This book has brought together analyses of twenty-three global problems, and proposals for their solution or mitigation. The authors have used economic cost–benefit analysis to provide a coherent framework for evaluation. This approach has been criticized by some people, especially by those who argue that climate change policy should have a higher priority. This is hardly surprising because, when you ask what you should do first, you also have to work out what should have a lower priority, and this inevitably means displeasing some people.

But we have to make choices. Not prioritizing explicitly means we still make choices, we just don't talk as clearly about them. We believe that putting prices on the world's solutions make our decisions better informed. They are still hard, but perhaps we have a sounder basis to get them right. This is not a perfect approach. But, like Winston Churchill's description of democracy, we believe that this is the least bad option. In particular, it gives a common framework of reference for comparing programs which might otherwise seem so far apart that comparison was not possible.

The other big plus of cost–benefit analysis is that it enables you, the reader, to make your own comparisons and judgments. There is no one right answer. Our aim is to encourage debate and to get people to think in a different way about the world's biggest problems. The challenges covered by our contributors have been with us for a long time.