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.