The Medieval Warm Period and Little Ice Age in San Francisco Bay

Was there a Medieval Warm Period anywhere in addition to the area surrounding the North Atlantic Ocean, where its occurrence is uncontested? This question is of utmost importance to the ongoing global warming debate, since if there was, and if the locations where it occurred were as warm then as they are currently, there is no need to consider the temperature increase of the past century or more as anything other than the natural progression of the persistent millennial-scale oscillation of climate that regularly brings the earth several-hundred-year periods of modestly higher temperatures (such as the Medieval Warm Period) and lower temperatures (such as the Little Ice Age) that are unrelated to variations in atmospheric CO₂ concentration.

In a study that sheds additional light on this subject, McGann (2008) analyzed a sediment core retrieved from the western portion of south bay near San Francisco International Airport (37°37.83'N, 122°21.99'W) for the presence of various foraminifers, as well as oxygen and carbon stable isotopes and numerous trace elements found in the tests of Elphidium excavatum.

The U.S. Geological Survey researcher reports that "benthic foraminiferal abundances, stable carbon and oxygen isotopes, and Mg/Ca ratios suggest that the climate of south bay has oscillated numerous times between warm and dry, and cool and wet conditions over the past 3870 years," and that "both the Medieval Warm Period [MWP] and the Little Ice Age [LIA] are evident." More specifically, she identifies the MWP as occurring from AD 743 to 1343 and the LIA as occurring in two stages: AD 1450 to 1530 and AD 1720 to 1850. In addition, she states that the timing of the MWP "correlates well with records obtained for Chesapeake Bay (Cronin et al., 2003), Long Island Sound (Thomas et al., 2001; Varekamp et al., 2002), California's Sierra Nevada (Stine, 1994), coastal northernmost California (Barron et al., 2004), and in the San Francisco Bay estuary in north bay at Rush Ranch (Byrne et al., 2001), and south bay at Oyster Point (Ingram et al., 1996)," and that the cooler and wetter conditions of the LIA have been reported "in Chesapeake Bay (Cronin et al., 2003), Long Island Sound (Thomas et al., 2001; Varekamp et al., 2002), coastal northernmost California (Barron et al., 2004), and in the San Francisco Bay estuary at Rush Ranch (Bryne et al., 2001), Petaluma Marsh (Ingram et al., 1998), and in Richardson Bay (Ingram and DePaolo, 1993)."

As for the more recent past, McGann notes that "near the top of the core" foraminiferal abundances suggest that, "once again, regional warming has taken place." However, that warming does not appear to have returned the region to the level of sustained warmth it enjoyed during the peak warmth of the MWP.

The new results and their concurrence with results obtained by many other researchers -- both nearby and across the country on the east coast of the United States -- continue to strengthen our contention that (1) the warming of the past century or so has been nothing more than the natural and only-to-be-expected recovery of the earth from the extremely cold conditions of the LIA, which phenomenon has yet to return the planet to the level of sustained warmth characteristic of the MWP, and that (2) this transition has had nothing to do with the increase in the air's CO₂ concentration that has occurred over the same time period.

Additional References
Barron, J.A., Heusser, L.E. and Alexander, C. 2004. High resolution climate of the past 3,500 years of coastal northernmost California. In: Starratt, S.W. and Blumquist, N.L. (Eds.), Proceedings of the Twentieth Annual Pacific Climate Workshop. U.S. Geological Survey, pp. 13-22.

Byrne, R., Ingram, B.L., Starratt, S., Malamud-Roam, F., Collins, J.N. and Conrad, M.E. 2001. Carbon-isotope, diatom, and pollen evidence for late Holocene salinity change in a brackish marsh in the San Francisco estuary. Quaternary Research 55: 66-76.

Cronin, T.M., Dwyer, G.S., Kamiya, T., Schwede, S. and Willard, D.A. 2003. Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay. Global and Planetary Change 36: 17-29.

Ingram, B.L., DeDekker, P., Chivas, A.R., Conrad, M.E. and Byrne, A.R. 1998. Stable isotopes, Sr/Ca, and Mg/Ca in biogenic carbonates from Petaluma Marsh, Northern California, USA. Geochimica et Cosmochimica Acta 62: 3229-3237.

Ingram, B.L. and DePaolo, D.J. 1993. A 4300-year strontium isotope record of estuarine paleosalinity in San Francisco Bay, California. Earth and Planetary Science Letters 119: 103-119.

Ingram, B.L., Ingle, J.C. and Conrad, M.E. 1996. Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California. Earth and Planetary Science Letters 141: 237-247.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during Medieval time. Nature 369: 546-548.

Thomas, E., Shackeroff, J., Varekamp, J.C., Buchholtz Ten Brink, M.R. and Mecray, E.L. 2001. Foraminiferal records of environmental change in Long Island Sound. Geological Society of America, Abstracts with Program 33(1), A-83.

Varekamp, J.C., Thomas, E., Lugolobi, F. and Buchholtz Ten Brink, M.R. 2002. The paleo-environmental history of Long Island Sound as traced by organic carbon, biogenic silica and stable isotope/trace element studies in sediment cores. Proceedings of the 6th Biennial Long Island Sound Research Conference, Groton, CT.