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Q. I keep on hearing that there is one part of the Antarctic that is melting, and that this is catastrophic. Then recently I heard that, in fact, the total amount of ice in the Arctic has stayed at about the same level for quite a while. Which is true? (Tom, by e-mail)
The first is true: part of the Antarctic ice sheet is melting and it is very likely that this has contributed to recent sea level rise. Other parts of the Antarctic are not melting (even though the rest of the world is warming, most of Antarctica remains well below freezing).The Greenland ice sheet is also melting and again it is likely that this has contributed to sea level rise.
By "ice sheets", I mean ice on land. The other issue is sea ice, ie: ice floating in the sea. Melting sea ice does not contribute to sea level rise because it is already in the sea, but it is an indicator of a warming climate. Antarctic sea ice grows and shrinks significantly from year to year and it is not possible to say whether it is reducing in the long term (this may be what you have heard about, although this refers to the Antarctic rather than the Arctic).
Arctic sea ice is melting though: although it grows every winter and shrinks again in summer, satellite data shows that the annual average Arctic sea ice cover has reduced by about 7% since 1978. The summertime coverage has reduced by over 20% in that time.
Q. Has global warming only occurred since global industrialisation or have there been previous warm decades/centuries? (Gary Burrows, Southport)
A. Before global industrialisation, there were only a very few systematic records of temperature and even they only go back as far as the invention of the thermometer. So estimates of pre-industrial temperatures rely mostly on examining "proxy" evidence such as tree rings, and since most such studies have been done in the northern hemisphere, your question can only be answered with high confidence for that half of the world and for the last 1,300 years. The records show that the latter half of the 20th Century is likely to have been warmer than any 50-year period in at least the past 1,300 years. So the current climate change is unusual compared to the millennium.
Analysis of the chemical composition of air bubbles trapped in ice suggest that the polar regions were 3 to 5 degrees warmer around 125,000 years ago (before the last ice age). This was due to changes in the Earth's orbit (which happen too slowly to explain current global warming). This warming is similar to that predicted for the end of the 20th Century under the higher greenhouse gas emissions scenarios. It is likely that, 125,000 years ago, global average sea level was 4 to 6 metres higher than at present.
Q. How did science reach the conclusion that global warming is caused by man? I have a theory that this is actually nature’s way of cleansing itself – an event that has occurred every once in a while though the millennia. (Neil Croydon)
A. It can be measured in the laboratory that certain gases such as water vapour, carbon dioxide, methane and nitrous oxide absorb a large proportion of energy from heat radiation passing through them. These greenhouse gases (GHGs) are present naturally in the Earth's atmosphere and exert a warming effect which keeps the global climate from freezing (no scientist argues with that). The concentrations of carbon dioxide, methane and nitrous oxide have been proven beyond doubt to be increasing in the atmosphere (no scientist argues with that) and we know for certain that human actions such as burning fossil fuels, deforestation and agriculture emit these gases (no scientist argues with that). We also know for certain that there are more than enough human emissions to account for the observed GHG rises (this is also indisputable). There is an uninformed argument that the rise in greenhouse gases is due to natural processes, but this is based on a half-truth which considers natural emissions of GHGs (which do occur) but ignores natural uptakes (which are larger than the natural emissions). Although the sun does exert a small warming influence, the warming influence of man-made increase in GHGs is more than 10 times larger. The scientific consensus of the IPCC (www.ipcc.ch) is that there is "very high confidence that the global average net effect of human activities since 1750 has been one of warming", and that "most of the observed increase in global average temperatures is very likely due to the observed increase in anthropogenic greenhouse gas concentrations". Natural factors alone cannot explain recent warming, so the evidence does not support your theory.
Q. Do climate change models take into account major tipping point events, such as, for example, permafrost melting enough to cause a huge release of methane, or ocean temperature rising far enough to cause large scale releases of methane hydrates? Also, to what extent do computer models of climate change address the problem of carbon sinks becoming sources of carbon through forest fires? (Helmuth Osborne, Worksop)
A. Current climate change models assessed in the recent IPCC report do not take into account the methane release from permafrost thawing or methane hydrates. The first is an area of very active study and should be in the models soon. Methane hydrate release is more difficult. The leading models (such as that of the Met Office Hadley Centre www.metoffice.gov.uk) do address the issue of carbon sinks becoming sources. Our model, and some others (but not all), suggests that particular patterns of warming in the Atlantic and Pacific Oceans could cause wind patterns to change such that less rain falls on Amazonia, causing the forest to be at increased risk of drought and fire and leading to emissions of CO2 as the forest is lost. But more significant than that is the potential for increased releases of carbon dioxide from soils worldwide as a result of warming. This would also contribute to an acceleration of the CO2 rise.
