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:

-     

•  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

C   

      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…

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.

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!!