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Q. OK, after 65 years of observing the climate, I am convinced that it is getting warmer. Climate change has always happened: why blame CO2? Specifically, how do you explain the fact that, during the cretaceous period, the CO2 levels were many times today’s and yet the ice caps did not melt? (Fred, by e-mail)
A. The climate can change naturally, and has changed greatly in the past. But we understand the reasons for these changes (they include continental drift, changes in the Earth's orbit or tilt of its axis, and natural changes in CO2 concentration) and know that these processes are not occurring at a significant rate now. However, the concentration of CO2 is now changing at a significant rate and there are more than enough human-caused emissions to account for this, and the change this exerts to the Earth's energy budget (radiative forcing) is more than 10 times greater than the most significant natural change (small changes in solar irradiance).
It is true that the evidence shows that in the cretaceous period (144-65 million years ago, when dinosaurs dominated), CO2 concentrations were several times higher than at present. However, the evidence also shows that the cretaceous was actually warm, with both poles being temperate and tropical conditions extending further than they do today. The ice caps were not present, and indeed Antarctica was not even at the south pole. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago, before the last ice age), reductions in polar ice volume led to 4 to 6 metres of sea level rise.
I find it very interesting when people use the argument of past climate changes to cast doubt on the established causes of current climate change. How do you know that climate change has always happened? Because of scientific evidence! So if you accept the scientific evidence for that, why not accept the scientific evidence that current climate change is largely caused by the CO2 rise? It is often the same scientists that study both past and current climate change. For a recent overview of past climate change to put current human-made changes into context, see http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch06.pdf.
Q. Assume we take action now (ie: within the next 10 years) to reduce fossil fuel usage and the amount of greenhouse gases emitted: how long will it take before the effect of this is passed on in a reduction in temperatures? (Julian, Staffordshire)
A. If greenhouse gas concentrations were stabilised at present-day concentrations immediately, global climate would continue to warm by between 0.3 and 0.9 degrees Celsius over the next hundred years. So cooling is not expected this century, and indeed some level of further warming is now inevitable. However, this warming is smaller than that expected if we continue to emit greenhouse gases (the IPCC scenarios project warming of 1.1 to 6.4 degrees Celsius, depending on how extensively emissions continue and how strong various climate feedbacks turn out to be).
Q. How much carbon dioxide do volcanoes produce in comparison to man-made emissions? (Craig, Hull)
A. Volcanoes produce between 0.02 and 0.05 billion tonnes of carbon per year, whereas man-made emissions from fossil fuel burning, deforestation and cement production are around 8-10 billion tonnes of carbon per year. So, man-made emissions are between 160 and 500 times greater than emissions from volcanoes. Furthermore, volcanoes have always emitted carbon dioxide, so are part of the natural background climate. Man-made emissions are a new addition on top of this.
Q. Could global warming be occurring as a result of the increase in the temperature of the sun? (Ian, Sidcup)
A. The sun has played a minor role in global warming over the 20th century, but much less than the human-caused increase in greenhouse gases. There is some evidence that increases in solar heating may have led to some warming in the early part of the 20th century, but direct satellite measurements show no appreciable change in solar heating over the last three decades. In contrast, the rate of global warming has been more rapid over the last three decades in comparison with the average over the last century. Greenhouse gas concentrations have continued to rise over the last three decades.
Solar heating can be compared with other effects on the Earth's energy balance, such as the enhancement of the greenhouse effect by carbon dioxide, methane and nitrous oxide increases, and the reflection of solar radiation back to space by aerosol particles, using a quantity called "radiative forcing". The radiative forcing due to increases in CO2 is 1.66 Wm-2, more than ten times that due to solar heating which is 0.12 Wm-2. The total radiative forcing due to all human-made factors (greenhouse gases, aerosols, and land cover change) is 1.6Wm-2. Further details are in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, Working Group 1 Volume Chapter 2, which you can find online at http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch02.pdf
