In order to elucidate interactions between climate change and biogeochemical processes and to provide a tool for comprehensive analysis of sensitivity, uncertainty, and proposed climate change mitigation policies, we have developed a zonally averaged two-dimensional model including coupled biogeochemical and climate submodels, as a part of an integrated global system model. When driven with calculated or estimated trace gas emissions from both anthropogenic and natural sources, it is designed to simulate centennial-scale evolution of many radiatively and chemically important tracers in the atmosphere. Predicted concentrations of chemical species in the chemistry submodel are used interactively to calculate radiative forcing in the climate submodel, which, in turn, provides winds, temperatures, and other variables to the chemistry submodel. Model predictions of the surface trends of several key species are close to observations over the past 10–20 years. Predicted vertical distributions of climate-relevant species, as well as seasonal variations, are also in good agreement with observations. Runs of the model imply that if the current increasing trends of anthropogenic emissions of climate-relevant gases are continued over the next century, the chemical composition of the atmosphere would be quite different in the year 2100 than that currently observed. The differences involve not only higher concentrations of major long-lived trace gases such as CO₂, N₂O, and CH₄ but also about 20% lower concentrations of the major tropospheric oxidizer (OH free radical), and almost double the current concentrations of the short-lived air pollutants CO and NO_x .

Copyright 1998 by the American Geophysical Union

Measures of reference evapotranspiration are essential for applications of agricultural management and water resources engineering. Using numerous esoteric variables, one can calculate daily reference evapotranspiration using the Modified Penman-Monteith methods. In 1985, Hargreaves developed a simplified method for estimating reference evapotranspiration. Similarly, Droogers and Allen improved upon Hargreaves’ method in 2002. Both methods provide excellent estimates of average daily rates for a given month, based on monthly climatology. The Hargraeves method also estimates daily rates based on daily data, though the Modified Hargreaves approach developed by Droogers and Allen is largely accepted as a stronger metric. Here efforts are made to improve the functionality of Droogers and Allen’s approach and to adapt it to provide daily estimates of reference evapotranspiration based on daily weather. The Hargreaves and Modified Hargeaves are used to calculate daily reference evapotranspiration based on daily data. The coefficients in these equations are then optimized to reduce the root mean squared difference between each estimate and the baseline value calculated by the Modified Penman-Monteith approach. The adapted method for daily reference evapotranspiration proves promising; estimating rates near a root mean squared difference of 1.07 mm/day. These results are validated with data from 1976-1980; here the root mean squared difference is 1.06 mm/day. Results are evaluated spatially and temporally. Weaknesses are seen in the estimates around clearly-defined summers. Further weaknesses are seen in pole-ward regions. Still, at the 1% significance level, the daily optimization of the Modified Hargreaves equation is found to be the best replica of the Modified Penman-Monteith method, globally. Finally, specific caveats and further avenues of research are noted. Overall, the daily Modified-Hargreaves method is advocated for general use in global studies where daily data and variation is of the utmost concern.

Possible climate change caused by an increase ingreenhouse gas concentrations, despite having been asubject of intensive study in recent years, is stillvery uncertain. Uncertainties in projections ofdifferent climate variables are usually described onlyby the ranges of possible values. For assessing thepossible impact of climate change, it would be moreuseful to have probability distributions for thesevariables. Obtaining such distributions is usuallyvery computationally expensive and requires knowledgeof probability distributions for characteristics ofthe climate system that affect climate projections. A fewstudies of this kind have been carried out with energybalance/upwelling diffusion models. Here wedemonstrate a methodology for performing a similarstudy with a 2 dimensional (zonally averaged) climatemodel that reproduces the behavior of coupledatmosphere/ocean general circulation models morerealistically than energy balance models. Thismethodology involves application of the DeterministicEquivalent Modeling Method to derive functionalapproximations of the model's probabilistic response.Monte Carlo analysis is then performed on theapproximations. An application of the methodology isdemonstrated by deriving the uncertainty in surfaceair temperature change and sea level rise due tothermal expansion of the ocean that result fromuncertainties in climate sensitivity and the rate ofheat uptake by the deep ocean for a prescribedincrease in atmospheric CO₂ concentration. Wealso demonstrate propagation of correlateduncertainties through different models, by presentingresults that include uncertainty in projected carbonemissions.

© Springer

A thorough analysis of the inter-hemispheric Rooth 3-box model is performed in order to study the stability of the present northen sinking regime of the Thermohaline Circulation (THC) against perturbations to the atmospheric freshwater and heat fluxes. The perturbations lead the system to a newly established equilibrium that is either qualitatively similar to the initial one, i.e. a northern sinking equilibrium, or radically different i.e. presenting a reversed THC.We first analyze the uncoupled version of the model. High rates of increase in the poleward moisture flux in the Northern Hemisphere lead to a THC breakdown at smaller moisture fluxes than low rates, while increases in the poleward moisture flux in the Southern Hemisphere strongly inhibit the breakdown Similarly, heat flux increases in the Northern Hemisphere destabilize the system more effectively than slow ones, and again the heat flux increases in the Southern Hemisphere tend to drive the system towards stability. Thus in the case of realistic perturbations, e.g., in global warming scenarios, it is necessary to perturb the system in both hemispheres at the same time. Although there is an overall similarity between the effects of atmospheric freshwater and heat fluxes perturbation, a detailed comparison where the intensity of the two perturbations are rescaled in density unity shows that the thermal and saline way of destabilizing the THC are not fully equivalent. In the coupled version of the model (the Scott et al. model) the atmospheric freshatwater and heat fluxes depend of the temperatures of the boxes and are parametrized in a physically based way. The preliminary results obtained for this version of the model present interesting features both resembling and not resembling the results obtained for the uncoupled model. Complete results will be presented at the conference.