will nonetheless have the same latitudinal and seasonal response of surface temperature. For example, Crutzen (2006) cites the results of Govindasamy and Caldeira (2000), whose model results indicate that, at least for the surface temperature response, roughly a 1.8% decrease in the solar constant would quite closely offset the latitudinal and seasonal temperature response to a doubling of the CO2 concentration, even though there are significant differences in the seasonal and spatial patterns of forcing. Indeed, this assumption is implicit in the global-scale summation of forcings made by the IPCC (2007a), which, although the regional patterns of response can somewhat vary, treat as equivalent, at least in a policy planning sense, the forcings caused by increasing concentrations of greenhouse gases, sulfates and other aerosols, tropospheric ozone, contrails, and land cover change.

That this may well not be the case would seem to be demonstrated by the correlation of the waxing and waning of ice ages with the cycling of orbital elements. Essentially, all that the orbital element changes do in terms of radiative forcing is to alter the seasonal and latitudinal pattern of incoming solar radiation; they do not significantly change the integrated annual solar flux to the Earth system. As a result, orbital forcing would be assigned a near zero value on the IPCC forcing diagram (attempts to explain ice age forcing typically do so by counting the albedo influence of the ice sheets as a forcing, but this seems to be confusing a forcing with a feedback). Factors (such as changes in solar irradiance) that are given small values on the IPCC chart of forcings typically have small effects on global average temperature, yet orbital variations appear to be the drivers of the ice age cycling that the Earth has experienced over the past million years and more, causing changes in global average temperature of 5-6oC. If geoengineering is going to have to be used over several centuries, it is going to be necessary to go beyond the few decade simulations by climate models that have suggested the offset can be calculated on a global basis.

The point to be made is that the IPCC chart is a simplification—the actual model simulations show the situation is a good deal more complex. For example, the cooling influence of the greater loading of sulfate in the Northern than in the Southern Hemisphere in the mid-20<sup>th</sup> century allowed a slightly larger warming in the Southern Hemisphere, and regional modification of sea surface temperatures during an El Niño, can affect the atmospheric circulation around much of the world. Thus, before proposing that geoengineering might be able to really counterbalance the climatic response from increasing greenhouse gas and aerosol concentrations, a much clearer explanation and much more simulation is going to be needed to investigate the long-term effects on climate of differences in the latitudinal and seasonal patterns of the forcing, and how changes in the oceans and in snow and ice cover might affect the response.

While analysis has begun for some of the geoengineering approaches (Caldeira and Wood, 2008; Latham et al., 2008; Robock et al., 2008), much more consideration is needed regarding how the approaches might be implemented over time and in association with other types of changes in the climate and environment. There is already some indication that volcanic aerosols can, by altering the atmospheric circulation, produce winter warming over Northern Hemisphere continents (e.g., see Robock, 2002), making it likely that stratospheric aerosols intended to cool the climate could warm the climate in some regions and at some times of the year, adding to the warming from the higher concentrations of greenhouse gases. However, much more effort will be needed, not only seeking to understand the consequences of geoengineering, but also to better understand the 29