Many uncertainties surround the future effects of climate change. Keith, a physicist, argues that humankind would be wise to do some contingency planning, in case those effects prove to be more powerful than now predicted or come faster than expected. The earth has experienced many different climates, and human beings are highly adaptable. But we adapt more easily to slow processes than to rapid ones, so we should learn how to slow down global warming through climate engineering -- just in case we need to. Keith focuses on techniques to reduce the amount of sunlight that reaches the earth’s surface by reflecting some of it back into space, such as placing aerosols into the stratosphere. Keith does not recommend that anyone try this yet, nor does he suggest that it would be a good substitute for reducing emissions of greenhouse gases. But he wants scientists to learn how to do it effectively and efficiently while minimizing any unwanted side effects.