Researchers at the UK’s National Oceanography Centre in Southampton have found that greenhouse gas concentrations similar to the present have been associated with sea levels at least nine metres above current levels.
The researchers compiled more than two thousand pairs of CO2 and sea level data points, spanning critical periods within the last 40 million years. They used these values to find the “natural equilibrium” sea level for CO2 concentrations ranging between values of 180 parts per million and more than 1,000 parts per million.
For values between 400 and 450 parts per million – which is the target range for global warming of 2ºC – the natural equilibrium level is 9 metres above the present (or, in scientist-speak, a sea level at 24 +7/-15 metres).
For concentrations between 450 and 650 part per million, there is not much change in the natural equilibrium sea level. Above 650 parts per million, sea level rise rapidly. At 1,200 part per million, the planet is ice-free with sea levels 65 metres above the present.
These changes take hundreds, or even thousands, of years – at a rate of roughly one metre per century.