Did the Medieval Warm Period (MWP) and Little Ice Age (LIA) occur only in Europe, or were they global in scope?
This is a hotly debated question, because it is harder to make the case that the warmth of recent decades is “unusual,” “extraordinary,” or “unprecedented” and therefore something to stress about if global climate oscillates naturally between warming and cooling periods. The catastrophic anthropogenic global warming (CAGW) crowd tend to write off the MWP (~1000-1200 A.D.) and LIA (~1300-1850 A.D.) as regional phenomena, largely confined to Northern Europe. A new study finds evidence of the MWP and LIA in a region 10,000 miles south of Northern Europe: the Antarctic Peninsula.
WattsUpWithThat provides a lengthy excerpt from the new study, “An ikaite record of late Holocene climate at the Antarctic Peninsula,” which will be published in Earth and Planetary Science Letters. A Syracuse University press release summarizes the study’s methodology and findings:
A team of scientists led by Syracuse University geochemist Zunli Lu has found a new key [to climate history] in the form of ikaite, a rare mineral that forms in cold waters. Composed of calcium carbonate and water, ikaite crystals can be found off the coasts of Antarctica and Greenland.
“Ikaite is an icy version of limestone,” say Lu, assistant professor of earth sciences in SU’s College of Arts and Sciences. “The crystals are only stable under cold conditions and actually melt at room temperature.”
It turns out the water that holds the crystal structure together (called the hydration water) traps information about temperatures present when the crystals formed. This finding by Lu’s research team establishes, for the first time, ikaite as a reliable proxy for studying past climate conditions.
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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.
Ikaite crystals incorporate ocean bottom water into their structure as they form. During cooling periods, when ice sheets are expanding, ocean bottom water accumulates heavy oxygen isotopes (oxygen 18). When glaciers melt, fresh water, enriched in light oxygen isotopes (oxygen 16), mixes with the bottom water. The scientists analyzed the ratio of the oxygen isotopes in the hydration water and in the calcium carbonate. They compared the results with climate conditions established in Northern Europe across a 2,000-year time frame. They found a direct correlation between the rise and fall of oxygen 18 in the crystals and the documented warming and cooling periods.
Although the authors do not claim to have “unambiguously established” the MWP in the Antarctic Peninsula (AP), they conclude that their “ikaite record builds the case that the oscillations of the MWP and LIA are global in their extent and their impact reaches as far South as the Antarctic Peninsula, while prior studies in the AP region have had mixed results.” Their research also indicates that the AP may have been warmer during the MWP than in recent decades: “Our most recent crystals suggest a warming relative to the LIA in the last century, possibly as part of the regional recent rapid warming, but this climatic signature is not yet as extreme in nature as the MWP” (emphasis added).
Although the Lu team is the first to use akaite as a proxy, they are far from the first to find evidence of the MWP outside of Europe. The Medieval Warm Period Project of the Center for the Study of Carbon Dioxide and Global Change reviews (by my count) 20 studies in Africa, 8 in Antarctica, 68 in Asia, 6 in Australia/New Zealand, 92 in North America, 31 in various Ocean areas, and 19 in South America, in addition to 97 in Europe — all indicating a period of climatic warmth approximately one thousand years ago. Many of those studies indicate that the MWP was warmer than the Current Warm Period (see the chart at the top of this post).