IT'S A FACT: the chemical composition of theatmosphere is changing rapidly, worldwide. Sincethe pre-industrial era, the concentrations ofimportant greenhouse gases (see panel) have allincreased significantly. NIWA maintains thelongest continuous measurements of atmosphericcarbon dioxide (CO

2) in the Southern Hemisphere at

Baring Head (see Water & Atmosphere 8(1):14–16).In the 30 years since measurements began there,atmospheric CO

2 has increased by about 15%.

But even the longest available dataset foratmospheric CO

2 goes back less than 50 years. So

how do we know how CO2 concentrations were

changing before this? And what were the levels ofgreenhouse gases in the atmosphere beforeemissions from modern industry and agriculturestarted to change things?

Drilling for iceFortunately, over 20 years ago, scientists made animportant discovery. Near the surface of ice sheets,snowfall eventually turns into ice containing airtrapped in tiny bubbles (above, right). This iceprovides a remarkable museum of the chemicalstate of the atmosphere.

The most successful investigations so far have beenat the polar ice caps. Here, snow traps air in thefirn, a permeable surface layer about 40–100 mthick. As more snow falls, the firn is buried deeperand deeper until it is compressed into solid ice,with the trapped air enclosed in bubbles. Layerafter layer of ice is built up, burying in the ice capa history of past atmospheric composition. Becausethe ice is both impermeable and inert, it is aremarkably good storage container for many of theimportant gases in air.

To retrieve this atmospheric information,researchers drill into the ice cap and remove coresof ice. The cores are kept frozen until the airtrapped in them can be analysed.

Methane increases over 1000 yearsAustralian researchers have been particularlysuccessful with cores extracted from Law Dome,an ice cap 200 km across and up to 1200 m thick

near the coast of the Australian Antarctic Territory.This area was especially suitable because of itsrapid ice formation and very cold temperatures,averaging –20ºC. Fast ice formation means thattrapped air bubbles cover a shorter time span.Changes over as little as 10 years can be detected.Also, the most recent air available is quite young,allowing comparison with modern atmosphericmeasurements.

The ice cores on Law Dome extended right downto the bedrock. At this depth the trapped air is over1000 years old.

Analysis of the methane (CH4) content of the

trapped air (below) shows small variations inatmospheric methane since 1000 AD. However,from 1800 to the present atmospheric methane has

ATMOSPHERIC RESEARCH

Antarctic ice: the world’s air museum

Dave Lowe

David Etheridge

Air bubblestrapped in icehundreds oreven thousandsof years ago areproviding vitalinformationabout past levelsof greenhousegases in theEarth'satmosphere.

A sliver of icefrom a core drilledfrom a depth ofseveral hundredmetres at LawDome inAntarctica. Thebubblescontaining the airare clearly visible.

Atmospheric methane over the last 1000 years. The plotshows small variations in atmospheric methane during thenatural climatic events: the medieval warm period (1000–1300 AD) and Little Ice Age (1550–1800 AD). Thesecontrast with the dramatic increase in methane over the last200 years. The blue symbols are measurements from ice; thered line at the end of the record is from recent atmosphericmeasurements from Cape Grim, Tasmania. (Adapted fromEtheridge et al. 1998. Journal of Geophysical Research103(D13): 15,979–15,993.)

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600

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to see how thisarticle links to theNZ Curriculum,visit:www.niwa.co.nz/pubs/wa/resources/

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more than doubled. This dramatic rise has beencaused by increased emissions associated withhuman activity.

A different climateWe currently live in a warm inter-glacial periodknown as the Holocene, which has lasted about10,000 years. Before this, the earth was gripped inglaciation for about 100,000 years. Temperaturesaveraged 10–15ºC cooler than today and ice sheetswere much more extensive. How did theatmosphere change during these different naturalclimatic events?

The answer has come from an extraordinary seriesof ice cores drilled at Vostok, a Russian station inthe remote centre of Antarctica. French, Russianand American scientists have worked together toproduce a climate history from these cores,spanning over 400,000 years and covering fourcomplete glacial and inter-glacial periods (seegraph, right).

Atmospheric CO2 and CH

4 data are clearly

correlated with temperature through the record. Weknow that the climate changes were initiallytriggered by changes in the amount of solarradiation (insolation) reaching the earth due towobbles in its orbit. The response of the greenhousegases, however, was to feed back and amplify thetemperature changes by a factor of about two.Nowhere in the over 400,000 years of record dothe CO

2 or CH

4 concentrations approach today's

levels. Current greenhouse gas levels, which have

Greenhouse gases and predictingclimate changeGreenhouse gases – mainly carbon dioxide(CO

2), methane (CH

4) and nitrous oxide (N

2O) –

make up a relatively small proportion of theearth's atmosphere. However, because theyabsorb strongly in the infrared part of the solarspectrum and retain heat, they play a significantrole in the radiation balance of the earth andhence its climate. Increases in their concentrationare viewed with concern, and intenseinternational activity is focused on predicting theconsequences of changing atmosphericcomposition and its impact on the earth's climate.Many of these studies are based on the use ofglobal climate models to predict, for example,the effects of doubling atmospheric CO

2. The

models use measurements of greenhouse gases asinput data but are restricted because data areneeded over long time periods for long-livedgases like CO

2.

The FRSTprogrammefunding this workis "GreenhouseGases and ClimateChange",C01X0034

Simultaneous recordsof solar radiation orinsolation,temperature, andatmospheric CH4 andCO2 concentrationsover the last 400,000years derived fromAntarctic ice atVostok. The parallelchanges in CO2 andCH4 are believed tohave caused about halfthe amplitude of thetemperature changes,with the other halfprobably due tochanges in solarinsolation. Today'slevels are indicated bythe solid lines at theright-hand side.(Adapted from Petit etal. 1999. Nature 399:429–436.)

developed in only the last 200 years, mean thatthe world now faces a climate different fromanything during the last 400,000 years.

Isotopes, gas measurements, modelsA joint NIWA/CSIRO project is currentlymeasuring air extracted directly from the Antarcticfirn layer to look at sources of atmosphericmethane over the last 100 years. NIWA has usedisotopic techniques to distinguish different sourcesof methane (see Water & Atmosphere 5(2): 16–17).Levels of the naturally occurring radioactiveisotope carbon 14 in methane, for example, canindicate whether the methane is derived from fossilfuels or from natural sources like swamps. Alreadythe work has shown that 60 years ago up to 15% ofatmospheric methane was derived from fossil sources.

The study of atmospheric greenhouse gas recordsin Antarctic ice will clearly provide a large amountof information about past climatic changes.However the data are also essential for testing howwell global climate models can simulate observedclimate change in the past.

The study of air bubbles trapped in Antarctic icegoes hand in hand with measurements of currentgreenhouse gas levels and the development ofclimate models to predict the future state of theatmosphere.

All three kinds of research are currently beingcarried out by NIWA and CSIRO. ■

Dave Lowe is based at NIWA in Wellington;David Etheridge is at CSIRO AtmosphericResearch, Aspendale, Victoria, Australia.

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