Posted by: ericgrimsrud | July 27, 2019

Denier’s claim concerning natural climate cycles demolished

The deniers of man-caused warming have made a major issue out of the unusual climates observed in specific regions of the northern hemisphere during the last millennium and have claimed that these unusual climates of the past prove that natural variations of climate are more important than any man-caused changes and are responsible for recently observed warming trends.

As shown in the figure below, two of these periods are known as the Medieval Warm Period (MWP)and the Little Ice Age (LIA) occurring over the years indicated in the figure. Also shown are the temperatures observed over the Modern Period (approximately the last two centuries).

T over 2000 years

The deniers have claimed that some of these measurements prove that the temperature variations observed recently during in the Modern Period are very likely due to natural variations such as those that caused the MWP and the LIA, both of which occurred long before large amounts of greenhouse gases were emitted by mankind beginning with the onset of the Industrial Age in about 1850. The blue line, showing the temperatures observed specifically in London region of central England, provided the evidence used by the deniers to argue for the existence of large natural cycles. Note however that the measurements of the average annual temperatures observed over the entire Northern Hemisphere (red line) do not show unusual temperatures during either the MWP or the LIA. Thus, a remaining question has been: do the MWP and the LIA represent periods of total global warming and cooling or were they caused merely by regional weather patterns along with no net changes in the average total global climate.

Three new research papers have very recently been published that directly address this question and each of them provide strong evidence for no anomalous periods of unusually low or unusually high average global temperatures over an extended period from 2,000 years ago up to the beginning of the Industrial Age. That is, these new measurements of total Northern and Southern Hemispheric annual average temperatures most closely resemble those of the red line shown above for the last millennium and also showed no such anomalous periods during the previous millennium. This new research suggests that there have been no significant natural cycles of global climate for the last 2,000 years and that the warming observed throughout our planet during the recent Modern Period is due to a new and different, unnatural phenomenon, such as that caused by the emission of greenhouse gases by mankind.

For an account of and references to the three new research papers that has provided this new insight, I will refer you to an article in the Science News magazine.


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