I think climate change should have been left outside of what they were looking at altogether for lots of reasons.
The projects are too uncertain to be ranked. Putting an extra $1b towards existing climate change projects may result in absolutely no effect on climate change. We are talking about risky, expensive projects. An economist doesn't really know if a propaganda campaign to convince americans or Chinese to sign some deal is better then some geo-engineering technology project to directly cool the earth. It is outside the realm of what smart economists are good at.
- It is coming out of different pockets. Climate change affects rich countries too. It is self interested spending. Certainly, there are tragedy of the commons like problems. But it is still in the interest of those spending to spend.
- The effects of climate change are uncertain and indirect. One of the potential climate change issues is decreased glacial melt in India resulting in less summer river flows in to Pakistan. India is entitled (by treaty) to a fixed (in litres) amount of water. Pakistan needs this water to grow food without which it will be in famine. This could result in, mass refuge crisis, nuclear war, etc. It is virtually impossible to link something like that directly to an increase or decrease of $500m in spending by some NGO.
*But... If you are a millionaire or in charge of an NGO or a foreign spending budget or give money to a poverty related charity, the Copenhagen Consensus seems like an important list to consider. If decide to act against its recommendations, at least have a reason.
If you look at the numbers, global warming are not nearly one of the biggest challenges.
Yes, there are some source issues that might solve a number of the problems (as asdlfj2sd33 noted earlier). But sometimes it's quicker to deal with symptoms first, and when millions of lives are on the line I'm happy with that kind of shortcut.
That Global Warming ranks so low warrants attention. What sacrifices are we in the developed world causing by focusing on Global Warming and overlooking the other issues?