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Medieval Warm Period Rediscovered

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Doug L. Hoffman
The Resilient Earth
Thursday, April 9, 2009

A recent article in the journal Science has provided a new, detailed climate record for the Medieval Climate Anomaly (MCA), also know as the Medieval Warm Period. It was the most recent pre-industrial warm period, noted in Europe and elsewhere around the globe. The researchers present a 947-year-long multi-decadal North Atlantic Oscillation (NAO) reconstruction and find a persistent positive NAO during the MCA. The interesting thing is that the MCA had basically been removed from the climate record by Michael Mann’s infamous “hockey stick” history graph that was adopted by the IPCC a decade ago.

More interesting, Trouet et al., based their work in part on a tree-ring–based drought reconstruction for Morocco (1049–2002) and a millennial-length speleothem-based precipitation proxy for Scotland (900–1993), a methodology similar to Mann’s work. Unlike Mann, these researchers found significant climate warming during the MCA. According to the report: “The Morocco and Scotland reconstructions contain substantial multi-decadal variability that is characterized by antiphase oscillatory behavior over the last millennium.” Their reconstruction can be seen in the figure from the article seen below.

The fugure shows a proxy-derived long-term NAO reconstruction. (Top) Reconstructed winter precipitation for Scotland and February-to-June Palmer Drought Severity Index for Morocco. Records were normalized over the common period (1049–1995) and smoothed with the use of a 30-year cubic spline. The bottom graph shows a winter North Atlantic Oscillation (NAO) reconstruction (black curve). The gray area is the estimated uncertainty. The researchers’ conclusions?

The persistent positive phase reconstructed for the MCA appears to be associated with prevailing La Niña–like conditions possibly initiated by enhanced solar irradiance and/or reduced volcanic activity and amplified and prolonged by enhanced AMOC. The relaxation from this particular ocean-atmosphere state into the LIA appears to be globally contemporaneous and suggests a notable and persistent reorganization of large-scale oceanic and atmospheric circulation patterns.


Here AMOC stands for the Atlantic Meridional Overturning Current (i.e. the ocean conveyer belt) and LIA for Little Ice Age, the period of global cooling that followed the Medeival Warm Period and lasted until the mid 1800s. What they are saying is that both the MCA and the LIA were real and had identifiable root causes. This result stands in stark contrast with the hocky stick result where the Medieval Warm Period and Little Ice Age almost disappeared, replaced by a largely benign, slight cooling trend that lasted until ~1900.

This is just the latest in a series of reports that quietly contradict some of the more outlandish untruths spread by the anthropogenic global warming extremists of the IPCC. Another recent Science article, by Brierly et al., addresses the Pliocene warm interval, a period of warm climate conditions that preceded the current Pleistocene Ice Age. Occurring some 4 million years ago, the Pliocene warm interval has been difficult to explain.

The early Pliocene epoch from 5.3 to about 3 million years ago was much warmer than today. The early Pliocene climate was very much like preindustrial conditions during the Holocene (our current interglacial warm period). Similarities include the amount of solar radiation Earth received, the concentration of atmospheric CO2, and a nearly identical geographic environment. But there were also significant differences. For example, there was no permanent ice sheet in the Northern Hemisphere, and global sea level was 80 feet (25 m) higher. Why is our climate today so different? Quoting from the article:


Our reconstruction shows that the meridional temperature gradient between the equator and subtropics was greatly reduced, implying a vast poleward expansion of the ocean tropical warm pool. Corroborating evidence indicates that the Pacific temperature contrast between the equator and 32°N has evolved from ~2°C 4 million years ago to ~8°C today. The meridional warm pool expansion evidently had enormous impacts on the Pliocene climate, including a slowdown of the atmospheric Hadley circulation and El Niño–like conditions in the equatorial region. Ultimately, sustaining a climate state with weak tropical sea surface temperature gradients may require additional mechanisms of ocean heat uptake (such as enhanced ocean vertical mixing).


This says that the difference in ocean temperatures between the equator and higher latitudes is much more pronounced today. The tropics were not warmer but the temperate zones were. Because of this, the researchers found a significant decrease in the heat transport by the atmosphere, along with an implied increase in ocean heat transport. This result contradicts studies using ocean general circulation models (GCM) with low vertical diffusion. Such models, heavily used by climate scientists to try and predict the future course of global warming, suggest that a permanent El Niño should be associated with reduced heat transport by the ocean. When simulating Earth’s climate with current GCM, the ocean typically gains a large amount of heat over the tropical Pacific cold tongue.

The region of the tropical Pacific that scientists call the “equatorial cold tongue,” is a band of cool water that extends along the equator from the coast of South America to the central Pacific Ocean. Departures from average of sea surface temperatures in this region are critically important in determining major shifts in the pattern of tropical rainfall, which influence the jet streams and patterns of temperature and precipitation around the world. Attempts to simulate the Pliocene climate with coupled atmosphere-ocean GCM have not succeeded in replicating the collapse of the sea surface temperature (SST) gradient along the equator, possibly because of this issue.

In summary the researchers conclude: “[I]t may be necessary to incorporate additional mechanisms for increased ocean heat uptake when simulating the early Pliocene climate and, potentially, the response of the tropics to contemporary global warming. The enormous impacts of changes in the warm pool (such as shifts in global precipitation patterns and cloud cover), as well as tentative evidence that the tropical belt has been expanding poleward over the past few decades, make our findings especially relevant to current discussions about global warming.”

The bottom line? Once again the climate models used by the IPCC and other climate catastrophists are shown to be inaccurate, incomplete and not up to the job of predicting future climatic conditions. What does the IPCC have to say about all of this? Here is a quote from Paleoclimate, chapter 6 of the 2007 IPCC report:

Palaeoclimate science has made significant advances since the 1970s, when a primary focus was on the origin of the ice ages, the possibility of an imminent future ice age, and the first explorations of the so-called Little Ice Age and Medieval Warm Period. Even in the first IPCC assessment (IPCC, 1990), many climatic variations prior to the instrumental record were not that well known or understood. Fifteen years later, understanding is much improved, more quantitative and better integrated with respect to observations and modelling.


Meaning all of the earlier IPCC predictions were wrong because they really didn’t know what they were doing. Of course that didn’t prevent them from predicting a coming climate catastrophe with great confidence. The thing that they don’t tell us is that their current “improved” predictions, which are also based on computer models, simply can not be considered accurate either. If the Holocene truly marks the end of the Pleistocene Ice Age and a return to the conditions that prevailed during the Pliocene, parking your SUV and buying carbon credits won’t do a thing to stop it.

On the bright side, ocean temperatures were much warmer then and distributed differently than today, so climate change still has a way to go before real global warming kicks in. The eventual rise in sea level will play havoc with oceanfront property around the world, but that will happen if the ice age is over regardless of what we mortals do. If the Pleistocene hasn’t come to an end the ice will eventually return and all the current “runaway global warming” will only be a fondly remembered historical age—much like the Medieval Warm Period.

As always, enjoy the interglacial and stay skeptical.

This article was posted: Thursday, April 9, 2009 at 3:46 am

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