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By Root 480 0

unrealistic. In one IPCC future, world population reaches fifteen billion by 2100, nearly double what demographers expect. In another, the poorest countries grow their per capita income four times as fast as Japan grew in the twentieth century. All the futures use market exchange rates instead of purchasing power parities for GDP, further exaggerating warming. In other words, the highend projections have pretty wild assumptions, so the 4°C warming, let alone the unlikely 6°C, will only happen if it is also accompanied by truly astonishing increases in human prosperity. And if it is possible to get so prosperous, then the warming cannot have been doing much economic harm along the way.

To this some economists such as Martin Weitzman reply that even if the risk of catastrophe is vanishingly small, the cost would be so great that the normal rules of economics do not apply: so long as there is some possibility of a huge disaster, the world should take all steps to avoid it. The trouble with this reasoning is that it applies to all risks, not just climate change. The annual risk of collision with a very large asteroid, such as wiped out the dinosaurs, is put at about one in 100 billion. Given that such an event would greatly reduce human prosperity, it seems to be rather cheap of humankind to be spending as little as $4m a year to track such asteroids. Why are we not spending large sums stockpiling food caches in cities so that people can survive the risks from North Korean missiles, rogue robots, alien invaders, nuclear war, pandemics, super-volcanoes? Each risk may be very unlikely, but with the potential harm so very great, almost infinite resources deserve to be spent on them, and almost nothing on present causes of distress, under Weitzman’s argument.

In short, the extreme climate outcomes are so unlikely, and depend on such wild assumptions, that they do not dent my optimism one jot. If there is a 99 per cent chance that the world’s poor can grow much richer for a century while still emitting carbon dioxide, then who am I to deny them that chance? After all, the richer they get the less weather dependent their economies will be and the more affordable they will find adaptation to climate change.

Warmer and richer or cooler and poorer?

So much for the outlying risks. Now consider the IPCC’s much more probable central case: a 3°C rise by 2100. (I say more probable, but note that the rate of increase of temperature will have to be double that experienced in the 1980s and 1990s to hit this level – and the rate has been decelerating, not accelerating.) Count the cost – and benefit – of the extra warmth in terms of sea level, water, storms, health, food, species and ecosystems.

Sea level is by far the most worrisome issue, because the current sea level is indeed the best of all possible sea levels: any change – up or down – will leave ports unusable. The IPCC forecasts that average sea level will rise by about 2–6 millimetres a year, compared with a recent rate of about 3.2 millimetres a year (or about a foot per century). At such rates, although coastal flooding will increase slightly in some places (local rising of the land causes sea level to fall in many areas), some countries will continue to gain more land from siltation than they lose to erosion. The Greenland land-based ice cap will melt a bit around the edge – many Greenland glaciers retreated in the last few decades of the twentieth century – but even the highest estimates of Greenland’s melting are that it is currently losing mass at the rate of less than 1 per cent per century. It will be gone by AD 12,000. Of course, there is a temperature at which the Greenland and west Antarctic ice caps would disintegrate, but according to the IPCC scenarios if it is reached at all it is certainly not going to be reached in the twenty-first century.

As for fresh water, the evidence suggests, remarkably, that, other things being equal, warming will itself reduce the total population at risk from water shortage. Say again? Yes, reduce. On average rainfall will increase in