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The Mid-Holocene "Warm Period"


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Mid-Holocene Warm Period - About 6,000 Years Ago

Paleoclimatologists have long suspected that the "middle Holocene" or a period roughly from 7,000 to 5,000 years ago, was warmer than the present Image courtesy of Kerwin et al., 1999.  Click here for larger viewing image. day. Terms like the Alti-thermal or Hypsi-thermal or Climatic Optimum have all been used to refer to this warm period that marked the middle of the current interglacial period. Today, however, we know that these terms are obsolete and that the truth of the Holocene is more complicated than originally believed.

What is most remarkable about the mid-Holocene is that we now have a good understanding of both the global patterns of temperature change during that period AND what caused them. It appears clear that changes in the Earth's orbit have operated slowly over thousands and millions of years to change the amount of solar radiation reaching each latitudinal band of the Earth during each month. These orbital changes can be easily calculated and predict that the northern hemisphere should have been warmer than today during the mid-Holocene in the summer AND colder in the winter. The paleoclimatic data for the mid-Holocene shows these expected changes, however, there is no evidence to show that the average annual mid-Holocene temperature was warmer than today's temperatures. We also now know from both data and "astronomical" (or "Milankovitch") theory that the period of above modern summer temperatures did not occur at the same time around the northern hemisphere, or in the southern hemisphere at all.

In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere. More over, we clearly know the cause of this natural warming, and know without doubt that this proven "astronomical" climate forcing mechanism cannot be responsible for the warming over the last 100 years.
For larger viewing version of the graph, please click here or on graph. Graph courtesy of Kerwin et al., 1999, complete scientific reference located here.

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Last Updated Wednesday, 20-Aug-2008 11:23:45 EDT by paleo@noaa.gov
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