The Ocean Engine: The Finale

It is now time to wrap up and conclude this blog. As stated, the aim of this blog was to explore past changes in ocean circulation; past, present and future. At the beginning of this blog, we sought to define the ocean’s role in the climate system. In particular, I stressed through the concept of the conveyor belt that whilst analogies are good for framing our imagination of ocean circulation, it is important to reconcile that the ocean is a complex mechanical engine in reality. Furthermore, this blog also reviewed techniques in palaeooceanography, to understand how past changes in ocean circulations are reconstructed. Upon reflection, given more time, I feel I could have perhaps added a few more detailed post attached to this topic as it is of crucial importance to understand past changes in ocean circulation.

This blog then explored paleaoclimatological evidence for past changes in ocean circulation. In particular, several episodic were analysed in relation to abrupt climate change; The Younger Dryas, 8.2 ka event and the Late Cretaceous. In this respect, I did narrow my research stand of the “ocean circulation” with respect to the North Atlantic circulation. However, this strand of blog posts could be nicely conceptualised by the posts I did on millennial-scale climate change. I found this strand of research particularly fascinating, given the complexity of feedback processes revealed from ice core studies such as the bipolar seesaw mechanism.

In this respect though, I feel I potentially could have explored two more strands of research in The Ocean Engine; notably the Southern Ocean and the El-Nino southern oscillation, both key are important components of the ocean system in relation to the past and present components. However, I feel that my definition of The Ocean Engine evolved over the course of this blog to run along the following theme; despite popular views advocated in films or by climate skeptics, ocean circulation is a complex science in reality. Thus through focusing on the North Atlantic thermohaline circulation, this blog sought to disentangle the myths attached to this component of the climate system by adopting a palaeo-approach and trying to understanding potential future impacts associated with anthropogenic climate change.

One of the key take-home messages from this blog is that by adopting a palaeo perspective, this can provide a critical position to interrogate questions surrounding climate change. The study of paleoclimatology differs somewhat in ideology to that of modelling, as emphasised in various posts. This divergence is a crucial point to convey a very key message; that anthropogenic climate change puts our understanding of the earth’s climate system to the limit, and we are currently at a stage to make fairly accurate predictions at the impacts but a significant distance away from the goal of quantification. I exemplified this point nicely in the last post, through the concept of noise, which I feel is useful analogy to help explain why uncertainty exists in climate change science; a crucial ingredient to any debate on climate change.

It has been an absolute pleasure writing this blog, and it has enriched my understanding of ocean circulation in relation to debates on climate change. To conclude, whilst we only are beginning to understand the components of The Ocean Engine, we do not understand its interior mechanics and complexities as of yet. Arguably, there are surprises in store as our understanding of the system develops and this warrants the seed for further research!

Conceptualising the ocean’s role in the climate system

Whilst we have been attempting to understand changes in ocean circulation due to climate change, it is useful to present a brief sketch of the earth’s climate system. The earth’s climate demonstrates variation on a variety of timescales, scaling from interannual, interdecadal, interannual, millennial and geological scales (Mann, 2007). Thus, the variation of the Earth’s climate can be conceptualised as the product of exogenous (factors independent of anthropogenic change and/or changes in other variables) and endogenous variables (factors affected by anthropogenic change and/or changes in other variables).

In relation to this conceptualisation, ocean circulation can be understood as an endogenous factor in the earth’s climate system.  Given that the current timescale of the Holocene (for purposes of simplicity this term is invoked) is shorter than that of orbital variations (eccentricity, precession and obliquity), termed “Milankovitch pacemakers”, events which occur at “sub-milankovitch periodicity” must be invoked to account for Holocene climate variability. Solar radiation and volcanic eruptions can be understood as examples of exogenous forcing factors and greenhouse gas emissions, EL Nino and Southern Oscillation (ENSO) are examples of endogenous forcing factors.

An interesting point in the debate is that of attribution; Is climate-change natural and /or anthropogenic driven? Crowley (2000) running a model similar to that of the IPCC, found that over the last 1000 years, solar forcing and volcanic forcing explained 41 – 59 % variance. However, when including greenhouse gas emissions and tropospheric aerosols in that scenario, it explained 41-64% of variance.

A question you may ask then is what accounts for the remaining 36 %?

I would argue that most of the uncertainty in the climate change debate is that it is difficult to attribute any degree of climate change conclusively to internal or external factors due to “noise” in the climate system. This can be defined by the fact that multiple possibilities exists for conceptualising the response of the climate system to a specific exogenous or endogenous forcing factors (Maslin and Christensen, 2007) (Figure 1). The climate system may respond directly in response to a variable (Figure 1a), in a delayed fashion (Figure 1b), a muted fashion (Figure 1c) or after passing a threshold (Figure 1d). To complicate this further, bifurcations exist, whereby forcing required to go through a threshold is different to the reverse, implying that once a threshold has crossed, it is difficult to reverse. This can be inferred from previous posts in relation to the impact of meltwater on the deep-water circulation.

Figure 1: Four Possible responses of the global climate system to forcing factors a) Linear b) Muted C) Non-Linear and D) Threshold.

Source: Maslin and Christensen (2007).

Whilst each element of the climate system may respond in variety of ways described above, this is compounded by the fact that each factor in the climate system responds on a different timescale. In this sense, the climate system is anti-essentialist in a way, the nature of climate system respond is multiple and complex. 

This conceptualisation illustrates the complexity of climate change science in particular in response to the question of attribution; detangling the greenhouse signal from that of natural climate variability.

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. 

Six Degrees: A worthwhile read...

Whilst this blog does not cover the science of climate change in its entirety, I would like to draw your attention to a very worthwhile read. Six degrees written by Mark Lynas in 2007, explores the scientific rationale underpinning the climate change debate that global temperatures will increase between one and six degrees.  Lynas (2007) undertakes a thorough review of the academic literature and rather masterfully as the temperature increases by 1 °C, on a chapter by chapter basis, takes the reader through six stages of global warming according to projections.  I believe it is an excellent read for anyone interested in climate change.  

This book was particularly inspiring for me and was one of the key factors for pursuing my interest in the science of climate change.

As Lynas (2007: xxii) writes:

“Climate change is the canvas on which the history of twenty-first century will be painted. Forewarned and forearmed”.

Surpises part 3: A Collapse of the thermohaline circulation?

Given the potential of the  north atlantic thermohaline circulation (NATHC)  to influence climate change as evident from study of abrupt climate changes from paleo-records, the following question is thus posed; will the NATHC collapse due to a rise in greenhouse gas emissions? The concern over the impact can be understood by the logic that an increase in sea surface temperatures due to thermal expansion of the oceans, may in turn reduce the solubility of CO₂ in warmer waters (Stocker et al. (2000 in Seidov et al., 2000). This in turn, may create a positive feedback effect whereby warmer waters hold less inorganic carbon, thereby causing a CO₂ release into the atmosphere (Stocker et al. (2000 in Seidov et al., 2000).

One issue in trying to understand this prediction through modelling is that it is difficult to achieve such a timescale. However, one of the earlier studies; undertaken by Manabe and Stoffer (1994), use a comprehensive atmospheric-ocean general circulation model (AOGCM) to simulate the impact over many centuries. Their results appear to indicate that a critical threshold lies between a doubling and four-fold increase in CO₂ concentrations. However only models of reduced complexity can be used to aid quantification of threshold, as this may allow for long-term simulations.

It has found that in addition to the stabilization level of greenhouse gas concentration in atmosphere, rate of greenhouse gas emission increase may also help determine a threshold. This is illustrated in Figure 1 with a simulation run by Stocker et al. (2000 in Seidov et al., 2000). They set climate sensitivity at 3.7 ° C for doubling of CO₂, and simulations of run concerning the rate of CO₂  increase a in 1 %/yr, 2 % year and 0.5 %/yr scenario (Figure 1a)

Figure 1a) Atmospheric CO₂ simulations for five experiments b) Simulated global mean temperature c) Simulations of maximum meridional overturning of the North Atlantic in Sverdrup (1 Sv= 10⁶ m³/s).

Source: by Stocker et al. (2000 in Seidov et al., 2000)

The global mean surface air temperature in simulations is not dependent on emission history for maximum CO₂ concentration (Figure 1b). However, a bifurcation appears apparent concerning the maximum meridional overturning of the North Atlantic. Reduction is apparent in all scenarios with the level dependent on maximum CO₂ concentration and CO₂ increase according to scenario. The circulation collapses at 750 ppmv with increase at rate of 1%./ yr. It then subsequently, appears to recover and settle to reduce value if increase is slower (0.5 %/yr ) or if the CO₂ level is reduced to 650 ppmv. At a rate of 2 %/yr, the circulation appears to collapse. The simulations appear to indicate that once the THC collapses it may settle a new equilibrium, with irreversible changes occurring, independent of CO₂ concentrations.

However, it must be noted that there is huge degree of complexity in understanding the impact of future changes in the thermohaline circulation due to anthropogenic climate change. These include; a lack of understanding concerning the variety of feedback mechanisms associated with changes in the THC and palaeoclimate modelling. There is a need to quantify components of climate-related components to a signal and test hypotheses regarding science of abrupt climate change. Changes in THC are likely in future and it is known the slowing down of THC moves system closer to thresholds. The uncertainty and importance attached to this as detailed in past blog posts highlight the need for further research.

Surprises part 2: The collapse of the Antarctic Ice Sheet

In contrast to Greenland ice sheet (GIS), potential melting of the Antarctic Ice Sheet (AIS) may have implications for ocean circulation. If you may recall from the posts on millennial-scale climatic changes,  potential freshwater outburst  (in particular from ice shelves around the Ross and Weddell seas) may affect the production of Antarctic Bottom Water (AABW) which in turn may level deep-water formation and in turn potentially affect global climates (Swingedouw et al., 2008). Swingedouw et al. (2008) attempted to quantify future AIS melting on climate by running 5 different experiments using the model LOVECLIM, a three-dimensional earth system model of intermediate complexity (Figure 1). These included a control simulation (CTRL) whereby forcing was set constant to pre-industrial conditions, notably with the CO₂ concentration in the atmosphere set to 277.6 ppm, where CO₂ concentration increased by 1 % per year until it reaches 4 times initial values, remaining unchanged till the year 3000. Scenario iAiG has fully interactive ice sheets over Antarctica and Greenland, whilst scenario fAfG, components are forced with a fixed ice-sheet component with simulated warming. Scenarios fAiG and iAfG show fixed and interactive GIS respectively.

Figure 1. Time series of annual mean value of the minimum of oceanic global meridional overturning streamfunction at 30°S ( 1 Sv= 10⁶ m³/s) emphasising the export of Antarctic and Circumpolar Deep Water (AABW and CDW) at 30°S and the maximum of Atlantic meridional overturning streamfunction at 30 °S, representing the export of North Atlantic Deep Water (NADW) at 30°S.

Source: Swingedouw et al. (2008).

The results are of particular interest. Without AIS melting, AABW export at 30 °S weakens during first 300 years and recovers, enhanced by changes compared to CTRL scenario after 1000 years. This can be related to sea ice changes due to freshwater forcing relating to the retreat of sea ice cover. Net annual mean sea ice melting in Weddell and Ross seas are lower in fAfG compared to CTRL. This in turn increases sea surface salinity and sea surface density, counter to density loss from projected temperature loss, increasing AABW formation in seas in fAfG compared to CTRL after 3000 years. However I would like to focus on particular observation in the above, that with AIS melting, this affects NADW export in all the scenarios but then this recovers after 1000 years in iAfg in contrast to the fAfg, indicating a potential stabilizing effect of AIS melting on the weakening of the NADW cell. AIS melting cause NADW cell weakening by 1.2 SV in iAiG compared to fAiG. This stabilization effect of the AIS, the authors note, is similar to that of the bipolar seesaw; a mechanism for explaining millennial-scale climate change (explained in blog post 17). This can be explained by the fact that a reduction in AABW density appears to enable NADW to penetrate further and deeper south in the Atlantic with the associated cell.

The findings of this study are important for two reasons. Firstly, the findings of the study highlight the importance of using palaeoscience to understand environmental problems; through the application of mechanisms used to explain millennial-scale climate change to help understand and try to predict future impacts of changes projected with  anthropogenic climate change (such as AIS melting) on changes in ocean circulation. Finally, the study highlights the importance of using models in addition to palaeoscience, to not forecast but to offer an insight into the feedbacks between ice sheets, ocean processes under a warming scenario. This point is important to communicate (to climate skeptics!) the manner in which science of climate change is being developed and understood; in this case through understanding the impact of the Antarctic Ice Sheet on changes in ocean circulation.  

Surprises part 1: The collapse of the Greenland ice sheet

Adopting a paleoclimatic perspective in our study of ocean circulation has highlighted the concern over the stability of the North Atlantic meridional overturning circulation (AMOC) given the discovery of millennial-scale climatic changes. To add to this, further concern must now be raised over the impact of greenhouse gas emissions on the collapse of the Greenland Ice sheet (GIS). It has been well documented that the GIS has been losing mass since the early 1990s, approximately 200 gt net loss (Wu et al., 2011). However, the retreat of this ice mass poses great concern for freshwater balance in the North Atlantic and in turn the AMOC. As can be inferred from the study of abrupt climate change through millennial-scale climate processes, a weakening of the AMOC has implications for global climate.

A vast body on literature exists on projections of the collapse of the Greenland ice sheet on AMOC, However, a study by Wu et al. (2011) is particularly of interest. They use a national centre for atmospheric research community climate system model to assess the influence of a shrinking GIS on AMOC, surface climate and sea level rise. However, the impacts on AMOC are of focus here. The authors use the IPPC A1B scenario, a mid greenhouse gas emissions scenario whereby CO₂ concentrations change from 368.5 ppm at year 1999 to 688.5 ppm at the year 2099, then a constant is assumed from the year 2099 to 2199 in conjunction with three other scenarios to predict the impact on global mean temperature.   This  scenario was used as a baseline which do not included GIS melting and subsequently scenarios were developed where rates of melting were 1 % and 3 % until the year 2099 and then kept at 2099 level until 2199 ( termed 1 % and 3 % exp) and then 7 % till 2050 and kept constant till 2199 ( termed 7 % exp) (Figure 1.)

Figure 1. The global mean surface temperature

Source: Wu et al. (2011).

The projections of the scenario indicate that a weakening of AMOC has a moderate effect on global climate. Mean global climate increases by 2. 43 °C in the last 20 years in the 21^st century, relative to last 20 years of 20^th century and an additional increase of 0.43 °C by end of the 22^nd century is observed in the A1B exp. scenario. Upon incorporating GIS melt-water, global mean temperature is not significantly affected in the first half of the 21^st century in all cases. However, at the end of the 21^st century, small fraction of a degree less is shown in the 7 % exp. scenario and the end of 22^nd century shows warming is 0.25 °C  and 0.50°C in the 3 % and 7% exp. scenarios. The 1 % exp. scenario seems to be in a similar time step to the A1B exp scenario. This has implications for our understanding of the impact of a collapse of the GIS on AMOC. That is that upon melting intensification of the GIS, AMOC weakens, global mean temperature is projected to rise due to increase in greenhouse gas emissions and that a weakened MOC will not reverse the current trend and trigger a global cooling trend.

However, the impact of melting GIS on global sea level means that the issue is still of pressing concern. We now turn to a more complex issue; predicting the impact of a potential collapse in the Antarctic ice sheets..

Predicting the impact of climate change in the oceans: Surprises

A warm and happy 2012 fellow bloggers! As we proceed to the closure of this blog, attention will now turn to predicting future impacts of climate change in the oceans. After reviewing paleoclimatological evidence for past changes in ocean circulation, it is useful to set out the context to which the latter end of this blog is concerned. From a paleoclimatological perspective, it is useful to appreciate that we are in a geologically unique epoch; owing to a significant rise in greenhouse gas emissions. Crutzen (2000) coined the term ‘Anthropocene’ to describe this phenomena.  Air temperatures on Earth are projected to rise between 1.1- 6.4°C for the 21st century (Tyrell, 2011). As stated earlier in the blog; even if we achieve climate stabilization, we are committed to approximately 0.5 °C due to the thermal inertia of the oceans.   

Reviewing past changes in ocean circulation, have revealed the potential of non-linear responses of the oceans in response to climate change. To add to this complexity, we shall now consider the impact of anthropogenic climate change on changes in ocean circulation.

Predicting the impact of anthropogenic climate change on ocean circulation thus, will involve the study of what I consider key “surprises” in the climate system. These are defined as the following; the collapse of the Greenland and Antarctic Ice sheets and the impact of emission scenarios on the thermohaline circulation. These will form the basis for the concluding thoughts to this blog.

Past Climate Change in the oceans: Test your knowledge!

Past Climate Change:in the oceans: Test your knowledge!

After our exploring past changes in ocean circulation, it is now time to put the events and theories to the test. Test your knowledge!

1. The cause behind Younger Dryas (YD) has been described as a 'hosing scenario'. What is this?
Large freshwater input into N. Alantic ceases thermohaline circulation, causing cooling.

Large freshwater input into Southern Ocean causing a shutdown of the thermohaline circulation.

Method of Geoengineering to combat global warming.

2.    In what way did, Murton et al. (2010) offer a new perspective on the Younger Dryas?

YD was triggered by freshwater outburst into the Arctic Ocean.

YD was triggered by freshwater discharge into the St Lawrence valley.

No idea.

3.   The Late Cretaceous has described as a useful analogue for predicting future climate change. Why?

 A greenhouse gas interval

High Volcanic Activity

Dinosaurs return

4. What is Milennial-scale climate variability?

That which occurs on timescales of 1000's of years.

That which occurs ever millenium.

That which occurs every million years.

5. Name two-processes of Millennial-scale climate change?

Dansgaard-Oeschger Cycles (D/O) and Heinrich Events.

Heinrich cycles and Dansgaard-Oeschger Cycles (D/O).

The Sun and volcanoes.

6. D/O events occur on a 1500 year cycle. How are Heinrich events different

Low frequency, High amplitude (Global Impact).

Ice-rafting debris events.

There have only occured six times in the last glacial. From 70,000 to 14,000 yr ago.

7. Name one perspective on theorizing milennial-scale climate variability

Internal ice-sheet dynamics.

External climate change.

Global Warming.

8. What do ice cores reveal about the North and South Hemispheres?

They are out of phase.

They show synchronous behaviour.

 They show asynchronous behaviour.

9. What greenhouse gas is used as a tool to help address this problem with the ice core records?

Carbon Dioxide

CFC

Methane

10. Name a mechanism that accounts for the behaviour of the hemispheres

Bipolar Climate Seesaw

Thermal Bipolar Climate Seesaw

The Day After Tommorow

11. The Deep Ocean is postulated to have 3 modes of ocean circulation. Describe the

North Atlantic Deep Water ceases, Antartic Bottom Water fills Atlantic basin-Heinrich event.

North Atlantic Deep Water forms in the Nordic Seas.

North Atlantic Deep Water forms in the North Atlantic

12. What is the term used to describe this behaviour of the North Atlantic?

Bifurcation

Linear

Climate Skeptic

13. What is a major problem in quantifying abrupt climate changes in deep-water ocean circulation

We do not know the magnitude of the freshwater threshold

We do not know the state of the climate system in relation to the threshold

Dennis Quaid isn't leading the research

‘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).

Bipolar Climate Seesaw and Deep Ocean Circulation

In response to the conundrum: it is better to think in terms of asynchrony.

The behaviour of the North and South hemispheres, would suggest that the nature of the climate system is analogous to a seesaw, with each hemisphere taking turns to drive the system. (Note; this change is in response to millennial-scale climate variability). To revisit, it has been acknowledged that iceberg discharges into the North Atlantic caused a disruption of the Atlantic meridional overturning circulation (AMOC),  causing a cooling of northern hemisphere and warming of Antarctica (Rahmstorf,2002) .  This one of two schools of thought on mechanisms of millennial-scale climate variability (externally forced climate change and internal ice-sheet dynamics, the former is what we will focus on in this post).

There has been a substantial volume of literature written on the subject of ‘seesaw’. Broecker (1998) coined the term; in light of the thesis that abrupt cooling in the North Atlantic would leave a distinctive signal in the South Atlantic; with changes in the north and south occurring in synchrony. Stocker (1998), also suggested an oceanic seesaw whereby changes are driven by high-latitudinal or near-equatorial changes or, a combination of both.

However, the original bipolar seesaw concept has been subject to scrutiny (Stocker and Johnsen, 2003).  Firstly, the original bipolar seesaw model suggests that changes in north and south occurred in phase. This is out of course divergence from records in Antarctica and Greenland and also for the fact that this requires very fast heat transmission in the oceans (which as we now, is difficult given the thermal inertia of the oceans). It also must be noted that the bipolar seesaw must not be used as an all-encompassing term to describe all climate changes where interhemispheric connections exist; it is a term to describe responses to millennial-scale climatic changes.

Stocker and Johnsen (2003) postulate a ‘thermal bipolar seesaw’, by coupling a heat reservoir to the original bipolar seesaw model (Figure 1).  They apply this conceptual model to GRIP and Byrd data (shown in last post) and highlight that correlation can be achieved on a timescale of 1000-1500 years.

Figure 1. Schematic model of the thermal bipolar seesaw. The bipolar seesaw is joined to a heat reservoir (potentially the Southern Ocean). Double arrow indicates that heat exchange with the reservoir is taken to be diffuse.

Source: Stocker and Johnsen (2003).

In short, the model suggests that the asymmetric response of both hemispheres can be explained by a bipolar seesaw mechanism in which changes in the strength of AMOC can contribute to changes in interhemispheric heat transport.

However, what is the likely to drive changes in this seesaw?

 Due to large volume and thermal inertia, it is likely the deep ocean is likely (Rahmstorf, 2002). There is however, an additional complexity; the notion of thresholds in north-atlantic deepwater system. Based on Greenland and the Iberian margin, Rahmstorf (2002) shows there are 3 modes; stadial, interstadial and Heinrich mode (Figure 2). In the interstadial mode (warm, modern and interglacial), North Atlantic Deep Water (NADW) forms in the Nordic seas. In the stadial mode; it forms in the subpolar open North Atlantic (leading to a cold, glacial D-O stadial) and in the Heinrich mode, NADW formation all but ceased and waters of Antarctic origin filled deep Atlantic basin.

Figure 2: Schematic version of the three modes (‘warm’, ‘cold’ and ‘off’) of ocean circulation as observed during different times of the last glacial period. Shown is the section along the Atlantic, with NADW highlighted by the red line and AABW by the blue line.

Source: Rahmstorf (2002).

There is an enormous difficulty in quantifying the risk of abrupt changes in deep-water circulation. This is due to the possibility of a NADW bifurcation, coupled with the fact that we do not know the state of the current climate system with regards to the threshold (a point I made all the way back in post 2).

These 3 modes of deep water circulation and the bipolar seesaw mechanism highlight the non-linearity and uncertainty in our understanding of millennial-scale climate variability.  This in turn, has significant implications when trying to understand and predict the impacts of climate change.

Interhemispheric comparisons

Returning to the conundrum in post 13, one of the most important finds in the study of millennial-scale climate variability is that the response of the North and South hemispheres are out of phase.

The Southern hemisphere has not received much attention so far in the blog post, but as we shall we see in this post and the next post, it holds vital importance for our understanding of global ocean circulation) (Blunier et al., 1998). Blunier et al. (1998) showed that Greenland warming around 36 and 45 kyr before present (BP) lagged Antarctica by 1 kyr. Therefore, in order to synchronize both records, the authors use methane (CH₄) records from the last glaciation from 2 Antarctic ice cores (Byrd station 80°S, 120°W, Vostok 78.47°S, 106.80°E ) and 1 Greenland ice core (GRIP ice core summit, 72.58 °N, 37.64 °W). CH₄ has an approximate residence times of c. 10 years in the atmosphere. The advantage of using CH₄ as a tool for synchronisation is that it has a residence time that is long enough to become globally homogenous but is also short enough to react quickly to budget imbalance linked to climate changes. Therefore any changes in concentration should be synchronous between Antarctica and Greenland (Figure 1).

Figure 1: GRIP, Byrd and Vostok isotopic and CH₄ records on the common timescale (GRIP timescale in years before 1989).Antarctic warming as indicated by A1 and A2, with vertical dashed lines  indicating the location of Greenland warmings 1,8 and 12 in the Antarctic cores.

Source: Blunier et al. (1998)

On average, the authors found that the change in Antarctic climate leads Greenland on an average of 1 1±2.5 kyr across the period 47±23kyr BP. One of the greatest findings of Blunier et al. (1998)’s observation is that the records dispelled any notion of a coupling between Northern and Southern hemispheres via the atmosphere, an idea postulated by Bond et al. (1993). Crucially, the findings potentially favour an ocean connection in order to conceptualise this behaviour.

One of the greatest challenges in climate science is the ability to derive a mechanism that accounts for the observed behaviour of high-latitude north and south hemispheres.

Any such mechanism would need to address the following conundrum (forgive my use of intertextuality with reference to arguably one of the most famous authors in English literature):

To be in Phase or To be in Antiphase?

That is the question…

What does Durban mean for the world?

So the news is out. After a frantic extra day of negotiations that continued late into Saturday night, a deal has been struck. So what exactly is this all about?

In a nutshell:

• The EU came to Durban calling for a mandate to negotiate a legally binding deal on climate change by 2015.
• However, EU clashed with India and China over the legality of a new agreement, threatening to put the talks between 194 countries into jeopardy.
• The EU wanted to push a “roadmap”, which would establish a new over-arching agreement that would commit countries to emission cuts. India and China, expressed concerns at the legality of such proposals, preferring to adopt the term ‘legal outcome’ as oppose to “legal instrument” in the agreement (The Guardian, 2011).
• After the South African President urged the EU and India to go into a huddle to resolve the language dispute, a Brazilian compromise saw an agreement between both parties to negotiate another legal instrument or an ‘agreed outcome with legal force’ (The Guardian, 2011).
• The treaty will be negotiated by 2015 and implemented from 2020. It will also allow action to address the emission deficit between voluntary reductions and those experts state are needed to tackle climate change.
• As a side issue, ministers had agreed that by 2020, a $ 100 bn fund to help countries move to a green economy and tackle the effects of climate change in addition to measures to protect forests and develop global markets.

These talks are part of a series of wider issues; and I have neatly summarised them below. The figures are referenced by Mark Maslin’s ‘Global Warming: A Very Short Introduction’, a very accessible text exploring all dimensions of the issue.

Why do we need a legally binding agreement?

-     Legally-binding multilateral agreements through the United Nations Framework Convention on Climate Change to cover emissions from all countries are the only viable way to keep an increase in global average temperatures since pre-industrial period below 2 °C,  a threshold widely agreed to mitigate "dangerous" climate change.

Why the urgency for a legally binding agreement?

• To summarise climate science, the earth has a thermal lag meaning so even if emissions stopped today; the earth’s temperature will still rise (Current estimates are we have experienced 0.5- 1°C warming already).
• The political issue is essentially a game of risk; what temperature and hence, CO₂  concentration do we aim for?
•   A ‘two-degree’ world is seen as the threshold as scenarios based on temperature rises beyond this to 3, 4 , 5 and 6 °C will involve a complex series of feedbacks that will affect humanity’s ability to adapt to climate change. 
• In order to reach 2°C, we need to peak global emissions by 2015 at a CO₂ concentration of 400 ppm (we are currently at 390.91 ppm). Thus, a legally binding treaty is required to give any hope of achieving this (or staying in the 2-3 °C margin).

Why the difficulty?

As witnessed at the Durban talks with the conflict over the use of language between the EU, China and India, there are some key issues in such global negotiations I would like to point out:

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•  Trust: The developing countries and ‘fast-emitters’ (e.g. India and China) are concerned by ‘green colonialism’, the concept that developed countries should dictate to developing countries how they should develop and what action they should take, actions they feel will supress their right to a fair development trajectory.

•  Geopolitics: As China rose at the talks, they felt the EU’s roadmap proposals would be mostly beneficial to the US. Multilateral talks need a global political view. The US’s importance, economically, politically and as a large emitter, requires its participation in order for a global community to develop on climate change.

•  Leadership: Tied in with the above, a united long-term vision is required from political leaders across the globe. That even in an era of economic austerity, a long-term vision and pathway must be clearly set to adapt to climate change to prevent a future social, economic (potentially greater than the recession), environmental and political catastrophe.

What should an agreement include?

To briefly conclude. The Durban summit could be looked back on as a success if the negotiations on a treaty outlined in the talks are implemented. So what could this include?

-          Contraction and Convergence

      

      This is the idea proposed by Meyer (2000) that the largest emitters of greenhouse gases contract the level of pollution towards an agreed per capita emissions total. For example, the US CO₂ emissions/person are 10 times that of China. As a global community, we need to try and keep the amount emitted per person the same. Therefore, every country has to contract their emissions, some more than others. For the developed world, this means a low-carbon economy is a serious alternative. This agreement should be open and transparent and include the developing world in order to invoke the concept.

-          A Green Economy

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      Carbon markets, initiated in Kyoto, need to become global carbon markets in order to accelerate capital flows from the developed to developing world. I believe the concept of a green economy can act as a framework by developing global institutions that allow the development of such markets, renewable energy technologies and capital flows between the developed and developing world. This would accelerate the capitalist system to accelerate a positive response by encourage investors and other actors in the private sector, and minimise the regulatory risk that has been witnessed for more than a decade as the world just watches conference after conference of inaction.

Chris Huhne, the UK climate change secretary, said the deal represented a “significant step forward” (The Guardian, 2011) . Let us hope he is right….

Part two: Mechanisms

After introducing D/O cycles and Heinrich events, we now turn to review the mechanisms associated with millennial-scale climatic processes. In a nutshell, there are two competing theories in the literature; externally forced global climate change and internal ice sheet dynamics (Maslin et al., 2001 in Seidov et al., 2001).   The former shall be the subject of an upcoming post, as mechanisms of iceberg discharges are the subject of the current post.

The prevailing hypothesis is the Binge-Purge hypothesis caused by internal stability of the Laurentide Ice Sheet, formulated by MacAyeal (1993) (Figure 1). Essentially, ice sheets on unconsolidated sediments when frozen acts analogous to cement, supporting the weight of the growing ice sheet. Upon ice sheet expansion, geothermal heat from the earth’s crust in conjunction with that from ice-ice friction, trapped by the overlying ice sheet, causes an insulating effect. This causes the temperature of the sediment to increase until a critical threshold is reached. Here, sediment becomes soft causing the base of the ice sheet to allow ice overflow through the Hudson strait to the North Atlantic. This in turn, can lead to a sudden loss of ice mass and reduce the effect of refreezing to the point when ice reverts a phase of slow-build up. MacAyeal (1993) observes that this system of progressive ice build-up, melting and surge followed by renewed build up have a periodicity of approximately 7,000 years when compared to intervals between the last Heinrich events (outlined in the last post).

Figure 1. A simple kitchen oscillator as an analogy to describe the Heinrich event cycle of the Laurentide Ice Sheet. Initially, the container sits upright as the centre of mass is assumed to position between the bottom of the container and the axle. As water drips slowly (binge phase), the centre of mass rises to a point where it exceeds the axle, causing the container to become unstable and purge the contents onto the floor (purge phase). Once the container has been emptied, it reverts back to the initial upright position and slowly fills with water, thus repeating the cycle.

Source: MacAyeal (1993)

Buttressing ice shelves and ocean forcing can also be attributed as another mechanism (Hulbe, 2010). This can be seen in the form of a pervasive coupled ocean, ice-shelf and ice-sheet mechanism of mass flow (Alvarez- Solas et al., 2010). Snow accumulation on ice sheet encourages glacial flow via an ice shelf, and is lost via melting. When this is forced to occur against ocean-driven changes, the ice sheet responds with iceberg discharge. The frequency of this response is modulated by the rate of snow-accumulation and ice-stream sliding. Changes in the rate of snow-accumulation could attribute for the production of irregular Heinrich events.

These concepts highlight the complexity of debate in internal ice sheet dynamics as processes of millennial-scale climate variability. We now turn to questions of impact and one of the most remarkable findings in the study of millennial-scale climate events…..

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…