Nicholas Stern writes: The Intergovernmental Panel on Climate Change last September pointed to a changing pattern of extreme weather since 1950, with more heatwaves and downpours in many parts of the world, as the Earth has warmed by about 0.7C.
The IPCC has concluded from all of the available scientific evidence that it is 95% likely that most of the rise in global average temperature since the middle of the 20th century is due to emissions of greenhouse gases, deforestation and other human activities.
The upward trend in temperature is undeniable, despite the effects of natural variability in the climate which causes the rate of warming to temporarily accelerate or slow for short periods, as we have seen over the past 15 years.
If we do not cut emissions, we face even more devastating consequences, as unchecked they could raise global average temperature to 4C or more above pre-industrial levels by the end of the century.
This would be far above the threshold warming of 2C that countries have already agreed that it would be dangerous to breach. The average temperature has not been 2C above pre-industrial levels for about 115,000 years, when the ice-caps were smaller and global sea level was at least five metres higher than today.
The shift to such a world could cause mass migrations of hundreds of millions of people away from the worst-affected areas. That would lead to conflict and war, not peace and prosperity.
In fact, the risks are even bigger than I realised when I was working on the review of the economics of climate change for the UK government in 2006. Since then, annual greenhouse gas emissions have increased steeply and some of the impacts, such as the decline of Arctic sea ice, have started to happen much more quickly.
We also underestimated the potential importance of strong feedbacks, such as the thawing of the permafrost to release methane, a powerful greenhouse gas, as well as tipping points beyond which some changes in the climate may become effectively irreversible.
What we have experienced so far is surely small relative to what could happen in the future. We should remember that the last time global temperature was 5C different from today, the Earth was gripped by an ice age.
So the risks are immense and can only be sensibly managed by reducing greenhouse gas emissions, which will require a new low-carbon industrial revolution. [Continue reading…]