During the 20th century, we’ve seen three distinct global temperature trends:
1) A warming period from 1910 to 1942,
2) A cooling period from 1942 to 1978, and
3) A warming period from 1978 to the present.
The graph below is of global temperature (as collected by the GISS) from 1880.
The warming in the early 20th century is largely seen as natural. CO2 levels in the atmosphere were not increasing nearly enough for anyone to claim that it caused the extent of warming recorded. Similarly, the mid-century cooling can’t have its roots in CO2 output, and instead was likely caused by increased particulate pollution and decreased solar activity. The warming seen since 1978 is what is so contentious. The purpose of this post is to compare zonal trends during the two warming trends.
Below are two graphs that show zonal trends in temperature during the perid 1910-1942.
Below are two more graphs, representing zonal trends over the period 1978-2007.
As you can see the zonal temperature signal for both periods are nearly the same. This is important because anthropogenic global warming promoters use the high lattitude warming as evidence for the effect of increased CO2 levels on temperature. Yet, this same pattern of warming, best demonstrated by the zonal mean vs lattitude charts, was observed during a completely natural period of warming. There is one notable difference between the trends observed during the two periods, though. Warming seen in Northern Asia and Europe during the modern warming did not occur during the period of natural warming. As I see it, there are a few explanations: 1) This Siberian warming has been caused by increased CO2 levels in the atmosphere. 2) The magnitude of the modern warming has been somewhatlarger, which might have caused a warming signal that the smaller, natural warming did not exhibit. 3) The warming periods are both natural, though they were not caused by the same climate shift. For example, warming during the first half of the century may have been caused by increases in total solar irradiance, while this modern warming may have its origins in oceanic circulation. This would result in different regional trends.
So, there are two points to be made about this zonal trend analysis.
1) Arctic warming may be natural. It has occured before with equal intensity, and thus Arctic warming is not evidence for a CO2 forcing.
2) The two periods of warming are not entirely the same. The modern Siberian warming that some attribute to rising CO2 levels did not occur during the natural warming. When I saw Harvard astrophysicist Willie Soon, Ph.D. speak at the International Conference on Climate Change, he attributed this warming to changes in regional oceanic currents. I am sure that there are other possible explanations, and I may post something about it in the future.