Was Wally Right?

As the end of this blog approaches, I would like to draw a rather interesting paper for my last discussion on ocean circulation. Alley (2007) provides a substantial literature review on the evidence to support Wally Broecker’s ‘conveyor belt’ hypothesis which has advanced the study of abrupt climate change, particularly in relation to millennial-scale events.  Earlier posts in this blog have explored the scientific basis (see post 3) in addition to evidence used to support Broecker’s hypothesis (e.g. The 8.2 ka event and The Younger Dryas), these are not of the main concern here.

Rather, I would like to focus on what Alley (2007: 242)  describes as the ‘predictive power’ of Broecker’s hypothesis.  Alley (2007) argues that Broecker’s hypothesis can allow us to make certain predictions about the impact of anthropogenic climate change in relation to abrupt climate change. These predictions are particularly of relevance to the last few blog posts on the notion of “surprises”.

 This is that shutdown of the meridional overturning circulation (MOC) although unlikely, is still a crucial issue. This argument can be related to the last post on the a potential collapse of the thermohaline circulation, and that typical global warming “runs” over the next century appear to produce a slowing down of the MOC without it shutting down.  Secondly, Alley (2007) notes that meltwater inputs from the melting of the Greenland Ice Sheet could be sufficient to trigger a slowdown or shutdown. The findings from the Antarctic blog post, to highlight demonstrated a bipolar seesaw effect whereby melting from the Antarctic Ice Sheet seemed to have a stabilization effect on the MOC.  The key observation from that blog is the notion that a shutdown of the MOC will not be sufficient to trigger an ice age (As the Day after Tomorrow had us believe!). This reason will be explained in a forthcoming blog post.

Predicting the impacts of future anthropogenic climate change on ocean circulation is complex. As demonstrated, through the examples of the Antarctic, Greenland and thermohaline circulation; providing definitive and quantifiable answers are difficult. This uncertainty can be attributed to the fact that these questions seem to test our understanding of ocean circulation in using the analogue of the past. Thus, Broecker’s analogy of the conveyor belt can used as a sensible analogy to fill this void to conceptualise future impacts of global warming on ocean circulation. However, the non-zero possibility and potential large of abrupt climate change sows the seed for warrant research which is likely to in the long-term provide crucial knowledge to policy makers. 

‘Climate is an ill-tempered beast, and we are poking it with sticks’

As shown over the last few posts, abrupt climate changes are very unique in the study of past environmental change. That atmosphere-ocean-sea ice coupled changes that take place highlight the importance of feedbacks in the earth’s system and of an earth science approach to understand this change. Whilst conceptual advances have provided frames for theories and models about how signals in the climate system are transmitted, many questions remain. For instance, which element of the climate system is responsible for the millennial-scale climatic changes that appear in ice-core data? High-resolution synchronized ice-core data, in conjunction with high-resolution marine-sediments from the Atlantic and Southern Oceans are two ways which will approve our understanding. Furthermore, not one element (i.e. external vs internal ice-sheet dynamics debate) but a combination of factors may be responsible for millennial-scale climate variability. However, more crucially than the above, is awareness and recognition of a point.

That a divergence exists between palaeoclimate data and models (I highlighted this point all the way back in point 2, but to elaborate here, I refer to resolution). Whilst palaeoclimate records have beyond doubt increased our understanding of the earth’s climate system (which is not a point I’m at all contesting), a divergence is still apparent between these records and the processes taken into account in models used to predict the impact of increases in global mean temperature projected due to climate change. This has led to the acknowledgement of uncertainty in the context of climate change which has fuelled scepticism and potentially inaction, in regards to the appropriate policy response, at the global scale.

(The above reference can be found in Mark Maslin’s (2004) book: Global Warming: A very short introduction, to paraphrase of Wally, S. Broecker, a notable scholar who has made a significant contribution to the advancement of palaeoceanography).

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…