Climate change

The topic of climate change has been given a great deal of prominence recently. While many people consider this as some kind of new threat facing our planet, you should be aware that climate change is the normal state of affairs on Earth. The present warming phase started around 1910, with a partial reversal from 1940 to 1985. This could legitimately be regarded as a recovery period from a distinctly chilly 19th century, and is not, as many imply, a recent phenomenon. The real debate is whether human activity, specifically burning fossil fuels which produce carbon dioxide, has had a significant effect on climatic warming over and above what would have been natural. That said it is difficult to be precise about that human contribution since it is a complex matter to quantify natural climate change.  

 

Chemistry can help 

Of all the scientific disciplines, chemistry equips us best to grasp the essentials of the global warming debate. After all global warming comes down to the absorption of infrared radiation by organic molecules, coupled with the mole concept which allows us to convert tonnes of fossil fuels into tonnes of carbon dioxide. As a starting point for anyone wanting to engage with this debate here are three fundamental questions with my comments attached. 

• By how much will the level of atmospheric CO₂ increase in the future?

For the past decade CO₂ levels have been increasing, fairly linearly, by approximately 0.5 per cent per annum (1.8 ppm, 14.5 billion tonnes)¹  which, if that continued, would mean a rise of ca 60 per cent by 2100, not a doubling as is so often routinely quoted. To double the 2005 CO₂ level by 2100 we would need to extract and burn a total of ca 1750 billion tonnes of fossil fuels, a rate of slightly over 18 billion tonnes per year which is about twice the present rate.²  This would mean sinking new coal mines, building new gas pipe lines, and locating new oil wells because the capacity does not exist at present to produce and distribute this amount of fuel. Extracting that amount of fuel would begin to exhaust known supplies quite quickly and limits of cost and availability would begin to impinge at some point. China is the only country whose use of fossil fuels is increasing rapidly, with a massive 80 per cent increase since 1999.³  However, it is unimaginable that such a rate of increase can be sustained for a prolonged period. 

• How much warming might a doubling of CO₂ produce?

Using a purely radiative model for the greenhouse effect, the energies involved may be calculated by using the Stefan-Boltzmann equation (E =sT⁴) which relates the temperature of an object to the heat radiating in units of watts per square metre of its surface. Greenhouse gases slow down heat loss from Earth to space and the net greenhouse warming effect is on average about 150W for each square metre of the Earth's surface. A figure⁴  for the additional warming from a doubling of carbon dioxide alone has been estimated at 3.5Wm^-2 but the true figure for the atmosphere may be different since account has to be taken of the profile of the Earth's infrared emission spectrum and one other very important factor - the effect of water vapour. Some scientists argue that water vapour reduces the effect of doubling carbon dioxide to ca 1Wm^-2,⁵  others say that it would magnify the warming from carbon dioxide many times over.

• What temperature rise does 1Wm^-2 produce, ie what is the climate sensitivity?

The greenhouse effect is ca 150Wm^-2 which heats the Earth by ca 33K so it would appear that ca 0.22KW^-1m^-2 is the climate sensitivity but this is an average figure over the full temperature range from 0K. Taking the Stefan-Boltzmann equation and applying it to the average Earth temperature of 288K, adding 1Wm^-2 would increase the temperature by the lower figure of 0.18K. If we take the optimistic rather than the catastrophic view of carbon dioxide increase (ie 1Wm^-2 for a doubling, a 60 per cent increase, and very little contribution from other greenhouse gases), this means that the Earth might warm up by ca 0.12K over the next 100 years, owing to additional greenhouse gases, an increase of only 0.0012K per year. Even taking less optimistic projections, those claiming that the effects of global warming from additional greenhouse gases can already be detected, I believe, are deluding themselves. It would take 5.5Wm^-2 to produce a rise of 1K and an 11K rise (sometimes claimed) would need a massive 55W of additional energy for every square metre of the Earth's surface. There simply is not that amount of energy available still to be absorbed from the Earth's spectrum, most of which is largely saturated anyway owing to absorption by carbon dioxide and water vapour. Those who promote apocalyptic global warming claim that the sensitivity is much higher than 0.18K, some claiming 0.75K and even 1.5K.⁶  These claims are mainly based on a postulated magnifying effect of water vapour but, from a consideration of infrared absorption spectroscopy in relation to the spectrum emitted by a body at 288K, it is not clear how such large values can be achieved.  

Hidden agendas 

Fossil fuels are a limited resource and conservationists argue that present generations should conserve fossil fuels for future generations, though many countries would have difficulty selling this idea to their citizens. It would be easier to frighten the population into compliance with talk of catastrophic global warming. Richard Lindzen, professor of meteorology at Massachusetts Institute of Technology, encapsulated the complex political situation surrounding the emphasis on the greenhouse gas hypothesis for anthropogenic global warming when he said: 'The remarkable centrality of carbon dioxide means that dealing with the threat of warming fits in with a great variety of agendas - some legitimate, some less so: energy efficiency, reduced dependency on Middle Eastern oil, dissatisfaction with industrial society (neo-pastoralism), international (economic) competition, governmental desires for enhanced revenue (carbon and environmental taxes), bureaucratic desires for enhanced power .' and one could add 'desires by unelected pressure groups for controlling governmental decision making'. 

Dr Wilson Flood is a chemistry education consultant. He can be contacted at wilson@flood6.fsnet.co.uk.

References

1. Mauna Loa Observatory, Hawaii. 
2. BP Statistical review of world energy 2005. London: BP, 2005. 
3. ibid. 
4. G. Myhre et al, Geophysical Res. Lett., 1998, 25, 2715. 
5. J. Barrett, Energy and Environ., 2005, 16, 1037. 
6. See, for example, various IPPC (International Panel on Climate change) reports published by United Nations.