The Ocean Engine: 2011

Thursday, 22 December 2011

‘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).

Monday, 19 December 2011

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.

Saturday, 17 December 2011

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…

Sunday, 11 December 2011

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:

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

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….

Saturday, 10 December 2011

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…..

Friday, 9 December 2011

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…

Wednesday, 30 November 2011

In the News..

This week's climate change conference at Durban (COP17) is of increasing importance given the failure at Copenhagen in 2009. Coverage can be followed here: http://www.guardian.co.uk/commentisfree/2011/nov/28/durban-climate-talks-plan-b. Essentially the concern is that it has been recognised that the multilateral approach is in deadlock; with conflict between developed and developing countries on how to sustain Kyoto commitments whilst balancing immediate economic concerns. This conference (im not counting on much success) I believe, represents a key opportunity for countries to advocate and see the benefits in creating green economies. However, only time will tell....

The Late Cretaceous- Macleod et al. (2011)

Continuing our understanding of past ocean circulation, we move to the Late Cretaceous. This greenhouse interval can serve as a useful analogue for predicting future climate change. Macleod et al. (2011) utilise an innovative technique for reconstructing past circulation patterns. They use neodymium (Nd) which in contrast to traditional proxies as discussed in post 2 (e.g. benthic ¹⁸ O), directly tracks water masses as the ¹⁴³Nd/¹⁴⁴Nd ratio of seawater (expressed as ƹ_Nd) is used as a tracer. The authors present ƹ_Nd measurements from Cretaceous to Palaeogene sediments from four cores in the North Atlantic on the Demerara rise of the Northeast coast of South America (Figure 1.)

Figure 1. Late Cretaceous temperature and ƹ_Nd records. The grey bars indicate times of correlated shifts in both variables.

Source: Macleod et al. (2011)

From 69 to 62 Myr ago, ƹ_{Nd (t)} shift from -16 to -11 was observed, values which appear to correlate to temperature record found in the North Atlantic during the same time period.

They suggest that this may the possibility of increasing northern-sourced water mass, indicating intensification of deep or intermediate water in the North Atlantic 69 Myr ago. They argue that this emphasised a heat piracy model whereby increased export of cool intermediate or deep waters in the North Atlantic are balanced by increased import of warm water from the South Atlantic. (Figure 2).

Figure 2. Schematic representation of circulation in the Atlantic during the late Maastrichtian. Arrows indicate the circulation described in the heat piracy model.

Source: Macleod et al. (2011)

It is important to reconcile that the termination of the Late Cretaceous greenhouse climate may have terminated primarily due to volcanic CO₂ forcing and that changes in nature and depth in ocean gateways may have profoundly affected circulation patterns. The shifts in ƹ_Nd measurements appear tightly correlated with changes in Late Cretaceous climate at a resolution of approximately 10⁶ years.

The paper has implications for our understanding of the mechanics of ocean circulation in greenhouse gas intervals. This is important to take into account when predicting the impact of future climate change on ocean circulation.

Tuesday, 22 November 2011

Frozen Planet

Before I head back into our exploration of the paleoclimate record, I came across a fascinating clip from Frozen Planet, a documentary narrated by Sir David Attenborough shown every wednesday on BBC:, which I highly recommend: http://www.bbc.co.uk/nature/15835014

This clip shows the process whereby sea water is excluded from sea ice and sinks to the ocean floor, a process which has key implications for ocean circulation as has been highlighted by previous posts. I thought a visual representation would be nice to supplement the content on this that has been previously discussed. Enjoy!!

Sunday, 20 November 2011

A Digression: "Climate Change Interactive" and the importance of Climate Communication

I would like to take a brief pause in our exploration of the paleoclimate record to introduce a rather neat interactive tool I came across. The Guardian have an 'Ultimate Climate Change FAQ', which explores some of the popular debates on climate change, with ocean circulation included (briefly and reductionist, one might say).

I feel this is important just to place this blog in the context of the wider picture. It is important to not take some of the information provided at "face value" per se but instead read it critically (particularly the stuff about ocean circulation in relation to blog post two).

The key take home message (ironically which the "All you need to Know" tool does not actually address!) is the importance of communication in issues regarding climate change. During the summer I undertook an internship with UCL Environment Institute as a researcher for ClimateCom Strategies. This emphasised the importance of communication (via framing and need for a singular positive discourse for policy momentum etc.) to me, particularly through the experience of working with the Department of Energy and Climate Change.

If any of you are interested, UCL Environment Institute are holding a 'Climate Communications event' on the 30th November. This event is based on the topic "Climate Communications 2.0", a research theme developed over the course of the summer and the basis of a current working paper (tbc) which I was heavily involved in.

I shall be exploring the theme of climate skeptics and communication with reference to ocean circulation towards the closure of this blog. As a fun exercise to test your imagination of the climate change debate, are there any themes/topics/issues which the "All you need to know" interactive tool does not take into account?

I look forward to hearing your responses!

The ‘8.2-kyr’ Event

The Younger Dryas is not the only episode of abrupt climate change documented in the transition to the Early Holocene (Oldfield. 2005). High-latitude records widely show a sharp dip in temperature at 8200 yr BP, temperature decreases by 4-8 °C in c. Greenland and 1.5- 3°C at marine and terrestrial sites around N. E. North Atlantic Ocean (Barber et al., 1999)

Barber et al. (1999) argue the cooling may hint that ocean-atmosphere heat transfer was reduced in the North Atlantic via a freshwater outburst from Lake Agassiz and Ojibway as the Hudson Bay Ice mass disintegrated (Figure 1).

Figure 1. Northeast Canada and adjacent seas. Former ice-sheet margins are shown for 8.9 cal. kyr ago and 8.2 cal. kyr ago (vertical hatched line and thick grey line respectively), before and after disintegration of ice in Central Hudson Bay. At the same time, northward drainage is shown via the Hudson Bay and Hudson Strait. Horizontal hatching shows Lake Agassiz and Ojibway.

Source: Barber et al. (1999: 345).

Based on determining of ice-core layers using radiocarbon dating, they present evidence of a freshwater pulse at 8.470 cal. yr BP which they believe coincided with the 8,400 cal. yr BP climate cooling observed in Greenland and around the North Atlantic (Figure 2).

Figure 2. Proxy records of the 8.2 kyr event. ¹⁴C (top) and calendar (lower) timescales are shown. Upper curve shows Cariaco basin greyscale record, with reduced values indicating high zonal-speed wind attributed to high-latitude cooling. Lower curve shows bidecadal ¹⁸ O values of ice from Greenland Ice Sheet Project 2 (GISP 2), to reflect temperature of precipitation, with negative values indicating cooler temperature. The age for lake drainage event is shown; 8.470 cal. yr BP is shown (vertical dashed line).

Source: Barber et al. (1999: 347).

The logic being that the freshwater pulse reduced sea surface salinity in the North-West Atlantic; reducing Intermediate Water in Labrador seas and North Atlantic Deep Water, causing reduced northward heat transport associated with the meridional overturning circulation in the North Atlantic. The idea that abrupt climate change caused a shift in North Atlantic freshwater balance represents a significant point for our understanding of ocean circulation.

From a paleoclimatic perspective, significance of this event for future predictions lies in the attribution and proof that such a large freshwater pulse can disrupt ocean circulation and climate under interglacial conditions (Oldfield, 2005). Incidentally. as we will see in the next post and through examination of ‘Heinrich Events’ and ‘Dansgaard- Oescheger Cycles’ in future posts, events have been documented whereby changes in freshwater content cause episodic changes in patterns of ocean circulation. However, it is the difficulty of attributing the mechanics whereby a large freshwater pulse can disrupt ocean circulation that is a key issue in the science of climate change.

Monday, 14 November 2011

The Younger Dryas: Part 2- Murton et al. (2010)

Following on from the last post which offered a conventional take on the Younger Dryas, a paper I’ve recently read by Murton et al. (2010) offers a fascinating new insight into the relationship between ocean circulation and abrupt climate change. The conventional view to recall, is that an outburst from Lake Agassiz led to freshwater discharge via the St. Lawrence valley into the North Atlantic ocean, suppressing the Atlantic meridional overturning circulation (AMOC), which in turn caused abrupt climate change via cooling experienced in the North Atlantic, during the Younger Dryas (Broecker et al., 1989). However, to gauge an accurate understanding of the link between freshwater input and abrupt climate change in the context of the changes in ocean circulation hypothesised during the Younger Dryas case requires all the potential pathways and timing of freshwater discharges to be identified.

Murton et al. (2010) conducted a field investigation on northeast Richards Island, an intervalley between the Channel- Kugmallit Trough valley (east) and Middle Channel- Mackenzie Trough systems (west) . They estimated the age of intervalley erosion and gravel deposition through optical stimulated luminescence dating eleven samples above and below erosion surface at six locations. Through determination of the age of the Mackenzie River flood into the Arctic Ocean after 13,000 yr ago near the onset of the Younger Dryas, the authors attribute the flood to a boulder terrace near Fort McMurray with radiocarbon dates of 11,500 yr ago. The geomorphological evidence presented is intriguing; the Lake Agassiz overflow via St. Lawrence route into the North Atlantic Ocean appears to not be the only flow at the time (Figure 1).

Figure 1: Large proglacial lakes along the retreating Laurentide Ice Sheet at 12.65-12.75 cal.kyr BP, near the start of the Younger Dryas. Three outlets are shown; northwest along the Mackenzie River to the Arctic Ocean, east via the St. Lawrence River to the North Atlantic Ocean and south along the Mississippi River to the Gulf of Mexico (Glacial ice= white, Proglacial lakes=dark blue and land= grey).

Source: Murton et al. (2010)

They postulated that a shift of approximately 50 km in the Laurentide Ice sheet (LIS) at 12.9 cal. kyr BP would open a corridor from Lake Agassiz to the Arctic Ocean during the Younger Dryas (Figure 3). In turn, the advance of the LIS at approximately 11.5 cal. kyr BP closed this corridor and the glacier dam subsequently retreated. A second outburst from Lake Agassiz embedded a boulder terrace in Athabasca valley, depositing younger and boulder terraces, supported by fluvial gravels downstream in Richards islands dated between 11.7 kyr – 9.3 kyr by optically simulated luminescence.

Figure 3: Modelled palaeotopgraphy of the Fort McMurray region at the Herman beach stage of Lake Agassiz. Stage age= 10.9 ¹⁴C kyr BP- 12.9 cal.kyr BP with selected radiocarbon dates shown. The following Glacial boundaries are shown; 11.0 ¹⁴C kyr BP= 12.9 cal.kyr BP, 10.5 ¹⁴C kyr BP= 12.5 cal. kyr BP, 10.25 ¹⁴C kyr BP= 12.0 cal. kyr BP and 10.0 ¹⁴ C kyr BP= 11.5 cal. kyr BP. Yellow-red= areas above Lake Agassiz level at 50m intervals. Blue colours= areas below level of Lake Agassiz at 50m intervals to -600m. Overflow from Lake Agassiz is shown to have occurred into Clearwater Valley across the ‘Modern Divide’, and flowed west to Fort McMurray, where it entered Athabasca Valley, flowed north to Lake Athabasca and to the Mackenzie River and Arctic Ocean.

Source: Murton et al. (2010)

A point to note is that several dates exceed 10¹⁴ Ckyr BP, equivalently 11.5 cal. kyr BP, around the onset of the Younger Dryas. This has led the authors to suggest a Younger Dryas link between Lake Agassiz and the Arctic Ocean.

The study offers an atypical conclusion; that the Younger Dryas was triggered along Artic route contrary to previous thought. It can be acknowledged that this study extends our understanding of freshwater forcing and abrupt climate change. Murton et al. (2010) suggest their hypothesis would consolidate the fact that overflow (via this route) would cause a suppression of the AMOC, triggering a negative feedback leading to rapid cooling.

Interestingly, I think the key point is that this debate (between the papers discussed in part 1 and 2) over a particular event in the paleoclimate record raises the issue of thresholds, defined in this sense as the level of freshwater needed to cause a suppression of the AMOC. This has telling implications as we shall see in future blogs to come, for predictions of abrupt climate change.

Thursday, 10 November 2011

The Younger Dryas: Part 1

The transition from the last glacial maximum (C. 1800 ¹⁴c yrs ago) to present interglacial (10,000 ¹⁴C yrs ago), involved episodes of abrupt climate change (Oldfield, 2005). An example of this is the extended cold period, termed the Younger Dryas, which, led to key changes in global ocean circulation and a decline in temperatures in the North Atlantic from 11,000 to 10,000 yr BP (Oldfield, 2005).

Rooth (1982) hypothesised that the Younger Dryas was caused by a diversion of water from glacial Lake Agassiz from the Mississippi drainage to St. Lawrence drainage system. Prior to 11,000 yr BP, meltwater to Lake Agassiz overflowed to Gulf of Mexico via Mississippi drainage system (Broecker et al., 1989) (Figure 1a.)

Figure 1a: Laurentide ice sheet and overflow route from Lake Agassiz basin to the Gulf of Mexico pre- Younger Dryas. (note: the area outlined is enlarged in Figure 1b)

Source: (Broecker et al., 1989)

By 11,000 yr BP, the Laurentide ice sheet retreated to create channels to Lake Superior basin via Great Lakes and St. Lawrence valley to N. Atlantic with a discharge estimated at 30,000 m³ (Broecker et al., 1989) (Figure 1b.)

Figure 1b: Overflow route from Lake Agassiz through the Great Lakes to the St. Lawrence valley and Northern Atlantic during the Younger Dryas.

Source: Broecker et al. (1989)

Broecker et al. (1989) postulated that a meltwater pulse would have affected ocean circulation by causing cooling of adjacent land and atmosphere over the North Atlantic. As recalled in post two, freshwater input will lead to a reduction in sea surface salinity (SSS) and density of water, leading to a decline in North Atlantic Deep Water (NADW) formation, thus suppressing the Atlantic meridional overturning circulation (Wunsch, 2002). Given the difference in heat capacity between the oceans and atmosphere (due to the chemical properties of air and water), this would create a delay before cooling in the North Atlantic, causing the climate in Western Europe to cool by several degrees (Broecker et al., 1989).

In a classic paper, Broecker et al. (1989) investigated the link between the freshwater outburst and ocean circulation. Data concerning O¹⁸ and radiocarbon on planktonic foraminifera were taken from the Orca Basin core GW21- PC4 (Broecker et al., 1989). Negative spikes in cores support the concept of major influx in meltwater via Mississippi river to Gulf of Mexico 11,200- 10,000 radiocarbon yr ago, appear broadly consistent with the Lake Agassiz record (although the authors were unclear at the how much of the change in the white form of G. ruber is due to meltwater) (Broecker et al.1989) (Figure 2).

Figure 2: Oxygen isotope changes in two species of Globigenniodes ruber(white and pink, left and right) from core EN32-PC4. Ages are accelerator radiocarbon ages. Both show a major meltwater spike from 320-200 cm.

Source: (Broecker et al., 1989)

Broecker et al. (1989)’s observation, in relation to the relationship between ocean circulation and abrupt climatic change has been described as the ‘hosing scenario’ (Wunsch, 2010: 7). This is the view that that the North Atlantic largely controls the climate system, with freshwater inputs largely contributing to a shutdown of the North Atlantic thermohaline circulation (NATHC) (Wunsch, 2010). However, re-diversion of meltwater from Mississippi to St. Lawrence after 10,000 yr BP failed to produce a cold event comparable to that observed in the Younger Dryas (Broecker et al., 1989). Broecker et al. (1989) and subsequently, Broecker (2006) acknowledged the possibility of alternative meltwater diversion routes.

As shown in Part 2, this has led to the traditional view of the Younger Dryas to come under scrutiny. This highlights the problems that begin to emerge when examining links between past changes in ocean circulation and climate. Stay tuned to find out further….

An Update

This is a brief post to highlight the direction the blog will be taking over the course of the next few weeks.
So far we have:

1) Introduced the ocean's role in the climate system.

2) Explored the fundamental components of ocean circulation.

3) Investigated the range of techniques used to infer past changes in ocean circulation.

By providing a critical commentary on several key papers, the blog will explore the impact of changes in past ocean circulation on climate by examining key events found in the paleoclimate record.

Friday, 4 November 2011

In the News recently.....

Elaborating on the last post, satellites are another high-resolution method used to reconstruct past changes in ocean circulation and processes more broadly. The US have launched the National Polar-orbiting Operational Environmental Satellite System Preparatory Project (NPP). The goal of the project is to test new instrumental observations on Earth, monitoring ocean, atmospheric and terrestial processes. Here is the link to the BBC article: http://www.bbc.co.uk/news/science-environment-15488016 (Note the hint of American exceptionalism in the commentator's language when describing the take off).

Reconstructing past changes in the oceans

Today will focus on the main methods used to reconstruct past changes in ocean circulation. A key point to consider is the timescales of ocean processes. The oceans have large response time, the surface ocean taking from months to years and deep oceans taking decadal to centennial time scales (Mackay et al., 2003). Given the range of timescales involved in processes of ocean heat transfer, historic records are thus too short to provide records of ocean system prior to anthropogenic intervention, a goal crucial if we are to place our understanding of past ocean circulation into the context of today’s climate (the role of paleoceanography) (Mackay et al., 2003; Oldfield, 2005).

What do Paleoceaographers use to reconstruct past changes in oceans?

Proxies

-Descriptions for ‘target parameters’ such as past ocean temperatures and salinity, to reveal a greater insight into the past ocean and bordering continents (Mackay et al., 2003; 192).

- The proxies are calibrated in some many to provide a quantification of changes in the target parameters (Mackay et al., 2003).

Marine surface sediments

- Reconstruct changes in ocean as well as surrounding continents (Mackay et al., 2003). Can reconstruct a range of parameters using marine sediments (e.g. SST, surface and deep circulation patterns). Also act as storage of information concerning continents (e.g. ice volume (Mackay et al., 2003).

- Two components; biogenic and lithogenic (Mackay et al., 2003). Former originates in surface water but can receive contribution from ocean bottom water, consists of organic matter (e.g. pollen grains), calcium carbonate (from organisation such as coccolithophores and planktonic foraminifera (Mackay et al., 2003). The latter is composed of clays, but larger material such as boulders can be deposited, such as iceberg melt. The origin can be found on continents where rock and soil are eroded, and deposited via rivers or icebergs for example, in the ocean.

Corals

- Provide second major source of evidence for past changes in sea surface temperature (SST) and sea surface salinity (SSS) (Oldfield, 2005).

(Note: with corals and marine sediment, estimates are determined from assemblages of marine organisms such as foraminifera, calculated to modern species distribution)

Stable Isotope ratios (Δ ¹⁸O)

- As ice forms, sequesters a higher proportion of the lighter isotope (Δ ¹⁶O), which becomes depleted in seawater. During glacial intervals, the Δ ¹⁸ O values in marine organisms increases, with the converse occurring during interglacials (Mackay et al., 2003)

- Can be inferred from bubbles in ice cores, relating to changes in ice volume and global sea-level BUT.. links with sea are complex (Mackay et al., 2003)

Paleoceanographers use a range of proxies in order to obtain target parameters to gain a more detailed insight into past oceans and adjacent continents (Figure 1) (Mackay et al., 2003).

Figure 1: Proxies and the corresponding target parameters used by Paleoceanographers

Source: (Mackay et al., 2003).

Are there any issues?

Resolution (Marine sediments)

- Temporal: Deep- Ocean sedimentation rates are approximately 2-5 cm/kyr, with productive areas producing a maximum of 20 cm/kyr . Thus, temporal resolution is thus limited to 200 yr/cm (Mackay et al., 2003). Normal marine sediments, active benthic community can mix approximately the top 20 cm of sediment via bioturbation, reducing resolution to an estimated 1000 years/ cm, leaving an estimated 10 data points for the Holocene (Mackay et al., 2003).

- Spatial: Marine sediments may contain localized environment and climate information, in comparison to high-resolution records such as ice cores, which offer global climatic information (Mackay et al., 2003).

Recommended Read: Maslin, M., Pike, J., Stickley, C. and Ettwein, V. (2003) 'Evidence of Holocene Climate Variability in Marine Sediments' in Mackay, A, Batterbee, R., Birks, J. and Oldfield, F. (eds) (2003) Global change in the Holcene, Hodder, London, 185-209.

The Ocean Engine

Wednesday, 28 December 2011

Past Climate Change in the oceans: Test your knowledge!

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