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Q. In Al Gore's movie, An Inconvenient Truth, when depicting the climate following rises and falls of CO2, the spikes and troughs of emissions followed the rise and fall of temperature, rather than the other way round. Why has this not been looked at further, particularly as the similarities in rises and falls were decades apart, if not centuries? Also, if human contribution to atmospheric carbon dioxide is less than 5% of the total, why do scientists believe we are having such a massive effect on the planet? (David Simpson, Edinburgh)
A. You are correct that at some times over the last few hundred thousand years, the rises and falls of CO2 have lagged behind those in temperature, and indeed variation changes in temperature due to natural causes (such as changes in the Earth’s orbit or tilt of its axis) have resulted in changes in CO2 which themselves have exerted further changes in climate (a feedback effect). This has actually been looked at in great depth: see http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch07.pdf for an extensive review of this and other areas of palaeoclimatology. However, we know that these natural processes are not the cause of the current temperature rise or CO2 rise (changes in the Earth’s orbit and tilt of its axis are too slow to account for the changes seen over the 20th century, and there is more than enough CO2 being emitted by human activity to account for the rise measured in the atmosphere).
In actual fact, humans have contributed more than 25% of the current atmospheric carbon dioxide concentration (before the industrial revolution the CO2 concentration was 278 parts per million, but in 2005 it was 379 parts per million).
Science is not a matter of “belief”, it is a matter of scrutinising the evidence.
Q. The total amount of CO2 in the atmosphere equates to 0.035% of atmospheric gas: the main gases being nitrogen (78.084%), oxygen (20.946%), argon (0.934%) and other gases (0.002%). Man contributes an insignificant amount of CO2 into the atmosphere when you compare the world’s CO2 emissions with CO2 already in the atmosphere. So why do we only ever hear about CO2 emissions? And what is the main factor contributing to an increase in world temperatures? (Tim Wiggins, by e-mail)
A. Nitrogen, oxygen and argon are not greenhouse gases, ie: they do not significantly absorb heat radiation emitted by the Earth’s surface. However, carbon dioxide is a greenhouse gas. Analysis of bubbles of air trapped in layers of Antarctic ice formed at the time of the industrial revolution reveals that the atmospheric CO2 concentration at that time was 278 parts per million. Now the CO2 concentration is measured as 379 parts per million. Therefore, we have increased the CO2 concentration by more than one-third of its pre-industrial value. I don’t think that is insignificant. This human-caused increase in the concentration of CO2 and other greenhouse gases (such as methane and nitrous oxide) is very likely to be the main factor contributing to an increase in world temperatures.
Q. Projections suggest the Amazon rainforest will be destroyed through lack of rainfall. If this happens, where will we get our oxygen from? (Roy, York)
A. Although the Amazon rainforest is huge, it still only covers a relatively small part of the overall surface of the Earth and only contributes a small amount to the total production of oxygen. If the Amazon rainforest was destroyed, either through lack of rainfall or by direct deforestation by humans, this would add to the CO2 rise (and hence global warming) and also possibly reduce its local rainfall and cloud cover – however, the effect on global oxygen would be minimal.
Q. No-one disagrees that climate change is occurring, but the cause seems to generate some debate. I recently watched a programme that suggested global warming was caused by increased activity of the sun. The evidence that supported this claim seemed to warrant some discussion. What is your opinion on this? (Alan Grier, by e-mail)
A. See my answer to Ian from Sidcup on the minor role of the sun compared to human emissions of greenhouse gases (below). It seems some people have chosen to use old data and incomplete information in order to put forward an argument which may seem plausible to the non-expert, but which in fact is not supported by a complete view of all the evidence.
In my opinion, if you want scientific facts on an issue of global importance, it makes more sense to rely on an objectively-written document based on an exhaustive review of thousands of peer-reviewed scientific papers and subject to open scrutiny by hundreds of experts than it does to rely on a TV programme. For such a document, see http://ipcc-wg1.ucar.edu/wg1/wg1-report.html. For the most succinct summary, see http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_SPM-v2.pdf. You can also see the reviewers' comments and the authors' responses to them at http://ipcc-wg1.ucar.edu/wg1/Comments/wg1-commentFrameset.html.
