“It’s around one metre, bang there in the middle”. Professor Tim Lenton, professor of Earth Systems Science at the University of East Anglia in Norwich, sounds very pleased when he hears that policy-makers now officially revise the estimated sea level rise for London to around +94 cm within a century. Indeed, his own sea level rise projections for the city span between a very conservative 55 cm and one metre and a half by the year 2100. But if the dreaded one-metre rise threshold can be managed by London, it will spell disaster in many other parts of the world. In terms of adapting London’s defences, what is going to be the critical sea level rise that is going to pose a major problem? In general, sea level rises of about one metre per century, or more, are going to be very hard to adapt to, because we will have to build and renew our defences all the time. The main problem is not just the absolute sea level rise, but the rate at which it is changing. I don’t know whether it is right to state an absolute amount, but I believe a metre per century will be very challenging to deal with. It’s worth remembering that the sea level rise over the past century was on the order of 18-20 centimetres. London is lucky because it already has the Thames barrier, which should protect it for at least a century, but elsewhere, especially in low-lying developing countries, a metre in sea level rise will pose major problems. What are your specific, model-based projections about sea level rises and how would these affect London? With our GENIE model we have made global projections of temperature and sea level rise assuming known fossil fuel will be burnt and we have translated them to London and the Thames estuary region. Assuming that business goes on as usual, the model says, quite frighteningly, that the world is going to get about 8 degrees centigrade warmer around the year 2300, and the temperature will then stay there until the year 3000, with the sea level rising at nearly 1 metre per century. But we all know this is monstrous, as an overall temperature increase of just 4 degrees would be already capable of bringing turmoil and destruction. Exactly. This is indeed twice what we now think will cause serious disruption and challenges to adaptation. It is arguable that we could never get there, because the disruption along the way would be too much. What is your model saying about the year 2100, which people consider as a benchmark now? The temperature is going to be about 3 and a half degrees centigrade warmer that it is now. Our sea level projections, unlike those of the Intergovernmental Panel on Climate Change, do include contributions from Greenland’s ice sheet melting, and the resulting increase from now will be a minimum of 55 centimetres, but I personally think that’s an underestimate, because it doesn’t factor in what could be quite a significant contribution, such as the melting of West Antarctica’s ice sheet. If that also started to break down, I think it’s conceivable that the sea level rise could exceed a metre in this century, which would spell bad news for many people. Since the local projections do vary, how would this affect London’s flood risk by the year 2100? Few people may know that the whole crust of the UK is still adjusting to the retreat of the ice sheet at the end of the last Ice Age. Unfortunately London and the South East are sinking, irrespectively of climate change, at the rate of about 2 millimetres per year, which makes around 20 centimetres a century. So you have to add this isostatic adjustment, whereby Scotland is rising instead, to the figures of sea level rise. I would say the figures for London’s sea level rise will span between a minimum of 55 centimetres and a maximum of a metre and a half. It may get to 2 metres by 2200, and to more than 7 metres by the year 3000. What do you think of certain geo-engineering countermeasures, such as putting structures up in space in order to cool the climate down? I am not a fan of that. Some people have suggested interposing mirrors between the sun and the earth in order to reduce incoming solar radiation. I am writing a critical review of these geo-engineering ideas, because we don’t understand the earth system and the consequences of such vast-scale measures well enough, which means that we could get things badly wrong.