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.



  1. You will not want to deal with this information, will you?
    Clarifying springtime temperature reconstructions of the medieval period by gap-filling the cherry blossom phenological data series at Kyoto, Japan
    We investigated documents and diaries from the ninth to the fourteenth centuries to supplement the phenological data series of the flowering of Japanese cherry (Prunus jamasakura) in Kyoto, Japan, to improve and fill gaps in temperature estimates based on previously reported phenological data. We then reconstructed a nearly continuous series of March mean temperatures based on 224 years of cherry flowering data, including 51 years of previously unused data, to clarify springtime climate changes. We also attempted to estimate cherry full-flowering dates from phenological records of other deciduous species, adding further data for 6 years in the tenth and eleventh centuries by using the flowering phenology of Japanese wisteria (Wisteria floribunda). The reconstructed tenth century March mean temperatures were around 7°C, indicating warmer conditions than at present. Temperatures then fell until the 1180s, recovered gradually until the 1310s, and then declined again in the mid-fourteenth century.

  2. You will not allow this to be seen, will you, because you have jo acquaintance with the truth.
    Investigation of peat sediments from Daiyun Mountain in southeast China: late Holocene vegetation, climate and human impact
    “In addition, major climatic events, such as the warm period from ad 670–960, the Medieval Warm Period (ad 1050–1520) and the Little Ice Age (ad 1580–1850), could be identified within the peat sediments in this study, with climatic conditions at these times being characteristically warm and wet, warm and dry, and cold and wet, respectively.”

  3. The evidence produced in this report goes against what you are trying to get people to believe. If the carbon dioxide levels were less than what it is today, how did this world wide warming occur?
    “The scientists studied ikaite crystals from sediment cores drilled off the coast of Antarctica. The sediment layers were deposited over 2,000 years. The scientists were particularly interested in crystals found in layers deposited during the “Little Ice Age,” approximately 300 to 500 years ago, and during the “Medieval Warm Period,” approximately 500 to 1,000 years ago.  Both climate events have been documented in Northern Europe, but studies have been inconclusive as to whether the conditions in Northern Europe extended to Antarctica.”
    “We showed that the Northern European climate events influenced climate conditions in Antarctica,” Lu says. “More importantly, we are extremely happy to figure out how to get a climate signal out of this peculiar mineral. A new proxy is always welcome when studying past climate changes.”

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