FAQ

This is one of a number of popular myths regarding temperature variations in past centuries. At hemispheric or global scales, surface temperatures are believed to have followed the “Hockey Stick” pattern, characterized by a long-term cooling trend from the so-called “Medieval Warm Period” (broadly speaking, the 10th-mid 14th centuries) through the “Little Ice Age” (broadly speaking, the mid 15th-19th centuries), followed by a rapid warming during the 20th century that culminates in anomalous late 20th century warmth. The late 20th century warmth, at hemispheric or global scales, appears, from a number of recent peer-reviewed studies, to exceed the peak warmth of the “Medieval Warm Period”. Claims that global average temperatures during Medieval times were warmer than present-day are based on a number of false premises that a) confuse past evidence of drought/precipitation with temperature evidence, b) fail to disinguish regional from global-scale temperature variations, and c) use the entire “20th century” to describe “modern” conditions , fail to differentiate between relatively cool early 20th century conditions and the anomalously warm late 20th century conditions.

This is yet another oft-repeated but problematic assertion based in this case on the mis-characterization of the so-called Mid-Holocene Climatic Optimum” or “Mid-Holocene Warm Period”. Paleoclimate experts now know that the mid-Holocene warmth centered roughly 8000 to 6000 years ago was probably restricted to high latitudes and certain seasons (summer in the Northern Hemisphere and winter in the southern hemisphere). Because much of the early paleoclimate evidence that was available (for example, fossil pollen assemblages) came from the Northern Hemisphere extratropics, and is largely reflective of summer conditions, decades ago some scientists believed that this was a time of globally warmer conditions. More abundant evidence now demonstrates, for example, that the tropical regions were cooler over much of the year.

[Read more…] about OK, But we do know it was warmer than present 6000 years ago, don’t we?