By Sebastian Lüning
Geoscientist and co-author of ‘The neglected Sun’
The climate of the pre-industrial past is of greatest importance to the ongoing climate discussion. Current climate can only be understood when interpreting it in the paleoclimatological context of the past few thousand years. Until not too long ago it was thought that the pre-industrial climate was monotonous and constant. This idea was e.g. promoted by Mann et al. whose famous hockey stick curve featured prominently in the IPCC report of 2001. Over the last 15 years, however, a large number of studies changed this view by providing robust evidence for the existence of significant natural climate variability. Of particular interest are the past 1000 years which commenced with the generally warm ‘Medieval Climate Anomaly’ (MCA, aka ‘Medieval Warm Period’, MWP), that eventually passed into the ‘Little Ice Age’ (LIA), before returning to the warm climate of the current ‘Modern Warm Period’ of the 20th and early 21st centuries.
There have been controversial debates about the existence of the MWP, the level of warming compared to the current climate as well as on regional differences in timing. Meanwhile, the MWP has been documented from all seven continents, as documented by literature syntheses prepared by the CO2Science group. (Australia & New Zealand here and here). In this article we present an independent follow up project, coordinated by geologist Dr Sebastian Lüning, in which we map the MWP across the globe, visualizing the existing literature on a zoomable Google Maps platform. A click on the respective data point opens an information panel which summarizes the results of the study using a common, simplifying scheme. Links to the journal abstract and key figure allow quick access to the data. The MWP Online Map is freely accessible for anyone.
Results are now available for Africa, Antarctica and Australia/Oceania. In the following, we want to present our findings for Australia/Oceania, hoping for reader feedback on additional studies or alternative interpretations. Up to now we have identified 15 study locations in the region (Figs. 1 and 14). Red symbols on the map mark studies where the MWP was characterized by a warming phase, green dots show areas which were wetter during the MWP, yellow dots regions where the MWP saw a reduction in rainfall.
At first sight, green symbols dominate Australia and New Caledonia, indicating that the MWP brought additional rain into the region, compared to proceeding and subsequent times (Fig. 1). This also includes Hawaii (Fig. 14). Temperature reconstructions are only available in the southernmost temperate part of Australia and New Zealand. All available studies show a prominent warming during the MWP (red dots). No studies were found in which the MWP would have been colder than the climate before or afterwards.
Figure 1: Studies covering the MWP in Australia/Oceania that are included in the MWP Online Map and are discussed in this article.
1. Cave KNI-51, Ningbing Range, eastern Kimberley region
Denniston et al. 2013 in Quaternary Science Reviews
The study covers the last 9,000 years and is based on delta18O isotopes in stalagmites. A strong increase in monsoon precipitation was found for 1000-1200 AD, corresponding to the Medieval Warm Period (Fig. 2).
Figure 2. Delta18O isotopes in stalagmites in Cave KNI-51, Ningbing Range, eastern Kimberley region, Australia. From Denniston et al. 2013.
2. Swamps on the Atherton Tableland in north Queensland
Burrows et al. 2014 in The Holocene
The study covers the last 4,000 years and is based on peat humification, lithostratigraphy and magnetic susceptibility. A series of wet phases occurred during 900-1400 AD (Fig. 3). The climate was drier before (Dark Ages Cold Period) and after (Little Ice Age) this phase.
Figure 3. Proxy records of wet phases in Bromfield Swamp, north Queensland. From Burrows et al. 2014.
3. Fournier Swamp, New Caledonia
Wirrmann et al. 2011 in Quaternary Research
The study covers the last 7000 years and is based on pollens and foraminifera. A wet phase is documented for 920-1250 AD, preceded and followed by dry phases.
4. Lake Callabonna
Cohen et al. 2011 in Geology
Cohen et al. 2012 in Quaternary Science Review
Gliganic et al. 2014 in The Holocene
The studies cover the last 50,000 years and feature a paleoshoreline chronology, dated by optically stimulated luminescence (OSL) ages. Cohen et al. 2011 and 2012 found a major pluvial / wet episode in southern central Australia between 1050-1100 AD associated with the Medieval Climatic Anomaly, that briefly interrupts a generally arid playa-dominated period. During this pluvial interval, Lake Callabonna filled to 10-12 times the volume of the largest historical filling (1974) and reached maximum depths of 4-5 m, compared to the 0.5-1.0 m achieved today. Also Gliganic et al. 2014 found an elevated lake level, centred around 1000 AD (Fig. 4).
Fig. 4. Lake-filling curve for Lake Callabonna. From Gliganic et al. 2014.
5. Blue Lake
Gouramanis et al. 2010 in Paleo3
Climate reconstruction of the past 6000 years based on ostracod assemblages, stable isotope (delta13C, delta18O), Mg/Ca, Sr/Ca and Na/Ca analyses on ostracod valves. The Lake level begins to rise 1000 AD with peak lake filling reached 1300 AD (Fig. 5). High lake level indicates wet phase. Subsequent fall of lake level.
Fig. 5 Modelled lake level for Blue Lake, South Australia. From Gouramanis et al. 2010.
6. Lake Surprise, Victoria
Barr 2010 in a PhD thesis at the University of Adelaide (unpublished elsewhere?)
The study covers the last 15000 years and is based on diatom flora and diatom conductivity (high values=arid, low values=humid). Barr found a series of droughts 680-1400 AD (with a wet interlude 800-900 AD). Wet conditions 1400-1900 AD were associated with Little Ice Age. This is the only study from Australia/Oceania so far that found a drying trend during the MWP in the region. Nearby studies #5 and #7 did not find similar droughts. We are not aware that the results from Barr 2010 have been published in a peer-reviewed journal, therefore interpretations have to be treated with caution.
7. Lake Keilambete
Wilkins et al. 2013 in The Holocene
The study covers the last 10,000 years and looks at lake levels reconstructed from sediment particle size and ostracod valve chemistry (delta18O and Sr/Ca). A major phase of lake filling, i.e. wet phase was found for 1000-1300 AD (Fig. 6).
Fig. 6. Lake level history of Lake Keilambete, South Australia. From Wilkins et al. 2013.
8. Core MD03-2611, Murray Canyon, S-Australia
Moros et al. 2009 in Quaternary Science Reviews
The study covers the last 10,000 years and looks at delta18O in foraminifera. A warm period prevailed 1000-1600 AD, flanked by cold events 600 AD (Cold Period of the Migration Period) and 1600 AD onwards (Little Ice Age).
Fig. 7. Oxygen isotope curve as temperature proxy for the last 10.000 years for foraminifera in a sedimentary core from Murray Canyon, South Australia. From Moros et al. 2009.
9. Mt. Read, western Tasmania
Cook et al. 2000 in Climate Dynamics
The study covers the last 3500 years and is based on tree rings which allowed reconstruction of warm season temperatures. A warm phase occurred 950-1500 AD, followed by Little Ice age cold phase (Fig. 8).
Fig. 8. Warm season temperature development for Mt. Read in western Tasmania, reconstructed based on tree rings. From Cook et al. 2000.
10. Fiordland, New Zealand (Aurora, Doubtful Xanadu, Waiau, and Calcite Caves)
Lorrey et al. 2008 in Quaternary International
The study covers the last 4000 years and is based on delta18O in stalagmites. A warm phase was found for 900-1400 AD (with a short cool phase 1150-1250 AD) (Fig. 9).
Fig. 9. Oxygen isotope curve for cave stalagmites in Fjordland, New Zealand. From Lorrey et al. 2008.
11. Oroko Swamp, New Zeland
Cook et al. 2002 in the Geophysical Research Letters
The study covers the last 1100 years and reconstructs summer temperatures based on tree rings. A warm phase has been reported for 950-1500 AD, with a short cold interlude 1000-1050 AD (Fig. 10). A cold phase 1500-1800 AD corresponds to the Little Ice Age.
Fig. 10. Summer temperatures in Oroko Swamp, New Zealand. From Cook et al. 2002.
12. Cave in NW Nelson District, South Island, New Zealand
Wilson et al. 1979 in Nature
The study covers the last 900 years and is based on delta18O isotopes in stalagmites. The authors report a warm phase 1000 AD (start of data set) to 1400 AD (Fig. 11). Subsequent cooling towards Little Ice Age, with brief renewed warming episode centred around 1500 AD.
Fig. 11. Oxygen isotope curve for a cave in NW Nelson District, South Island, New Zealand. From Wilson et al. 1979.
13. Caves in Waimoto district, North Island
Williams et al. 2004 in The Holocene
The study covers the last 12,000 years and is based on d13C and d18O isotopes in stalagmites. The authors report a warm phase 1000-1550 AD, peaking around 1250 AD (Fig. 12). Subsequent to 1550 AD, significant cooling occurred associated with the Little Ice Age.
Fig. 12. Oxygen isotope development in cave stalagmites in Waimoto district, North Island. From Williams et al. 2004.
14. Hawkes Bay (Disbelief and Te Reinga Caves)
Lorrey et al. 2008 in Quaternary International
The study covers the last 4000 years and is based on delta18O in cave stalagmites. A warm phase occurred 750-1250 AD (Fig. 13).
Fig. 13. Oxygen isotope development in Disbelief and Te Reinga Caves in Hawkes Bay, New Zealand. From Lorrey et al. 2008.
15. Kealia Pond, Maui Island, Hawaii
Pau et al. 2012 in Annals of the Association of American Geographers
The study covers the last 2600 years and is based on palynology, charcoal, and sedimentological analysis of a lake sediment core. The authors report a wet phase 850-1200 AD, plus subsequent short wet spikes around 1350 AD and 1550 AD (Fig. 15). Climate was more arid during other times.
Fig. 14. Location of the Hawaiian Kealia Pond study by Pau et al. 2012.
Fig. 15. Precipitation evolution in Kealia Pond, Maui Island, Hawaii. From Pau et al. 2012.
Previous regional syntheses
The PAGES 2k Consortium (2013) has recently published a climate reconstruction on a nearly global basis where they present a clear warming for the Medieval Warm Period in Australasia, followed by cooling of the Little Ice Age and warming of the Modern Warm Period (Fig. 16). Closer inspection of the database shows, however, that only a very small number of studies have been used from Australia/Oceania that actually reached back to MWP times. Results therefore need to be treated with caution.
Fig. 16. Temperature reconstruction on a continent basis for the past 2000 years, as proposed by PAGES 2k Consortium (2013).
The existing studies document, that the MWP is clearly developed in Australia/Oceania. Temperatures have been elevated 950-1500 AD, with only short cooler interludes. Clear subsequent cooling is reported towards the Little Ice Age. Renewed warming occurred during ramp-up towards Modern Warm Period. There is currently no basis to say that the Modern Warm Period might be much warmer than the MWP in the region.
The MWP Mapping Project
The MWP Mapping Project is supported by crowd funding and has now reached a funding level of nearly 50%. If you like what you see, you may consider supporting us with a donation, small or large, which will allow us to reach our funding target and complete this important project. More information here.
The author would be very interested in any reader suggestions, additional literature from the region or alternative interpretations. Contact Email here.