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!!

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.

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.

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.

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.