Sunday, January 10, 2010

2. Global temperature records

The blue curve and the left scale in the figure are the same as in my last post, showing the increase in atmospheric CO2 since 1890.  
The red curve and right scale show the variations in the annual average global surface temperature, as recorded by the Climate Research Unit (CRU) at the University of East Anglia in Britain. (The values shown are the deviations from the average value from 1950 to 2000.) The overall temperature rise is about 0.7ºC (1.3ºF). The heavy curve, indicating the temperature trend, is a ten-year moving average of the data.

These two curves are basically the only actual evidence for anthropogenic global warming (AGW). While the temperature and CO2 concentration have both increased since 1890, it is obvious that the correlation between these curves is not very impressive: the CO2 increased steadily, but the temperature decreased after 1900, after 1940 and since 2002. It is immediately clear that CO2 is not the only significant factor affecting the climate.

In any case, the upward trend of both curves in the figure does not prove that CO2 causes warming. As an example of the dangers of reading too much into correlation, I offer another theory of climate change: it is an undisputed fact that the climate cools each year after deciduous trees drop their leaves, and warms again when they grow new ones. Should we conclude from this that tree leaves cause warming? Should we mount a worldwide campaign to cut down evergreen trees, which never lose their leaves and thus presumably cause sustained global warming?

Most people would agree that this absurd hypothesis gets the facts backward: it is the temperature variation that affects tree leaves, not the other way around. If it is true that only a fraction of the growth in CO2 is anthropogenic, it is highly probable that the AGW theory has also confused cause and effect. There are plenty of possible external reasons for the rise in temperature, but what could have caused a spontaneous increase in CO2?  Perhaps the observed warming is merely a return to normal from the Little Ice Age, an abnormally cold period from the 16th to the mid-19th century. Perhaps it is due to some other natural cause (e.g.,variations in solar activity). Perhaps the CO2 concentration would have remained roughly constant, despite manmade emissions, were it not that the ability of the oceans to absorb CO2 decreases when the surface temperature rises. Choosing between these possibilities requires more information, from an independent source.

These uncertainties are compounded by the fact that the surface temperature measurements shown in the figure are subject to many significant errors. They were mostly obtained from thousands of recording thermographs in the little white louvered boxes called Stephenson Screens, which typically become dirtier with time. Without regular, meticulous cleaning, the screens gradually absorb more sunlight and thus produce a spurious rising trend in temperature. Moreover, many of them are located in or around urban heat islands, which get warmer as cities grow. Some of them are very poorly located next to heat sources (e.g., air conditioning exhausts or blacktop parking lots). There are large areas of the planet, especially at sea, where measurements are rare or nonexistent; in these cases, the values are interpolated from the nearest stations, which may be thousands of miles away (in other words, the estimates are simply guesswork). The CRU team claim that the data have been corrected for these effects, but have declined to specify precisely how this was achieved. Unsurprisingly, few independent observers are convinced that the corrections are reliable.

There is a second repository for surface temperature data at NASA's Goddard Institute for Space Science (GISS) in New York. The raw data are basically the same as those used by the CRU, but the algorithms employed for geographic interpolation and for correction of known measurement errors are apparently different  (although here again they have not been specified). The GISS record differs in detail from that in the figure, but shows the same general trends.

Since 1979, instruments aboard NOAA and NASA satellites of the Earth Observing System (EOS) have provided an independent record of global temperatures at low altitudes in the atmosphere. The measurements are archived at the University of Alabama in Huntsville and at Remote Sensing Systems, Inc., in Santa Rosa, CA. From 1979 to 2001, the data were very controversial, because there was little indication of the warming that is shown in the surface record. Various corrections were then proposed, mostly related to perturbations of the orbits of the satellites, which have now been applied to the entire archive. These adjustments have substantially reduced the discrepancies compared to the surface data, although the satellites still show significantly less average warming during the last thirty years.


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bigjohn1 said...

I am thankful that you posted here. I read your article and told my wife about it. Two years ago I started telling my friends and family that the real danger we face is global cooling not warming then I found your warnings. Sometimes you have to wonder if global warming is dis-info.

bigjohn1 said...

Global cooling is indeed the thing we should fear most. Only one warming period in the last 250,000 years has lasted this long. The glaciers are coming.