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Q. How much will the sea level rise, per degree C, due to the warming? It must present a serious threat to coastal resorts and harbours, etc... (Jeremy, Colwyn Bay)
A. This is difficult to predict because there may be sudden changes in the gradual flows of ice out of the Greenland and Antarctic ice caps. If these sudden changes do not happen, global average sea level is expected to rise by between approximately 10 and 16cm per degree celcius of global warming by the end of the 21st Century, with likely warming being anywhere between 1.1 and 6.4°C, according to different scenarios of future greenhouse gas emissions – so that's a sea level rise of about 18 to 59cm. However, even if temperatures were stabilised at that time, sea level would continue to rise by about another 30 to 80cm by 2300, and continue for several centuries more because it takes a long time for large ice caps to melt and for warming to penetrate into the deep oceans (expansion of water as it warms is another factor causing sea level rise). In fact, even if we stopped emitting all greenhouse gases immediately, there would still be a warming of 0.3 - 0.9°C by the end of the century and sea level would continue to rise to some extent: we are already committed to further consequences of the gases we have already emitted. Sudden accelerations of ice flow would cause sea level to rise more rapidly (see www.ipcc.ch working group 1 summary).
Yes, this does present a threat to coastal resorts, habours and indeed the many large centres of population in low-lying coastal areas such as the mega-deltas of Asia and Africa. The greatest threat is actually from storm surges (short-term rises in sea level caused by low atmospheric pressure and high winds during storms). Storm surges are more damaging if occurring on top of an increase in the underlying average sea level. Many millions more people are projected to be flooded every year due to sea-level rise by 2080 (see www.ipcc.ch working group 2 summary).
Q. I am old enough, unfortunately, to remember when the perceived threat was of an impending rapid onset of the next ice age. The two things that really worry me are the slow down in the Gulf stream, which would throw us rapidly into winter woolly weather (must use alliteration as per media speak), and the potential for runaway feedback of the Arctic ice melt/tundra defrost/methane clathrate, causing it to get a trifle warm rather rapidly. Just look at the global warming rate at the end of the Younger Dryas. What is your opinion? (Bill Fuller, by-email)
A. The Gulf stream (which keeps the UK warmer than it would otherwise be for this close to the North Pole) is expected to slow down, possibly by up to 50% by the end of the 21st Century. However, the cooling effect of this is still expected to be more than offset by the warming by greenhouse gas increases, so we still expect a warmer climate in the UK. An overall cooling of the UK would require much larger changes in the Gulf stream, and the research suggests that this is very unlikely.
Melting of Arctic sea ice is already included in computer model projections of future warming (of about 1.1 to 6.4°C by 2100 depending on our emissions). Release of carbon dioxide from thawing tundra soils is included in these projections to some extent, but more work is needed on this and on the release of methane from these regions. Release of methane from clathrates (a kind of methane ice in the ocean floor) is not included at all. The risks of methane clathrate release due to global warming are not really known.
Q. Exactly how safe are we? And what can we really do to reduce the risks of global warming/ climate change? (Vanita, Mansfield)
A. That depends on what you mean by "safe"! The IPCC report on "Impacts, adaptation and vulnerability" (www.ipcc.ch working group 2) assesses a wide range of projected impacts of climate change, and while some of these are positive (such as increased crop yields in colder regions – for a limited time, anyway), the majority are negative (eg: flooding, drought, reduced crop yields in the tropics). Reducing greenhouse gas emissions (by cutting fossil fuel use and reducing deforestation) would reduce these risks, but clearly this relies on massive reductions in either our demand for energy and agricultural land or changes in our production of energy and food. Big decisions need to be made. Risks can also be reduced by being prepared for change via "climate proofing": for example, improving flood defences, increasing the resilience of food supply in vulnerable regions, planning ahead for higher temperatures and more extreme events when designing buildings, cities and national infrastructure. This is not giving up on the idea that we can reduce our contributions to climate change: some further warming and sea level rise is already inevitable even if we somehow stabilised greenhouse gas concentrations immediately, and realistically we are not going to stabilise immediately anyway.
