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The nature of New Zealand's atmosphere and climate

New Zealand's atmosphere is most tangible to us in the air we breathe and in the weather patterns we experience near the ground-the sunshine, wind, cloud, and rain. But this is a highly selective view. It only applies to the lowest reaches of the atmosphere, a zone of just a few kilometres where oxygen is dense and water vapour is abundant. The atmosphere actually extends for more than 1,000 kilometres out into space, gradually becoming thinner and thinner until it merges with the solar wind. It is layered into a series of discrete zones where gases of different densities tend to cluster (See Figure 5.3).

Two zones of prime importance to life on Earth are the troposphere and the stratosphere. Together they wrap the Earth in a protective cloak less than 80 km thick. Without them, the Earth would be a sterile ball of ice and rock bombarded by killer radiation. These zones contain tiny amounts of trace gases which have a powerful effect on the amount of solar radiation reaching the Earth and the amount of heat leaving it. These, in turn, influence ocean currents, climate patterns, plant growth and the health of humans and other animals.

The troposphere is the atmospheric zone that is closest to Earth. It is the zone in which our weather occurs and where atmospheric gases are densest. It is bun-shaped, extending out about 18 kilometres at the equator and about 6 kilometres at the poles. Above New Zealand, it is about 12 kilometres thick. Within it, greenhouse gases form an insulating blanket and contribute to the Earth's climate patterns. Outside it, the stratosphere forms a larger protective shell which rises to about 50 kilometres and has much lower concentrations of gases. The stratosphere's main significance lies in the fact that it contains the ozone layer-Earth's thin but effective bullet-proof vest which shields us from harmful solar radiation.

Figure 5.3: Layers of the atmosphere, and ozone concentrations measured over Lauder (Central Otago) during Spring and Autumn.

Ozone concentrations in the layers of the atmosphere - Troposphere, Stratosphere (Ozone layer), Mesosphere - measured over Lauder (Central Otago) during spring and autumn. Ozone concentrations are greatest in the Stratosphere and are higher in spring than they are in autumn.

Profiles up to 35 km have been plotted from NIWA balloon sonde data and extrapolated above that point from data from other sources.

Box 5.1: Out of thin air - the evolution of life and the atmosphere

Clues to the history of our atmosphere are scattered throughout the Earth's crust, in fossil and mineral deposits, in ancient ice layers, in the air around us, and even on neighbouring planets. Scientists are ingeniously sifting these clues in an effort to understand how the atmosphere has changed and how it might change in the future. The picture is becoming clearer every year, though much remains tentative, theoretical, and hotly debated, particularly for the earliest periods. However, there is broad agreement on the following scenario. Earth's original atmosphere of hydrogen and helium was stripped away shortly after the planet's birth 4.6 billion years ago. A secondary atmosphere arose from volcanic eruptions. It was dominated by carbon dioxide and prevailed for some 2-3 billion years. Today's atmosphere was created within the last 1-2 billion years by photosynthesising organisms. It is characterised by abundant nitrogen and free oxygen, an ozone layer, and comparatively low levels of carbon dioxide which maintain a moderate greenhouse effect that keeps Earth about 33°C warmer than it would be if it had no atmosphere.

At first, the Earth was all atmosphere and nothing else. The nine planets of the Solar System started out 4.6 billion years ago as a vast cloud (or nebula) of particles and gases orbiting the Sun (Harper and Jacobsen, 1996; Hecht, 1996; Hunten, 1993). This cloud, like the Sun itself, was composed mostly of the two lightest elements in the Universe-hydrogen and helium-with a small sprinkling of the 91 other natural elements (most notably, carbon, nitrogen, oxygen, iron and sulphur). Within the first 10 million years or so, these gases and particles condensed into hundreds of spinning lumps which, in turn, aggregated into planets. The four inner planets became small compact balls of dense rock and ice surrounded by hydrogen and helium gas (Mercury, Venus, Earth, and Mars). The five outer planets, formed much larger spheres known as gas giants (Jupiter, Saturn, Uranus, Neptune and Pluto).

In those early days, the Sun radiated about 30 percent less heat than it does today. However, for a relatively brief period of several million years the faint young Sun became a raging fireball which blasted vast amounts of ultraviolet radiation and extreme solar winds into space. This solar holocaust, which scientists call the Sun's T-Tauri phase, stripped the inner planets of their gas shrouds. In its wake came a sustained series of asteroid storms that rained on the planets for 700 million years, from around 4.5 to 3.8 billion years ago. Our Moon is believed to have formed at this time from an impact between Earth and a Mars-sized asteroid. Smaller asteroids with diameters as large as 100 kilometres were commonplace. Their impacts still scar the waterless surfaces of Mars and the Moon.

Volcanoes were also more active on the young planets than they are today. Soon new atmospheres were forming from the erupting clouds of volcanic steam and gas. These 'steam atmospheres' were dominated by water vapour (H2O) and carbon dioxide (CO2), but also included nitrogen (N2), carbon monoxide (CO), hydrogen sulphide (H2S), and a number of trace gases (Kasting, 1993; de Duve, 1995). Whenever temperatures dropped below 100°C on Earth or Mars the steam condensed into rain, and oceans formed - only to be vaporised by the next asteroid impact. When the asteroid storms finally abated, most of the atmosphere on Mars had been blasted into deep space, well beyond the pull of the planet's weak gravity. With the faint sunlight and very little greenhouse gas to trap heat, temperatures on Mars fell to below freezing point, and its remaining water turned to ice.

In contrast, the atmosphere on Venus survived the asteroids but was so dense that a runaway greenhouse effect took over, keeping temperatures forever above boiling point. The planet's water vapour was driven outward and upward by the intense heat until eventually the water molecules were split by solar radiation and dissipated into space. Only on Earth did temperatures settle at a level that allowed permanent oceans to form-about 85°C initially (Kasting, 1993). Apart from water, the atmospheres of all three planets had a fairly standard assortment of gases: about 95 percent carbon dioxide, 3 percent nitrogen, and a sprinkling of trace gases. On Venus and Mars, it is still like that. But on Earth, all that was to change.

Because it is further from the Sun, Earth was spared the runaway greenhouse effect that enveloped Venus. However, in the first few hundred million years, its dense concentrations of carbon dioxide and carbon monoxide may have produced an atmospheric pressure as high has 11,000 millibars (or hectopascals), compared to today's 1,000, and an average temperature of around 85°C (Kasting, 1993). When the first bacteria appeared more than 3.5 billion years ago, the atmosphere was probably still heavier and warmer than it is now. Earth's ancient atmosphere still had no free oxygen (O2) and no ozone layer (O3), and populations of bacteria were confined to the radiation-safe zone on the sea-floor, several metres below the surface (de Duve, 1995). The high carbon dioxide levels, perhaps 300 times current levels, would have ensured a mild greenhouse climate during this era of faint sunlight.

The atmosphere did not start changing until one group of bacteria (the cyanobacteria) developed the ability to 'eat' sunlight and carbon dioxide in the process called photosynthesis (see Chapter 9). In effect, they used the Sun's energy to separate the carbon from the oxygen. While the carbon was 'welded' into big useful molecules, energy-rich carbohydrates (e.g. oils, starches, sugars), the unwanted oxygen was simply released to the environment. In fact, the free oxygen was not merely unwanted, it was toxic. Being so chemically reactive, it was a danger inside living cells, and still is-a fact now borne out by medical researchers who are finding that oxidation of body tissues by molecules called free radicals (OH) is related to ageing, heart disease and cancer, while antioxidants (e.g. vitamins E and C) appear to delay these processes (Nesse and Williams, 1995).

It is not known when photosynthesis started. Although fossil bacteria resembling cyanobacteria are known from nearly 3.5 billion years ago (Schopf, 1993), recent genetic research suggests that cyanobacteria actually arose only about 1.5 billion years ago (Doolittle et al., 1996; Morell, 1996). Geological evidence from iron deposits and ancient soils indicates that free oxygen started coupling up with other substances to form oxides around 2 billion years ago, and that carbon dioxide began to be extracted from the air and buried in sediments with dead organisms about 2.5 billion years ago (Kasting, 1993). On this basis we can only say that photosynthesis probably began between 1.5 and 2.5 billion years ago.

As oxygen accumulated in the atmosphere, its uppermost layer was bombarded by ultraviolet radiation which transformed a small but significant proportion of the oxygen molecules (O2) into ozone (O3). By perhaps 1.5 billion years ago total oxygen (O2 plus O3) still made up less than 1 percent of the atmosphere's volume - well below today's 21 percentbut this was sufficient to form an effective ozone layer which allowed life to come out of the shadows and evolve into more genetically complex forms (Kasting, 1993). An entire new group of hybrid organisms now began to appear: the algae. Part protozoan and part cyanobacteria, algae became floating oxygen factories on the ocean surface. Their vast blooms caused global oxygen levels to surge upward as CO2 fell (Kasting, 1993). Between 1 billion and 600 million years ago, there was a pronounced oxygen increase and the stage was set for oxygen breathing animals to evolve (Canfield and Teske, 1996; Knoll, 1996). This momentous stage was reached only in the last 12-20 percent of Earth time.

By the time the world's earliest animals began to proliferate in the sea 530-570 million years ago (an event known as the Cambrian Explosion) oxygen probably comprised some 15 percent of the atmosphere (Graham et al., 1995). A further oxygen boost came about 70 million years later, when green algae colonised the land and gave rise to the plant kingdom. Oxygen levels soared even more when trees appeared about 100 million years after the first plants. The trees could make a new kind of carbohydrate-wood-and on such a scale that, by 300 million years ago, forests were everywhere and oxygen made up 35 percent of the atmosphere, an all-time high (Graham et al., 1995). At the same time, carbon dioxide levels had plummeted to an all-time low as the trees voraciously sucked the carbon from the air. Over tens of million of years, the CO2 had been reduced from concentrations perhaps ten times higher than today, to levels below ours (Appenzeller, 1993a; Kaiser, 1996). This appears to have coincided with a shift in the Earth's orbit and axis, plunging the planet into an ice age.

Then came a spectacular, and drastic, atmospheric reversal. Continuous volcanic eruptions clouded the skies above the supercontinent of Pangea, CO2 levels rose, and oxygen began to decline (Renne et al., 1995). The ocean became anoxic and stagnant (Wignall and Twitchett, 1996). A vast upwelling of CO2 from layers of dead sea organisms belched from the sea depths, killing marine life and adding to the clouds of greenhouse gas (Knoll et al., 1996). By 250 million years ago, oxygen levels had fallen by two-thirds (Graham et al., 1995) and life was nearly annihilated in Earth's greatest ever mass extinction (Kerr, 1995d and 1995e). From the ashes eventually arose the dinosaurs whose reign lasted until the next great extinction, 65 million years ago. Atmospheric oxygen levels slowly climbed back from their 12 percent low, and have fluctuated between 15 percent and 25 percent throughout the past 200 million years. Today's level of 21 percent is part of a small declining trend that began 50 million years ago (Graham et al., 1995).

Today's CO2 levels are also part of a declining trend that began about 100 million years ago (Appenzeller, 1993a). In the past few million years, these levels have stabilised at around 200 parts per million during ice ages, and about 280 ppm in the warm interglacial periods between. Earth has been in an interglacial period for the past 12,000 years or so, with CO2 levels remaining at around 280 ppm-until the present century. While some scientists have taken comfort from the fact that ancient carbon dioxide levels were once much higher than they are now without any apparent devastating effects (Emsley, 1994), others note that the Sun was weaker for much of that time and also that the speed of atmospheric change was much slower, giving living things much more time to adapt. While much remains to be learned, it is clear that our oxygenated atmosphere was a late development in Earth's history, built by living things whose fate is now closely tied to its state. Awareness of this fact is what underlies the current concern about rapid atmospheric change.

Box 5.2: The natural formation and destruction of ozone

The Natural Formation of Ozone

1 O2 + UV light in a reaction changes to O + O

2 O + O2 + M in a reaction changes to O3 (ozone) + M

M = any other molecule.
(The reaction in step 2 will not proceed without a third molecule.)

The net result from the 2 steps is that two oxygen molecules are converted to an ozone molecule and an oxygen atom.

3 O2 + O 2in a reaction changes to O3 (ozone) + O

Oxygen is a versatile atom which can combine with many others to form a bewildering variety of different molecules. When oxygen atoms combine with each other though, they take only two forms: 'molecular' oxygen, which has two atoms (O2); or ozone, which has three (O3). When either of these is broken apart, the separated oxygen atoms quickly latch onto the first things they meet, usually other oxygen atoms, to form new molecules. In some cases, however, they join up with non-oxygens to form oxides or other compounds.

The Natural Destruction of Ozone

1 O3 (ozone) + UV light in a reaction changes to O + O2

2 O3 (ozone) + O in a reaction changes to O2 + O2

OR

1 O3 (ozone) + UV light in a reaction changes to O + O2

2 O3 (ozone) + Y in a reaction changes to YO + O2

3 YO + O in a reaction changes to Y + O2

Y = atoms and molecules that can combine with oxygen, such as chlorine atoms (Cl), nitric oxides (NO), or hydroxyl radicals (OH).

The result from each of these processes is that an ozone molecule (O3) is converted into common oxygen (O2). In the second process, however, the destructive 'Y' molecule is set free to break down more ozone.

In the stratosphere, a delicate but stable balance exists between the destruction and formation of ozone. The Sun's ultraviolet rays constantly smash the oxygen and ozone molecules into separate atoms which then career wildly into each other. If a free atom happens to join another free atom, molecular oxygen is formed (O+O=O2). If the free atom collides with molecular oxygen, the duo becomes a trio and a new ozone molecule is born (O + O2 = O3). However, if the free atom hits an ozone molecule, the ozone's third atom breaks off to join it and two molecular oxygens (O + O3 = O 2 + O2) result.

As the concentration of chlorine (Cl) increases through the breakdown of chlorofluorocarbons, the natural balance is upset. Chlorine atoms are extremely effective at snatching oxygen atoms away from ozone molecules and then losing them to free oxygen atoms which couple up to form molecular oxygen rather than ozone (see Box 5.4). The net result is that ozone is now being destroyed at a faster rate than it is being created (QBO).

Ozone and the ozone layer

Ozone is a pale blue gas which is toxic to humans and other animals, yet crucial to our existence. Ninety percent of it is concentrated in the stratosphere where it is under constant bombardment from the sun's radiation. Fortunately, by being such an easy target for the damaging ultraviolet-B (UV-B) rays, it stops many of them reaching Earth. However, at ground level, ozone is not so benign. It still absorbs ultraviolet light, but it is also a pollutant, being a key ingredient of big city smog (see Chapter 6).

In the stratosphere, ozone is constantly being broken down and reconstituted by the incessant radiation barrage (see Box 5.2). Under natural conditions it can sustain this pressure indefinitely, but, in recent years, it has also had to cope with a new enemy attacking from belowhuman-made chemicals, such as CFCs and halons. When these chemicals are hit by radiation, they lose chlorine and bromine atoms which dart wildly through the stratosphere, destroying ozone molecules. The main battle zone is in the 'ozone layer' at an altitude of 15-25 kilometres above the poles and 25-35 kilometres above the equator. This is where the ozone molecules are most densely concentrated (see Figure 5.3).

Even here, though, ozone is still a relatively rare commodity whose natural density never exceeds about 10 parts per million (ppm). In fact, typical ozone levels around the world range from 230 to 500 Dobson Units (DU), with a world average of about 300 DU. If all this ozone were brought down to sea level, it would automatically compress into a very thin gas layer of only 3 millimetres at standard pressure and temperature, one millimetre for every 100 DUabout the same thickness as an average pane of glass. This is the 'thin blue line of defence' that stands between us and the Sun.

The amount of ozone over any one place varies considerably in response to stratospheric winds. These fluctuate from day to day, week to week, and season to season. They also vary on a two-yearly cycle called the Quasi-Biennial Oscillation (QBO) in response to regular global circulation patterns. On average, the total ozone concentrations over New Zealand are highest in late winter and spring, when they often exceed 350 DU, and lowest in late summer and autumn when they fall well below 300 DU. The season of thinnest ozone also corresponds to New Zealand's season of skimpiest clothing. (See Figure 5.4).

Figure 5.4: New Zealand total ozone, 1965-95

The total level of Ozone over New Zealand has fluctuated seasonally, with the highest concentrations being in late winter and spring and the lowest in late summer and autumn.

Source: Nichol and Coulmann (1990); NIWA unpublished data

Solar ultraviolet (UV) radiation

The Sun is a natural nuclear reactor emitting radioactive rays into space. This radiation travels to Earth in waves of varying length. The rays fall into three broad bands or spectra: long wavelength infrared radiation; medium wavelength visible light, and short wavelength ultraviolet light. Infrared radiation is sensed by us as heat. It is strongly absorbed by the atmosphere and, without it, Earth would freeze. Visible light passes virtually unimpeded through the Earth's atmosphere until it encounters clouds or solid objects. In some cases, the light is absorbed by these objects and converted to heat energy. In others it bounces back. Different objects absorb specific wavelengths of light and reflect others. Much of this reflected light is visible to us as colours.

It is the radiation with the shortest wavelengths-ultraviolet (UV) radiation-that has captured the headlines over the past decade. Depletion of the ozone layer allows more of this radiation to reach Earth with possible harmful impacts on plants, animals and human health (see Box 5.6). Ultraviolet radiation spans wavelengths from 200 nanometres (nm) to 400 nm and is of three types: UV-A (320-400 nm); UV-B (290-320 nm); and UV-C (200-290 nm).

UV-A radiation is relatively harmless and reaches the Earth largely unobstructed by the ozone layer. UV-B radiation can be more of a problem. Although the ozone layer filters out most of it, UV-B can cause eye and skin damage to humans and other animals and can retard plant and algal growth. On the positive side, it is used in water treatment to kill bacteria. UV-C radiation is lethal to plants and animals but, fortunately, oxygen and ozone almost completely remove it from the atmosphere.

Solar radiation in New Zealand is 50 percent more intense than at comparable latitudes in Europe. This is partly because we are closer to the Sun during summer and partly because we have less air pollution and so less tropospheric ozone. Besides being more intense, direct sunshine is also more frequent in many parts of New Zealand than at comparable European latitudes.

Most of our sunlight shines on the east and north of both islands (Statistics New Zealand, 1996). The highest sunshine levels in the South Island are in Nelson-Marlborough and the inland Mackenzie country (averaging around 2,300 hours per year, or about half the daylight hours). In the North Island, Hawke's Bay, Gisborne, the Bay of Plenty and New Plymouth have high amounts of sunshine, averaging about 2,200 hours. In contrast, the cloudiest areas, averaging less than 1,800 hours of sunshine are in Southland, Otago, the Southern Alps and the North Island's central plateau. Clouds can both block solar radiation and reflect it. Continuous cloud layers may reduce solar radiation by as much as half in the course of a day, but patchy clouds on sunny days can actually intensify radiation exposure by reflecting escaping light back down to land and water surfaces.

Figure 5.5: Average annual sunshine hours in New Zealand.

Average annual sunshine hours in New Zealand:

  • Kaitaia 2065 hours (sunny)
  • Auckland 2070 hours (sunny)
  • Tauranga 2225 hours (very sunny)
  • New Plymouth 2175 hours (sunny)
  • Gisborne and Napier 2185 hours (sunny)
  • Palmerston North 1725 hours (cloudy)
  • Wellington 2025 hours (sunny)
  • Nelson 2370 hours (very sunny)
  • Blenheim 2470 hours (very sunny)
  • Hokitika 1835 hours (average)
  • Christchurch 2065 hours (sunny)
  • Tekapo 2225 hours (very sunny)
  • Queenstown 1885 hours (average)
  • Dunedin 1600 hours (very cloudy)
  • Invercargill 1580 hours (very cloudy)

The sunniest areas are Bay of Plenty, Gisborne, Hawkes Bay, Blenheim, Nelson.

Source: NIWA and Statistics New Zealand (1996)

New Zealand's climate

The influence of solar radiation on New Zealand is not just confined to the amount of sunshine and cloud directly overhead. The Sun's energy drives the Earth's climate patterns, causing the fluctuations which give us our daily weather. It does this by heating the air, water and land each day, causing temperatures to rise and fall and air and water pressure to change. Changes in temperature affect evaporation rates and hence cloud formation and rainfall patterns. Variations in air and water pressure create the wind and ocean currents which redistribute heat and moisture around the planet.

Most of the sunlight that stirs up Earth's climate lands in the tropics. From there, its heat is dispersed on winds and ocean currents toward the poles. These currents form weather belts around the globe.

New Zealand, with its elongated shape and north-south axis, straddles two of these (Salinger, 1988). The north of the country protrudes into the subtropical belt of anticyclones, receiving relatively dry settled weather with average temperatures of around 16°C. Its coastline is bathed with warm subtropical currents.

The south of the country is washed by cold sub-polar waters and lies in the path of the westerly wind belt. These winds travel over vast areas of ocean accumulating large amounts of moisture which is released as rain when the clouds roll over the high country. A series of westerly-flowing anticyclones and low pressure troughs give an alternating cycle of settled and unsettled weather, each lasting a few days. Average temperatures in the south are around 10°C.

Occasionally the westerly pattern breaks down with southerly 'cold snaps' bringing snow to low levels in winter-and sometimes spring-or tropical depressions moving down from the north bringing warm, moist air into the New Zealand region. The surrounding ocean means that New Zealand generally has a 'marine' climate, except in central Otago where the climate is more 'continental' with hot, dry summers and cold, dry winters.

Besides the north-south climate division, there is also an east-west division, defined by the long mountain ranges which run up the centre of the islands. These intercept the prevailing westerly winds, causing them to release more rain in the west and south than in the east and north. This division is particularly marked in the South Island where the mountains are higher. Temperature extremes also tend to occur in the drier areas east of the main ranges. The result is a tendency towards drought, or severe dry spells, in the east and a widespread vulnerability to flash floods (see Chapter 7).

Evidence from sediments, pollen and ice suggests that, for the past 14,000 years, New Zealand's climate has fluctuated between 10°C and 14°C (Salinger, 1988). Prior to that, the world was in a long Ice Age, during which New Zealand's average temperature is estimated to have been about 7°C. That Ice Age was the latest of about a dozen which have occurred over the past 1.2 million years. Each has lasted about 100,000 years, punctuated by a 10-20,000 year warmer interglacial period.

The current interglacial period, the Holocene, started about 14,000 years ago in New Zealand and a little later in the Northern Hemisphere. Temperatures doubled over about 4,000 years to reach a peak average of 14°C by 10,000 years ago (see Figure 5.6). This warmer climate was mild, with light winds and lush forests. However, temperatures fell sharply to average under 11°C about 5,000 years ago before climbing back up to where it is today. By 3,000 years ago, New Zealand's modern climate was established. Glacial advances and natural forest fires began to occur, indicating that winds from the west and southwest had strengthened, and that the east was periodically subject to extreme temperatures and dryness. These trends probably received periodic assistance from the El Niño-Southern Oscillation which continues to exert a strong impact on New Zealand's weather (see Box 5.3). The average temperature in the past 3,000 years has remained within 1°C of today's average temperature (12°C) which, in turn, is about 3°C below the global average.

Figure 5.6: New Zealand's estimated yearly temperatures since the last Ice Age

Estimated yearly temperatures have increased since the last ice age 25,000 year ago during which the average annual temperature was around 7 degrees Celsius. Temperatures peaked at about 14 degrees Celsius 10,000 years ago and 12 degrees Celsius today.

Source: Salinger (1988)

Box 5.3: El Niño and the Southern Oscillation

The El Niño Southern Oscillation (ENSO) climate pattern has a marked effect on New Zealand. The term El Niño was originally coined by Spanish-speaking Peruvians to describe an unusually warm current that periodically appears off the coast of Peru. Because it tends to show up around Christmas time, the current was dubbed 'the Boy Child', which, at that time of year corresponds to 'the Baby Jesus'. This warm current is caused by low air pressure settling over the eastern equatorial Pacific and preventing food-rich cold water welling up to the surface. In 1972, El Niño caused havoc in the Peruvian anchovy fisheries. It was the research into this event that linked the warm South American current to air pressure changes.

These changes, known as the Southern Oscillation, turned out to be a Pacific-wide phenomenon, in which low surface air pressures in the east are mirrored by high pressures in the west. Every few years the pattern reverses in see-saw fashion as pressures rise in the central and eastern Pacific and fall over the Indian Ocean, Western Australia, and Indonesia. These switches recur every 2 to 10 years. The Southern Oscillation Index (SOI) measures the ENSO effect by calculating the monthly air pressure difference between Darwin and Tahiti. When air pressure is high over Darwin and low over Tahiti, the SOI is negative and is said to be in the El Niño phase. At other times, when the pressure systems are reversed and the SOI is positive, the climate pattern is referred to as a La Niña event. These oscillations affect weather patterns around the world (see Figure 5.7).

During a strong El Niño event, New Zealand tends to be cooler and windier than at other times, with more droughts in the east and north of both islands and more rain in the south and west. The effects vary with the season. Winter has more frequent southerly winds, autumn and spring have more frequent south-westerlies, and summer experiences more westerlies. Temperatures over the country are cooled by the southerly airflow. The most recent El Niño event, from autumn 1991 to autumn 1995, contributed to: low rainfall in the South Island lakes during the autumn and winter of 1991; particularly severe South Island snowstorms in the winter of 1992; much cooler than average temperatures in 1992 and 1993; and extremely low rainfall in the north and east of the North Island in 1994. During a La Niña event, westerly winds are dominant, with fewer southerlies and more north-easterlies. Rain is also more widespread, and temperatures are warmer. The 1988-89 La Niña contributed to one of the warmest periods on record and brought high summer rainfall to Northland and Auckland, a pattern which was also forecast for 1996-97.

Recent research indicates that the ENSO climatic fluctuations began around 5,000 years ago, at a time when the world's climate entered a slight cooling phase (Sandweiss et al., 1996). Climate researchers are now trying to assess whether there is any link between recent climate warming and the ENSO phenomenon (Kerr, 1994; Wuethrich, 1995). In its summary of recent climate anomalies that may be associated with human-induced global warming, the IPCC's scientific working group noted that: "The 1990 to mid-1995 persistent warm phase of the El Niño-Southern Oscillation (which causes floods and droughts in many areas) was unusual in the context of the last 120 years" (Houghton et al, 1996).

A number of computer models have suggested that there is a connection. One recent computer 'experiment' investigated the impacts of rising carbon dioxide levels on Pacific sea surface temperatures and cloud cover (Meehl and Washington, 1996). The resulting climate pattern resembled some aspects of an El Niño event, pointing to "the possibility that CO2 -induced climate change in the Pacific could have this signature." The researchers also noted that El Niño-associated droughts in the Australasia/western Pacific region could intensify with climate warming, disrupting water resources on small islands dependent on rainfall for fresh water, and contributing to long-term depletion of freshwater resources. This concern could also apply to some parts of New Zealand (see Chapter 7).

Greenhouse gases and the 'greenhouse effect'

The idea that carbon dioxide helps warm the planet was very controversial when first put forward last century (Tyndall, 1861; Idso, 1984). It is now known that this and several other 'greenhouse gases' provide the necessary insulation to maintain Earth's climate within a relatively stable temperature range. Without these gases, the heat which beams down each day would evaporate back into space by night, causing huge fluctuations. Temperatures on the moon, for example, can vary from -150°C to +200°C in a single lunar day (Lowe et al., 1988).

The most important greenhouse gases are water vapour (H2O) first, followed by carbon dioxide (CO2), and methane (CH4). In natural concentrations these gases trap just enough heat from the Sun to keep Earth about 33°C warmer than it would be without them (15°C instead of -18°C). This is the 'natural greenhouse effect' (see Figure 5.8). In the 600 million years since animal life evolved, carbon dioxide levels have fluctuated considerably, but the general trend has been a decline from levels that were sometimes 10-20 times higher than today (Appenzeller, 1993a; Mora et al., 1996). The lows generally coincided with ice ages and the highs with warm periods, but the relationship is not a straightforward one (Barron, 1994; Culotta, 1993; Kerr, 1993b; Emsley, 1994; Sellwood et al., 1994).

In nature, carbon dioxide is absorbed from the atmosphere by photosynthesising organisms, particularly trees, other vegetation, and marine algae, and by the ocean. Things that remove carbon dioxide from the atmosphere are referred to as 'carbon sinks'. Things which emit carbon dioxide into the atmosphere are referred to as 'carbon sources'. Natural sources include living things (when they decay, or when animals exhale), the ocean, volcanoes, soils and natural forest fires.

Vast quantities of carbon which were absorbed by forests and animals millions of years ago, were removed from the atmosphere when these organisms were buried in sediments. Today, these fossil deposits have been dug up and their carbon is familiar to us as the fossil fuels, coal, petrol and oil, and the sedimentary rock, limestone (calcium carbonate, CaCo3). Human use of fossil fuels and the burning of limestone in cement manufacture are now the major sources of carbon dioxide emissions into the atmosphere. As a result, CO2 levels are now higher than they have been for millions of years.

Figure 5.7: The Southern Oscillation during an El Niño Event.

During an El Nino event there are low surface air pressures in the east of the Pacific Ocean, mirrored by high surface air pressures in the west.

Adapted from Trenberth and Shea (1987)

Figure 5.8: The Greenhouse Effect.

The 'natural greenhouse effect' is caused by greenhouse gases in the Earth's atmosphere trapping heat from the Sun at the Earth's surface. Incoming Solar Radiation is 342 W m2and outgoing longwave radiation is 235 W m2. Both the Earth's surface and the atmosphere reflect, absorb and emit radiation.

Source: Houghton et al. (1996)

The widespread deforestation that has accompanied the rise of modern agriculture is the other main source of today's CO 2 build-up, not just because carbon dioxide escapes when trees are burnt, but also because a significant carbon sink has been removed. New Zealand was once a wall-to-wall carbon reservoir, with about 85 percent of its surface covered in native forest. Today, only about 28 percent of the country is forested, including both native forests and planted exotic forests (see Chapter 8).

Figure 5.9: Carbon dioxide concentrations past, present, and future.

Prior to the industrial revolution carbon dioxide concentration was around 280 parts per million by volume. Today it is over 360, and it is predicted to rise to between 600 and 700 over the next century.

Source: M. Manning, NIWA

Like carbon dioxide, methane is emitted by biological processes. Before a.d. 1800, the dominant methane source was decaying organic matter in wetlands. Other sources included termites, the belching of ruminant animals (e.g. buffalo, bison, cattle, sheep, deer, goats), wildfires and oceans (Brook et al., 1996). The major sink is oxidation in the air, which converts the methane to other gases. In the past century, natural sources of methane (e.g. wetlands) have declined markedly. New Zealand's wetland area has been reduced by about 85 percent (see Chapter 7). All things being equal, this should have reduced methane emissions, but, in fact, methane levels have risen because human-induced sources of methane are more prolific than the natural sources were. They include livestock, waste disposal, and fossil fuels.

Carbon dioxide and methane are not the only greenhouse gases however. Nitrous oxide is released from many small natural and human-induced sources. These are difficult to quantify but levels are rising (Houghton et al., 1996). The halocarbons, such as CFCs, simply did not exist before this century, but they too, are rising. Halocarbons are manufactured chemicals that combine carbon atoms with atoms of the halogen group (i.e. fluorine, chlorine, bromine or iodine). Besides being potent greenhouse gases, many halocarbons are also potent destroyers of the ozone layer.

Today, all the greenhouse gases other than water vapour are more abundant in the atmosphere than they have been for at least 200,000 years (Raynaud et al., 1993). Even water vapour is predicted to become more abundant as temperatures rise and evaporation rates increase. Pre-industrial levels of carbon dioxide were around 280 parts per million by volume (ppmv)(see Figure 5.9). Today they are over 360 ppmv. In the next century, the total amount of CO2 discharged into the atmosphere by human activity will exceed the amount released from the deep ocean over a period of 7,000 years at the end of the last Ice Age (Sundquist, 1993). Methane concentrations were once about 700 parts per billion by volume (ppbv). Now they stand at around 1,720 ppbv. Nitrous oxide has risen from around 275 ppbv to about 312 ppbv, and the halocarbons have risen from nothing at all to levels in excess of 600 parts per trillion by volume (pptv) (Houghton et al., 1996).