Over the last eight years, it has been interesting to experience first-hand how public attitudes have changed. Initially, there was a good deal of scepticism about whether there were indeed any observed changes in the global climate. This was followed by a couple of years in which people seemed to think the issue had been settled and it was obvious that the global climate was changing. In recent years, amongst a wide acceptance that ‘something is happening’, people have been concerned about whether there is anything we can do. But amongst this concern – partly stimulated by the ‘Climate-Gate’ furore – there has been an emergence of a group with an almost militant sceptical stance that I simply don’t understand. So why am I am concerned?
First, never mind about ‘warming’ or ‘cooling’ – why is the Earth’s surface the temperature that it is? This has been understood for more than a century, and was the subject of primary school science songs in the 1950s. The greenhouse effect makes the Earth’s surface ca 33°C warmer than it would be if the atmosphere contained no water vapour or carbon dioxide. Most of the warming (ca 31°C) comes from the water vapour and liquid water in the atmosphere. The warming arises because water – and to a lesser extent carbon dioxide – absorb the infrared radiation emitted from the Earth. It is the emission of this radiation that cools the Earth – balancing the warming by the Sun.
Secondly, the amount of carbon dioxide in the atmosphere is increasing. There is no doubt about this, or about its origin: basic calculations and the isotopic signature of the changes indicate that it comes from burning carbonaceous fuels. The corresponding decrease in oxygen concentration has also been detected. The amount of carbon dioxide being put into the atmosphere is staggering. The atmosphere already contains around 2800bn t of carbon dioxide, and each year we add roughly another 30bn t – approximately 1% of the total. The rise in atmospheric concentrations is around half this – roughly 0.5%/year – because carbon dioxide is absorbed by growing plants and by the oceans. This has resulted in a measurable decrease in the pH of seawater.
So what will be the effect of the extra carbon dioxide? At this point I could refer to any number of publications and models, which indicate that we should expect an increase in the average surface temperature of the Earth. But actually, I don’t think that anybody knows what will happen. Although climate prediction is easier in some ways than weather forecasting, there are many uncertainties.
However, in broad terms there are just three possible outcomes. Either the carbon dioxide will have no effect; or it will cause a cooling; or it will cause a warming. It would be astonishing if changing the concentration of an infrared absorbing gas by 30% had no effect at all – unbelievable in fact. And although it is not predicted, a cooling of the global climate is conceivable – the climate system has many non-linear elements and feedback responses. But a global cooling would have serious consequences too. However, every published calculation I have seen predicts that the additional carbon dioxide will warm the Earth.
Sceptics can argue whether the changes we have observed so far are real. Personally, I find the disappearance of 2m km2 of Arctic sea ice quite convincing. But I understand that others may not.
Sceptics can argue whether the air temperature above the land surface of the Earth is really increasing. The four teams that have tried to average the meteorological records agree that, since 1970, the Earth has warmed at a rate of more than 2°C/century. But some dispute this.
Sceptics can argue that observed changes are just ‘natural cycles’ and point out that the ‘Little Ice Age’ was not caused by humans. To me, this shows the sensitivity of the climate system to small changes, but I understand that others view things differently.
Sceptics can argue that it is better to enjoy the benefits of an economy based on carbon fuels and to just wait and see what happens. Personally, I think it would be prudent to switch to renewable energy sources, but I understand there are other arguments.
Sceptics can argue that the near unanimity of the scientific community is evidence of a conspiracy. I think there is an alternative – and simpler – explanation.
Sceptics can disagree with predictions of future temperature rises and sea-level changes. I sympathise: it is possible that these could be wrong.
Being sceptical about these latter points is understandable, but these are just details. We are putting colossal quantities of an infrared active gas into the atmosphere and we don’t know what the effect will be. We should all be concerned.
Michael de Podesta, principal research scientist, National Physical Laboratory
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