16. August 2010

Menschen im Gespräch: Teil V



copyright Dr. Barron

Dr. John A. Barron

Today, I am proud to introduce Dr. John A. Barron as my next interview partner. Dr. Barron has a PhD in Geology and works as a researcher (Geologist, Micropaleontologist) for the US Geological Survey, Western Region, Southwest area (USGS Professional Pages). The scientific importance of his work lies inter alia in helping to "refine diatoms as a biochronologic tool for dating Cenozoic and Late Cretaceous rocks and sediments from polar waters and coastal upwelling regions like California. This biochronology was used to date tectonic events in California and the North Pacific, determine periods of widespread deep-sea hiatuses in the world's oceans, and detail the Cenozoic history of the Antarctic ice sheet"(cf., Synopsis of Research).

Dr. Barron has published dozens of papers in renowned scientific journals like the Journal of Paleontology or "Palaeogeography, Palaeoclimatology, Palaeoecology". In one of his papers: "Santa Barbara Basin diatom and silicoflagellate records suggests coincidence of cooler SSt with widespread occurrence of drought in the west during the past 2,200 years for PACLIM Proceedings volume" (="Santa Barbara ..."), Dr. Barron et al. found in the "assemblage data from the ODP 893A record evidence for a broad interval of generally cooler SSts between ~AD 800 and 1350, which corresponds to the Medieval Climate Anomaly (MCA)"(Abstract).

W.v.B.: Dr. Barron, thank you for taking the time for this interview. Let me start the interview by focussing on your paper "Santa Barbara ...". Could you first, in general talk about the paper before you point out the main results?

Dr. Barron: The Santa Barbara Basin is unique, because it contains a varved sediment record that accumulated at fairly high sediment accumulation rates. It has a well developed time scale that is necessary for detailed correlation with the high-resolution time scales developed in ice cores and by tree rings. The aim of the paper was to determine paleoclimatic conditions during the Medieval Warm Period (MWP) and the Little Ice Age (LIA) in waters off California in order to test the hypothesis that cool-water, La Niña-like conditions occurred during the MWP and caused widespread drought in the southwestern parts of the US. Diatoms and silicoflagellates are phytoplankton and respond to surface water temperatures better that planktonic foraminifer, which live at greater depth ranges in the water column.

W.v.B.: Your paper shows evidence for a MWP/MCA. It seems to me that there is a lot of evidence for the existence of a MWP/MCA in the Northern Hemisphere and especially in North America. For all parts of North America and the US, peer-reviewed papers, presenting evidence for a MWP/MCA can be found (Alaska, Canada, Greenland and the US [without Alaska])[1]. During the last years, there has even been an increasing number of papers supporting the conclusion that there was a MWP/MCA in the US [2]. What is your view on the existence of a MWP/MCA in the Northern Hemisphere and especially in the US.

Dr. Barron: I believe that there is good evidence for a MWP/MCA in North America. For western regions dominated by Pacific weather that evidence stems from the persistence of La Niña-like conditions and the absence of strong El Niña events during the interval between about 900 and 1300 AD. The persistence of these conditions results in a stable North Pacific high pressure system being established during the late spring and summer at ~ 40 degrees North, causing northward deflection of storm tracks and drought throughout the southwestern US. Similar to the present summer of 2010, cooler conditions in coastal California coincide with warmer conditions throughout much of the eastern US. It is likely that high solar radiance during this period contributed to the persistence of these climatic conditions.

W.v.B.: Dr. Böhm responded to my question on the possible occurence of a MWP in the Southern Hemisphere in short: Yes, but less quantifiable. What ist your point of view on this topic?

Dr. Barron: My opinion is based on reading the literature. I know that there is good evidence for the persistence of La-Na-like conditions in Pacific coastal regions of South America during the MWP/MCA that resemble those of the western US. Mohtadi et al (2007) argue for a sustained northward shift of zonal systems (i.e., the South Westerly Wind belt and Antarctic Circumpolar Current) in the SE Pacific between ~1300 and 750 yr BP that contributed to reduced ENSO activitiy throughout South America.

Mohtadi, M., Romero, O., Kaiser, J., and Hebbeln, D., 2007. Cooling of the southern high latitudes during the Medieval Period and its effect on ENSO. Quaternary Science Reviews 26: 1055-1066.

W.v.B.: Recently I have read a paper, published by Graham et al.[ 3]. I encountered an interesting statement. The statement was: ... if not a "Medieval Warm Period", then a substantial "Medieval Climate Anomaly". What do you think, is there enough evidence for the claim that the Medieval Warm Period was indeed a global phenomenon or do you prefer the statement by Graham et al.?

Dr. Barron: I tend to argue with Graham et al. because strict use of the term Medieval Warm Period implies that temperatures were warmer everywhere on earth at the same time. This is climatically impossible, yet the documentation of anomalously warm conditions in Europe, the arctic regions, and the North Atlantic during the MWP/MCA are important observations that influence our understanding of the MWP/MCA.

W.v.B.: Thanks for the interview!

copyright W.v.B.



[1] Alaska:
Barclay et al.: Tree-ring crossdates for a first millennium AD advance of Tebenkof Glacier, southern Alaska, in: Quaternary Research 71 - 2009, 22-26.

Loso: Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska, in: Journal of Paleolimnology 41 -2009, 117-128.

McKay et al.: Biogenic silica concentration as a high-resolution, quantitative temperature proxy at Hallet Lake, south-central Alaska, in: Geophysical Research Letters 35 - 2008, 10.1029/2007GL032876.

Loso et al.: A 1500-year record of temperature and glacial response inferred from varved Iceberg Lake, southcentral Alaska, in: Quaternary Research 66 - 2006, 12-24.

Rolland et al.: Evidence for a warmer period during the 12th and 13th centuries AD from chironomid assemblages in Southern Island, Nunavut, Canada, in: Quaternary Research 72 - 2009, 27-37.

Cook et al.: Five thousand years of sediment transfer in a high arctic watershed recorded in annually laminated sediments from Lower Murray Lake, Ellesmere Island, Nunavut, Canada, in: Journal of Paleolimnology 41 - 2009, 77-94.

Zabenskie, Gajewski: Post-glacial climatic change on Boothia Peninsula, Nunavut, Canada, in: Quaternary Research 68 - 2007, 261-270.

Luckman, Wilson: Summer temperatures in the Canadian Rockies during the last millennium: a revised record, in: Climate Dynamics 24 - 2005, 131-144.

Vinther et al.: Climatic signals in multiple highly resolved stable isotope records from Greenland, in: Quaternary Science Reviews 29 - 2010, 522-538.

Lloyd: Late Holocene environmental change in Disko Bugt, west Greenland: interaction between climate, ocean circulation and Jakobshavn Isbrae, in: Boreas 35 - 2006, 35-49.

Lassen et al.: Late-Holocene Atlantic bottom-water variability in Igaliku Fjord, South Greenland, reconstructed from foraminifera faunus, in: The Holocene 14 - 2004, 165-171.

Dawson et al.: Late-Holocene North Atlantic climate 'seesaws', storminess changes and Greenland ice shee (GISP2) palaeoclimates, in: The Holocene 13 - 2003, 381-392.

US (without Alaska):
St. Jacques et al.: A 900-year pollen-inferred temperature and effective moisture record from varved Lake Mina, west-central Minnesota, USA, in: Quaternary Science Reviews 27 - 2008, 781-796.

Millar et al.: Late Holocene forest dynamics, volcanism, and climate change at Whitewing Mountain and San Joaquin Ridge, Mono County, Sierra Nevada, CA, USA, in: Quaternary Research 66 - 2006, 272-287.

Lund, Curry: Florida Current surface temperature and salinity variability during the last millennium, in: Paleoceanography 21 - 2006, 10.1029/2005PA001218.

Cronin et al.: Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay, in: Global and Planetary Change 36 - 2003, 17-29.

[2] Brunelle et al.: A 8000 year fire history from an Arizona/Sonora borderland ciénega, in: Journal of Arid Environments, Volume 74, Issue 4, April 2010, Pages 475-481.

*Barron et al.: Santa Barbara Basin diatom and silicoflagellate response to global climate anomalies during the past 2200 years, in: Quaternary International, Volume 215, Issues 1-2, 15 March 2010, Pages 34-44.

Schmeisser et al.: Modern and late Holocene wind regimes over the Great Plains (central U.S.A.), in: Quaternary Science Reviews, Volume 29, Issues 3-4, February 2010, Pages 554-566.

Hanson et al.: Megadroughts and late Holocene dune activation at the eastern margin of the Great Plains, north-central Kansas, USA, in: Aeolian Research, Volume 1, Issues 3-4, January 2010, Pages 101-110.

Knight et al.: A bimillennial-length tree-ring reconstruction of precipitation for the Tavaputs Plateau, Northeastern Utah, in: Quaternary Research, Volume 73, Issue 1, January 2010, Pages 107-117.

[3] The paper was titled "Tropical Pacific - mid-latitude teleconnections in medieval times", published in Climate Change 2006, DOI 10.1007/s10584-007-9239-2. The whole statement (p. 38) was: It (the nexus between long-established evidence for medieval drought in the western US ... and recent evidence suggesting a cool tropical Pacific, W.v.B.) also offers a partial response to the query posed by Hughes and Diaz more than a decade ago (Hughes and Diaz 1994 [Was there a "medieval warm period" and if so, where and when?, published in Climate Change 26 - 1994, 109-142, W.v.B.],) if not a "Medieval Warm Period", then a substantial "Medieval Climate Anomaly", a significant transient in Holocene climate that affected a large area of the Pacific Sector during medieval time, from about 500-1300 A.D.