Digging up the weather

Climate change lessons from the past

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 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.75_DigginguptheWeather_01

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 thousand cu km 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.

[Chapter Break]

The atmosphere is Earth’s blanket. It insulates the planet from extremes of temperature, shields it from ultraviolet radiation (which is harmful to life), and, along with the oceans, acts as an air-conditioning sys­tem, distributing heat around its surface. The evolution of complex life was intimately bound up with the progressive development of the modern atmosphere, starting some 2.5 billion years ago. So what happens when we abruptly tweak the thermostat on our blanket? When two trillion tonnes of carbon is abruptly (in geological terms) released into the atmosphere?

Exactly this happened 55 million years ago, in an episode of cataclysmic warming known as the Initial Eocene Thermal Maximum (IETM). New Zealand has an outstanding geolog­ical record of the IETM and the only well-exposed rocks in the Pacific deposited in a range of marine environments near the edge of a landmass. This continental margin cross-sec­tion is very important for three reasons. First, it allows us to relate changes on land with those in the deep ocean. Second,computer modelling suggests that, during the IETM, 80 per cent of global oceanic heat transport occurred in the Pacific Ocean. Third, climate models indicate that at this time New Zealand straddled a critical oceanographic boundary—much as it does today—with warm subtropical waters sweeping past northern parts of the country and cool subantarctic waters bathing the south.

For several years now New Zealand and international re­searchers have sought to understand how the IETM affected the New Zealand region. Of course, 55 million years ago New Zealand looked nothing like it does today. It lay further south and was cruising slowly northwards, at a few centimetres per year, courtesy of continental drift, and was very different in shape and form from its contemporary incarnation. The modern Alpine Fault, marking the boundary between the Australia and Pacific Plates, did not exist, so the land hadn’t been scrunched-up by tectonic activity into the great chains of mountains we see today.

At the North Greenland Ice Core Project (NGRIP) site at latitude 75.1degrees N, drilled each summer between 1996 and 2003, neighbours were hundreds of kilometres away and all laboratories were excavations in the ice cap so there was no danger of ice cores melting. With a mean annual temperature of -31.5degrees C, palm trees were sparse and therefore had to be enjoyed when weather permitted.
At the North Greenland Ice Core Project (NGRIP) site at latitude 75.1degrees N, drilled each summer between 1996 and 2003, neighbours were hundreds of kilometres away and all laboratories were excavations in the ice cap so there was no danger of ice cores melting. With a mean annual temperature of -31.5degrees C, palm trees were sparse and therefore had to be enjoyed when weather permitted.

Instead, sediments were being deposited on the sea-bed immediately north of a subdued, relatively tame landscape, in a place that would later be compressed and heaved upwards to form the Kaikoura ranges of coastal Marlborough. Because there were no substantial mountains, there was relatively little erosion, hence little sand and gravel being deposited offshore. Rather, the sea-floor was blanketed with the calcium and silica shells of microscopic algae and animals that flourished in the water. Over countless millennia these shells accumulated to form great thicknesses of fine sediment that slowly compacted and, through the addition of natural cements, hardened into rock. Thus calcium turned into limestone and silica became chert or flint, sedimentary rocks exposed in layers, or strata,in the Clarence Valley, Marlborough.

Along its meandering course the Clarence River has carved a deep valley that runs parallel to, and just south of, a con­torted ridge of Eocene limestone. In their rush to join the Clarence, numerous tributaries have cut deep and spectacular gorges through the limestone, gorges that provide perfect ex­posures through the IETM. The northern gorges reveal stra­ta that were deposited some distance from shore in perhaps 1000–2000 m of water, whereas the southern-most gorges reveal strata that were deposited closer to the shore at a depth of perhaps 200 m. Further south, along the Waipara River of northern Canterbury, are sandy strata that were deposited in just tens of metres of water. Elsewhere, such as in Tawanui Stream, southern Hawke’s Bay, are more muddy strata laid down at intermediate depths, perhaps 500–1000 m. Together, these localities form the ancient continental margin cross-section noted above.

To obtain a clear picture of the IETM as recorded along these waterways, geologists painstakingly measure and ana­lyse the strata, from oldest to youngest, taking fossil and chemical samples as they go. From these samples, estimates can be obtained of ocean temperatures, atmospheric compo­sition, oceanic ecosystem dynamics and the age of the strata. The research is still incomplete but much has already been learned.

During the Eocene, Earth was generally much warmer than it is now: it was a time of “greenhouse” climate. Carbon dioxide levels in the atmosphere were much higher—roughly 10 times higher—than they are today. Such were conditions before the onset of the IETM, which was precipitated by a massive and abrupt release of additional carbon into the environment that ended up as carbon dioxide in the atmosphere, causing the average global temperature to rise by 5–6ºC. This occurred over a period of just 10,000 years—a blink in geological time. As the temperature rose, the ocean currents around New Zealand changed, pushing subtropical waters southwards into subantarctic regions.

Where did all the extra carbon come from? The favoured hypothesis is that it was released from submarine methane ice, or clathrate. Methane ice is found just below the sea-bed over large areas of the deep ocean and beneath the permafrost in polar regions. It is estimated that as much as 10 trillion t of carbon is currently locked up in this form. Methane ice is unstable: slight alterations in water temperature or pressure (caused by changes in sea-level) can cause the release of meth­ane gas. Many scientists think this is what caused the IETM: a gigantic methane burp. According to this model, long-term warming of the oceans reached a critical threshold, at which point there was cataclysmic destabilisation of sea-floor clath­rate in a runaway feedback reaction of methane release caus­ing warming causing further methane release.

Not surprisingly, the IETM had a profound effect on life around the world. In the oceans, about 60 per cent of all bot­tom-dwelling species became extinct. Dramatic changes oc­curred also among surface-dwelling, single-celled algae and animals, which form the first link in the marine food chain. For example, several New Zealand localities record massive blooms of the single-celled alga Apectodinium, which appar­ently multiplied in shallow waters to the exclusion of other species. First detected in New Zealand, these blooms were global events that may have resembled, on a vastly magnified scale, the toxic algal blooms that have occurred locally around New Zealand coastlines in recent summers. In contrast, lo­calities in the Clarence Valley reveal that further offshore the rate of algal growth declined as nutrient-poor, subtropical wa­ters encroached southwards.

On land, the effects of the IETM were equally striking. For example, changes in the composition of sedimentary rocks in New Zealand point to a pulse of erosion due to in­creased rainfall under warm and humid conditions. In the Northern hemisphere, the IETM is linked to abrupt changes in mammal communities, changes that led, eventually, to the ascendancy of modern groups such as the ungulates (hoofed mammals) and primates. In fact, one particularly interesting observation of this climate event is that, although it was itself geologically brief, some effects on animal communities were permanent.

A polarised light image of ice crystals from deep in the Siple Dome core in West Antarctica. Crystals increase in size with depth and can grow to several centimetres.
A polarised light image of ice crystals from deep in the Siple Dome core in West Antarctica. Crystals increase in size with depth and can grow to several centimetres.

Analyses of fossils and environmentally sensitive iso­topes from New Zealand and elsewhere show that, after the methane belch, atmospheric carbon dioxide and global tem­peratures returned to pre-IETM levels within about 100,000 years. Somehow, the ocean–climate system mitigated the impact of the carbon release by locking carbon away in the ocean, soil and sea-floor sediments, although the mechanisms involved remain little understood and of more than passing interest. Natural mitigation mirrors some approaches being considered by scientists today, such as the injection of anthro­pogenic carbon dioxide back into exhausted oil and gas fields.

It is sobering to consider the IETM in the context of modern ocean warming. Although the global climate in the Eocene was so much warmer than the current “icehouse” cli­mate, there is growing evidence of a much more recent release of methane gas when the planet was even cooler than it is to day—during the last ice age. There is still a great deal to learn about this wild card in the climate-change pack.

Following the IETM, Earth’s climate remained warm for another 20 million years. Under these greenhouse condi­tions the planet was largely ice-free and New Zealand enjoyed subtropical climates. Around the end of the Eocene, some 35 million years ago, the global climate began to cool. As it did so, the first Antarctic ice sheets formed, but these were relatively unstable, coming and going in response to regular slight changes in the planet’s orbit. Conditions during this “doubthouse” period of instability were intermediate between the greenhouse and icehouse states. Fifteen million years ago global cooling accelerated, and Earth slid towards the ice ages of the past few million years. The first ice cap formed in the Arctic about 2.6 million years ago.

For many years, scientists thought that initial cooling in Antarctica had been a consequence of continental drift, which had opened up seaways around the continent and thus “ther­mally isolated” it with a circumpolar oceanic current. Such a current does indeed exist (the Antarctic Circumpolar Cur­rent), and acts like the insulation in a fridge, reducing the flow of heat into Antarctica. Nevertheless, evidence from both computer climate models and paleoclimate records now sug­gests that Antarctic cooling predated thermal isolation of the continent. In fact, it seems that the change from a greenhouse to an icehouse climate was related to a very long-term decline in atmospheric carbon dioxide, which was itself a result of changes in the rate of continental drift, of the weathering of rocks (which releases carbon dioxide) and of volcanic activity.

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Sine Sir Edmund Hillary’s exploits in Antarctica, New Zealanders have had a special relationship with the frozen continent, one that is clearly evident in their country’s scientific work there. It is widely recognised that New Zealand “punches above its weight” in the arena of Antarctic research. For example, it has been a pioneer of various scientific drilling projects in Antarctica. Because of this, US and European collaborators are happy for Antarc­tica New Zealand to take control of the logistics for a major new multinational initiative to probe Antarctica’s ancient climate archives, a project dubbed ANDRILL. Due to begin in earnest in 2006, this will entail a consortium of nations drilling at least two holes through Antarctica’s fringing ice shelf and sea ice in the Ross Sea region and deep into the sea­bed to extract sedimentary records of climate change going back, hopefully, 50 million years. New Zealand researchers will also be at the forefront of scientific interpretation of the drill core.

Why is Antarctica important from a paleoclimate point of view? The continent is covered by an ice sheet up to 4 km thick that contains 70 per cent of Earth’s freshwater. Each winter, a sea-ice apron extends to about 60° S, in effect doubling the area of the continent. This is one of the major “pumps” in the great ocean conveyor mentioned above.

As the sea freezes, salt is excluded from the ice and goes towards the formation of very cold, very salty brine. Denser than normal seawater, this sinks to the bottom of the South­ern Ocean, where it circulates Antarctica in the Antarctic Cir­cumpolar Current. A strand of this current, the Pacific Deep Western Boundary Current, peels off and flows northwards along the eastern edge of New Zealand. Another strand, the Humboldt Current, flows up the western side of South Amer­ica and is forced to the surface—or upwells—along the edge of the continent, where it warms and mixes with surface water. From there, as the South Equatorial Current, it flows west­wards along the equator, through the Indian Ocean, round South Africa and, as the Gulf Stream, up into the North At­lantic.

At NGRIP, 3085m of core was drilled, giving a detailed record of climate for the last 123,00 years. Cores were cut into manageable lengths before temporay storage on site, but were destined for permanent storage in frozen core libraries, such as the US National Ice Core Laboratory in Denver.
At NGRIP, 3085m of core was drilled, giving a detailed record of climate for the last 123,00 years. Cores were cut into manageable lengths before temporay storage on site, but were destined for permanent storage in frozen core libraries, such as the US National Ice Core Laboratory in Denver.

Similar pumps operate around Greenland, re-cooling the water and causing it to sink before pushing it back down the Atlantic towards the Antarctic.

The importance of the great ocean conveyor in transfer­ring heat can be gauged from the fact that the Gulf Stream, which bathes north-western Europe in warm water, delivers 27,000 times the energy of all Britain’s power stations com­bined. The Stream is the reason why Europe is habitable and why it is possible to enjoy a side-walk coffee in Oslo at 60° N.

The conveyor’s northern pumps have been well studied, and many ideas about Earth’s changing climate are based on de­tailed data from the highly populated Northern hemisphere. Only in the last decade have scientists realised that, to under­stand climate fully, they need to devote much more effort to studying the other end of the globe. The central role the Ant­arctic ice sheets have played in global climate change over the past 35 million years has only recently been appreciated.

Two issues are of particular interest. First, there is increas­ing evidence that, during periods of past climate change, the Southern and Northern hemispheres have behaved in rather different ways. It seems that some changes, while manifestly abrupt in the north, may have been ameliorated and/or oc­curred out of step in the south. This “interhemispheric see­saw” has been revealed by comparing paleotemperature records preserved in ice cores from deep within the Green­land and Antarctic ice caps (see figure page 78), which also pro­vide details of past atmospheric composition.

The longest ice core from the Antarctic, the European Project for Ice Coring in Antarctica (EPICA) core, was ex­tracted near the French station of Concordia, the drill pen­etrating to a depth of over 3000 m. It is a record that extends back more than 800,000 years, through the last eight major ice ages, or glacial periods, and the intervening warm inter­glacial periods. These 100,000-year glacial–interglacial fluc­tuations were caused by regular wobbles in Earth’s orbit, known as Milankovitch Cycles, which result in small but sig­nificant changes in the amount and distribution of solar heat the planet receives.

At the glacial–interglacial scale, climate changes in the Northern and Southern hemispheres were essentially syn­chronous. However, closer inspection reveals that, as the world warmed up following the last major glaciation, there was a 2500-year-long cool spell in Antarctica between 14,500 and 12,000 years ago that wasn’t matched in the Northern hemisphere. Rather, Greenland was relatively warm until 12,900 years ago, then cooled abruptly once the Antarctic had started warming again (this cooling event is described in more detail below). Subsequently, Greenland warmed rapidly as the global climate adjusted fully to the present interglacial phase.

This see saw effect is poorly understood, but it seems that pulses of freshwater caused by partial melting of ice sheets in one hemisphere inhibit oceanic circulation and heat transport in that hemisphere, causing cooling, while invigorating circu­lation and heat transport, and consequently causing warming, in the other hemisphere. Whatever the cause of this process, it is clear that we cannot expect future global warming to be uniform around the globe: it is quite possible that warming in one part of the globe will precipitate cooling in another. The only way to understand the dynamics involved is to con­tinue probing paleoclimate records in Antarctica and New Zealand.

The second key issue is the long-term stability of the Ant­arctic ice sheet and implications for future sea-level rise. If all the ice on Antarctica melted, it would raise sea level by 65 m. This is a remote possibility only: the ice sheet has existed in roughly its present form for about 15 million years, and it would take many hundreds, even thousands, of years to melt entirely. Few people believe the whole ice sheet is in jeopardy, at least over the next few hundred years.

At sea, the Joides Resolution recovers cores from sea floor sedimentary rocks. Aboard, they are split, and one half used for scientific analysis immediately, while the other is filed in a core library in Texas.
At sea, the Joides Resolution recovers cores from sea floor sedimentary rocks. Aboard, they are split, and one half used for scientific analysis immediately, while the other is filed in a core library in Texas.

During the doubthouse period, however, between 35 and 15 million years ago, the ice sheet fluctuated in volume by up to 80 per cent, as evidenced by drill cores of sedimentary rock extracted from beneath the floor of the Ross Sea. These cores were extracted in the 1990s by the multinational Cape Rob­erts Project using a drill rig parked on the floating sea ice and supported, in part, by giant balloons inflated beneath the ice.

Cape Roberts cores reveal evidence of repeated cycles of glacial advance into the sea and across the continental shelf in response to cooling, and subsequent glacial retreat during warming. Each advance is marked in the cores by a break in sedimentation, where layers of sea-floor mud and sand were bulldozed by the glaciers as they ground their way seawards. The retreats are represented in the cores by layers of chaotic gravel—sediment that was pushed in front of the glaciers and then left behind as they retreated. Each advance–retreat epi­sode took about 41,000 years and was a consequence of a Mi­lankovitch Cycle.

Importantly, the Cape Roberts cores proved that changes in Antarctic ice volume were responsible for sea-level fluctua­tions that had been inferred from other lines of evidence gath­ered elsewhere on the globe.

What is the relevance of these doubthouse changes in the volume of Antarctic ice? We know that before 15 million years ago the globe was still much warmer than it is today, so perhaps it can be argued that they are of little significance in the modern context. But—and this is important—projec­tions of future global warming suggest that within the next 100–200 years the average global temperature will rise to be the same as during the doubthouse period. Just how quickly the Antarctic ice sheet might react to such a development is anyone’s guess.

The Antarctic ice sheet is divided into two parts by the Transantarctic Mountains: the large East Antarctic Ice Sheet, containing 85 per cent of the total (26 million cu km of ice), and the much smaller West Antarctic Ice Sheet (3 million cu km). Because of its smaller size and, therefore, lower inertia,the West Antarctic Ice Sheet is likely to be the less stable of the two. If it melted entirely, global sea level would rise by 5 m.

Disturbingly, it has been found that the flow rates of some glaciers that drain this ice sheet have accelerated dramatical­ly in the past few years. These glaciers are now discharging over 60 per cent more ice into the sea than they accumulate in their catchments—250 cu km per year. This acceleration has followed the well-publicised break-up of large segments of coastal ice shelf, which appear to have buttressed the glaciers, acting as dams and slowing glacial discharge into the sea.

These observations are exactly what would be expected if the West Antarctic Ice Sheet had begun to collapse. So, while we monitor the behaviour of Antarctic glaciers and ice shelves, we must continue to explore paleoclimate archives that can provide a historical record of the speed and frequency of ice-sheet collapse. This is, in part, the motivation behind AN-DRILL. In particular, ANDRILL will explore the behaviour of the largest ice shelf, the Ross Ice Shelf, during past warm periods. By relating records of previous ice advance and re­treat to changes in global temperature, we should be better able to predict the effects of future global warming on the Antarctic ice sheets.

[Chapter Break]

Many paleoclimate archives document changes that took place over hundreds of thousands, even millions, of years, and thus appear irrelevant to us, destined as we are to live for no more than a few dec­ades. But is there evidence that significant changes can oc­cur within a human lifespan or less? There is growing con­cern that climatic response to increasing levels of carbon dioxide may be non-linear, i.e. that progressive small in­creases may result, at some critical threshold, in an abrupt and major change in global climate—a flip of the switch, as it were, as in the case of the IETM. Concern about such “climate surprises” has been widespread only in the past few years and represents something of a paradigm shift for climate scientists and politicians alike.

Given the time resolution required for accuracy, the best paleoclimate records for studying abrupt climate changes are those from the last one or two million years—the Quaternary epoch, in geological parlance. New Zealand has some excep­tional Quaternary paleoclimate records that support and ex­tend findings on the subject from elsewhere. Such windows into the past are significant for three reasons.

First, several well-dated terrestrial records from bog and lake deposits can be calibrated with and corroborate evidence from deep-sea cores from the adjacent ocean floor. Because of New Zealand’s small size and maritime climate, the two sets of data correlate well as regards timing, duration and struc­ture. Second, New Zealand’s remote location in the south­west Pacific means that records of changes in the ocean–cli­mate system are free of the complexities associated with large continental areas. Third, New Zealand’s small size and lack of a large ice sheet during the last ice age mean that climatic change at that time resulted in rapid changes to flora and fau­na—changes that are preserved in local paleoclimate records.

In Mead Stream, Clarence Valley, Marlborough, layers of white limestone deposited during the Eocene Epoch about 55 million years ago reveal details about a catastrophic and short-lived warming of the climate.
In Mead Stream, Clarence Valley, Marlborough, layers of white limestone deposited during the Eocene Epoch about 55 million years ago reveal details about a catastrophic and short-lived warming of the climate.

The best-studied climate surprise is a mini-ice age known as the Younger Dryas (named after a European alpine wild­flower). This occurred between 12,900 and 11,500 years ago, as the world was gradually warming out of the last major ice age (which peaked about 20,000 years ago).

The start of the Younger Dryas was marked by a sud­den cooling over perhaps as little as a decade or so. Across Greenland, from where ice cores have revealed much about the event, the temperature fell an average 15° C. In England it fell about 5° C. At the end of the Younger Dryas, there was an abrupt warming of 5–10° C over about a decade—or, ac­cording to some well-respected paleoclimatologists, over as little as three years.

To put this in perspective, 5° C is the difference between the mean average temperatures in Invercargill (10° C) and Kaitaia (15° C). Such a change of temperature would have profound impacts on modern society and agriculture.

Evidence suggests that the effects of the Younger Dryas were less severe in the Southern hemisphere than in the Northern—a consequence of the interhemispheric seesaw and the dampening effect of the vast southern oceans on such abrupt climatic swings. But there have been many other sur­prises for the antipodes, including substantial cooling events, although we still have a poor understanding of most of these and cannot yet resolve the timing of them in great detail.

Much of the evidence of these events comes from ma­rine records around New Zealand. For example, a drill core taken from the sea-bed 1100 km east of New Zealand, at Ocean Drilling Project Site (ODP) 1123 (see New Zealand Geographic, Issue 41), has provided an amazing succession of fossil foraminifera (see “Aquatic Microfossils” in sidebar). The drill penetrated a huge deposit of sediment eroded from the Southern Alps, transported to the coast by rivers and eventu­ally deposited on the sea-floor by the Pacific Deep Western Boundary Current as it flowed northwards and slowed around the end of the Chatham Rise.

By comparing assemblages of foraminifera from ODP 1123 with communities living in the sea today, and by using a bit of computer cleverness known as neural network modelling, it is possible to reconstruct the sea temperatures around New Zealand over the past 1.2 million years. The result is a re­markable timetable of geologically rapid glacial–interglacial fluctuations in sea-surface temperature of up to 6° C.

Given New Zealand’s maritime climate, these fluctuations translate directly into onshore climatic changes, as shown by corresponding fluctuations in pollen counts in bog and lake deposits. Thus, during the cold spells, when glaciers advanced across large parts of the South Island, the flora was dominated by southern-beech forests and species such as celery pine, bog pine and coprosmas. During the warm spells, glaciers retreat­ed to the highest mountains and the forests were dominated by rimu, totara and matai. These changes in flora point to a temperature fluctuation of about 5° C, closely mirroring the marine record.

Working on a much finer scale than in the case of these fluctuations or the Younger Dryas, New Zealand paleocli­mate scientists have been looking at abrupt changes in cli­mate related to the phenomenon known as El Niño/Southern Oscillation (ENSO). This is the most pronounced natural climatic fluctuation on a one-to-ten-year timescale to have been observed anywhere on the planet. Although driven by variations in temperature and pressure in the tropical Pacific, ENSO affects the climate in New Zealand and in many other parts of the world.

El Niño and its converse, La Niña, represent end states in the cycle. In El Niño summers, western New Zealand expe­riences average or higher-than-average rainfall and westerly winds of above-average strength, while eastern regions suffer drought. In contrast, La Niña delivers higher-than-average temperatures, relatively common easterly to north-easterly winds, and greater exposure to tropical cyclones.

High-resolution paleoclimate records retrieved from east­ern parts of the North Island—from the Waipaoa flood plain, near Gisborne, and the sea-floor east of this—tell the history of ENSO fluctuations. Changes in the thickness and frequen­cy of relatively coarse storm-derived sediment layers docu­ment an increase in storminess about 4000 years ago. This in turn indicates an intensification of atmospheric circulation and the establishment of the contemporary climate, which is strongly ENSO-influenced.

In addition, a 2250-year-long storm history has been iden­tified in high-resolution sediment records from Lake Tu­tira, created by a landslide dam, north of Napier. Layers of sediment representing the products of individual storms are clearly visible in drill cores and have been attributed to vio­lent La Niña weather events that may have been similar to the devastating Cyclone Bola of March 1988. A recent, but as yet unstudied, core from Lake Tutira has extended the sediment record back to the time of the lake’s formation 6500 years ago, and should enable a direct comparison with the Waipaoa flood plain and offshore marine record.

Many important paleoclimate records in New Zealand re­main to be studied—in Tasman Glacier, for example. Here, scientists are drilling relatively short, but still potentially val­uable, ice cores. An initial core 53 m long has been taken, and longer ones are planned that will, it is hoped, detect ENSO patterns going back perhaps 200–300 years. These will be compared with tree-ring records and recent instrumental re­cordings from meteorological stations in the Southern Alps.

In an exciting development, reconnaissance work has demonstrated that Lake Pukaki, in the central South Island, con­tains a rich sedimentary archive that could provide one of the best terrestrial mid-latitude records of past climate change in the Southern hemisphere. It extends back 17,000 years so should detail the climate changes that have affected New Zealand since the last major glacia­tion. In particular, it should provide an unbroken record of rainfall and wind patterns in the Pukaki catch­ment, which could be used to assess the possible frequency, scale and magnitude of future droughts—of considerable importance, given that the enclosing Waitaki catchment produces almost 30 per cent of New Zealand’s electricity.

Interest in these and other geologically young records has resulted recently in the creation of a collaborative project in­volving scientists from many institutions that goes by the cu­rious acronym NZ-INTIMATE (New Zealand INTegration of Ice core, Marine And TerrEstrial records—who says sci­entists don’t have fun?). This is closely aligned with interna­tional programmes and aims to establish detailed knowledge of the nature, timing and extent, both regional and global, of climatic and environmental changes since the end of the last glaciation, 20,000 years ago.

What might New Zealand expect in the coming century? Current predictions suggest that within the next 80–100 years, the country will be subjected to a 30–50 cm rise in sea level and a 2–3º C increase in average temperature. Associ­ated with these developments, New Zealand may experience a moderate strengthening of El Niño weather patterns, con­tinuing the trend of the past 30 years. This will increase rain­fall in the west and drought in the east. It is also predicted that New Zealand will experience more extreme rainfall events and more heat waves.

These potential climate changes present both opportuni­ties and risks, issues that are discussed at length in a docu­ment produced in 2004 by the Ministry for the Environment and the New Zealand Climate Change Office entitled Climate Change Effects and Impacts Assessment. It may not be the catchi­est of titles, but if the nation is to be prepared for what lies ahead, it would be well advised read and pay heed.

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