Sandy Harrison Dr. Sandy Harrison School of Geographical Sciences Bristol University UK If you would like to interview this scientist, please contact Leah Christen. > biographical information > abstract > paper

Biographical Information:

I am a Reader in Geography at Bristol University, UK. I am a paleoclimate diagnostician, with an interest in the role of the land-surface, terrestrial biosphere and hydrological processes on modulating regional climate changes in the past and the future. My background is in Earth Sciences, and I have expertise in the creation of global paleoenvironmental data sets and their use in evaluation of Earth System models. I am on the Scientific Steering Committee for the Palaeoclimate Modelling Intercomparison Project (PMIP) and for the IGBP-GAIM initiative for an Earth System Atlas. I am also Vice-President of the INQUA Commission on Paleoclimatology and a member of the Terrestrial Observation Panel for Climate (TOPC) of the Global Climate Observing System (GCOS). I have had coordination roles in several paleoclimate synthesis initiatives sponsored by IGBP, including BIOME6000, the Global lake Status Data Base, DIRTMAP, 21ka TROPICS and SNOWLINE.

Abstract:

Towards modeling the ice-core record of atmospheric trace gas and aerosol variations between glacial and interglacial times

The large and systematic variations in atmospheric trace gases and aerosols between glacial and interglacial periods, documented by polar ice-core records, including the new EPICA core, provide a challenge to our understanding of the natural regulation of the atmospheric oxidizing capacity and greenhouse gas content. The problem is complex, and requires insight into changing terrestrial and marine sources and sinks, the physical and biological processes that control emissions from the ocean and land surface, and the complex physical and chemical transformations that take place during atmospheric transport. Recent developments in the field of biogeochemical and atmospheric chemistry modeling, coupled with the existence of global syntheses of paleodata documenting changes in climatic and environmental conditions, are already helping to provide a better understanding of the role of various competing or complementary processes in the natural regulation of atmospheric composition. While much of the work so far has focused explicitly on the carbon, methane and dust cycles, algorithms for the climatic control of biogenic emissions of other key reactive species are being developed. A push towards coupling these components into the next-generation Earth System Models will be required before we are able to fully answer the challenge posed by the ice core record.

Paper:

Explaining the changes in atmospheric composition through glacial-interglacial climate cycles

How has the composition of the atmosphere changed in the past?
Bubbles of air, trapped during the growth of the polar ice sheets, provide a continuous record of changes in the composition of the atmosphere through time. Analysis of the extremely long ice core extracted from the Antarctic Ice Sheet by the European Project for Ice Coring in Antarctica (EPICA) is providing a record of changes in the trace gas and aerosol composition of the atmosphere covering ca 740,000 years. This interval is long enough to cover multiple shifts from interglacial to glacial climates – from times when the climate was similar to today to time when the earth was covered by extensive ice sheets, the oceans were cold with large areas of sea-ice in winter, and the extent of forests was much reduced because the continents were cold and generally dry, and the climate was kept cold by very low concentrations of carbon dioxide and other “greenhouse gases” and by enormous amounts of dust in the air.

What controls changes in atmospheric composition ?
The ice-core record shows that, before the pre-industrial era, changes in atmospheric composition fluctuated within well-defined limits that scale with global temperature. Over the last four glacial-interglacial cycles, the upper and lower bounds on carbon dioxide and methane (the two most important greenhouse gases after water vapour) have been between 180-280 ppm and 350-700 ppb respectively. However, the causes of changes in climate, and of the changes in greenhouse gases and dust (which in turn affect the climate), are not fully understood. The only known external cause is the variation of the Earth’s orbit, which gradually alters the amount of the Sun’s energy received at different latitudes and seasons. The Earth apparently responds to this variation in a very complex way, and sometimes abruptly. The changes in the atmosphere, especially, give a clue that climate changes are not just a matter of physics; they also involve changes in “biogeochemical cycles” – the exchanges of carbon and other elements between the atmosphere, ocean and land, which are regulated by living organisms. Understanding these processes is particularly important because human activities are now changing the atmosphere and climate, and we need to be able to predict the long-term consequences of these changes.

Modeling changes in atmospheric composition
Numerical models provide one way of examining how biogeochemical cycling may have responded to changes in climate and how these changes may in turn have influenced climate. A variety of terrestrial models have been developed to explore changes in emission sources – for example, of methane from wetlands, trace gases and carbon-containing aerosols released by fires, and the control of dust emissions by vegetation. Models have been developed to look at marine emissions, especially of dimethyl sulphide, a gas that becomes oxidized to sulphate in the atmosphere and affects the formation of clouds. A major goal of climate modeling is now to produce models that include all of these components – so-called Earth System Models. In the next few years it should be possible to use such models to simulate the changes in atmospheric composition as seen in the ice-core record. If we succeed in doing so, it will be a proof that we really can predict the consequences of the continuing alteration of the natural system by human activities.