean and variability characteristics. The relevance of the first kind predictability aspect of climate change is that we do not know what the initial conditions of the climate system were at the beginning of the “industrialization experimentâ€? and this adds an element of uncertainty to the climate prediction.”

He also states that,

“To add difficulty to a prediction is the fact that the predictability of a system is strongly affected by non-linearities. A system that responds linearly to forcings is highly predictable, i.e. doubling of the forcing results in a doubling of the response. Non-linear behaviors are much less predictable and several factors increase the non-linearity of the climate system as a whole, thereby decreasing its predictability (e.g. Rial et al., 2004).”

Thus, if the climate system is both a boundary value and an initial value problem (which I agree with), it is an initial value problem!

I read the article as a transition in progress in the thinking, from the assumption that multi-decadal global climate model projections are a boundary value problem, to the recognition that the prediction of the evolution of the climate system is an initial value problem. Moreover, if the climate system is sufficiently nonlinear, as the observational evidence indicates that it is (see), then achieving skillful multi decadal climate predictions in response to the diversity of human and natural climate forcings is a daunting challenge, as has been emphasized on Climate Science.

Deliberate Global Climate Modification – Is This A Good Idea?

There have been proposals to engineer the global climate system so as to reduce a positive radiative imbalance associated with the well-mixed greenhouse gases, particularly CO2. This would be global climate modification.

This concept has been presented by Paul Crutzen (see “Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma? “), and is discussed in the August 2006 issue of Climatic Change (see). Paul Crutzen is a Nobel Prize winner and is a very well respected scientist. He suggested that large amounts of sulphates be ejected into the stratosphere in order to reduce the amount of solar radiation that reaches the Earth’s surface.

The issue has resurfaced in a December 18, 2006 Reuters news release by Ari Rabinovitch

It reads,

“TEL AVIV, Dec 15 (Reuters) – Nobel Prize laureate Paul Crutzen says he has new data supporting his controversial theory that injecting the common pollutant sulphur into the atmosphere would cancel out the greenhouse effect.

Though such a project could not be implemented for at least 10 years, the data is aimed at appeasing critics of the idea he first championed in the scientific journal Climatic Change in August.

The Dutch meteorologist showed what he calls the positive cooling effect of adding a layer of sulphates to the atmosphere at a global warming conference at the Porter School for Environmental Studies in Tel Aviv.

He said new, detailed calculations carried out since August showed the project would indeed lower global temperatures.

‘Our calculations using the best models available have shown that injecting 1 million tonnes of sulphur a year would cool down the climate so the greenhouse effect is wiped out,’ Crutzen told Reuters.

An added layer of sulphates in the stratosphere, some 10 miles (16 km) above the earth, would reflect sunlight into space and reduce solar radiation reaching the earth’s surface, Crutzen said.

He said he envisioned giant cannons or balloons dispersing the sulphur to offset the build-up of greenhouse gases such as carbon dioxide, largely released by burning fossil fuels in power plants, factories and vehicles.

The world has struggled for decades to reduce sulphur pollution, a component of acid rain that kills forests and fish, mainly through tighter controls on burning coal.

‘We are now entering a very intensive period of model calculations and following that we will conduct small experiments to test the sulphur oxidation mechanisms that we calculated,’ Crutzen said.”

I have enormous respect to Paul Crutzen. However, his proposal has not adequately considered the consequences of such a deliberate manipulation of the global climate system.

Besides the obvious issue of unintended consequences, the concept is one dimensional in its approach as it focuses on just one climate forcing; the radiative forcings, as well as ignores the spatial consequences to radiative forcing of inputting aerosols into the stratosphere.

Also, completely ignored in this proposal is the issue that the atmospheric composition of gases such as CO2 would still be different than in the natural atmosphere! The biogeochemical effect of higher atmospheric concentrations of CO2, for example, would have substantial, yet incompletely understood on the Earth’s ecology. Research by our group and others; e.g. see

Eastman, J.L., M.B. Coughenour, and R.A. Pielke, 2001: The effects of CO2 and landscape change using a coupled plant and meteorological model. Global Change Biology, 7, 797-815.

suggests that the biogeochemical of enhanced atmospheric concentrations of CO2 may have a greater effect on the climate system than that due to the radiative effect of added CO2.

The goal of the Paul Crutzen’s proposal which focuses on altering the global averaged radiative forcing is clearly too limited and incompletely thought through.

In answer to the question “Deliberate Global Climate Modification – Is This A Good Idea?â€? The answer is NO.

WMO Statement On The Status Of The Global Climate in 2006 – A Comment By Climate Science

The World Meteorological Organization (WMO) released a statement on the global climate in 2006 on December 14 2006.

Unfortunately, with respect to reporting on surface temperature trends, the Statement perpetuates the use of surface temperature trends as the metric to assess global warming (or cooling) [i.e. why not, at least, also include ocean heat content anomalies for 2006?]. Moreover, the Statement does not question the accuracy and spatial representativeness of the land surface temperature data.

The WMO Statement on the Status of the Global Climate in 2006 includes the information that,

“The global mean surface temperature in 2006 is currently estimated to be + 0.42°C above the 1961-1990 annual average (14°C/57.2°F), according to the records maintained by Members of the World Meteorological Organization (WMO). The year 2006 is currently estimated to be the sixth warmest year on record. Final figures will not be released until March 2007.

Averaged separately for both hemispheres, 2006 surface temperatures for the northern hemisphere (0.58°C above 30-year mean of 14.6°C/58.28°F) are likely to be the fourth warmest and for the southern hemisphere (0.26°C above 30-year mean of 13.4°C/56.12°F), the seventh warmest in the instrumental record from 1861 to the present.

Since the start of the 20th century, the global average surface temperature has risen approximately 0.7°C. But this rise has not been continuous. Since 1976, the global average temperature has risen sharply, at 0.18°C per decade. In the northern and southern hemispheres, the period 1997-2006 averaged 0.53°C and 0.27°C above the 1961-1990 mean, respectively.

Regional temperature anomalies

The beginning of 2006 was unusually mild in large parts of North America and the western European Arctic islands, though there were harsh winter conditions in Asia, the Russian Federation and parts of eastern Europe. Canada experienced its mildest winter and spring on record, the USA its warmest January-September on record and the monthly temperatures in the Arctic island of Spitsbergen (Svalbard Lufthavn) for January and April included new highs with anomalies of +12.6°C and +12.2°C, respectively.

Persistent extreme heat affected much of eastern Australia from late December 2005 until early March with many records being set (e.g. second hottest day on record in Sydney with 44.2°C/111.6°F on 1 January). Spring 2006 (September-November) was Australia’s warmest since seasonal records were first compiled in 1950. Heat waves were also registered in Brazil from January until March (e.g. 44.6°C/112.3°F in Bom Jesus on 31 January – one of the highest temperatures ever recorded in Brazil).

Several parts of Europe and the USA experienced heat waves with record temperatures in July and August. Air temperatures in many parts of the USA reached 40°C/104°F or more. The July European-average land-surface air temperature was the warmest on record at 2.7°C above the climatological normal.

Autumn 2006 (September-November) was exceptional in large parts of Europe at more than 3°C warmer than the climatological normal from the north side of the Alps to southern Norway. In many countries it was the warmest autumn since official measurements began: records in central England go back to 1659 (1706 in The Netherlands and 1768 in Denmark).”

Climate Science has three comments in this presentation of the temperature anomolies.

1. The summary of regional extremes included only one brief extreme cold period in 2006. If there were just this one, this would be remarkable, and would bolster those who have concluded the global climate system is on a rapid upswing of warming. However, if there were other extreme cold periods, the neglect of including these cold events is a clear example of cherry picking to promote a particular perspective on climate change.

The figure below presents the NCEP/NCAR Reanalysis of the surface temperature anomalies for January to November 2006 (thanks to Phil Klotzbach for this). As clear in this figure, it was significantly warmer than average in the polar latitudes, but there was regions of cooler than average temperatures (such over and near northern Australia and large parts of Siberia). Such regional spatial structure illustrates why a focus of regional trends and anomalies, rather than a global average linear trend should be the emphasis in multi-decadal climate assessments.

[Obtained from
http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl].

To provide examples of the regionally large anomalies on shorter time scales, the four figures below illustrate both significant cold and warm anomalies for the January-February and October-November 2006 time periods (for both the surface air and 700 hPa temperatures). The large winter cold anomaly is quite clear in the January-February figure, for example.

Other figures which document regionally large warm and cool anomalies are available from the excellent NOAA Climate Diagnostic website.

2. As we have documented most recently in

Pielke Sr., R.A., C. Davey, D. Niyogi, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, J. Angel, R. Mahmood, S. Foster, J. Steinweg-Woods, R. Boyles , S. Fall, R.T. McNider, and P. Blanken, 2006: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Research, submitted,

there are significant biases in the land surface component of the temperature trend record. This includes a warm bias in the nighttime minimum temperatures (if the intent is to monitor climate system heat content changes). The WMO is either ignoring or is unaware that any warming (or cooling) in the nighttime boundary layer results in near surface temperature anomalies that overstate the actual warming or cooling in the boundary layer; see

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.

3. The WMO Statement reports on where the surface temperature data used to prepare the Statement comes from; i.e.

“Information sources

This preliminary information for 2006 is based on observations up to the end of November from networks of land-based weather stations, ships and buoys. The data are collected and disseminated on a continuing basis by the National Meteorological and Hydrological Services of WMO Members. However, the declining state of some observational platforms in some parts of the world is of concern.

It should be noted that, following established practice, WMO’s global temperature analyses are based on two different datasets. One is the combined dataset maintained by the Hadley Centre of the UK Met Office, and the Climatic Research Unit, University of East Anglia, UK. The other is maintained by the US Department of Commerce’s National Oceanic and Atmospheric Administration (NOAA). Results from these two datasets are comparable: both indicate that 2006 is likely to be the sixth warmest year globally.”

The two data sets that are referred to, however, are NOT different and are NOT independent assessments of temperature anomalies. As we report in the Pielke et al. paper,

“The raw surface temperature data from which all of the different global surface temperature trend analyses are derived are essentially the same. The best estimate that has been reported is that 90-95% of the raw data in each of the analyses is the same [Phil Jones, personal communication]. That the analyses produce similar trends should, therefore, come as no surprise.”

Thus the WMO Statement that there are two data sets is misleading, and provides a reader of the Statement with an inaccurate assumption on the robustness of the assessment of the surface temperature trends.

The preparation of the WMO Statement, therefore, is not a balanced presentation of climate system heat content changes (e.g. “global warming”), or climate, in general, in 2006. What should be of concern to everyone is that peer-reviewed issues that have been reported in the scientific literature concerning the robustness of the land surface temperature data to assess multi-decadal trends and anomalies to tenths of a degree are being ignored. Moreover, there is a clear emphasis on warm events rather than also including the colder than average episodes that occurred during the year. It is encouraging, however, that the WMO Statement had a regional focus in part of their Statement, as has been urged by Climate Science.

For their Final Statement for 2006, all of us should encourage the WMO to prepare a summary of the climate which includes each of the major regional temperature anomaly events, even if they conflict with the multi-decadal global climate models predictions.

Land Surface Station Density Change Over Time

There is an excellent illustration of land surface station locations and their change over time on a website at the University of Delaware (thanks to Ross McKittrick for alerting me to this). Cort Willmott of the University of Delaware is the original source for this information.

The information includes

“Station Locations (1950 – 1999) by year”

and

“Station Locations (1950 – 1999) by month”.

There is a remarkable reduction in station density after about 1990. An appropriate research question is whether this loss of stations is part of the reason for the greater increase in land surface surface air temperatures in the 1990s, since, as documented in Climate Science (e.g. see), a singinficant number of the current stations are poorly sited with respect to monitoring tenths of a degree C per decade trends.

The Need For Sustained and Freely Available Access to Real-Time and Archived Climate Data

A very important Editorial appeared on December 15 2006 in Science entitled “Taking the Pulse of the Oceans” by Keith Alverson and D. James Baker. It reads,

“Understandng human impact on the global environment requires accurare and integrated observations of all of its interconnected systems. Increasingly complex models, running on ever more powerful computers, are being used to elucidate dynamic links among the atmosphere, ocean, earth, cryosphere, and biosphere. But the real requirement for integrated Earth system science is a systematic, sustained record of observations, starting from as early as we can get quantitative information and extending reliably into the future. In particular, the ocean is critically undersampled both in space and time, and national and intergovernmental observational commitments are essential for progress.

Ocean basins cover most of the planet and are filled with circulating turbulent fluid whose behavior can be modeled only by approximation. For instance, we talk of a “conveyor belt,â€? but this is an unrealistic cartoon of actual turbulent circulation, which by transporting heat and fresh water affects the planet’s climate. Knowledge about the true variability of the circulation remains elusive because long-term systematic observations are lacking.

Any seafarer knows that although one can look up from the deck of a ship and see the Moon clearly through 100 km of atmosphere, one cannot look down and see further than 1 m. Because the ocean is opaque to all wavelengths of electromagnetic radiation, Earth-observing satellites can’t see below the surface either. Thus, much of the ocean must be observed from a patchwork of drifting and moored buoys, neutrally buoyant floats, coastal installations, and ship-based measurements.

Great recent progress has been made with each of these individual observing-system components. The launch of the 1250th drifting surface buoy in Halifax Harbor last year completed a network that is vital for tropical storm track prediction. The rapidly expanding international network of Argo floats has rewritten our knowledge of the temperature and salinity of the upper oceans. Moored buoy arrays in the tropics have made seasonal climate and El Niño prediction a real possibility. With tide gauges reporting in real time, not only can we predict coastal inundation hazards, but we can also disentangle the myriad processes involved in changing global sea level. Although observing the ocean is challenging, in particular cases it can be done well.

For 15 years, a global ocean-observing system under the auspices of the Intergovernmental Oceanographic Commission (IOC) of the United Nations’ Educational, Scientific, and Cultural Organization (UNESCO) has been meeting important needs of global society. However, surprisingly little progress has been made toward a truly global system with long-term funding commitments. Lacking such a system and commitments, critical scientific hypotheses will remain untested.

The IOC is now working with the Global Earth Observation System of Systems (GEOSS) to identify national focal points for ocean observation efforts and to integrate these efforts into a truly global system. Unfortunately, there is still no plan for sustaining individual measurement programs, for integrating them into a coherent observing system, or for supporting them with stable funding. With a few notable exceptions, substantial multilateral government support for coordination and integration remains elusive.

To address this flaw, we propose the development of a UNESCO convention that commits nations to sustaining an integrated ocean-observing system that will lead to better understanding of the ocean and at the same time enable the provision of hazard warnings, monitoring of climate change, and management of marine and coastal resources. UNESCO’s IOC stands ready to broker the development of such a convention. Preliminary discussions, including completion of the initial GEOSS tasks in ocean observation, begin at the next meeting of the Intergovernmental Committee for the Global Ocean ObservingSystem in June 2007 in Paris. Will your nation be at the table?”

As has been discussed on Climate Science, and also indicated in the Op-Ed by Keith Alverson and D. James Baker, climate involves the atmosphere, ocean, earth, cryosphere, and biosphere. Climate Science endorses this Editorial with respect to ocean data and adds that we also need real-time internet access to the ocean measurements as well as all climate data. Real-time and archived climate data should be open and freely available to everyone.

The Relevance of Nonlinear Effects In the Climate System

There is an interesting and important comment posted by Tas at December 2, 2006 at 02:31 AM on Prometheus with respect to the nonlinearity of the climate system in that small perturbations can result in critical threshold shifts in climate with resultant important environmental and social impacts.

The comment states in part,

“…we have strong indications that we live in a non-linear world. The climate system almost certainly has thresholds which when crossed cause an essentially irreversible mode-switch. For example, once the Greenland ice sheet collapse passes a certain point, it is unlikely to regrow in the current regime.

So the argument that incremental changes don’t matter because they are “insignificant”, fails at some point – an unknown point, a priori – because once crossed such thresholds cannot be “uncrossed”. This is compounded enormously by the inherent lags in the system. It is the potential for committing now, to a non-linear change that will only be realized sometime later, that really underlies the need for caution. To dissect changes to their minutest components and argue that any or all are not significant is a convenient rhetorical device, but, to paraphrase Richard Feynman, nature will not be fooled. Whether it is an air conditioner in Melbourne, or an oil furnace in Madison, the final mole of CO2 may push us to a point where the world is no longer the same.”

This comment is correct that the natural climate system is nonlinear. We present evidence of this in our paper

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.

Climate Science agrees that the human disturbance of the climate system certainly could also result in threshold changes (as well as, inadvertantly move us away from a natural climate change threshold).

However, what is missed in the discussion is that it is not only the radiative effect of CO2 that is being perturbed by human activity, as has been extensively documented on Climate Science (see the weblogs in the Category “Climate Change Forcings and Feedbacks”), and in the 2005 National Research Council Report “Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties“.

Thus if we accept that small perturbations can result in significant changes in the climate system through nonlinear interactions, then all of the human- and natural climate forcings need to be assessed in this context. Moreover, since the radiative forcing of CO2 is more dispersed than a number of other climate forcings, as we have documented for aerosols (see) and land cover/land use change (see), it reasonable to propose that they pose a greater threat of causing threshold changes in the climate system since they are more concentrated in their spatial forcing. [as a simple example of this, we find a greater impact of sunlight on a piece of paper when we focus it with a lens!).

The more heterogeneous climate forcing of land cover/land use change is why NASA reported, based on our peer reviewed research, that “Landcover changes may rival greenhouse gases as cause of climate change“. The comment on Prometheus inadvertently provides further reasoning on why the IPCC needs to move beyond its focus on the radiative effect of the well-mixed greenhouse gases, and its treatment of the multi-decadal behavior of the climate system as a quasi-linear system.

The subject of the importance of the nonlinearity of the climate was also discussed in the Climate Science weblog of December 24 2005.