8.3.4.3 Surface Fluxes

Despite considerable effort since the TAR, uncertainties remain in the representation of solar radiation in climate models (Potter and Cess, 2004). The AMIP-2 results and the recent model results in the MMD provide an opportunity for a major systematic evaluation of model ability to simulate solar radiation. Wild (2005) and Wild et al. (2006) evaluated these models and found considerable differences in the global annual mean solar radiation absorbed at the Earth’s surface. In comparison to global surface observations, Wild (2005) concluded that many climate models overestimate surface absorption of solar radiation partly due to problems in the parametrizations of atmospheric absorption, clouds and aerosols. Similar uncertainties exist in the simulation of downwelling infrared radiation (Wild et al., 2001). Difficulties in simulating absorbed solar and infrared radiation at the surface leads inevitably to uncertainty in the simulation of surface sensible and latent heat fluxes.

8.3.4.4 Carbon

A major advance since the TAR is some systematic assessments of the capability of land surface models to simulate carbon. Dargaville et al. (2002) evaluated the capacity of four global vegetation models to simulate the seasonal dynamics and interannual variability of atmospheric CO₂ between 1980 and 1991. Using off-line forcing, they evaluated the capacity of these models to simulate carbon fluxes, via an atmospheric transport model, using observed atmospheric CO₂ concentration. They found that the terrestrial models tended to underestimate the amplitude of the seasonal cycle and simulated the spring uptake of CO₂ approximately one to two months too early. Of the four models, none was clearly superior in its capacity to simulate the global carbon budget, but all four reproduced the main features of the observed seasonal cycle in atmospheric CO₂. A further off-line evaluation of the LPJ global vegetation model by Sitch et al. (2003) provided confidence that the model could replicate the observed vegetation pattern, seasonal variability in net ecosystem exchange and local soil moisture measurements when forced by observed climatologies.

The only systematic evaluation of carbon models that were interactively coupled to climate models occurred as part of the Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP), where Friedlingstein et al. (2006) compared the ability of a suite of models to simulate historical atmospheric CO₂ concentration forced by observed emissions. Issues relating to the magnitude of the fertilization effect and the partitioning between land and ocean uptake were identified in individual models, but it is only under increasing CO₂ in the future (see Chapter 10) that the differences become large. Several other groups have evaluated the impact of coupling specific models of carbon to climate models but clear results are difficult to obtain because of inevitable biases in both the terrestrial and atmospheric modules (e.g., Delire et al., 2003).