Science Blog

Comment on Phil Chapman's opinion piece "Sorry to ruin the fun, but an ice age cometh"
Professor David Karoly
School of Earth Sciences at The University of Melbourne.
23 April 2008

David Karoly was a lead author of the IPCC Fourth Assessment Report (Working Group 2). Below, David comments on an opinion piece by Phil Chapman published in the Australian (23 April 08) titled "Sorry to ruin the fun, but an ice age cometh." To read the original article, click here.

It is excellent to have well-informed opinion pieces published in our newspapers. It is a pity when opinion pieces contain significant errors or misleading information. This opinion piece written by Phil Chapman appears to have a number of factual errors, incorrect conclusions, and misleading statements.

Paragraph 2: All four agencies that track Earth's temperature (the Hadley Climate Research Unit in Britain, the NASA Goddard Institute for Space Studies in New York, the Christy group at the University of Alabama, and Remote Sensing Systems Inc in California) report that it cooled by about 0.7C in 2007. This is the fastest temperature change in the instrumental record and it puts us back where we were in 1930. If the temperature does not soon recover, we will have to conclude that global warming is over.

The above paragraph is based on some information on web sites in the US. It is true that these data sets show a pronounced cooling from January 2007 to January 2008 of a little less than 0.7C. It is an error to state that this is unprecedented, as similar dramatic falls occurred from 1998 to 1999, and from 1973 to 1974. It should also be noted that in those data sets that provide monthly estimates, the global average temperature has warmed substantially, by more than 0.3C from January 2008 to March 2008. In addition, the annual average temperature for 2007 was within 0.1C of the average temperature in 2006 and 2005; no dramatic cooling there.

So what caused this rapid cooling over 2007, and also from 1998 to 1999, and from 1973 to 1974; what was common to all those periods? In each case, the common factor was a rapid change from El Nino to La Nina conditions, warm temperatures in the eastern equatorial Pacific Ocean to cold temperatures in the same region, which has a major effect on global climate patterns and global average temperature. La Nina is associated with below normal global average temperature, and 2008 is likely to be about 0.3C cooler than the average of the previous few years, because of the influence of La Nina.

The new cycle, No.24, was supposed to start soon after that, with a gradual build-up in sunspot numbers.
It didn't happen. The first sunspot appeared in January this year and lasted only two days. A tiny spot appeared last Monday but vanished within 24 hours. Another little spot appeared this Monday.

I don’t know where these sunspot numbers came from but they are in error. The best source of data for current sunspot numbers is the World Data Center for Solar Terrestrial Physics at the National Geophysical Data Center in Boulder CO. That gives an average number of sunspots per day in January 2008 of 3.4, 2.1 in February and 9.3 in March 2008. The minimum was in October 2007.

So, are variations in global average temperature directly related to sunspot numbers on a monthly, annual or decadal timescale? Certainly not on a monthly timescale and the effect, if any, on a year-to-year time scale is very small, as can be found by correlating the variations of global average temperature on monthly or annual timescales with the sun spot numbers. Any relationship between sunspot numbers and global average temperatures is much much smaller than the clear relationship between interannual variations of equatorial Pacific sea surface temperatures and global average temperatures, showing the effect of the El Nino-La Nina cycle.

However, the major flaw in this opinion piece is trying to infer understanding of long-term climate trends from short term (one year) variations of global temperature. It is well known (by climate scientists) that there are very large interannual variations of global temperature caused by a number of factors, including El Nino, or major volcanic eruptions, or just the chaotic variability of the climate system. It is not possible to make conclusions about long term climate trends from interannual climate variations. Many lines of evidence support the conclusion in the IPCC Fourth Assessment Report in 2007 that “warming of the climate system is unequivocal”, referring the warming over the last 100 years. Even when we consider only the global average temperatures during La Nina episodes, such as the current cool period, we find that this La Nina is experiencing the warmest global temperature of any strong La Nina episode in the last 100 years, again showing a clear long term global warming.

Analysis of climate variations over the 20th century shows:
• the strongest influence on year-to-year variations in global temperature is El Nino cycles, sunspot cycles have a much smaller influence on global temperature,
• the recent decrease in global temperature is due to the occurrence of La Nina, and
• we can infer almost nothing about longer term temperature trends from variations over one year.

There has been a clear warming trend in global average temperature over the last 100 years, with most of this warming due to the increase in greenhouse gases in the atmosphere. This long-term increase in global average temperature will continue throughout the 21st century due to increasing concentrations of greenhouse gases in the atmosphere.

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