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It is excellent to have well-informed opinion pieces published in our media. It is a pity when opinion pieces contain significant errors or misleading information, and then draw mischievous conclusions from them.

The opinion piece by Phil Chapman ("Sorry to ruin the fun, but an ice age cometh’’) warns of an approaching ice age but contains a number of factual errors, misleading statements and incorrect conclusions.

Chapman reports a cooling of global average temperature of 0.7C in 2007 and says “If the temperature does not soon recover, we will have to conclude that global warming is over”.

It is true that global 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, as Chapman does, that this is unprecedented, as similar dramatic falls occurred from 1998 to 1999, and from 1973 to 1974. It should also be noted that the global average temperature has warmed by about 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 during 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 Niño to La Niña conditions, from 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 Niña 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 Niña.

Chapman did not consider La Niña as a cause of the cooling in 2007 and instead linked it the minimum in the 11-year cycle in sunspot numbers: “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 daily number of sunspots 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 pot 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 Niño-La Niña cycle.

While those errors are bad enough, the major flaw in Chapman’s opinion is trying to infer long-term climate trends from short-term (one-year) variations of global temperature. It is well known (by climate scientists) that there are large interannual variations of global temperature caused by a number of factors, including El Niño, 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 to the warming over the last 100 years. Even when we consider only the global average temperature during La Niña episodes, such as the current cool period, we find that we are experiencing the warmest global temperature of any strong La Niña episode in the last 100 years, again showing clear long-term global warming.

Most of the warming in global average temperature over the last 50 years is 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 further increases of greenhouse gases. Yes, there will be year-to-year natural climate variations, with some colder years, but the long-term warming will continue.

An ice age is definitely not going to occur in the 21st century! Instead, we will all need to make very large reductions in emissions of greenhouse gases if we are to minimise dangerous anthropogenic climate change.

So why would a former astronaut based in the United States write mischievous misinformation in the form of an Opinion piece in Australia? I don’t know. Perhaps you should ask Phil Chapman or draw your own conclusions.

A version of this article was first published in The Australian on April 29, 2008.

David Karoly is a professor in the University of Melbourne's school of earth sciences and a member of the Wentworth Group of Concerned Scientists.