Millennial-scale climate change: an introduction

Before we turn to questions of predictions of future impacts of climate change, it is important to conceptualise the last several blog posts.  Earth systems science is one such framework, defined as an integrated earth system including human activities (Barron and Seidov, 2001 in Seidov et al., 2001). In relation to ocean circulation this can be understood two-fold, ocean, atmospheric and cryosphere feedback and the timescales over which such processes occur. The last several blog posts concerned events when meltwater forcing may have caused a series of changes in the North Atlantic Meridonal Overturning Circulation. These events are examples of abrupt climate change (decades) which occur on millennial timescales (10³ yr) with significant changes in air temperature and sea surface temperatures observed. This is a primer for a series of posts which shall explore the mechanisms behind such processes, records available and they have in turn have influenced our conceptualisation of ocean forcing.

Two processes are thought to contribute to abrupt climate change experienced in the ocean during the last glacial period; Heinrich Events and Dansgaard-Oeschger Cycles (hereinafter referred to as D/O cycles). First identified in the Greenland ice core, D/O cycles are a succession of warm events lasting decades (interstadials), which characterize Greenland ice core records of the last glacial episode and cold events (stadials), as found in ice rafting records of the North Atlantic, which last for centuries (Dansgaard et al. 1993). These events are thought to be of high frequency and low amplitude, in a 1500 year cycle. Heinrich events on the contrary, are of low frequency and high amplitude.  These ice-rafting debris (IRD) events, thought to be global in impact, have occurred six times in the last glacial, from 70,000 to 14,000 years ago (Figure 1). (Hulbe, 2010).

Figure 1. Heinrich events during the last glacial. Glacial North Atlantic cycles of warming and cooling, shown

in the oxygen isotope record from Greenland ice cores, are punctuated with iceberg discharge events (represented by blue bars) lasting approximately 500 ± 250 years.

Source: Hulbe (2010: 81)

Heinrich events coincide with D/O cycles, though the connection is tenuous. They do not occur during the cool phase of a D/O cycle per se, but can be thought of as extreme D/O stadials. Essentially, a Heinrich event requires three essential conditions; source of sediment, mechanism whereby sediment is moved up into glacial ice and transported into the ocean and a process that varies the rate of iceberg production (which are debated to be either processes internal to the ice sheet or forcing from other factors in the climate system) (Hulbe, 2010).

Stay tuned for part two…