Learn how plants respond to higher atmospheric CO2 concentrations

How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Medieval Warm Period (Australia and New Zealand) -- Summary
Climate alarmists have claimed for quite some time that late-20th-century and early-21st-century global temperatures were so high as to merit the word "unprecedented" when comparing them to temperatures of the past millennium or two; and they also claim that this achievement was both driven and sustained by the carbon dioxide or CO2 released to the air by mankind's burning of fossil fuels such as coal, gas and oil. But to maintain this dual contention, they have been forced to further contend that the well-known Medieval Warm Period was neither hemispheric nor global in scope, but merely confined to the much smaller region surrounding the North Atlantic Ocean. And they have also had to contend that the MWP was never really as warm as it had long been believed to be.

In introducing their paper of quite some time ago, Wilson et al. (1979) wrote that one of their main objectives in conducting the study it described was to compare the temperature record from New Zealand - which they emphasized is "in the Southern Hemisphere and a region meteorologically unrelated to Europe" - with the climate record of England, where the MWP had already made its mark on that country's and the surrounding region's climatic history. Their contribution to this endeavor was to decipher the 18O/16O profile from the core to the surface of a stalagmite obtained from a cave in New Zealand, which was dated by the 14C method. And in doing so, they found that the proxy temperature record provided by the stalagmite was broadly similar to the climate record of England, exhibiting a period in the early part of the past millennium that was about 0.75°C warmer than it was in the mid-20th-century. And based on that finding they made the broader conclusion that "such climatic fluctuations as the Medieval Warm Period and Little Ice Age are not just a local European phenomenon."

A quarter of a century later, Williams et al. (2004) wrote that their new paper on the subject revises and builds on results that were derived by Williams et al. (1999) from stable isotope stratigraphy found in caves at Waitomo, which is located at 38.3°S latitude about 35 km from the west coast of the central North Island of New Zealand. More specifically, they enhanced three existing speleothem (stalactite, stalagmite or flowstone cave deposit) records "by adding another chronology, increasing the subsample resolution of existing records, and by much improving the temporal control of all chronologies by basing it entirely on uranium series TIMS dating." And with these improvements, Williams et al.'s speleothem master chronologies revealed a warmer-than-present late-Holocene warm peak located between 0.9 and 0.6 ka BP that they equated with the Medieval Warm Period of Europe, further noting that this period "coincided with a period of Polynesian settlement (McGlone and Wilmshurst, 1999)." Thereafter, they further reported that temperatures "cooled rapidly to a trough about 325 years ago," which they said corresponded to "the culmination of the 'Little Ice Age' in Europe."

In clear contradiction of the claims of climate alarmists, these findings are but another example of the unending stream of studies from all around the world that continue to document the global presence of a warmer-than-present Medieval Warm Period; and they are becoming ever more difficult to deny, because they demonstrate that the Medieval Warm Period was warmer than the Modern Warm Period, even though the air's CO2 concentration of that earlier period was about 115 ppm less than it is currently. This being the case, there is absolutely no reason to attribute the planet's current level of warmth to its current elevated atmospheric CO2 content, as there is an historical precedent (the MWP) for even higher temperatures than those of the present with much lower-than-current atmospheric CO2 concentrations.

A short four-year hiatus later, Lorrey et al. (2008) developed two master speleothem δ18O records for New Zealand's eastern North Island (ENI) and western South Island (WSI) for the period 2000 BC to about AD 1660 and 1825, respectively. The WSI record was a composite chronology composed of data derived from four speleothems from Aurora, Calcite, Doubtful Xanadu and Waiau caves, while the ENI record was a composite history derived from three speleothems from Disbelief and Te Reinga caves. This work revealed that for both the ENI and WSI δ18O master speleothem histories, their warmest periods fell within the AD 900-1100 time interval, which is also where the peak warmth of a large portion of the temperature records found in co2science.org's Interactive Map and Time Domain Plot of their Medieval Warm Period Project falls.

Not wanting to acknowledge that the earth of a thousand or so years ago was likely as warm as, or even warmer than, it is currently - when the atmosphere's CO2 concentration was much lower than it is today (285 vs. 400 ppm) - the world's climate alarmists have been loath to admit there was an MWP anywhere other than in countries surrounding the North Atlantic Ocean. And so it is that the results of the several studies described above are of great importance to the ongoing global warming debate, as they greatly advance the thesis that the MWP was indeed a global phenomenon, wherein temperatures throughout the world were significantly warmer than they have been anytime subsequently, and that there is thus nothing unusual or unprecedented about earth's current level of warmth, with the obvious implication that the maximum temperatures of the present simply cannot be attributed to the historical increase in the air's CO2 content.

Lorrey, A., Williams, P., Salinger, J., Martin, T., Palmer, J., Fowler, A., Zhao, J.-X. and Neil, H. 2008. Speleothem stable isotope records interpreted within a multi-proxy framework and implications for New Zealand palaeoclimate reconstruction. Quaternary International 187: 52-75.

McGlone, M.S. and Wilmshurst, J.M. 1999. Dating initial Maori environmental impact in New Zealand. Quaternary International 59: 5-16.

Williams, P.W., King, D.N.T., Zhao, J.-X. and Collerson, K.D. 2004. Speleothem master chronologies: combined Holocene 18O and 13C records from the North Island of New Zealand and their palaeo-environmental interpretation. The Holocene 14: 194-208.

Williams, P.W., Marshall, A., Ford, D.C. and Jenkinson, A.N. 1999. Palaeoclimatic interpretation of stable isotope data from Holocene speleothems of the Waitomo district, North Island, New Zealand. The Holocene 9: 649-657.


Last updated 25 September 2013