WHAT HAPPENS to Earth’s climate when 2,000,000,000,000 tonnes of carbon is released into the atmosphere over a period of a few hundred years? This question is far from academic. Since the start of the Industrial Revolution, humans have put about this amount of carbon into the atmosphere in the form of carbon dioxide and methane, much of it from the burning of fossil fuels and the clearing of forests. Many scientists and, increasingly, politicians are concerned about the effects this so-called anthropogenic carbon may have on climate, while the news media regularly report on global warming, unusual weather patterns, sea-level rise, melting ice caps and disappearing species.

Can we determine what impact this huge release of carbon will have on climate and, consequently, on plant and animal life? Using sophisticated computer programs we can model the climate as we think it operates; however, Earth’s ocean–climate system is enormously complex and difficult to model comprehensively. An alternative approach is to go back in time.

There has been climate change throughout Earth’s history. Sometimes the change has been gradual and sometimes it has been frighteningly fast. Any future climate scenario we might envisage will have occurred at some time in the past, and past changes are recorded in rock, sediment and ice deposits. The successive layers of sedimentary rock, marine mud and sheet and glacier ice have been compared to the pages of a diary, chronicling the environmental conditions at the time of deposition. Depending on the nature of the rock, mud or ice, these “pages” can reveal details of biological communities and their habitats, air and ocean temperatures, wind, rain and snowfall, and oceanic currents and chemistry—all of which tell us about climate. Air bubbles trapped in ice provide samples of the atmosphere itself at the time the snow from which the ice is formed accumulated.

Ancient climates—or paleoclimates—can be recorded in other layered deposits, too: those of which stalactites and stalagmites (known collectively as speleothems) are formed, and the growth rings of ancient tree trunks.

The importance of paleoclimate records can be illustrated by a simple example. Scientists studying modern ocean current systems had not considered the possibility that deep-ocean circulation could change until it was shown that circulation patterns during the last ice age, 20,000 years ago, were radically different from those in operation today. Ocean circulation is now known to have a major influence on climate.

Paleoclimate records are, therefore, an essential resource in the quest to unravel the workings of Earth’s ocean–climate system, and the region between New Zealand and Antarctica is a very important archive. The great ice sheets of Antarctica, which contain 29 million cubic km of ice, have been major drivers of Earth’s climate, although just how and why are still poorly understood. New Zealand sits adjacent to the planet’s largest single source of deep, cold oceanic water: the Pacific Deep Western Boundary Current. Carrying a staggering 16 million cu m of water a second past New Zealand’s shores—80 times the flow of the Amazon River in flood—this forms part of the “great ocean conveyor” that plays a major role in the movement of heat around the planet.

The 35-million-year history of this current is preserved in New Zealand’s paleoclimate annals. And for the past 70 million years, New Zealand has sat in glorious isolation in the south-west Pacific, far from the well-studied paleoclimate records of the Northern hemisphere. For this reason it has a unique perspective to contribute to the study of climate research. For example, it remains unclear whether past climate events in the Northern hemisphere have affected southern regions in the same way or at the same time. Both New Zealand and Antarctica have scores of rich paleoclimate records, many of which remain unexamined and might well shed light on this and on other important matters.

Further reading:

Richard B. Alley, 2002. The Two-Mile Time Machine: Ice cores, abrupt climate change, and our future. Princeton University Press, 240 pp.

Mark Maslin, 2004. Global warming: A very short introduction. Oxford University Press, 162 pp.

World Meterological Organization, 2003. Climate into the 21st Century. Cambridge University Press, 240 pp.

New Zealand Paleoclimate website: http://www.paleoclimate.org.nz/

Ministry for the Environment climate change website: http://www.mfe.govt.nz/issues/climate/

National Institute of Water and Atmospheric Research climate website:
http://www.niwa.co.nz/ncc/clivar/

Manaaki Whenua/Landcare Research climate change website:
http://www.landcareresearch.co.nz/research/greenhouse/climate_change.asp

Institute of Geological & Nuclear Sciences paleoclimate website:
http://www.gns.cri.nz/research/programmes.html#global

Intergovernmental Panel on Climate Change website: http://www.ipcc.ch/