Cryoconite has been studied intensively, but we have only touched upon the redeposition of incumbent microbes to other glacial zones – something we expect to happen more as the climate continues to warm. Whether microbes that fix and respire carbon on glacier surfaces continue to do so when they are washed elsewhere has been pondered but not properly studied, nor have the potential effects on downstream ecosystems. This is an increasingly important question because as glaciers retreat there will be less available ice surface for microbes to inhabit, greater washaway of microbes by melt runoff and increased delivery to other environments (Irvine-Fynn et al, 2013).
Wilhelm et al (2013) engaged with this problem using ecological concepts. They focussed on proglacial streams. Much of the water feeding these streams comes from melting glaciers, and along with it come microbes (predominantly from cryoconite holes). In addition, however, water and microbes are delivered from groundwater, snowmelt and atmospheric deposition. Wilhelm et al (2013) showed that as glaciers retreat, the relative input from each source can change, and the result is markedly different downstream microbial assemblages.
Biodiversity in biofilms in the streams was linked to elevation, glacier coverage and hydrological inputs from icemelt, snowmelt and groundwater, as well as local geomorphology and physicochemistry. These are variables likely to change as glaciers retreat. Furthermore, biodiversity in biofilms was found to differ from both stream water and glacier ice, largely due to environmental ‘harshness’ and the size of each zone’s ‘metacommunity’. The metacommunity refers to all of the microorganisms which interact ecologically in spite of geographical separation. For example, microbes in cryoconite holes are part of the metacommunity of the proglacial streams despite being geographically distinct because some end up transcending the geographical divide and adding to the stream’s biodiversity.
Glacier retreat will reduce the available ice surface area available for habitation. Therefore, ice surface microbes will form a decreasingly important part of the metacommunity for downstream organisms as glacier recede. Soils and groundwater will become more hydrologically important, they will also become increasingly important components of the stream metacommunity.
Which particular microbes from a metacommunity form an assemblage in a specific habitat is controlled predominantly by environmental pressures. However, in stream water these pressures have less impact because cells generally have very short residence times. Biofilms, however, represent selections of organisms from stream water – which microbes form a biofilm is very much dependent upon both upstream metacommunity size and environmental factors. The implication of this is that as glaciers retreat, accompanied by both shifts in metacommunity and environmental pressures, downstream community structures will change markedly.
It is thought that glacier retreat could increase ‘alpha’ biodiversity (biodiversity within a specific habitat) whilst decreasing ‘Beta’ biodiversity (biodiversity between habitats within a metacommunity). This suggests that each habitat will support more diverse microbial assemblages, but that these will be more homogenous across the various locations. Species which are geographically isolated or only sporadic in their occurrence face greater risk of extinction.
In summary, then, Wilhelm et al’s (2013) study has further enlightened us to the potential for multidisciplinary approaches to glaciology, the sensitivity of glacial ecosystems and the potential impacts of glacier retreat on polar biodiversity.
Irvine-Fynn, T.D.L., Edwards, A., Newton, S., Langford, H., Rassner, S., Telling, J., Anesio, A., Hodson, A.J. 2013. Microbial cell budgets of an Arctic glacier surface quantified using flow cytometry. Environmental Microbiology, 14 (11): 2998 – 3012
Nutrient cycling has been a central theme of glacier microbiology in the twenty-first century. Here is a run-down of the fundamentals, focussing on the major ones: nitrogen and carbon. Nitrogen’s up first…
The Nitrogen Cycle:
Nitrogen is a key nutrient required for synthesising crucial organic molecules such as nucleotides, proteins, and chlorophyll. Nitrogen availability also impacts upon rates of primary production (crucially photosynthesis) and respiration in microbial communities.
Nitrogen makes up the majority of earth’s atmosphere, but in its atmospheric form (dinitrogen, N2) organisms cannot use it. It has to be converted, or ‘fixed’ before it can be used to build organic molecules. Just as vinyl records cannot be played on an mp3 player – the analogue information has to be digitised and formatted first – so it is with nitrogen! Organisms cannot use gaseous N2, but when it is fixed into ammonia (NH3) it becomes ‘formatted’ for biota. Once it is ‘bio-available’, it can go through various permutations for incorporation into different biological pathways. Being a cyclic process, organic nitrogen is also ultimately converted back into atmospheric N2.
Nitrogen fixation (N2 -> NH3) requires lots of energy due to dinitrogen’s triple bond and the few organisms (prokaryotes) able to do it do so via complex biochemical mechanisms, sometimes involving host organisms. Nitrogen fixers are diverse but they all use an enzyme called nitrogenase to fix nitrogen. Humans have also learned to force dinitrogen to react with hydrogen to form ammonia through the Haber-Bosch process and release of various bio-available nitrogenous compounds also occurs during combustion of fossil fuels.
For plants to take-up nitrogen through their roots, ammonia must be converted into nitrate (NO3–), primarily via the action of nitrifying bacteria (NH3 -> NO2– -> NO3–). Nitrates are either converted directly back to atmospheric dinitrogen by denitrifying bacteria, or taken up by plants and assimilated into a range of crucial biomolecules. Ultimately, the plants die and decompose, and the assimilated nitrogen undergoes ammonification (back to NH3) and then nitrification (back to NO3–).
In oceans and lakes the cycle is essentially the same, except that some of the sources and transfers are modified to reflect the aquatic environment. This probably better reflects nitrogen cycling in most supraglacial habitats. Cyanobacteria fixes most of the N2 which enters the system via runoff, precipitation, snowmelt and atmospheric exchange. Bacteria convert ammonia (NH3) into ammonium (NH4+) which can then be nitrified into nitrite (NO2–) and nitrate (NO3–) for assimilation into organic molecules.
Nitrogen Cycling on Ice Surfaces:
Nitrogenous material on ice was first identified by Bayley (1891), but N cycling in the supraglacial environment was largely overlooked until relatively recently. N cycling was examined in polar rivers (Tockner et al, 2002), oceans (Dittmar, 2001), soils (Nordin et al, 2004), ice cores (Olivier et al, 2006), subglacial sediments (Wynn et al, 2007) and under seasonal snow (Williams et al, 1996; Hodson et al, 2005; Hodson et al, 2006; Jones, 1999), but organisms directly inhabiting ice surfaces were not considered in detail until very recently.
Tranter et al (2004) examined Antarctic cryoconite hole biogeochemistry, reporting high inorganic : organic N ratios. This suggests recycling of organic compounds has dominated over fixation of N2, probably due to surface freezing isolating holes from nitrogen inputs. In contrast, Hodson et al (2008) explained that Svalbard cryoconite holes generally show net NH3 production, suggestive of fixation of atmospheric N2 by cyanobacteria. Since nitrogen fixation is so energy demanding, microbes use allochthonous (delivered from elsewhere) bio-available nitrogen in preference to fixing it. Therefore, active fixation indicates that allochthonous bio-available nitrogen is not meeting microbial demand. Sawstrom (2009) confirmed that Svalbard cryoconite communities were not limited by nitrogen availability, highlighting the action of nitrogen-fixing bacteria as important components of microbial assemblages.
In 2009 and 2010, Jon Telling and others undertook detailed investigations into nitrogen cycling across the ablation zone of glaciers in Svalbard and on the Greenland ice sheet. Nitrogen fixation was found to be active in cryoconite hole communities in both regions, implying that there is not enough bio-available nitrogen delivered to these habitats via abiotic processes (predominantly wind and snowmelt inputs) to sustain microbial growth. As mentioned earlier, nitrogen fixation requires a large amount of energy and may therefore exert some limits on cell proliferation.
Some evidence of denitrification and ammonification was also identified in Greenland cryoconite holes by Telling et al (2011), although not quantified. These processes are crucial to nitrogen cycling and are expected to occur in these habitats.
Human Impacts on Glacial Nitrogen Cycling
Greater allochthonous (delivered from elsewhere) nitrogen deposition unburdens microbial nitrogen fixers from the task of providing bio-available nitrogen, meaning more energy is available for microbial growth and proliferation in the community (although nitrogen limitation is unlikely to be the primary factor limiting biomass production in cryoconite holes; Telling et al, 2011). Human derived nitrogenous compounds such as those produced by fossil fuel combustion are being deposited in polar snowpacks in ever increasing concentrations such that Arctic cryoconite communities often no longer need to fix atmospheric nitrogen at all; microbial activity can be sustained by allochthonous inputs (Telling et al, 2011). Hodson et al (2010) monitored nutrient budgets in Svalbard after a particularly intense period of pollutant deposition in Svalbard and showed that even isolated deposition events can have huge impacts for nutrient cycling which permeate through entire glacial catchments.
Anthropogenic activities are increasing the concentration of bio-available nitrogenous compounds in the atmosphere. These can be carried over long distances and deposited on glacier surfaces (Kozak et al, 2013) and have a significant impact upon nutrient cycling and possibly rates of activity and biomass production in these areas (although more research is required to confirm or deny this). Outside of the cryosphere, human activity has a well-known and marked effect upon ecosystem dynamics, particularly resulting from industrial emissions, road vehicles and the use of agricultural fertilisers.
There is likely to be significant variation in nitrogen cycling in supraglacial habitats in the Arctic and Antarctic, largely due to the freezing of cryoconite hole ‘lids’ in Antarctica. These lids are formed when the surface of the melt water overlying cryoconite sediment in cryoconite holes freezes. This isolates the cryoconite hole from atmospheric exchanges and input from snowmelt. The process of nitrogen cycling in these pseudo-closed systems is uncertain, but is likely to be characterised by organic molecule recycling rather than fixation of dinitrogen, and may lead to extreme hydrochemical conditions (Tranter, 2004). There may also be significant differences between stable large glacier / ice sheet locations and alpine / ice-marginal zones where inputs and processes are more dynamic.
Bayley, W.S. 1891. Mineralogy and Petrography. The American Naturalist, 25 (290): 138-146
Dittmar, T. , Fitznar, H. P. and Kattner, G. (2001): Origin and biogeochemical cycling of organic nitrogen in the eastern Arctic Ocean as evident from D- and L-amino acids , Geochimica et Cosmochimica Acta, 65 (22), pp. 4103-4114 .
Hodson A.: Biogeochemistry of snowmelt in an Antarctic glacial ecosystem, Water Resources Res., 42, doi: 10.1029/2005WR004311, 2006.
Hodson A.J., Mumford P.N., Kohler J. and Wynn P.M.: The High Arctic glacial ecosystem: New insights from nutrient budgets, Biogeochemistry, 72, 233-256, 2005.
Hodson A., Anesio A.M., Tranter M., Fountain A., Osborn M., Priscu J., Laybourn-Parry J. and Sattler B.: Glacial ecosystems, Ecological Monographs, 78, 41-67, doi:10.1890/07-0187.1, 2008.
Hodson A., Roberts T.J., Engvall A.-C., Holmen K. and Mumford P.: Glacier ecosystem response to episodic nitrogen enrichment in Svalbard, European High Arctic, Biogeochemistry, doi: 10.1007/s10533-009-9384-y, 2009.
Hodson, A., Roberts, T.J., Engvall, A-C., Holmen, K., Mumford, P. 2010. Glacier ecosystem response to episodic nitrogen enrichment in Svalbard, European High Arctic. Biogeosciences, 98 (1-3): 171-184
Jones, H.G. 1999. The ecology of snow-covered systems: a brief overview of nutrient cycling and life in the cold, Hydrological Processes, 13, 2135 – 2147.11
Kozak, K., Polkowska, Z., Ruman, M., Kozioł, K., Namieśnik, J. 2013. Analytical studies on the environmental state of the Svalbard archipelago provide a critical source of information about anthropogenic global impact. Trends in Analytical Chemistry, 50: 107 – 126
Nordin, A., Schmidt, I.K., Shaver, G.R. 2004.Nitrogen uptake by Arctic soil microbes and plants in relation to soil nitrogen supply. Ecology 85: 955–962
Olivier S., Blaser C., Brütsch S., Frolova N., Gäggeler H.W., Henderson K.A., Palmer A.S., Papina T., and Schwikowski M.: Temporal variations of mineral dust, biogenic tracers, and anthropogenic species during the past two centuries from Belukha ice core, Siberian Altai, J. Geophys. Res., 111, D05309, 2006.
Sawstrom, C., Karlsson, J., Laybourn-Parry, J., Graneli, W., (2009), Zooplankton feeding on algae and bacteria under ice in Lake Druzhby, East Antarctica. Polar Biology, 32(8), 1195-1202
Telling, J., A. M. Anesio, M. Tranter, T. Irvine-Fynn, A. Hodson, C. Butler, and J. Wadham (2011),Nitrogen fixation on Arctic glaciers, Svalbard, J. Geophys. Res., 116, G03039,
Telling, J., Stibal, M., Anesio, A.M., Tranter, M.L., Nias, I., Cook, J., Lis, G., Wadham, J.L., Sole, A., Nienow, P., Hodson, A.J. 2012. Microbial nitrogen cycling on the Greenland Ice Sheet. Biogeosciences,
Tockner, K., Malard, F., Uehlinger, U. and Ward, J. V. (2002) Nutrients and organic matter in a glacial river-floodplain system (Val Roseg, Switzerland). Limnology and Oceanography, 47, 266-277.
Tranter, M., FountainA., FritsenC., Lyons, B., Priscu, J., Statham, P., and Welch, K., 2004. Extreme hydrochemical conditions in natural microcosms entombed within Antarctic ice. Hydrological Processes, 18, 379-387.
Williams, M., Brooks, P.D., Mosier , M. 1996. Mineral nitrogen transformations in and under seasonal snow in a high-elevation catchment, Rocky Mountains, USA, Water Resources Research, 32, 3175-31856
Wynn P.M., Hodson A.J., Heaton T.H.E. and Chenery S.R.: Nitrate production beneath a High Arctic glacier, Svalbard, Chem. Geol. 244, 88-102, 2007
Many thanks to Dr Bethan Davies from Aberystwyth University for contributing this excellent article to To The Poles. Not only is Bethan a prolific and ingenious palaeoglaciologist, she also manages Antarcticglaciers.org, a key resource for anyone wishing to learn about glaciology from the fundamentals right up to the cutting edge.
NB. This article about palaeo-glacier reconstruction will be of great interest to any of my Earth Surface Processes and Landforms or Physical Geography of the Human Realm students.
Reconstructing past ice sheets
By Dr Bethan Davies
Why are we interested in ancient glaciation?
Worldwide, glacial geologists are hard at work attempting to unravel the past behaviour of former ice sheets. They want to know how big these ice sheets were, how thick, how fast they retreated at the end of the Last Glacial Maximum, and what processes were active at the base of the ice sheet. Did the West Antarctic Ice Sheet collapse in the past? Did ice shelves collapse regularly throughout the Holocene? Is the rate of glacier recession around Antarctica accelerating? How big were the Antarctic and Greenland ice sheets in the Pliocene (+3°C warmer than today)? Did ice streams change and fluctuate in the past, as they do now, or were they constant in space and time?
In order to answer these questions, and many more, palaeoglaciologists spend hours studying minute slices of sediment impregnated with resin underneath a microscope, counting tiny marine microfossils like foraminifera, and collecting samples of sand and mud and rock from all over the world. But why are these scientists so interested in understanding past ice-sheet change?
The answer is simple. Climate change over the next few hundred years threatens the viability of our present-day glaciers and ice caps. Warming air temperatures and ocean currents worldwide will melt these reserves of ice, raising our sea levels, potentially changing ocean circulation, and threatening water supply in many arid regions. Sea level rise is one of the greatest threats posed by climate change to humanity, with the possibility of displacing hundreds of millions of people, swamping small island nations and forcing thousands of people to relocate. Wealthier countries will need new flood defences, and London will need a new Thames Barrier.
Politicians and policy makers want to know how much, and how fast, sea levels will rise. If they have accurate predictions, they can implement adaptation and mitigation strategies to avoid the worst damage. By studying ancient glaciation, glacial geologists hope to understand past ice-sheet response to environmental change.
The present is the key to the past… and the future
If we want to know how ice sheets will respond to future climate change, we must look to the past. In the 18th Century, geologists first suggested that ‘the present is the key to the past’ (Charles Lyell), stating that modern processes help us understand ancient sediments and landforms. We can turn this on its head: ‘the past is the key to the future’. Glaciers and ice sheets around the world have undergone huge cyclical, natural changes over the last two million years. By using various techniques to understand how quickly they thinned and receded during past periods of rapid change, we can understand better how they are likely to behave in the future.
We also know that ‘the past is the key to the present’. The base of ice sheets is hard to observe. There is a lot of ice in the way. However, ancient glacial sediments and glacial landforms are widely distributed across Britain, the North Sea, North America and Europe. They are easy to observe and study. We can look at these sediments, compare them to modern analogues, and form hypotheses about how they were made. This helps us to understand processes operating at the base of an ice sheet, which again helps glacial geologists understand modern changes in Antarctica.
So, glacial geologists study glacial sediments to understand the past, present and the future.
How can we reconstruct past ice sheets?
Fortunately, glacial geologists have many techniques at their disposal. Glaciers leave behind distinctive landforms. Moraines mark the end positions of ice. Drumlins, mega-scale glacial lineations and roche moutonnées indicate the direction of ice. Cross-cutting relationships of drumlins superimposed on one another tells us about shifting ice divides and changing ice-flow patterns. Trough-mouth fans form at the end of ice streams on the continental shelf edge. Trimlines on mountains tell us how thick the ice was. The advent of satellite images allows palaeoglaciologists to observe these landforms at massive scales from space, rapidly advancing our mapping of ancient glacial landforms.
Subglacial sediments indicate whether the ice was wet-based or cold. The structures of subglacial deformation tell us about pore-water pressure, ice-flow direction, depth of deformation and more. We can use classic ‘indicator erratics’ (such as a specific granite like the Shap Granite) to trace ice flow back to a specific point source.
We can also use sea level change to reconstruct past ice sheet volume. In some places, like Scandinavia, Scotland and Antarctica, the land is rebounding following the last ice age. The huge weight of ice sheets in these areas during the last deglaciation depressed the Earth’s mantle. It is slowly responding to the removal of this massive weight by rebounding. If we can measure the rate of past and present sea level rise due to this isostatic uplift, and we know the viscosity of the mantle, we can make estimates of former ice volume in these areas.
Dating past glaciations
And then we have a number of chronological techniques that tell us how old these sediments and landforms are. Radiocarbon dating is frequently used on organic matter outside ice sheet margins. For example, after ice recession, a lake may form in the lee of a moraine. Sediments will collect in this lake, including pollen and bits of vegetation. A lake sediment core will give us a record of the Late Glacial, and a radiocarbon age at the base of the lake will give us a minimum age for ice recession. In Antarctica, marine sediment cores have been taken all across the continental shelf. The base of these cores is typically subglacial sediments, followed by ice-marginal glaciomarine sediments and then distal glaciomarine sediments. Radiocarbon ages on these transitional glaciomarine sediments gives palaeoglaciologists a minimum age for ice recession.
Cosmogenic nuclide dating dates the length of time that boulders or bedrock has been exposed to the atmosphere. The yield an exposure age; the age since the boulder or rock was left behind by the glacier. Sandy sediments, often deposited by glaciofluvial rivers just in front of the glacier, can yield a burial age: optically stimulated luminescence gives the time since burial. Together, with these and other techniques, palaeoglaciologists can understand the dimensions and change through time of past ice sheets. By comparing this data to records of past environmental change, for example ice cores from Greenland or marine sediment cores, they can investigate past ice-sheet response to atmospheric and oceanic forcing – the golden key that will unlock the future.
The physics of glacier ice is reasonably well understood. We know the electrical conductivity of ice – so we know how its temperature changes with depth in the ice. We have laws that govern how ice deforms and moves. We have observations that tell us how much snow and ice melts for a given temperature. Together, we can input these mathematical equations into a computer programme. The computer programme will calculate the movement of ice through time. We can use this to investigate different ice flow under different environmental conditions.
Numerical modellers use these computer programmes to test hypotheses of past ice sheet behaviour generated by palaeoglaciologists. These palaeoglaciological data are used to improve these ice sheet models. Ultimately, the improved models are used to generate hypotheses of future ice sheet change under various carbon emissions scenarios. We can investigate how glaciers will behave under different air and ocean temperatures. By continuously working together to better understand past and present ice sheets, palaeoglaciologists reduce uncertainty and improve predictions of future sea level rise.
About the author
Dr Bethan Davies is a post-doctoral research associate at the Centre for Glaciology, Aberystwyth University, UK. She is currently analysing glacier response to climate change over centennial to millennial timescales, both in Antarctica and Patagonia. She uses a combination of field work, remotely sensed images and computer simulations to assess changes in glaciers. She blogs and writes about climate change and glaciers at www.AntarcticGlaciers.org.
Cryoconite holes represent the most active and biodiverse habitats in the supraglacial (ice surface) environment. Within cryoconite holes the majority of microbial life is concentrated in and around spheroidal granules of 1-10mm diameter, composed of mineral and organic matter, known as cryoconite. However, overlying cryoconite is almost always a column of meltwater centimetres to tens of centimetres deep, and a new paper published in Polish Polar Research has emphasised the importance of this water as a microbial habitat.
Mieczen et al (2013) focussed upon ciliates in cryoconite melt water. Ciliates are a type of protozoan. Protozoans are single celled, eukaryotic organisms which means they contain a membrane-bound nucleus, amongst other organelles. Ciliates are probably the most complex protozoans and are so called because they are covered in ‘cilia’ – small hair-like organelles on their outer membrane which they use as touch-sensors, for movement and for feeding. They are particularly important in cryoconite melt water because they metabolise the tinest primary producers, regenerate nutrients and provide a food source for higher organisms (Mieczen et al, 2013). However, no previous studies attempted to identify exactly which ciliates are present in cryoconite hole communities and what their role in ecosystem functioning might be.
Mieczen et al (2013) set about filling this research gap by visiting Ecology Island in West Antartica, initially proposing two hypotheses: firstly that the abundance of various ciliate species would vary according to depth in the vertical profile of a cryoconite hole; second that this is related to changes in physical and chemical conditions in the melt water.
At nine sites along a transect on Ecology glacier, water from near the surface and at the bottom of cryoconite holes was removed for analysis. The ciliates in each sample were identified and counted by eye using an optical microscope, which must have been an arduous and time consuming process! Waters were also tested using spectrophotometry to determine their hydrochemistry. Species distribution and water chemistry were then compared using the common statistical techniques PCA (principal component analysis) and RDA (redundancy analysis).
Sixteen types of ciliate were found in the cryoconite melt water samples. Ciliates were more abundant lower in the water column, probably because of closer proximity to food sources originating in cryoconite sediment, as supported by the chemical analyses: nutrients were far more abundant nearer the sediment; furthermore higher temperatures near the sediment may also promote ciliate growth and reporoduction. Both temperature and nutrient concentration were clearly related to cliliate abundance.
Species composition also changed with depth. Near the surface, ciliates in greatest abundance were those that can feed on a wide range of carbon sources (mixotrophs), or which specifically feed on algae (algiverous), whereas near the cryoconite sediment ciliates feeding on bacteria (bactiverous) dominated. This is related to the high bacterial production in the sediment.
This provides a fascinating example of the complexity of these microbial habitats – not only is there a heirarchical food web existing in the cryoconite sediment, but also in protozoa in the overlying meltwater. These tiny, hairy, single celled organisms relocate to find the most appropriate food source and thrive in micro-niches in what seems at first to be homogenous, empty water! Amazing!
Mieczan, T., Gorniak, D., Swiatecki, A., Zdanowski, M., Tarkowska-Kukuryk, M., Adamczuk, M. 2013. Vertical microzonation of ciliates in cryoconite holesin Ecology Glacier, King George Island. Polish polar Research, 34 (2): 201-212
Physical Geography of the Human Realm students: this post provides additional notes to accompany the ‘Cryosphere’ lecture on Friday 8th November!
Ice and Climate
Ice ages and glacial interglacial cycles are periodic fluctuations in earth’s ice cover over geologic time. An ice age is a period during which perennial ice is present on earth’s surface. We are in an ice age now – there are glaciers and ice sheets on earth’s surface. However, within that ice age we are in a relatively warm period – ice has retreated to high latitudes and high altitudes. We call these warm periods interglacials. Colder spells during which ice expands from the poles down to lower latitudes are known as glacial periods. There is some evidence to suggest that during previous glacials ice expanding from each pole has met at the equator, forming a complete coverage of the entire globe in multi-year ice – a scenario known as snowball earth (Hoffman, 2000). Earth is currently moving from a glacial period (which reached its maximum extent about 18,000 years ago) into an interglacial. There was certainly no snowball earth 18,000 years ago; however ice did extend far enough to cover much of northern Europe and North and South America.
But what is it that causes these changes in ice cover? The answer to this question was discovered in long ice cores removed from the Greenland and Antarctic ice sheets. From ice cores we can reconstruct past temperatures (see ice core review). Doing so revealed that past fluctuations in ice extent synchronised very closely with changes in earth’s orbit, as proposed by Milankovitch (Hays et al, 1976). These include periodic changes to the shape of earth’s path around the sun (eccentricity), tilt of earth’s axis (obliquity) and the ‘wobble’ of the earth axis (precession). Each of these changes the amount or distribution of the sun’s energy received by the earth, and therefore the global temperature. Simple…?
Not quite. It turns out that although orbital variations and ice ages have remarkable synchronicity, they do not change the earth’s temperature enough (i.e. their effect is of insufficient magnitude) to result in ice age or glacial-interglacial cycling. What is required is an amplifier. A guitar string can be picked hard, but the resulting sound will never fill a stadium if the amplifier is not plugged in. Similarly, orbital variations alone cannot result in ice ages and glacial cycles; however with amplification, they can. Earth’s glacial cycles are a guitar solo plucked by orbital variations – the amplifier is a complex set of positive feedbacks which are internal to the earth system!
What are these climate amplifiers? There are many, but the main ones are carbon dioxide, cloud, water vapour, and ice-albedo feedbacks. Primarily, we are interested in the ice-albedo feedback. It is slightly misleading of me to refer to ‘the ice albedo feedback’ as though it is a single, isolated phenomenon, because in fact there are many layers of ice-albedo feedback which operate on many superimposed spatial and temporal scales. At a global scale, and over geologic time scales, surface albedo is altered according to the extent of the cryosphere. Ice is more reflective than sea water or land, meaning that more ice cover leads to more reflection (Budyko, 1968). More reflection means less absorption of energy by the planet as a whole, and therefore lower global average temperatures and further ice accumulation. Conversely, if temperatures rise and ice melts the earth’s surface becomes overall less reflective (albedo decrease) and global temperatures increase – further reducing global ice extent. These positive feedback mechanisms are thought to amplify orbital variations and drive the earth into and out of ice ages and glacial periods.
Superimposed upon these long term, global scale ice-albedo amplifiers are smaller scale phenomena related to the changing colour of glacier surfaces. Ice is not constant in its reflectivity – rather it changes according to rates of deposition, rates of microbial activity and pooling of melt water. Lots of research is currently being carried out in this field, but in summary, higher temperatures mean more pooling melt water, higher rates of photosynthesis and faster delivery of weathered material, all of which darken glaciers and further enhance melting. There are, however, thresholds and superimposed positive and negative feedback mechanisms (for example, photosynthetic rates plateau and then decline with rising light intensity due to microbial photoinhibition (‘sunburn’) and high melt rates can wash microbes off glacier surfaces).
There is a distinction to be made, then, between processes which are external to the earth system (changes in received solar irradiance) and amplifiers which are internal to the earth system (e.g. ice-albedo feedbacks). Internal climate amplifiers operate at a range of scales, from long term changes in global ice coverage to daily fluctuations in microbial activity and rates of sediment deposition. Furthermore, these albedo processes are wove intricately into a wider system involving amplifiers in the hydrosphere, atmosphere and – in contemporary times – the anthrosphere.
Key Point: Changes in earth’s orbit cannot explain ice ages on their own. Variation in energy flux resulting from orbital changes requires amplification by processes internal to the earth system to alter temperatures enough to force ice ages.
Budyko, M.I. 1968. The effect of solar radiation variations on the climate of the earth. Tellus, XXI (5) 1969
Hays, J.D. John Imbrie, and N.J. Shackleton. “Variations in the Earth’s Orbit: Pacemaker of the Ice Ages.” Science. Volume 194, Number 4270 (1976). 1121-1132
Hoﬀman, P.F. and Schrag, D.P., 2000. Snowball Earth. Sci. Am., 282, 62–75.