book_cover_big.gifWe all know the facts: the earth is warming up. Al Gore has put quite some convincing facts together in his book “An Inconvenient Truth”. Right?

Mmmmmm…… sometimes you can find some counter arguments from credible scientists[1]. In an article from Salomon Kroonenberg[2], a geologist,  he argues for instance that for the last 10 years the average temperature on earth has not increased. It peaked in 1998, most probably because of an overactive El Nino. However, the CO2 levels did increase in that period but at the same time the activity of the sun was changing as well. And the activity level of the sun is known to have an impact on the temperature of earth. In the period between 1945 and 1975 the average temperature decreased while the concentration of green house gases increased. He brings forward several other examples where there is no correlation between the climate change and the greenhouse gases. For instance he points out periods in history where there have been dramatic changes in climate while the atmosphere did not change. Kroonenberg’s point is that there are many other mechanisms that have changed the climate in the past and at speeds much larger than what we observe now. His advise is therefore that we better adapt to the changing climate and find ways to cope with that.

But what about the average temperature of the earth?  How sure can we be about that crucial parameter? In fact it appears not to be such a trivial job to monitor the average temperature of earth and certainly not when you want to see one degree of difference over a long period. Temperature monitoring of the oceans was done for quit a while from ships. American ships typically measured the temperature of the water sucked into the inlet of the cooling funnels of the ship whereas English ships simply would use buckets to take a sample of the water and stick a thermometer in it. It proved that the American method gave a too high of a value (about 0.3°C ) whereas the English method gave a too low of a value. Temperatures that are measured at the surface of earth can have many flaws such as changing environment (urbanization) or changing station locations. See for more details on this reference.

There are also measurements taken by satellites. Satellite data have the advantage that they are truly covering the entire globe but do span a relatively short period, something like only 30 years. One of the institutes that generates satellites temperature data is the University of  Alabama of Huntsville.[4] In the figure below you can find the temperature recorded at globe level since 1979, I have averaged the monthly numbers per year. The year 1998 looks indeed, as mentioned above, as a maverick year, but let’s for this discussion not spent time on that observation. I believe that it is more important to note that there appears to be two periods of different behavior. The period from 1979 till 1997 seem to be a stable period with an average temperature of -0.027°C and the period from 1999 till 2007 seem to show a still increasing trend with an average temperature in that period of 0.216°C (substantially higher than the previous period). So the claims found in the press today that the temperature of the globe did not increase within the last 10 years is perhaps true but on average the average temperature in the last 10 years is substantially higher then in the previous 20 years. But let’s not forget, the time span we are discussing here is still awfully short at geological timescales.  So be careful drawing conclusions one way or the other would be my advise.



Average Global anual temperature. Data taken from the University of Alabama at Huntsville

Copyright © 2008 John Schmitz