Retorts

An Answer

In the October issue, Paul King of Wainuiomata wrote: "I am having great difficulty matching the graphs in John Daly's (Sept) article with those in the August Scientific American in `Global Warming Trends'. Is there a conflict in the baseline data?"

John Daly replies:

In the Scientific American article, Wigley compared his data subset for the US with the National Oceanographic & Atmospheric Administration study of US temperatures since 1895. He stated that his was only 0.1C warmer than the NOAA result, but did not add that the NOAA study concluded that there was no change in US temperatures over the last 100 years. Wigley's overall results suggested a total global warming of 0.5C.

Wigley's methods fail to correct adequately for heat island effects, which affect over 90% of all measurements collected in the last 100 years. The US has meticulous meteorological records and well-documented data on heat islands and other error-producing effects. It is not surprising, therefore, that Wigley's US results matched the NOAA's. For the rest of the world the quality of data is significantly poorer, and this no doubt led to the 0.5C warming he ended up with.

The next problem is the small number of measuring stations Wigley analysed. He divided the Earth into 1,296 blocks of 600,000 square kilometres and obtained a mean temperature for each. He used an inadequate number of stations, especially in the Southern Hemisphere where 293 stations represent 648 blocks.

Schneider reports that the St Helena station was moved because of storm damage. This could lead to the whole South Atlantic apparently experiencing "instant" warming, and indicates the distortions possible from using a single station to represent large parts of the globe.

The results of several large-area temperature studies are as follows:

Wigley +0.5C globally 1850-1989

NOAA zero change in the US 1895-1987

Dept of Agri. -0.15C in US 1919-1989

MIT +0.2C globally 1860-1980

NASA zero change globally 1978-1988

MIT's result would appear to be highly accurate globally, as it was based on ocean surface temperatures. It is consistent with the shrinkage of glaciers over the same period (although these glaciers are now advancing again apparently) and also with the hotter, more active Sun this century.

Wigley claimed his results matched NASA's 10-year satellite study, but this is only evident in year-to-year variations, as he claims an overall warming trend and NASA's study suggests no change. Wigley's warming trend would be consistent with heat-island distortions --  a factor not applicable to NASA satellites.