Forecasting the Climate of the New Century.

• Author: J. Somerville, Richard C.

Computer simulations are our best window into the climate of the future, but our view is obscured by uncertainties in the models based on an incomplete understanding of how the climate system works. Recent research has led to a greatly increased understanding of the uncertainties in today's climate models. In attempting to predict the climate of the twenty-first century, we must confront not only computer limitations on the affordable spatial resolution of global models, but also a lack of physical realism in attempting to model key climate processes. Until we are able to incorporate adequate treatments of critical elements of the entire biogeophysical climate system, our models will remain subject to these uncertainties, and our scenarios of future climate change, both anthropogenic and natural, will not fully meet the requirements of either policy makers or the public. The areas of most-needed model improvements are thought to include air-sea exchanges, land surface processes, ice and snow physics, hydrolog ic cycle elements, and especially the role of aerosols (small particles), and cloud-radiation interactions.

Of these areas, cloud-radiation interactions are known to be responsible for much of the inter-model differences in sensitivity to green-house gases. In current research, scientists in many countries are diagnostically evaluating current and proposed model cloud-radiation treatments against extensive field observations. Satellite remote sensing provides an indispensable component of the observational resources. The researchers find that newly developed advanced schemes with explicit cloud-water budgets and interactive cloud-radiative properties are potentially capable of matching observational data closely. However, it is difficult to evaluate the realism of model-produced fields of cloud-radiative properties, cloud-liquid water content and effective cloud-droplet radius until high quality measurements of these quantities become more widely available. Thus, further progress will require a combination of theoretical and modelling research, together with intensified emphasis on both in situ and space-based rem ote sensing observations.

From far away, the Earth appears as a mainly blue ocean planet flecked with constantly changing intricate patterns of white clouds. The clouds, which cover about half the surface of the planet, are critical to climate. They cool the Earth by reflecting away sunlight. At the same time, they warm the...