The Irish Times have kindly published an article of mine on climate change today. Two apparently contradictory facts have recently emerged that I thought worth discussing in my regular column on the IT science page:
A. The buildup of greenhouse gases in the atmosphere has increased over the last few decades, despite the warnings of climate scientists (this month, the concentration of carbon dioxide in the atmosphere exceeded a value of 400 parts per million for the first time in millions of years).
B. There has been a slight slowing in the rise in average global surface temperatures in the last ten years in comparison with the preceding decade.
Some commentators have sought to reconcile these two facts by suggesting that scientists may have over-estimated our climate’s sensitivity to greenhouse gases. In particular, a recent article in The Economist making this argument has been widely cited. However, the Economist article makes heavy use of a single Norwegian study that has yet to be published (why?) and I don’t know any physicists who agree with the hypothesis , for several reasons:
(i) Climate sensitivity is a complex issue and measurements of global surface temperatures do not tell the whole story: about 90% of global warming is estimated to occur in the oceans
(ii) Heat and temperature are not the same thing: temperature response to heating can be significantly delayed, particularly in the case of large bodies of water
(iii) Sometimes significant heating can occur without any discernible change in temperature, for example when ice melts to water: just such a ‘change of state’ is happening on a massive scale at the poles and will result in inexorably lead to increased sea levels and reduced reflectivity, causing further warming
Images from the Skeptical Science website
Some climate scientists fear that we may be entering a new phase of global warming, where a dangerous amount of heat will be stored in the ecosystem without an appreciable change in temperature at first (known as latent heat). Significant temperature rise will eventually follow, but we can expect climate change skeptics to use this delay to deny the reality of climate change.
You can read my full article on the subject on the Irish Times website
Update and correction
In a follow-up letter to The Irish Times, Tony Carey points out that the figures I gave in the article for the increase in greenhouse gas concentrations are actually those for CO2 only. He is absolutely right in this, the error crept in at the very last draft of the article, aargh. He is also right to point out that while the concentrations of other GHGs are also rising, the rate of increase has in fact slowed for these gases. Well spotted.
However, this doesn’t really affect the argument I make in my article simply because the CO2 concentration is much larger than that of the other long lived greenhouse gases. Indeed, figures for CO2 are often quoted as a proxy for all longlived greenhouse gases for this reason, although this is not strictly accurate. The diagram below from NOAA illustrates the dominant effect of CO2 very well (turquoise line). Note that ‘radiative forcing’ relates to the effect of GHGs on climate, involving calaculations I won’t go into here. Note also that climate sensitivity is defined in terms of a doubling of CO2, because of its dominance.