Milankovitch cycles and the Arctic: insights from past interglacials

The Arctic is warming at a rate greater than the global average. End-of-summer minimum sea ice extent is declining and reaching new minimums for the historical record of the last 4 decades. The Greenland ice sheet is now losing more mass than it is gaining, with increased surface melting. Earth System Models suggest that these trends will continue in the future. The geologic past can be used to inform what could happen in the future. Emiliani in his 1972 Science paper commented on the relevance of paleoclimate for understanding our future Earth.

 

Interglacials of the last 800,000 years, including the present (Holocene) period, were warm with low land ice extent. In contrast to the current observed global warming trend, which is attributed primarily to anthropogenic increases in atmospheric greenhouse gases, regional warming during these interglacials was driven by changes in Earth’s orbital configuration. Although the circumstances are different, understanding the behavior, processes, and feedbacks in the Arctic provides insights relevant to what we might expect during future global warming.

 

Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (Last Interglacial, ~129 to 116 ka) was globally strong. The Last Interglacial (LIG) is characterized by large positive solar insolation anomalies in the Arctic during boreal summer associated with the large eccentricity of the orbit and perihelion occurring close to the boreal summer solstice. The atmospheric carbon dioxide concentration was similar to the preindustrial period.

 

Geological proxy data for the LIG indicates that Arctic latitudes were warmer than present, boreal forests extended to the Arctic Ocean in vast regions, summer sea ice in the Arctic was much reduced, and Greenland ice sheet retreat contributed to the higher global mean sea level. Model simulations provide critical complements to this data as the they can quantify the sensitivity of the climate system to the forcings, and the processes and interplay of the different parts of the Arctic system on defining these responses. As John Kutzbach explained in a briefing for science writers, "climate forecasts suffer from lack of accountability. Their moment of truth is decades in the future. But when those same computer programs are used to hindcast the past, scientists know what the correct answer to the test should be."

 

Significant attention and progress have been made in modeling the LIG in the last 2 decades. Earth System Models now capture more realism of processes in the atmosphere, ocean, and sea ice, can couple to models of the Greenland ice sheet, and include representations of the response of Arctic vegetation to the NH high-latitude summer warming. Increases in computing power has allowed these models to be run at higher spatial resolution and to perform transient simulations to examine the evolving orbital forcing during the LIG.  The international PMIP4 simulations for 127 ka illustrated the importance of positive cryosphere and ocean feedbacks for a warmer Arctic. A CESM2-Greenland ice sheet, transient LIG simulation from 127 ka to 119 ka, established a key role of vegetation feedbacks on Arctic climate change.