The relation between the stock of GHGs in the atmosphere and the resulting temperature increase is at the heart of any risk analysis and discussion of the economics. It is the clearest way to begin and anchors most of the discussion. While the greenhouse effect was discovered and its causes identified during the 19th century, science has in recent years begun to provide us with probability distributions of temperature outcomes for different stocks of GHGs.

Current concentrations of GHGs are around 430 parts per million (ppm) CO2 equivalent (CO2e) - which aggregates carbon dioxide with other GHGs - and are increasing by around 2.5ppm CO2e per year. This rate appears to be accelerating, particularly as a result of rapid growth of emissions in the developing world. There seems little doubt that, under BAU (i.e. in the absence of any restraining policy), the annual increase in the overall quantity of GHGs would average somewhere above 3ppm CO2e, potentially 4ppm CO2e or more, over the next 100 years. This is likely to result in concentrations of around 750ppm CO2e by the end of this century.

This level of concentration, if there were no further growth, would result in a 50-50 chance of a temperature increase over 5°C. In contrast to when temperatures were 5°C below pre-industrial levels, the planet last experienced temperatures 5°C above pre-industrial levels around 30 million to 50 million years ago, long before the arrival of humans. The most recent warm period was around 3 million years ago when the world experienced temperatures 2°C or 3°C higher than today (Jansen et al, 2007). Humans have never experienced such high temperatures. Many areas, such as much of Bangladesh, would be inundated; others such as Southern Europe would become deserts; rivers would change course; hurricanes would batter the southern US. The physical and, thus, human geography (where we can live and how we live our lives) would be transformed.

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