Cenozoic greenhouse to icehouse transition
In the past 65 million years, the Cenozoic era, Arctic climate changes coincided with a series of global climate shifts. Earth’s climate transitioned from a warm “greenhouse” climate, roughly 60 to 40 million years ago (Ma), to the cold “icehouse” climate beginning about 34 Ma.
The greenhouse climate was an exceptionally warm period in Arctic climate history. Sediment records taken during the Integrated Ocean Drilling Program (IODP) Arctic Coring Expedition (ACEX) cruise to Lomonosov Ridge in the central Arctic Ocean (Backman et al., 2006 and Moran et al., 2006) show that during greenhouse conditions (55 to 45 Ma), Arctic Ocean temperatures reached 18 to 23°C (64 to 73°F). The highest temperatures occurred 55.9 Ma during the Paleocene-Eocene Thermal Maximum (PETM) (Sluijs et al., 2006), an intensely studied period of global warmth that is linked to massive injections of carbon into the ocean-atmosphere system.
Diatom microfossils from the ACEX cores suggest that sea ice in the Arctic first developed by 47 Ma, although land-derived icebergs may have existed also (Stickley et al., 2009). The ACEX sedimentation record is incomplete between about 44 and 18.5 Ma. However, various sea ice proxies for the last 18 milion years indicate continuous sea ice cover in the central Arctic for much of the last 17 to 12 Ma, though others suggest that the margins of the Arctic were ice-free during the Pliocene (4.5-2.8 Ma), at least during summers months (Cronin et al., 1993 and Matthiessen et al., 2009). Clearly, additional sediment records are needed to establish a detailed Cenozoic climatic, sea and land ice history of the Arctic.
Quaternary Glacial-interglacial climates
In contrast to periods of warm climate, extreme glacial conditions characterized the icehouse Arctic, especially during the Quaternary (2.6 Ma to present). During this period, Arctic climate has been dominated by glacial-interglacial cycles caused by changes in aspects of earth’s orbit: axial tilt (obliquity), precession (wobble) and eccentricity. During the last two glacial maxima, about 160 thousand years ago (ka) and 22 ka, ice sheets, ice shelves, and thick sea ice covered the Arctic Ocean and surrounding regions. Geophysical studies show that ice shelves or large icebergs calved from continental glaciers scoured the seafloor in several regions to depths as great as 1000 meters (Jakobsson et al., 2010).
The initial development of extensive glacial-age ice cover in the Arctic may have occurred during the Mid-Pleistocene Transition (MPT) about 1.2 to 0.5 Ma. The MPT marks a global climatic transition signifying a shift from obliquity-dominated climate cycles (~41 thousand-year) to eccentricity-dominated climate cycles (~100 thousand-year). Since the MPT, climatic cycles paced by orbital changes are characterized by amplified glacial-interglacial climatic extremes. It appears that the central Arctic retained perennial sea ice throughout these cycles (O’Regan et al., 2008 and Cronin et al., 2008); however, Arctic sea ice also varied on a millennial scale during interglacial periods including the current Holocene interglacial.
Interglacial climate variability
Decadal and centennial climate variability during interglacial periods such as the current Holocene interglacial can also impact the Arctic Ocean. For example, Arctic climate is linked to the Arctic Oscillation and North Atlantic Oscillation modes of sea level pressure variability, which influence wind, atmospheric circulation and sea ice thickness. Additionally, the influx of freshwater from large Siberian rivers can fluctuate, altering sea ice patterns and Arctic surface ocean circulation. Global-ocean meridional overturning circulation governs the strength of warm Atlantic Layer water, which enters the Arctic through the eastern Fram Strait and the Barents Sea. The Atlantic Layer circulates through the Arctic at depths of 100-500 meters below the surface and might affect the Arctic’s sea ice conditions, circulation and heat budget. Studies suggest large centennial to millennial variability in Arctic climate (Kaufman et al., 2004) and in sea ice cover (Vare et al., 2009; de Vernal et al., 2005; Cronin et al., 2010) during the Holocene interglacial.
The history of Arctic climate shows a dynamic environment with important connections to global climate. Despite the significance of climatic change in the Arctic, most paleoceanographic records provide low temporal resolution and little is known about the temperature and sea ice history in key parts of the Arctic Ocean during periods of climatic warmth and during abrupt climatic transitions. As a consequence, there is minimal baseline information about the speed, timing, causes and impacts of Arctic temperature change and sea ice decline or disappearance. Paleoclimatic studies can provide baseline information to better understand Arctic climate changes.
de Vernal, A., C. Hillaire-Marcel, D. A. Darby. 2005. Variability of sea ice cover in the Chukchi Sea (western Arctic Ocean) during the Holocene. Paleoceanography 20, PA 4018.
Jakobsson, M., J. Nilsson, M. O’Regan, J. Backman, L. Löwemark, J. A. Dowdeswell, L. Mayer, L. Polyak, F. Colleoni, L. G. Anderson, G. Björk, D. Darby, B. Eriksson, D. Hanslik, B. Hell, C. Marcussen, E. Sellén, Å. Wallin. 2010. An Arctic Ocean ice shelf during MIS6 constrained by new geophysical and geological data. Quaternary Science Reviews 29, 3505-3517.
Kaufman, D. S., et al. 2004. Holocene thermal maximum in the western Arctic (0-180°W). Quaternary Science Reviews 23, 529-560.
Matthiessen, J., J. Knies, C. Vogt, and R. Stein. 2009. Pliocene paleoceanography of the Arctic Ocean and subarctic seas. Philosophical Transactions of the Royal Society A 367, 21-48.
Stickley, C. E., K. St. John, N. Koç, R. W. Jordan, S. Passchier, R. B. Pearce, L. E. Kearns. 2009. Evidence for middle Eocene Arctic sea ice from diatoms and ice-rafted debris. Nature 460, 376-379.
Vare, L. L., G. Massé, T. R. Gregory, C. W. Smart, S. T. Belt. 2009. Sea ice variations in the central Canadian Arctic Archipelago during the Holocene. Quaternary Science Reviews 28, 1354-1366.