Cold climate during the closest Stage 11 analog to recent Millennia

The early part of marine isotopic Stage 11 near 400,000 years ago provides the closest analog to Holocene insolation levels of any interglaciation during the era of strong 100,000-year climatic cycles. The CH4 concentration measured in Vostok ice fell to ∼450 ppb, and CO2 values to ∼250 ppm. These natural decreases contrast with the increases in recent millennia and support the early anthropogenic hypothesis of major gas emissions from late-Holocene farming. During the same interval, δD values fell from typical interglacial to nearly glacial values, indicating a major cooling in Antarctica early in Stage 11. Other evidence suggests that new ice was accumulating during the closest insolation analog to the present day: a major increase in δ18Oatm at Vostok, a similar increase in marine δ18O values, and re-initiation of ice rafting in the Nordic Sea. The evidence permits extended (>20,000 year) intervals of Stage 11 interglacial warmth in the Antarctic and North Atlantic, yet it also requires that this warmth ended and a new glacial era began when insolation was most similar to recent millennia. The Holocene CO2 anomaly was produced only in part by direct anthropogenic emissions; over half of the anomaly resulted from the failure of CO2 values to fall as they had during previous interglaciations because of natural responses, including a sea-ice advance in the Antarctic and ice-sheet growth in the northern hemisphere.