Curt Covey

THE WCRP CMIP3 Multimodel Dataset: A New Era in Climate Change Research

A coordinated set of global coupled climate model [atmosphere–ocean general circulation model (AOGCM)] experiments for twentieth- and twenty-first-century climate, as well as several climate change commitment and other experiments, was run by 16 modeling groups from 11 countries with 23 models for assessment in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Since the assessment was completed, output from another model has been added to the dataset, so the participation is now 17 groups from 12 countries with 24 models. This effort, as well as the subsequent analysis phase, was organized by the World Climate Research Programme (WCRP) Climate Variability and Predictability (CLIVAR) Working Group on Coupled Models (WGCM) Climate Simulation Panel, and constitutes the third phase of the Coupled Model Intercomparison Project (CMIP3). The dataset is called the WCRP CMIP3 multimodel dataset, and represents the largest and most comprehensive international global coupled climate model experiment and multimodel analysis effort ever attempted. As of March 2007, the Program for Climate Model Diagnostics and Intercomparison (PCMDI) has collected, archived, and served roughly 32 TB of model data. With oversight from the panel, the multimodel data were made openly available from PCMDI for analysis and academic applications. Over 171 TB of data had been downloaded among the more than 1000 registered users to date. Over 200 journal articles, based in part on the dataset, have been published so far. Though initially aimed at the IPCC AR4, this unique and valuable resource will continue to be maintained for at least the next several years. Never before has such an extensive set of climate model simulations been made available to the international climate science community for study. The ready access to the multimodel dataset opens up these types of model analyses to researchers, including students, who previously could not obtain state-of-the-art climate model output, and thus represents a new era in climate change research. As a direct consequence, these ongoing studies are increasing the body of knowledge regarding our understanding of how the climate system currently works, and how it may change in the future.

An overview of results from the Coupled Model Intercomparison Project

Abstract The Coupled Model Intercomparison Project (CMIP) collects output from global coupled ocean–atmosphere general circulation models (coupled GCMs). Among other uses, such models are employed both to detect anthropogenic effects in the climate record of the past century and to project future climatic changes due to human production of greenhouse gases and aerosols. CMIP has archived output from both constant forcing (“control run”) and perturbed (1% per year increasing atmospheric carbon dioxide) simulations. This report summarizes results form 18 CMIP models. A third of the models refrain from employing ad hoc flux adjustments at the ocean–atmosphere interface. The new generation of non-flux-adjusted control runs are nearly as stable as—and agree with observations nearly as well as—the flux-adjusted models. Both flux-adjusted and non-flux-adjusted models simulate an overall level of natural internal climate variability that is within the bounds set by observations. These developments represent significant progress in the state of the art of climate modeling since the Second (1995) Scientific Assessment Report of the Intergovernmental Panel on Climate Change (IPCC; see Gates et al. [Gates, W.L., et al., 1996. Climate models—Evaluation. Climate Climate 1995: The Science of Climate Change, Houghton, J.T., et al. (Eds.), Cambridge Univ. Press, pp. 229–284]). In the increasing-CO2 runs, differences between different models, while substantial, are not as great as one might expect from earlier assessments that relied on equilibrium climate sensitivity.

Environmental perturbations caused by the impacts of asteroids and comets

We review the major impact-associated mechanisms proposed to cause extinctions at the Cretaceous-Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 104 Mt (i.e., impact frequencies less than one in 6 × 104 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 104 or 105 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 104–105 Mt (intervals up to 3 × 105 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 105 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 105 to 106 Mt (intervals up to 2 × 106 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild-fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock-heated air to destroy the ozone shield. Between 106 and 107 Mt (intervals up to 1.5 × 107 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 107 km², and the resulting smoke would reduce light levels even further. At energies above 107 Mt, blast and earthquake damage reach the regional scale (106 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the worlds ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 109 Mt (>108 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous-Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

The Coupled Model Intercomparison Project (CMIP)

Abstract The Coupled Model Intercomparison Project (CMIP) was established to study and intercompare climate simulations made with coupled ocean–atmosphere–cryosphere–land GCMs. There are two main phases (CMIP1 and CMIP2), which study, respectively, 1) the ability of models to simulate current climate, and 2) model simulations of climate change due to an idealized change in forcing (a 1% per year CO2 increase). Results from a number of CMIP projects were reported at the first CMIP Workshop held in Melbourne, Australia, in October 1998. Some recent advances in global coupled modeling related to CMIP were also reported. Presentations were based on preliminary unpublished results. Key outcomes from the workshop were that 1) many observed aspects of climate variability are simulated in global coupled models including the North Atlantic oscillation and its linkages to North Atlantic SSTs, El Nino–like events, and monsoon interannual variability; 2) the amplitude of both high– and low–frequency global mean surfa...

OVERVIEW OF THE COUPLED MODEL INTERCOMPARISON PROJECT

Abstract The Coupled Model Intercomparison Project (CMIP) involves study and intercomparison of multi-model simulations of present and future climate. The simulations of the future use idealized forcing in increase is compounded which CO2 1% yr−1 until it doubles (near year 70) with global coupled models that contain, typically, components representing atmosphere, ocean, sea ice, and land surface. Results from CMIP diagnostic subprojects were presented at the Second CMIP Workshop held at the Max Planck Institute for Meteorology in Hamburg, Germany, in September 2003. Significant progress in diagnosing and understanding results from global coupled models has been made since the time of the First CMIP Workshop in Melbourne, Australia, in 1998. For example, the issue of flux adjustment is slowly fading as more and more models obtain stable multi-century surface climates without them. El Nino variability, usually about half the observed amplitude in the previous generation of coupled models, is now more accur...

Radiative and Dynamical Feedbacks over the Equatorial Cold Tongue: Results from Nine Atmospheric GCMs

Abstract The equatorial Pacific is a region with strong negative feedbacks. Yet coupled general circulation models (GCMs) have exhibited a propensity to develop a significant SST bias in that region, suggesting an unrealistic sensitivity in the coupled models to small energy flux errors that inevitably occur in the individual model components. Could this “hypersensitivity” exhibited in a coupled model be due to an underestimate of the strength of the negative feedbacks in this region? With this suspicion, the feedbacks in the equatorial Pacific in nine atmospheric GCMs (AGCMs) have been quantified using the interannual variations in that region and compared with the corresponding calculations from the observations. The nine AGCMs are the NCAR Community Climate Model version 1 (CAM1), the NCAR Community Climate Model version 2 (CAM2), the NCAR Community Climate Model version 3 (CAM3), the NCAR CAM3 at T85 resolution, the NASA Seasonal-to-Interannual Prediction Project (NSIPP) Atmospheric Model, the Hadley ...

Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth

Abstract Impacts of comets and small ( ∼ 10 km) asteroids with Earth—frequent events over geologic time—would generate atmospheric dust in amounts orders of magnitude greater than historically large volcanic explosions. Three-dimensional atmospheric model simulations show that climatic effects of such impacts would include drastic cooling of land surfaces due to interception of sunlight by high-altitude dust. Unlike earlier one-dimensional calculations, the three-dimensional results indicate that large areas of the planet would escape freezing. They also suggest catastrophic changes in the hydrological cycle.

Intercomparison makes for a better climate model

Global coupled climate models are elaborate numerical/physical formulations of the atmosphere, ocean, cryosphere, and land which are “coupled” together and interact to simulate the three-dimensional distribution of the climate over the globe. Such models are used to make projections of future climate change due to human activity. Simulation results are widely used to identify vulnerabilities and to study societal impacts that have policy implications. It is clearly important for the scientific community to systematically assess the simulation capabilities of these models. The climate modeling community is doing so in the Coupled Model Intercomparison Project (CMIP) which is an assessment of the “state-of-the-art” in global coupled climate modeling. This activity is being organized by the World Climate Research Programme under the auspices of the Climate Variability and Predictability (CLIVAR) project.

Paleoclimate data constraints on climate sensitivity: The paleocalibration method

The relationship between paleoclimates and the future climate, while not as simple as implied in the ‘paleoanalog’ studies of Budyko and others, nevertheless provides sufficient constraints to broadly confirm the climate sensitivity range of theoretical models and perhaps eventually narrow the model-derived uncertainties. We use a new technique called ‘paleocalibration’ to calculate the ratio of temperature response to forcing on a global mean scale for three key intervals of Earth history. By examining surface conditions reconstructed from geologic data for the Last Glacial Maximum, the middle Cretaceous and the early Eocene, we can estimate the equilibrium climate sensitivity to radiative forcing changes for different extreme climates. We find that the ratios for these three periods, within error bounds, all lie in the range obtained from general circulation models: 2–5 K global warming for doubled atmospheric carbon dioxide. Paleocalibration thus provides a data-based confirmation of theoretically calculated climate sensitivity. However, when compared with paleodata on regional scales, the models show less agreeement with data. For example, our GCM simulation of the early Eocene fails to obtain the temperature contrasts between the Equator and the Poles (and between land and ocean areas) indicated by the data, even though it agrees with the temperature data in the global average. Similar results have been reported by others for the Cretaceous and for the Last Glacial Maximum.

Planetary-scale waves in the Venus atmosphere

Abstract A numerical model of planetary-scale waves in Venus’ atmosphere is used to simulate observed wave-like cloud features such as the dark horizontal Y. The model is based on the linearized primitive equations. Observed variations of static stability and mean zonal wind as a function of altitude are included in the basic state. Preferred modes of oscillation are found by imposing forcing over a range of frequencies, and determining the frequencies at which atmospheric response is greatly enhanced. Preferred responses exist at frequencies which are observed for the Y and other wave-like features. The Y shape can be produced by a linear combination of two model output waves: a midlatitude Rossby wave and an equatorial Kelvin wave. In order to preserve the relative phase between the waves and maintain the Y, nonlinear coupling between the waves is needed. Both waves are upward propagating, similar to the upward propagating planetary waves in Earths stratosphere. The Kelvin wave may be forced at any alt...