The Medieval Warm Period in Antarctica: How two one-data-point studies missed the target

Guest essay by Sebastian Lüning

A common claim by warmists in the climate debate is the alleged absence of the Medieval Warm Period (MWP) in the Southern Hemisphere. In a previous post we discussed the MWP in Australia, New Zealand and Oceania. In the following, we will take a look at Antarctica.

In 2012 a group led by Robert Mulvaney of the British Antarctic Survey published in Nature an ice-core record of deuterium variations from James Ross Island, off the northeastern tip of the Antarctic Peninsula, in which deuterium was used as a temperature proxy. Whilst they found indeed a slight warming centred around 1000 AD, later developments are puzzling. Unexpectedly, the highest temperatures of the past millennium occurred during the Little Ice Age (LIA) around 1750 AD (Fig. 1). And the coldest temperatures were found at 1400 AD, during the late MWP. Based on this apparent mismatch with the general MWP concept, Mulvaney and colleagues concluded in the paper that their result demonstrates the absence of the MWP on the Antarctic Peninsula:

Whereas SST [sea surface temperature] to the west of the Antarctic Peninsula shows similarities to Northern Hemisphere climate over the past 2,000 yr, the JRI record shows an opposing temperature excursion which demonstrates that the Antarctic Peninsula did not experience a widespread Medieval Warm Period/Little Ice Age sequence comparable to Northern Hemisphere climate at that time. Warming at JRI has been ongoing for several centuries, although the warming by 1.56°C over the past 100 yr (red lines in a and b) is highly unusual in the context of natural variability.

Figure 1: Temperature reconstruction for the JRI ice core from James Ross Island. From Mulvaney et al. 2012.

As part of our ongoing crowd-funded MWP Mapping Project we have recently studied the available literature of the region (Fig. 2). Is the ice core data of the Mulvaney group representative for the Antarctic Peninsula or at least the eastern part? A quick look at the map shows that the study represents the only known case with an anomalous temperature evolution during the past millennium (grey dot). All other data points show MWP warming (red dots).

Fig. 2: Overview of MWP studies in the Antarctica Peninsula region. Red dots indicate studies with MWP warming, grey dot marks study with mismatch to MWP concept (Mulvaney et al. 2012). Numbers refer to locations in text. Interactive online map here.

1. JRI ice core, James Ross Island, Mulvaney et al. 2012

2. Herbert Sound, off James Ross Island, Minzoni et al. 2015

A new paper published on 1 December 2015 by Rebecca Totten Minzoni and three colleagues in Quaternary Science Reviews sheds new light on the anomaly reported by the Mulvaney group. The Texan team from Houston and Austin studied diatoms and petrography in a sediment offshore core, located merely 25 km north of the ice core. Interestingly they found a distinct 200-year long peak in organic matter related to life-friendly warming conditions with a warm-water diatom assemblage (DA1) centred around 1000 AD (peak ‘5b’ in Fig. 3).

Fig. 3: Total Organic Carbon (“TOC”) content of a sediment core from Herbert Sound, north of James Ross Island. From: Minzoni et al. 2015

It becomes clear that the MWP is well represented also on the James Ross Island region. The Minzoni study confirmed that the area seems to have experienced an additional warming phase after 1600 AD which is indeed unusual. Is this local anomaly possibly related to volcanism in the region which is known to have interacted with the glaciers (e.g. Hambrey et al. 2008)? More likely, however, this is a more regional feature in Antarctica, because similar warming episodes during the LIA have been also reported in other studies in West and East Antarctica (see below). Detailed curve correlation will be necessary to exactly map out the area of these LIA anomalies. Regardless of the explanation, the LIA warm phase in the region of James Ross Island does not conflict with the MWP concept. MWP rejection by Mulvaney et al. therefore has no sound basis.

3: Core JPC2 in Firth of Tay, Lu et al 2012

From the abstract:

“This ikaite record qualitatively supports that both the Medieval Warm Period and Little Ice Age extended to the Antarctic Peninsula.”

Lead author Lu in the related press release from Syracuse University:

“We showed that the Northern European climate events influenced climate conditions in Antarctica,” Lu says. “More importantly, we are extremely happy to figure out how to get a climate signal out of this peculiar mineral. A new proxy is always welcome when studying past climate changes.”

4. Core A9-EB2, eastern Bransfield Basin, Khim et al. 2002

Pronounced minimum in magnetic susceptibility (MS) between 1050-1550 AD which indicates a section that is dominated by biogenic sediment (Fig. 4). Presence of biogenic sediment in this environment reflects high biological productivity associated with a warmer period and minimal sea ice. Strong increase in MS from 1550 AD onwards marks change to clastic-rich section and low productivity conditions associated with cold conditions. Little Ice Age conditions lasted 1550-1850 when another sharp drop in MS is recorded.

Fig. 4: Magnetic susceptibility in core A9-EB2, eastern Bransfield Basin. From Khim et al. 2002.

5. Piston Core JPC-24, Bransfield Basin, Barnard et al. 2014

Study of isotopes and geochemistry. Warm Phase 400-1500 AD, preceded and followed by cold phases (Fig. 5).

Fig. 5: Warm (pink) and cold (blue) periods in the Bransfield Basin. From Barnard et al. 2014.

6. Collins Ice Cap, King George Island, Hall 2007

The author studied radiocarbon dates of moss incorporated in glacier ice. Result: In 1300 AD the studied glacier was at or behind its present position. Temperatures have been similar to or possibly warmer than today. From 1300 AD onwards, a major glacier advance commenced, associated with the cooling of the Little Ice Age. This was the most extensive advance of the last 3500 years.

7. Anvers Island, Hall et al 2010

Hall et al. 2010 studied radiocarbon data from moss and shells recently uncovered by retreating ice. Result: Peat from sediments overrun by glacier ice dates between 707 and 967 B.P. (1033 AD and 1293 AD) which indicates that the ice edge was at or behind its present position 1030-1300 AD. During this period the area was at least as warm as, or likely even warmer than, the peak warmth of the Current Warm Period.

8. Piston Core PD92-30 & ODP 1098, Palmer Deep, Domack & Mayewski 1999, Shevenell et al. 2011

Domack & Mayewski 1999: Warm phases 900-1100 AD and 1250-1400 AD. The warm phase is marked by low magnetic susceptibility (MS) values, corresponding to an increase in biogenic sedimentation under warmer conditions. Entire climate record of past 3500 years is characterized by typical 200 years climate cycles, possibly corresponding to solar Suess-de Vries cyclicity.

Shevenell et al. 2011: Poorer temporal resolution than curve from Domack & Mayewski. General warm phase 400-1500 AD with abrupt cooling towards subsequent Little Ice Age. Warming at 400 AD equally abrupt, marking transition of Dark Ages Cold Period to Medieval Warm Period. See also follow-up work by Etourneau et al. 2013.

Fig. 6: Temperature reconstruction for ODP well 1098 (graph from Mulvaney et al. 2012, data from Shevenell et al. 2011.

Conclusion Antarctic Peninsula

All of the discussed studies from the Antarctic Peninsula provide robust evidence for significant warming during MWP times. In contrast to Mulvaney et al. 2012, there is no reason to assume the MWP might not have affected the entire peninsula.

East Antarctica

A similar case of likely over-interpretation of isolated data has also occurred in coastal East Antarctica. In October 2014 a Belgian/UK/Japanese research team led by Ines Tavernier published in Antarctic Science a study of a sediment core from Mago Ike Lake, Skarvsnes in Lützow Holm Bay (location A in Fig. 7). They plotted lots of parameters, but unfortunately did not provide a summarizing climate curve. Yet, they seem to be confident enough to conclude the absence of the MWP and LIA not only in their study area but quickly expanding to Antarctica and the entire Southern Hemisphere. In their abstract they write:

“There is no evidence for a Medieval Climate Anomaly, Little Ice Age or twentieth century warming in our lake sediment record suggesting that studies that have imposed Northern Hemisphere climate anomalies onto Southern Hemisphere palaeoclimate records should be treated with caution.”

As part of our global MWP mapping project we have compared the results of Tavernier et al. 2014 to other studies in the region. Astonishingly, all other ten studies document the MWP rather well (Fig. 7). Another case, where researchers appear to have extrapolated their results much too generously and quickly across the region, without thoroughly consulting the available regional literature.

Figure 7: Location of MWP studies in coastal East Antarctica. Red dots indicate studies with MWP warming (Tavernier et al. 2014), grey dot marks study with mismatch to MWP concept. Letters refer to locations in text. Interactive online map here.

A. Mago Ike Lake, Skarvsnes, Lützow Holm Bay, Tavernier et al. 2014

The group studied fossil pigments, sedimentological and geochemical proxies and carried out absolute diatom counts. According to the authors there is “no evidence for a Medieval Climate Anomaly, Little Ice Age or twentieth century warming in our lake sediment record”. Brief period of slightly increased primary production and warming occurred between 1510-1560 AD.

B. Lake Terrasovoje, Amery Oasis, Wagner et al. 2004

Study of diatoms and geochemistry. Result: Warm conditions between 500 to 1000 AD, indicated by high accumulation of organic matter and reducing bottom water conditions, preceded and followed by colder periods. Note that the study comprises a long 12,000 years (i.e. low time resolution) so that the warm peak may be somehow shifted.

C. AM02 core on Amery Ice Shelf, Hemer & Harris 2003

Peak in total diatom and Fragilariopsis curta species abundance at around 1250 AD indicates that the shelf ice edge had further retreated than its present position, indicating warmer conditions than present at the time. The peak coincides with a minimum in the abundance of the species Thalassiosira antarctica which fits with the warming interpretation around MWP times.

D. Kirisjes Pond, Larsemann Hills, Verleyen et al. 2004

Study of diatoms. High lake level 600-1200 AD triggered by warm conditions that led to increased humidity (Fig. 8). Subsequent cooling and lake level fall due to drier conditions. Another warm period centred around 1550 AD with subsequent cooling / lake level fall. Lake level rise during 20th Century warming.

Fig. 8: Reconstructed depth and salinity in the lacustrine zones of the Kirisjes Pond core based on diatoms. From Verleyen et al. 2004.

E. Core Co1010 in Rauer Group archipelago in eastern Prydz Bay, Berg et al. 2010

Berg et al. 2010 studied total organic carbon and C14 age dates. They found a peak in organic carbon content centred around 1000 AD, indicating more favourable / warmer conditions for organisms around this time (Fig. 9).

Fig. 9: Total organic carbon in core from Prydz Bay. From Berg et al. 2010.

F. Dome Summit South (DSS) – Law Dome ice core

Dahl-Jensen et al. (1999) and Roberts et al. (2001): Warm Period 700-1050 AD and 1350-1750 with cold phase in between. Significant cooling commences at 1750 AD, related to Little Ice Age which ends 1950 with major modern warming.

Morgan & van Ommen (1997): Illustrated ice core data start in second half of Medieval Warm Period and includes a significant winter warming event centred at 1425 AD lasting for 50 years. Winter temperatures dropped markedly from 1750 AD onwards, initiating the Little Ice Age. Warming from 1900 onwards marks beginning of Modern Warm Period.

Fig. 10: Oxygen isotopes and temperature interpretation from Dome Summit South (DSS) – Law Dome ice core. From Dahl-Jensen et al. (1999)

G. EPICA Dome Concordia (“Dome C”) Ice Core, Masson-Delmotte 2004, Masson et al. 2000

The ice core has been investigated by deuterium which reveals a warm Phase 700-1200 AD (Fig. 11).

Fig. 11: Deuterium curve as temperature proxy for Dome C ice core. From Masson et al. 2000.

H. Vostok Ice Core, Masson et al. 2000

Study based on deuterium which shows an extended warming peak 500-1500 AD (Fig. 12).

Fig. 12: Deuterium in Vostok ice core. From Masson et al. 2000.

I. Dome B Ice Core, Masson et al. 2000

Deuterium as temperature proxy. Warm phase 800-1500 AD (more positive deuterium values indicate warming, upward peak in curve), interrupted by short cooling episodes (Fig. 13).

Fig. 13: Deuterium in Dome B ice core. From Masson et al. 2000.

J. Dronning Maud Land, Graf et al. 2002

Delta18O in ice core as temperature proxy. Warm phase 1000 AD (start of data set) until 1250 AD when cooling started (Fig. 14). Maximum cold conditions of Little Ice Age centred around 1500 AD. Attached curve: More positive values in oxygen isotopes indicate warming (upwards peaks).

Fig. 14: Oxygen isotopes of Dronning Maud Land ice core. From Graf et al. 2002.

K. Plateau Remote Ice Core, Mosley-Thompson 1996, Leduc et al. 2010

Delta18O as temperature proxy. Warm Phase 950-1100 AD (Fig. 15). More positive oxygen isotopic values indicate warming.

Fig. 15: Oxygen isotope curve of the Plateau Remote ice core. Graph from Leduc et al. 2010.

Conclusion East Antarctica

The MWP is well documented in the vast majority of the discussed studies across East Antarctica. The exact timing and possible regional differences of the MWP warming still have to be worked out. At some stage in the future it is planned to download and/or vectorise all curves to plot them all in a consistent style and age-format. In some studies, the MWP warming appears to be front-loaded during the general 800-1200 AD MWP period, in others more back-loaded. Whether this is real or an age-dating artefact needs to be thoroughly checked case by case.

In some areas, an unusual LIA warming episode is recorded around 1550 AD which seems to be real. It has to be noted that Antarctica also today experiences a special development that does not necessarily follow the global trend. According to Wagner & Zorita 2015 there has been no warming in Antarctica during the 20^th Century, when at the same time global temperatures rose by nearly one degree C. Clearly, a robust documentation of Antarctic temperature changes during the past few thousand years is needed, specified by sub-regions.

The literature overview presented in this article indicates that it may not be sufficient to model the whole continent using one high resolution ice core or a single study. Regional variations do occur, nevertheless, the MWP warming signature seems to be present across Antarctica. It is unfortunate that Tavernier et al. 2014 have chosen to declare their one data point to be representative for a whole continent and in fact the entire southern hemisphere, when the real picture looks very different.