We apply a multiple regression method to estimate the response to anthropogenic and natural climate forcings simultaneously from a number of paleo‐reconstructions of Northern Hemispheric average temperature. These long records (600 to 1000 years) provide a unique opportunity to distinguish between different external influences on climate. The response to volcanic forcing is reliably detected in all reconstructions, and the simulated temperature response to volcanic eruptions compares favorably with observations. The response to solar forcing is detected in Hemispheric mean data only over some periods in some records, and appears weak. Although most records can be used only to the middle of the 20th century, the temperature response to CO2 can be detected by this time in most records.

Geologic studies indicate that prior to ∼40 Ma the Drake Passage was closed and the Central American Isthmus was open. The effect of these changes has been examined in an ocean general circulation model. Several sensitivity experiments were conducted, all with atmospheric forcing and other boundary conditions from the present climate, but with different combinations of closed and open gateways. In the first experiment, the only change involved closure of the Drake Passage. In agreement with earlier studies the barrier modified the geostrophic balance that now maintains the circumpolar flow in the southern ocean, with the net effect being decreased transport of the Antarctic Current and an approximate fourfold increase in outflow of Antarctic deep‐bottom waters. The very large increase in Antarctic outflow suppresses North Atlantic Deep Water (NADW) formation. In addition to corroboration of results from earlier studies, our simulations provide several new insights into the role of a closed Drake Passage. A more geologically realistic closed Drake/open central American isthmus experiment produces essentially the same pattern of deepwater circulation from the first experiment, except that Antarctic outflow is about 20% less than the first experiment. The resultant unipolar deepwater circulation pattern for the second experiment is consistent with paleoceanographic observations from the early Cenozoic. A third experiment involved an open Drake and open central American isthmus. In this experiment, Antarctic outflow is diminished to slightly above present levels but NADW production is still low due to free exchange of low‐salinity surface water between the North Pacific and North Atlantic. The low level of thermohaline overturn should have reduced oceanic productivity in the Oligocene (∼30 Ma), a result in agreement with geologic observations. Finally, simulations with an energy balance model demonstrate that the changes in surface heat flux south of 60°S due to breaching of the Drake barrier do not result in temperature changes large enough to have triggered Antarctic glaciation. This last result suggests that some other factor (CO2?) may be required for Antarctic ice sheet expansion in the Oligocene (∼30–34 Ma). Our results lend further support to the concept that even in the absence of changing boundary conditions due to ice sheet growth, variations in the geometry of the ocean basins can significantly influence ocean circulation patterns and the sediment record. The results also suggest that the primary polarities of the Cenozoic deepwater circulation may have been controlled by opening and closing of these two gateways.

Sea ice loss is accelerating in the Barents and Kara Seas (BKS). Assessing potential linkages between sea ice retreat/thinning and the region's ancient and unique social–ecological systems is a pressing task. Tundra nomadism remains a vitally important livelihood for indigenous Nenets and their large reindeer herds. Warming summer air temperatures have been linked to more frequent and sustained summer high-pressure systems over West Siberia, Russia, but not to sea ice retreat. At the same time, autumn/winter rain-on-snow (ROS) events have become more frequent and intense. Here, we review evidence for autumn atmospheric warming and precipitation increases over Arctic coastal lands in proximity to BKS ice loss. Two major ROS events during November 2006 and 2013 led to massive winter reindeer mortality episodes on the Yamal Peninsula. Fieldwork with migratory herders has revealed that the ecological and socio-economic impacts from the catastrophic 2013 event will unfold for years to come. The suggested link between sea ice loss, more frequent and intense ROS events and high reindeer mortality has serious implications for the future of tundra Nenets nomadism.

Observational studies show a significant increase in ocean heat content over the last half century. Herein we estimate heat content changes during the last millennium with a climate model whose forcing terms have been best‐fit to surface proxy data. The model simulates the observed late 20th century ocean heat content increase and a comparable Little Ice Age minimum. When glacial advances are factored in, these results imply a sea level fall after the Middle Ages that is consistent with some geologic data. The present ocean heat content increase can be traced back to the mid‐19th century, with a near‐linear rate of change during the 20th century.

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