Scientists have developed complex computer models to produce climate simulations. The models produce maps that are depictions of how the world of doubled CO2 in the computer differs from the present-day world. One thing that climate models must do skillfully to get the doubled-CO2 climate right is to simulate the present climate realistically. For example, the effect of CO2 in the polar regions is largely dominated by whether there is ice and snow to melt. If there is, then we get a feedback effect, because when it melts, the darker, newly exposed surface absorbs more sunlight. The main challenge of climate modeling is getting the feedbacks right. At present, we cannot blindly rely on the results of climate models, because we are not sure about the extent or magnitude of the feedbacks. In models, the virtual world warms in response to increasing atmospheric carbon dioxide concentrations. Different models with different treatments of important feedback mechanisms give somewhat different magnitudes for the warming.