Global Climate Models and Their Limitations Anthony Lupo (USA) William Kininmonth (Australia) Contributing: J

1 Global Climate Models and Their Limitations Anthony Lupo (USA) William Kininmonth (Australia) Contributing: J. Scott Armstrong (USA), Kesten Green (Australia) 1. Global Climate Models and Their Limitations Key Findings Introduction 1.1 Model Simulation and Forecasting 1.2 Modeling Techniques 1.3 Elements of Climate 1.4 Large Scale Phenomena and Teleconnections Key Findings Confidence in a model is further based on the The IPCC places great confidence in the ability of careful evaluation of its performance, in which model general circulation models (GCMs) to simulate future output is compared against actual observations. A climate and attribute observed climate change to large portion of this chapter, therefore, is devoted to anthropogenic emissions of greenhouse gases. They the evaluation of climate models against real-world claim the “development of climate models has climate and other biospheric data. That evaluation, resulted in more realism in the representation of many summarized in the findings of numerous peer- quantities and aspects of the climate system,” adding, reviewed scientific papers described in the different “it is extremely likely that human activities have subsections of this chapter, reveals the IPCC is caused more than half of the observed increase in overestimating the ability of current state-of-the-art global average surface temperature since the 1950s” GCMs to accurately simulate both past and future (p. 9 and 10 of the Summary for Policy Makers, climate. The IPCC’s stated confidence in the models, Second Order Draft of AR5, dated October 5, 2012). as presented at the beginning of this chapter, is likely This chapter begins with a brief review of the exaggerated. The many and varied model deficiencies inner workings and limitations of climate models. discussed in this chapter indicate much work remains Climate models are important tools utilized to to be done before model simulations can be treated advance our understanding of current and past with the level of confidence ascribed to them by the climate. They also provide qualitative and IPCC. quantitative information about potential future The following points summarize the main climate. But in spite of all their sophistication, they findings of this chapter: remain merely models. They represent simulations of the real world, constrained by their ability to correctly • Properties inherent in models make dynamic capture and portray each of the important processes predictability impossible. Without dynamic that operate to affect climate. Notwithstanding their predictability, other techniques must be used to complexities, the models remain deficient in many simulate climate. Such techniques introduce aspects of their portrayal of the climate, which biases of varying magnitude into model reduces their ability to provide reliable simulations of projections. future climate. 9 Climate Change Reconsidered II • To have any validity in terms of future Introduction projections, GCMs must incorporate not only the Global Climate Models (GCMs) have evolved from many physical processes involved in determining the Atmospheric General Circulation Models climate, but also all important chemical and (AGCMs) widely used for daily weather prediction. biological processes that influence climate over GCMs have been used for a range of applications, long time periods. Several of these important including investigating interactions between processes are either missing or inadequately processes of the climate system, simulating evolution represented in today’s state-of-the-art climate of the climate system, and providing projections of models. future climate states under scenarios that might alter the evolution of the climate system. The most widely • Limitations in computing power frequently result recognized application is the projection of future in the inability of models to resolve important climate states under various scenarios of increasing climate processes. Low-resolution models fail to atmospheric carbon dioxide (CO2). capture many important phenomena of regional At the core of a GCM is an AGCM that and lesser scales, such as clouds; downscaling to dynamically simulates the circulation of the higher-resolution models introduces boundary atmosphere, including the many processes that interactions that can contaminate the modelling regulate energy transport and exchange by and within area and propagate error. the atmospheric flow. The basic atmospheric flow is represented by fundamental equations that link the • The magnitude of the range of projected mass distribution and the wind field. These equations responses to a doubling of atmospheric CO2 by are represented on a spherically spatial grid field that itself establishes that large errors and limitations has many levels representing the depth of the in the models remain to be corrected. atmosphere. The flow equations are modified by the representation of processes that occur on a scale • Many GCMs fail to account properly for certain below that of the grid—including such processes as “multiplier effects” that may significantly amplify turbulence, latent heat of condensation in cloud the initial impacts of various biospheric formation, and dynamic heating as solar and infrared processes. For example, although the absolute radiation interact with atmospheric gases, aerosols, variations associated with some solar-related and clouds. phenomena are rather small, Several multiplier The oceans are at least as important as the effects may significantly amplify the initial atmosphere for the transport of energy. For that perturbation. reason, the GCM also includes an Ocean General Circulation Model (OGCM) that simulates the • Major imperfections in the models prevent proper circulation of the oceans. The OGCM is vital for simulation of important elements of the climate climate simulations because the oceans represent a system, including pressure, wind, clouds, dynamic thermal reservoir that, through energy temperature, precipitation, ocean currents, sea ice, exchange with the atmosphere, dominates the permafrost, etc. Large differences between model evolution of the climate system. The specification of predictions and observations frequently exist the processes that regulate heat, moisture, and when comparing these elements or features. In momentum exchanges between the ocean and some cases computer models fail to simulate even atmosphere is crucial to the integrity of a GCM. the correct sign of the observed parameters. Land surface, and how soil moisture and vegetation type regulate heat, moisture, and • Although some improvements have been noted in momentum with the atmosphere, plays a lesser but performance between the CMIP3 set of models nevertheless important role in the simulation of used in AR4 and the newer CMIP5 models climate. Soil moisture and vegetation respond to local utilized in AR5, many researchers report finding precipitation and affect the exchange of heat, little or no improvement in the CMIP5 model moisture, and momentum with the atmosphere over output for several important parameters and time. The soil moisture and vegetation (and their features of Earth’s climate. regulation of land-atmosphere exchange processes) respond to the climate on the shorter time-scale of weather systems but, due to the varying accumulation 10 Global Climate Models and Their Limitations of soil moisture, the influence of land surface on on a limited wavelength band, will affect the local climate is on seasonal and interannual time-scales. magnitude of infrared radiation loss to space. Apart Surface ice sheets also have an important role in from water vapor concentration these variations are the evolution of the climate system. Their formation not necessarily temperature-dependent. Thus a change and expansion represent a lowering of the total energy to the internal structure of the climate system for of the climate system as a whole because latent heat is whatever reason will—all else being equal—lead to lost as water changes from the liquid to solid phase. change in Earth’s equilibrium temperature. Likewise, contraction of surface ice sheets represents Within the AGCM there are many important an increase in the total energy of the climate system. processes that operate on scales below the resolution The representation of heat, moisture, and momentum of the computational grid (sub-grid scale processes) exchanges between ice surfaces and the atmosphere and regulate local temperature, moisture, and differs from that of land surfaces or ocean surfaces. momentum. Perhaps the most important of these is Dominating the climate system and its evolution convection. are the radiation processes that regulate the input and As described by Riehl and Malkus (1958), it is output of energy. The shape of the rotating Earth, its the “hot towers” of deep tropical convection that distance from the Sun, and the characteristics of its distribute the accumulating heat and latent energy of orbit determine the nature of diurnal (daytime) and the tropical boundary layer through the troposphere. seasonal solar heating, including its maximum over Correct specification of the mass flows within the the tropics. The shedding of energy by infrared cloud mass and its surroundings, including the radiation originates from the surface, from aerosols, updrafts and downdrafts, is essential for regulating from clouds, and from greenhouse gases of the the Hadley Cell circulation and the availability of atmosphere (CO2, H2O, O3, etc.). The latitudinal tropical

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