High-resolution analysis of rapid climate change from Greenland ice cores

Abstract

Extreme shifts in the climate system have long been recognized but the timescales for most events are large, occurring over millennia or longer. There is however, growing evidence for abrupt shifts in the climate system on much shorter timescales of centuries, decades or even years. It is these abrupt climate changes that would have the biggest impact on modern society with a potentially large and catastrophic climate shift occurring within the human lifespan. In this thesis I investigate two large and abrupt climate oscillations, as observed in the Greenland ice core record. The first is the most prominent cold event to have occurred during the Holocene, the cold event 8,200 years ago (the 8.2 kyr event) and the second is one of the strongest and longest glacial oscillations, Dansgaard-Oeschger event 8 (DO-8). I present a collection of high-resolution chemistry and stable isotope records from the plateau of the Greenland ice cap during the cold event 8,200 years ago. Using a composite of 4 records, the cold event is observed as a 160.5 year period during which decadal-mean isotopic values were below average, within which there is a central event of 69 years during which values were consistently more than one standard deviation below the average for the preceding period. The results show clear evidence for colder temperatures and a decrease in snow accumulation rate. However, the changes in chemical concentrations for the ions looked at here are small, suggesting only minor changes in atmospheric circulation for this event. Apart from the decrease in methane concentration, Greenland ice cores give only weak evidence for effects outside the North Atlantic region. A new high-resolution chemical and stable isotope record is presented, from the North Greenland Ice Core Project (NGRIP) ice core, during Dansgaard-Oeschger event 8. The onset of DO-8 is first observed as a rapid decrease in chemical deposition to Greenland, indicating a large and abrupt shift in oceanic and atmospheric circulation. The change in the chemical deposition is followed over a decade later by an increase in temperature of approximately 13°C, from extreme cold stadial conditions to warm interstadial conditions, accompanied by a 33 % increase in annual snow accumulation. The transition is observed in the deuterium excess record as an abrupt shift to warmer source water conditions in the period after the chemical transition but considerably earlier than interstadial temperatures have been reached.