Melting Permafrost - should I be worried?

Just before getting started, I found the video I really wanted to share with you guys before, but ended up using a replacement. It's from the fantastic series 'Earth: The Power of the Planet', presented by the brilliant Prof Iain Stewart, a Geologist at the University of Plymouth. He's like Geology's answer to Brian Cox! If you missed the series, I'd encourage you to have a catch-up. Interestingly, this video also features the same Dr Walter as the previous video..

Although it's the easiest and most spectacular way of showing the processes which are happening, it's not the lakes that are causing the most concern. Phrases like 'ticking time bomb' and 'unstoppable warming' are used

Using Quartz as an Ice Rafted Debris (IRD) record

In addition to IP25, Axford et al 2011 (as mentioned in the previous post) also tested the presence of Quartz in marine cores as a proxy for ice rafting in the North Atlantic. Both of these proxies allow a reconstruction of ice abundance in the Northern Atlantic regions. Unlike IP25, however, Quartz is not a biological proxy, rather it is sometimes present as a 'foreign' material assumed to have been transported by floating ice (icebergs). Click 'Read more...' for a short summary of the method and a couple of examples.

Using the Arctic Proxies (past 2Kyr)

As mentioned at the end of the last post, palaeo-records are not perfect, and the way to ensure the best picture is to compare multiple proxies in aptly-named multi-proxy studies. A recent such synthesis study focussing on average temperature changes in the arctic is Kaufman et al (2009) ‘Recent Warming Reverses Long-Term Arctic Cooling’. A discussion of this and related articles will hopefully go some way towards answering the question of how we know recent warming is unusual.

The authors of this study combined the results of 23 palaeo-records with at least a decadal resolution from the arctic region. Seven ice core, 4 tree-ring and 12 lake records were included, with the lake records incorporating sedimental and biological indicators*. These data were taken from various individual studies, allowing a synthesis reconstruction of past temperatures extending 2Kyr BP. A strong cooling trend was found until around AD1950, likely caused by decreasing solar insolation (here a product of precessional cycles, see past the jump), and assertion backed up by a Community Climate System Model run in the same study.

What’s clear post-1950, however, despite the continuous insolation trend, is an upward spike in most of the proxies. Tree-ring records exhibit the most sudden and significant response, with lake records showing a large but more gradual trend, likely the result of the differences in annual resolution between the two proxies:



'2000' relates to AD2000, therefore warming taken place in past 100yrs.



The ice core data, however, is more perplexing, and under initial investigation suggests a further cooling trend. Strangely, this feature is not commented upon by the authors in the published paper, although the nature of the journal may have limited the extent to which the results could be fully discussed. Despite this, combined results (in grey above) show an increase from a -0.5 to +0.2 (standardised to 900-1800yr period) temperature anomaly during the period 0 to AD1950, which increases further to +1.4 for the very latest records. The recent warming trend agrees with the global climate review articles shown below by Mann et al (2008) and Moberg et al (2005). The global reconstructions, however, show greater variability during the '1900 year cooling period', suggesting that the cooling trend appears to be confined to the Arctic caused by the dominance of solar forcing upon the regional climate.



(F) decreasing solar insolation recieved at the atmosphere in the arctic region caused by precession (G) synthesised reconstruction from this study shown in grey compared to global synthesis reconstructions.



In light of the past 2Kyr then, the recent warming trend in the Arcticis sudden at around 1950, and reverses the cooling trend of the past 2Kyr despite continued precessional forcing. It is clear from this study that the recent warming is unusual and has been clearly registered on a number of proxy records around the arctic. The extent of this synthesis is only 2Kyr, however. In the next post I'll look at a few papers to get an idea of the Arctic climate throughout the Holocene.

Where’s the evidence?

A key question to ask before discussing potential impacts of human activity upon Arctic ecosystems is:

How do we know that the recent warming trend in the Arctic is unusual when compared to long-term natural cycles?

As instrumental records are limited both in spatial and temporal extent (earliest records go back only a couple of centuries) they are not adequate to answer this question. The answer comes in the form of palaeo-records; naturally stored records of past environmental conditions. Before I plunge into reviewing scientific papers, I thought it would be a good idea to review some key concepts which we can refer back to later in the blog. As we'll see, the answer to 'where's the evidence?' is: everywhere!

I’ve heard about ice cores and seen the hockey stick diagram, is this what you’re talking about?

In part, yes! Ice cores contain a number of proxies (indicators) of past conditions around the Arctic, and contribute to understanding of global interactions. Most key ice-core records from the Arctic are from Greenland including GISP/Grip and NGRIP. A common proxy used from ice-cores is the δ¹⁸O:δ¹⁶O ratio, a comparison between two stable forms of oxygen isotope found bonded with hydrogen in the water molecules (H₂O, I’m trying not to insult your intelligence!).

As δ¹⁸O is two protons heavier than δ¹⁶O, it requires more energy to evaporate and condensates more readily. Therefore, by measuring this ratio in ice cores, it is possible to reconstruct past temperatures as well as a history of moisture transport. Cold temperatures generally are indicated by a lower concentration of the heavier isotope δ¹⁸O. Oxygen isotope records are also found in other proxies including in tree-rings and in the chemical composition of marine organism shells. Other proxies included in ice cores are deuterium ratios (heavy hydrogen), greenhouse gas concentrations (care is required due to a certain amount of gas exchange) as well as many others.

So, that’s it?

Fortunately (or I suppose unfortunately depending on your viewpoint) not, there’s a couple more key records used to reconstruct Arctic environments which will crop up a lot in this blog, proxies found in lake sediments and marine records as well as tree rings.

How can lake sediments show past environment?

Common to most palaeo-records is a build-up of material over time, as happens with lake sediments. All manner of organisms and materials build up with this sediment, and these themselves are the proxies. A common proxy is diatom frustules, siliceous shells of microscopic algae of which there are thousands of species. These all respond to different conditions favourably, so by recording the species community composition, it is possible to reconstruct the past conditions qualitatively (descriptively) or quantitatively (through the use of transfer functions). Diatoms can be identified relatively easily under a light microscope, if you know what you’re doing that is (as a side point, if you don’t, as I’ve experienced, the number of species can be quite overwhelming).

Other proxies include chironomidae (non-biting midge species), fossil pollen and marine ostracod records (bivalve crustaceans). Also, a recently developed but useful record is SCP (spheroidal carbonaceous particles) which are produced from burning. They indicate atmospheric contamination when found in lake sediments.

From these records and knowledge of the conditions individual species prefer today, it is possible to reconstruct many aspects of the environment including temperatures, pH, Total Phosporous (TP) and ice cover as well as many others.

What about tree rings?

Everyone knows you can count tree rings to find the age of a tree, as they build up in layers over time. Due to this quality of a ring layer growing during one year, tree-ring proxies have the useful feature of being annually resolved; having an environmental record for each individual year. The longest complete records extend past 10,000 years before 1950 (10Kyr BP) include fossilised tree remains, giving an almost complete record of the Holocene (they also allow the calibration of radiocarbon dates to calendar years as described here). Analysis of tree rings is called ‘dendrochronology’.

Fossilised tree records contain much information, including the dating of extreme events as well as climate records. Trees destroyed due to an earthquake, for example, can show the exact date of that event if patterns are widespread. Climatic information is revealed by the widths of tree rings themselves; narrow rings indicate drought or cold years. If these patterns are seen across a wide area, a regional climatic pattern may be inferred to a very high resolution.

Thanks! You’ve made everything absolutely clear and I have no further questions.

Great! (If, as is more likely, you have any issues with this post, please comment).

It's important to make the point at this stage that reconstructions from palaeo-records are not perfect and do not always agree. Natural systems are complicated and are not always fully understood, and the patterns observable today may not have always worked in the same way throughout all glacial-interglacial cycles. Therefore, assumptions are stretched for very old reconstructions. Multi-proxy studies are desirable and when possible will be preferentially referred to in this blog.

The next post will consider some key papers which have used these proxies, focussing on this article.

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For a much better and thorough summary of all of these proxies see the book ‘Global Change in the Holocene’ by Mackay et al eds. (2005) published by Hodder Arnold 528pp.