Thursday, 9 January 2014

Using “proxy” data to tell us about past climate change by Melanie Leng

One of the highlights of 2013 was publication of our research, in collaboration with the British Antarctic Survey and various UK Universities, on past climates along the Antarctic Peninsula, here Professor Melanie Leng tells us how climate change from 11,000 years ago to the last few decades has affected the Antarctic from 'proxy data'

Prof Melanie Leng with Dr Robert Mulvany
(British Antarctic Survey) examining ice from the Antarctic Peninsula
The Earth’s climate is always changing and shifts can be very dramatic especially during glacial (large Antarctic and Northern Hemisphere ice caps) and interglacial (smaller ice caps) periods. There is debate as to what causes the change between “icehouse” and “greenhouse” conditions, but the change is usually quite slow taking hundreds to thousands of years to wax and wane between the two conditions. At the moment we are experiencing probably one of the highest rates of change of CO2 concentrations in the atmosphere and as a result parts of the globe are experiencing warmer or more unpredictable climate than we have known for the past 150 years.  For example the Antarctic Peninsula is experiencing one of the highest rates of warming (3°C in the last 100 years) than anywhere on the Planet and freshwater from the melting of the Antarctic ice sheet is entering the ocean at unprecedented rates, ice shelves are collapsing and glaciers are retreating - all quite scary information when it has been calculated that the Antarctic ice sheet alone holds the equivalent of 70m of global sea-level rise. The Antarctic continent is in a key position to be influenced by global climate change being impacted by both global ocean and atmospheric circulation, and therefore a go “thermometer” for monitoring present and predicting future change. One question that faces us is what is causing the current high rates of melting? Current thoughts include increasing greenhouse gases and ozone depletion due to man’s activities but also changes due to natural cycles of climate variability in the South Westerly winds and ocean temperatures in the equatorial Pacific.  The key to unpicking natural from man made processes are comparing current changes (with our increasing concentrations of CO2 in the atmosphere) with past natural climate variability.
The Antarctic Peninsula is experiencing
one of the highest rates of warming on the
Planet over the last century

There are several ways to get evidence on past climates around the Antarctic continent, we can look at the amount of freshwater (from melting of the ice) entering the coastal environment from the chemistry of algae that live in the coastal zone, we can obtain past temperatures from the chemistry of coastal ice caps, and we can calculate  temperatures and CO2 changes preserved in small pioneering plants that are found in pockets around Antarctica. Here at the British Geological Survey we have used all these methods to obtain climate “proxies” back through time for the Antarctic Peninsula.

Mosses from the Antarctic Peninsula are being
used to look at modern climate change and
how it is affecting plant communities
What we are discovering from our research is that melting of the Antarctic ice cap has been very variable over the last 11,000 years since the ice cap retreated onto land as the world shifted from icehouse (large ice caps) to greenhouse (small icecaps) conditions. Before 11,000 years ago the ice cap was thought to have been much bigger, extending over the coastal margins. From our proxy data we have shown that over the last 11,000 years there has been changes between melting and build up of ice related to different processes through time. The chemistry of the algae that lived in the coastal zone and subsequently accumulated on the sea bed have shown that sometimes high rates of melting are due to the transfer of heat through the atmosphere during periods of warmer Equatorial Pacific ocean surface temperatures(1). Temperature data calculated from the chemistry of the snow and ice show that the Antarctic Peninsula has experienced very warm summer air temperatures over the last 600 years, but that since the mid twentieth century  the level of summer melting is unprecedented in parallel with rising greenhouse gases into our atmosphere(2).  The growth rates of moss plants over the last 150 years also show the most dramatic rises since the 1960’s alongside the greatest temperature and CO2 changes that we have seen in the last 150 years (3). Together the various proxies and different expertise of the scientists involved are showing that while climate is always variable, past changes have tended to be slower than the current rate of warming and therefore we are in a period of unprecedented rates of change.

Much of our proxy data confirms what we already know, there are natural and manmade influences on climate at different scales. Our studies help provide evidence for policy makers and Governments (like the Intergovermental Panal on Climate Change) but to you and I it helps demonstrate how fragile our planet is and how humanity is likely to profoundly transform the Earth in ways that we can only imagine by looking at proxy data from the geological archives.

Mel @MelJLeng

Prof Melanie Leng is an isotope geochemist and palaeoclimatologist at the BGS. This research is in collaboration with many scientists in the UK including colleagues at the British Antarctic Survey, and the Universities of Cambridge, Exeter, and Cardiff.

 (1) Pike, J., Swann, G.A.E., Leng, M.J., Snelling, A. 2013. Glacial discharge along the west Antarctic Peninsula during the Holocene. Nature Geoscience, 6, 199-202.
 (2) Abram,N.J.,  Mulvaney,R., Wolff, E.W., Triest,J., Kipfstuhl,S., Trusel,L.D., Vimeux,F., Fleet, L.  & Arrowsmith, C. 2013. Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geoscience, 6, 404-411.
(3) Royles, J., Amesbury, M.J., Convey, P., Griffiths, H., Hodgson, D.A., Leng, M.J., Charman, D.J. 2013. Plants and soil microbes respond to recent warming on the Antarctic Peninsula. Current Biology, 23, 1702-1706.

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