A note about long term trends..Posted: May 5, 2008
As mentioned on Deltoid, there is still the erroneous belief going around that there has been some sort of global cooling over the past few years and hence climate change isn’t happening. A brief mention of the fact that those whose hold this belief use dodgy or simply wrong data could be made and discussed but there is something I’d like to show regardless of this.
A currently accepted value for global warming is something like 3C per 100 years. This long term trend is meant to be on top of the annual variation of about 1C. To get some idea of what this looks like here is a graph made in Excel. The data on this graph is the the last 20 “years” of temperature with the annual variation superimposed on top of the long term trend.
As can be seen the temperature goes up and down each year and no long term trend is apparent. Now if this was real temperature it would be difficult to say anything. In fact it sort of looks flat and we could say the temperature has gone down after about the 20th year for a period of 5 years. And this is the problem of looking at this sort of data when the noise, the annual variability of 1C, totally swamps the long term trend of 0.03C. Now we’ll look at the 100 year graph this small graph was taken from.
Looking over the long term, in this case 100 years, the trend can clearly be seen. The right hand end of the graph shows the small section from the top graph.
Now if this was the real data one could see that an argument could be made that after about the 90th year the trend had ended because there is 5 years of lower temperature averages. However this would be wrong as another year later the temperature jumps up again.
This took about 5 minutes with Excel to show. It is not a complex argument. Yet we have people who are claiming that global warming has finished because of a run of lower temperatures. There is three things wrong with this argument, the data does not support it ,even if the data did show a few cooler it means SFA and thirdly what is happening to the climate is change, not just global warming.