Reference
Field, D.B. and Baumgartner, T.R. 2000. A 900 year stable isotope record of interdecadal and centennial change from the California Current. Paleoceanography 15: 695-708.
What was done
Noting there is a "problem of detecting effects of anthropogenic forcing on natural climate change," the authors developed "a robust time series of stable isotope [δ18O from Neogloboquadrina dutertrei] variability over the past millennium from the varved sediments of the Santa Barbara Basin," which they related to observed environmental variability within this part of the California Current over the past half-century, wherein they demonstrate that "thermal variability dominates the δ18O signal."
What was learned
Field and Baumgartner report that "an anomalously warm coastal ocean persisted at the multicentennial-scale from roughly AD 1200 to 1450," which time interval, as they describe it, "coincides with the age generally assigned to the 'Medieval Warm Period'." They also report that "the period of positive anomalies in the low-frequency series of δ18O from N. dutertrei that continues from ~AD 1450 to ~1800 is consistent with the dates associated with the cooling and neoglaciation of the 'Little Ice Age' in both the Southern and Northern Hemispheres." In addition, they note that "the long-term ocean warming and cooling of the California Current region appears to be in phase with the warming and cooling of the midlatitude North Atlantic described by Keigwin (1996)." Finally, they report that "persistent large interdecadal fluctuations in the upper ocean temperature are superimposed on these longer-period changes and conceal to some degree this lower-amplitude multicentury variability."
What it means
With the researchers' δ18O record indicating that California Current temperatures at the start of the 20th century were still far less than they were throughout most of the Medieval Warm Period, there is indeed, as they proclaim in the introduction to their study, a "problem of detecting effects of anthropogenic forcing on natural climate change," and that problem has remained in effect to the present day; it is essentially impossible to determine how much of the 20th century's global warming is natural and how much might be due to various anthropogenic influences.
Reference
Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1507.