Nobody knows what the global temperature is likely to be in the future, for the climate is a system of almost infinite complexity. Predicting how much hotter a particular level of carbon dioxide will make the world is impossible. It's not just that the precise effect of greenhouse gases on temperature is unclear. It may set off mechanisms that tend to cool things down (clouds which block out sunlight, for instance) or ones that heat the world further (by melting soils in which greenhouse gases are frozen, for instance). The system could right itself or spin out of human control. This uncertainty is central to the difficulty of tackling the problem. Since the costs of climate change are unknown, the benefits of trying to do anything to prevent it are, by definition, unclear. What's more, if they accrue at all, they will do so at some point in the future. So is it really worth using public resources now to avert an uncertain, distant risk, especially when the cash could be spent instead on goods and services that would have a measurable near-term benefit? If the risk is big enough, yes. Governments do it all the time. They spend a small slice of tax revenue on keeping standing armies not because they think their countries are in imminent danger of invasion but because, if it happened, the consequences would be catastrophic. Individuals do so, too. They spend a little of their incomes on household insurance not because they think their homes are likely to be torched next week but because, if it happened, the results would be disastrous. Similarly, a growing body of scientific evidence suggests that the risk of a climatic catastrophe is high enough for the world to spend a small proportion of its income trying to prevent one from happening.