Nelson's Column
Charlie Nelson's Blog
The correlation
between global temperature and atmospheric carbon dioxide
David Murray is the Chairman of Australia's Future Fund, which manages over $71 billion of taxpayers money. In an interview with the Australian Financial Review (10 June 2011, page 57) he stated that "there is no correlation between warming and carbon dioxide."
He did not share the analysis that led him to this conclusion with the Financial Review, buy I now share my analysis openly. David Murray is wrong!
Atmospheric carbon dioxide (CO2) has been measured since 1959. Global temperature (deviations from the 1961 to 1990 average) and CO2 parts per million (ppm) are shown in the chart. Statistical analysis shows a highly significant correlation between these two sets of data – there is less than one chance in a million that this is a fluke correlation. Over this period, for every 10 ppm increase in CO2, there is a temperature increase of 0.09 degrees C. In other words, approximately 1 degree warming for every 100 ppm increase in CO2.
Of course, we all know that correlation does not prove causation. It could be that rising temperature causes to CO2 concentration to rise or there could be a third factor causing both to rise.
But we know that human burning of fossil fuels is causing a measurable increase in the amount of CO2 in the atmosphere. And it was established by scientists over a century ago, both through experiments and theory that some gasses, such as CO2, block outgoing heat radiation from the earth. The more of such greenhouse gasses in the atmosphere, the more outgoing heat is blocked.
The temperature in the year 1998 was very much higher than expected, given the level of CO2 but that was mostly due to a very strong El Nino. Unfortunately, global warming deniers such as Ian Plimer have claimed that the earth has cooled since 1998. This is deliberately choosing a very hot year as a base and intended to cause confusion. If we choose 1997 or 1999 or any year in the 20th century other than 1998 as the base it is clear that the atmosphere has continued to warm. Indeed, temperatures so far in the 21st century are very close to those predicted by this model.
There are many factors, in addition to CO2 and the El Nino / La Nina cycle, which influence global temperature. These include other greenhouse gasses such as methane, other climate cycles such as the Pacific Decadal Oscillation (PDO), volcanic ash in the atmosphere (which reflects incoming heat) and sulphur dioxide emitted from coal-burning power stations (which also reflects incoming heat).
It is even plausible that the atmosphere could cool for decades before warming again while still being consistent with the fact that rising CO2 causes warming - see my scenario.
There is very little room for doubt that increasing the amount of CO2 in the atmosphere will cause the temperature to increase in the long-term. The only question is the influence of the many other factors on the overall rate of warming.
Some of Australia's taxpayers may wonder whether David Murray is fit to make long-term decisions about their money, given his willful ignorance about the future of the climate.
Charlie Nelson
June 2011