Since 1950, New Zealand’s average temperature has risen by 0.4°C, the number of cold nights and frosts each winter has fallen, sea level has risen by 70 mm, and a quarter of the ice in the alpine glaciers has melted. Similar changes are being seen around the world. Eleven of the last 12 years have been the warmest recorded since the 1850s. In the last 100 years, the global average temperature has increased by 0.74°C, mountain glaciers have lost volume and seasonal snow cover has declined. Permafrost is thawing, sea ice in the Arctic is shrinking fast and the oceans are warming up. There is no longer any doubt about global warming—it’s happening before our eyes. There’s little doubt about the cause, either. So-called greenhouse gases—carbon dioxide, methane, nitrous oxide and various halocarbons—are accumulating in the atmosphere because of human activity, and more greenhouse gas means a warmer planet.
Over the first half of 2007, global warming and climate change have been big news. The Intergovernmental Panel on Climate Change (IPCC) has been releasing the conclusions of its Fourth Assessment Report, an impressive overview of all the scientific work done on climate change in the last six years. It makes a compelling statement about the reality of the problem and the need for ac-ion. The report is divided into three main sections: the science of climate, the pacts of climate change, and what we might do to mitigate, or limit, the likely age. The news isn’t good.
Evidence for global warming is now unequivocal, according to the IPCC, which expresses “very high confidence” (defined as a greater than 90 per cent probability) that it’s being caused by human activity. Over the last 650,000 years, analysis of gas bubbles in ice cores drilled in Antarctica and Greenland shows that the atmospheric concentration of carbon dioxide (CO2) has varied from about 180 parts per million (ppm) in the depths of an ice age to a maximum of about 300 ppm during an interglacial (a warm interlude between ice ages). In the middle of the 19th century, before industrialisation had spread round the world, CO2 concentration stood at about 280 ppm. In 1850 global population was about 1.26 billion. Today there are 6.5 billion people, and industrial activity has spread worldwide. China and India are developing rapidly. China is said to be firing up a new coal-fired power station every week to 10 days to meet the energy demands of its rapidly growing economy. CO2 in the atmosphere has reached 382 ppm—36 per cent higher than pre-industrial levels—and the annual rate of increase has doubled in the last five years. Most of the carbon added to the atmosphere comes from the burning of fossil fuels—coal, oil and gas—for energy and transport, while around a third is produced by agriculture, land-use changes and deforestation.
This rise in atmospheric CO2 is accompanied by increases in the concentration of methane, up from about 715 parts per billion (ppb) in the 19th century to 1774 ppb in 2005, and nitrous oxide, up from 270 ppb to 319 ppb. These increases are caused largely by farming, although methane can be released also during oil and natural-gas recovery. Methane is emitted by bacteria in the stomachs of ruminant farm animals (more as burps than farts), and from rice paddies and wetlands. Nitrous oxide comes mainly from the use of nitrogen fertilisers. Both gases have a much more potent greenhouse effect than CO2—23 times more so in the case of methane—so are of much greater significance than their relatively low (though increasing) concentrations might suggest.
Greenhouse gases in the atmosphere trap heat at the surface of the earth. If the earth had no atmosphere, the surface would have an average temperature of -18°C. Carbon dioxide, methane, nitrous oxide and halocarbons, together with water vapour (the most important greenhouse gas) trap enough heat to warm the surface of the planet to an average 14°C. Energy from the sun arrives at the top of the atmosphere, and moves through the thin skin of gas to the surface, heating it up somewhat. The warmed surface radiates heat back up towards space, but this is intercepted by the greenhouse gases, which absorb longer wavelength radiation such as heat much more effectively than shorter wavelength radiation like light. This absorbed heat warms the atmosphere, which then radiates heat back down to the surface. The sky above us literally glows with heat, which is why astronomers who want to work at infra-red frequencies have to fly their instruments in orbiting spacecraft. Glass in a car or a greenhouse acts pretty much like a greenhouse gas in that it is transparent to light but blocks heat—hence the name.
John Tyndall, the Victorian polymath who was one of the first scientists to identify the greenhouse phenomenon, likened the effect of greenhouse gases to that of a dam in a river. The water running down the river is the energy arriving at the earth from the sun. When the energy arriving is balanced by the energy being radiated back into space, the earth is in thermal equilibrium and its temperature stable. Now we build a dam across the river. Once the dam is full, the flow downstream is once again the same as that upstream, but the water level—standing in for the temperature at the earth’s surface—is higher behind the dam. By adding carbon dioxide to the atmosphere, we’ve been building an ever-higher dam and, as the water level behind it has risen, increasing the temperature of the earth.
At the moment, we’re adding bricks to the top of the dam faster than the water can fill it. If we could somehow freeze greenhouse gas levels at where they stand today, it would take another 20–30 years for the earth to warm up enough to regain equilibrium. It requires a lot of energy to warm up the planet—especially the deep oceans, which cover 70 per cent of the earth’s surface. Global temperature is therefore playing catchup with greenhouse gas levels—with worrying climatic consequences.
The earth’s climate system is solar powered. Energy Efrom the sun flows through the various parts of the system—the atmosphere, the hydrosphere (oceans, lakes, water vapour, etc.), the cryosphere (ice and snow), the land surface and the biosphere (all living things)—in complex patterns. Movements of air and ocean shift heat from the tropics towards the poles. Great swirls of weather move around the planet. These weather systems are the mechanisms of climate. As weather patterns change, so does the climate.
Climate is what weather averages out to over time. Meteorologists and climate scientists define climate as the average weather statistics for any given place over the most recent 30-year period. 1971 to 2000 is the current baseline. Thus the figure for, say, the average daily maximum temperature in Christchurch in June (11.7°C) is derived from all the June daily maxima over those three decades.
As well as providing a statistical base for comparisons of temperature, precipitation, sunshine and so on, climate is about the frequency with which weather events occur. A severe storm might be described as a 100-year event, meaning that on average a storm of that severity would be expected to occur once in any 100-year period. As a result of climate change, such a storm could become a 50-year event, or a 10-year frost a 50-year event.
Over the last 4 million years, the earth has alternated between prolonged periods of cold climates—ice ages, or glacials—and shorter, warm spells—interglacials. The difference in global temperature between an ice age and an interglacial has been about 5°C, and the change from cold to warm has taken about 5000 years. As, during the current interglacial, the ice sheets over North America, Europe and southern New Zealand have melted and sea levels risen, the global average temperature has risen about 0.1°C every century. Over the last 150 years, human activity has boosted greenhouse gas levels by at least a third, and in the last 20 years the rate of warming has averaged 0.18°C per decade. In other words, the world is currently warming nearly 20 times faster than during the most recent prolonged natural change.
To understand how the climate system works and how it might change in the future, researchers have developed highly complex computer simulations of the global atmosphere. These take a mathematical representation of the atmosphere—very similar to the models that weather forecasters use—and link it to a representation of the surface of the planet. Models of ocean circulation, sea-ice formation and ice-sheet behaviour are also added. Run with all the factors that affect the climate set at current values, the models do a good job of reproducing the large-scale features of atmospheric circulation and global climate. Fed assumptions about how greenhouse gas and pollution levels will change in the future, they give an idea of how the climate system will respond.
Climate projections based on modelling are often unfairly compared with weather forecasts. If we can’t rely on weather forecasts to be accurate for more than a few days into the future, why should we expect climate models to provide accurate projections for decades to come? The answer is related to the difference between climate and weather. Meteorologists attempt to predict how weather systems will behave, so as to forecast rain, sun, frost or fog at a specific time and place. The success of a weather forecast depends critically on the accuracy of the information fed into it at the beginning. Small errors in what are called the initial conditions multiply over time until the forecast becomes useless. Climate modellers try to see how averages will change, not to make accurate weather predictions. The initial conditions do not determine the accuracy of their projections; that depends on the design of the models and changes in the factors that drive climate: increasing greenhouse gas levels, rises or falls in solar radiation, changes in atmospheric pollution, and so on.
Climate scientists check the accuracy of their models in several ways, one of the most important being to make them reproduce the climate change we’ve seen over the last century—making a “hindcast” rather than a forecast. We have a pretty complete picture of how greenhouse gas levels have increased since Victorian times from ice cores and direct measurement, and we also have a good idea of changes in solar output and atmospheric pollution. When the models are run with these inputs, they get very close to the pattern of change we can see in the historical record.
We can also look back to projections that were made 20 years ago and see how they turned out. In 1988, one of the top US climate scientists, James Hansen of the Goddard Institute of Space Studies (GISS), showed Congress a range of climate projections made by the GISS climate model. He used three scenarios to describe how greenhouse gases might increase over the next 20 years, allowing for one major volcanic eruption to cool things down a little for a year or two (which the eruption of Mt Pinatubo in 1991 certainly did). His middle scenario, the one he suggested as most likely, projected an increase in global average temperature of about 0.24°C per decade. The actual increase turned out to be exactly the same. This doesn’t mean the GISS model, or any other climate model, is perfect—they all have limitations and imperfections—but it does demonstrate that they can provide us with useful information.
So what do the models indicate about future climate change? One of the key features of all warming projections is that polar regions will warm more than the rest of the planet. Ice, which reflects heat, will be replaced by dark-coloured ocean or rock that absorbs heat. This can already be seen in the Arctic, which has warmed twice as fast as the rest of the globe, and on the Antarctic peninsula, which has warmed faster than anywhere else—a full 2.5°C in the last 50 years. Grass has started to colonise the tip of the peninsula, ice shelves have been shrinking and breaking off, as the Larsen B ice shelf did rather dramatically in 2002, and its glaciers are losing mass. In the Arctic, summer sea-ice cover has been declining by 7.5 per cent per decade, and there are suggestions that the North Pole could be ice-free in summer before the middle of the century.
Another consequence of this warming is that the Greenland and West Antarctic ice sheets are showing signs of significant loss of mass. There’s a lot of ice in those two sheets—enough to cause sea levels to rise by about 14 m if it all melts. Until recently it was assumed that big ice sheets melted slowly, from the outside in—like an iceberg in the ocean or an ice cube in a glass of water. At the same time, because the centre of each ice sheet is high and cold, it was thought that snowfall in the interior might be enough to offset melting at the edges. Unfortunately, ice-sheet melting appears to be a much more dynamic process.
At low levels on the Greenland ice sheet in summer, surface snow melts to form large pools and lakes. This water works its way down through cracks in the ice, melting passages called moulins. These drain the water down to the base of the ice, where it acts as a lubricant, speeding the ice on its way as it moves towards the outlet glaciers and the sea. As the huge masses of ice move, they cause glacial quakes that can be picked up on seismometers around the world. They have a characteristic pattern that allows them to be distinguished from earthquakes, and they’ve been increasing in frequency.
There have been signs of surface melting in West Antarctica too, although nothing like on the scale of Greenland. The main threat to the West Antarctic ice sheet lies below. The sheet stands on land that is lower than sea level, meaning it interfaces with the ocean; and it is fringed by ice shelves that float on the ocean surface. If, as the ocean warms, the ice shelves thin and break off, the glaciers draining from the centre of the sheet could speed up and pour into the sea.
Monitoring these two major ice sheets and working out how they might respond to future warming is a matter of urgent research. Their behaviour in a warming world will have important consequences for sea level. Even if no more ice were to melt, the sea would continue to rise because water expands as it warms. The IPCC suggests that the total rise could be between 18 cm and 59 cm by the 2090s, based on thermal expansion and allowing for some additional melting from glaciers and ice sheets. These numbers, however, specifically exclude “future rapid dynamical changes in ice flow” in the big ice sheets. The IPCC makes a pointed comparison with the last major interglacial period, 125,000 years ago, when polar temperatures were 3–5ºC warmer than now and sea level 4–6 m higher.
The part of the planet that will warm least is the Southern Ocean, which circles the earth between Antarctica and New Zealand. It is very big and very cold, so it will take an enormous amount of energy to warm up. After the polar regions, warming will be most pronounced in continental interiors far from the cooling effect of oceans. There will be fewer cold days in winter, and more frequent, and warmer, hot days and nights in summer. Warm spells and heat waves will become more common. The European summer of 2003 was the hottest for at least 500 years and caused 45,000 more deaths than a “normal” summer. Such a summer is likely to be normal by the 2030s.
The effect of warming on rainfall and drought is particularly interesting. As the atmosphere warms, it is able to hold more water vapour; and water vapour, being a greenhouse gas, contributes to further warming. This feedback not only increases the rate of warming, but also adds fuel to the hydrological cycle. With more water vapour available to transfer heat around the atmosphere, the intensity of rainfall events is expected to increase. Even if the total amount of rainfall in a given place doesn’t increase, the rain may fall in fewer but heavier events. As Erick Brenstrum noted in his column in the last issue of New Zealand Geographic (85), there are signs that this is already happening. It could exacerbate erosion and flooding problems, particularly in areas prone to drought, which are expected to increase significantly. The IPCC also expects intense tropical cyclones to become more common, although the overall number of storms may not change.
How big any of these effects will be depends on how much warming takes place. For the IPCC’s Fourth Assessment Report, climate modellers around the world used six emissions scenarios to make projections of greenhouse gas concentrations, atmospheric pollution and other climate drivers over the next century (see box). With the lowest-emissions scenario the models project a global temperature in the 2090s 1.8°C higher than the average for the period 1980–99; with the highest-emissions scenario they project a rise of 4°C. To those numbers add the 0.8°C rise in global temperature we’ve seen since the start of the industrial revolution. Best case: +2.6°C. Worst case: +4.8°C. As we have seen, the change in temperature on moving from the depths of an ice age to an interglacial has been an increase of about 5°C over 5000 years. We are now facing the possibility of an almost 5°C rise above the temperature of a normal interglacial over just 250 years, with most of that jump taking place over less than a century.
There is one major uncertainty in these projections, and that relates to the carbon cycle. Carbon moves round the climate system much as water moves round the hydrological cycle. Carbon dioxide in the atmosphere is absorbed and emitted by the oceans; plants take it in and turn it into sugars; soils emit and absorb it. The process is both complex and complicated; suffice to say that before people started burning fossil fuels and felling forests, all the various fluxes were more or less in balance—at least across short (not geological) time scales. In fact, since the industrial revolution, natural carbon sinks—oceans, plants and soils—have absorbed up to half of all the CO2 mankind has added to the atmosphere. Unfortunately, early research linking carbon-cycle and global-climate models suggests that, at some point, nature will stop performing this favour and, instead of absorbing carbon, start to put it back into the atmosphere.
Two of the biggest dangers are the thawing of permafrost and the drying of the Amazonian rainforest. As permafrost thaws, plant material that has been in cold storage warms up and begins to rot, giving off methane and CO2. This is already being observed in Siberia. Rainfall in the Amazonian rainforest is projected to drop by the middle of this century, which could cause it to die off and be replaced by savannah.
Together, these two dramatic changes could add as much as 100 ppm CO2 to the atmosphere by the end of the century, which would make a mockery of human attempts to limit carbon emissions. In keeping with its conservative stance on sea-level rise, the IPCC has excluded these carbon-cycle feedbacks from its projections because research into their workings is only in its earliest stages.
One of the most robust findings of climate modellers has already been mentioned: even if we could somehow fix greenhouse gas levels where they stand today, the planet would continue to warm for 20–30 years. For the planet to regain thermal equilibrium, the temperature has to rise commensurate with the quantities of greenhouse gases already added to the atmosphere. Whatever action is taken to restrict greenhouse gas emissions, the next two or three decades will see a rise of 0.6°C in the global average temperature. This virtually inevitable warming is known as climate commitment, and the only things that could reduce it are a series of major volcanic eruptions or a significant reduction in solar radiation reaching earth.
New Zealand is expected to warm at about the global average rate. The cool ocean around it will moderate the warming it experiences by acting as a massive heat sink. If the global average increase by the end of the century is 3°C, the northern polar regions will be more than 6°C warmer, while the middle of large landmasses will be 4°C or more warmer. Large parts of the Southern Ocean may warm by only 2°C. The sea will act as an air conditioner, keeping New Zealand cool—at least in comparison with the rest of the planet. While New Zealand has warmed by 0.4°C since 1950, the global average has risen by 0.6°C in just the last 30 years.
New Zealand’s climate is also very variable. Under the influence of the El Niño/La Niña cycle and the movement of large bodies of warmer or cooler water around the Southern Ocean, the annual temperature can fluctuate as much 1°C above or below the longterm mean. The summer of 2006–7, for instance, was notably cool, thanks in part to the body of cold water that accompanied icebergs up the east coast of the South Island. A few months later, warm water in the Tasman Sea helped to create the conditions that made May 2007 the warmest May since reliable records were first kept, in the 1860s. This sort of variability will be overlaid on the warming trend. New Zealand will continue to experience warm and cool years, but over time cool years will become warm and warm years hot. It’s worth noting that the difference in seasonal average temperature between a summer thought of as hot and a summer thought of as normal can be as little as 1°C. Little numbers can mean a lot.
The cool ocean to the south of New Zealand will have one especially important effect on the climate changes the country will experience. As the world warms, the great bands of westerly winds that circle the globe around Antarctica—the Roaring Forties, Furious Fifties and Screaming Sixties—will become stronger. This effect has already been observed, and its impact on New Zealand is likely to be profound. Stronger, and more frequent, westerly winds will bring increased rainfall to the west coast of both islands and create drier conditions on the east coast. At the same time, the general warming will spread south.
The National Institute for Water and Atmospheric Research (NIWA) is working on revised projections for New Zealand, based on the modelling done for the IPCC’s most recent report. Earlier NIWA studies, however, suggest that by the end of the century, Taranaki, Manawatu, West Coast, Otago and Southland will have higher average rainfall, while Hawke’s Bay, Gisborne and the eastern parts of Canterbury and Marlborough will be drier. In other words, the south and west of the country will be wetter, while the north and east will be drier. Spring in particular is expected to be drier in the north and east of the North Island.
Large rainfall events are expected to become up to four times more common by the end of the century, especially in regions where there is an increase in overall rainfall, and recent work using NIWA’s regional-climate model suggests that rainfall events generally will become more intense. There could be 20 per cent more rain on the three wettest days of the year, which on the West Coast translates to as much as 40 mm more on each of those days. In parts of Canterbury there could be 50 per cent more rain in a year’s heaviest falls. The potential for flooding and erosion is obvious.
Droughts are also expected to become more common as the climate changes. By the 2030s, water-availability problems will be intensifying in Northland and down the east coast of both islands. With low-to-medium warming, the risk of what is currently a 20-year drought might double by the 2080s in inland and northern Otago, eastern parts of Canterbury and Marlborough, and parts of the Wairarapa, Bay of Plenty and Northland. If warming is more pronounced, such a drought could become a 5-year event in the 2080s in the east of the South Island from Otago to Marlborough, much of the Wairarapa, Bay of Plenty and Coromandel, most of Gisborne and a large chunk of Northland. In Whangarei, what is currently a 20-year drought might occur every three years on average. Large parts of New Zealand could experience damaging droughts twice as often as at present.
It isn’t only the risk of severe drought that will increase. With eastern districts receiving less rainfall, the average moisture deficit—that is, the difference between the amount of water in soils available to plants and the amount plants need for optimum growth—will increase. Soils could go into moisture deficit earlier in the growing season, and the deficits could last longer into autumn than at present. What we think of today as a 1.0 medium-severity drought could be an almost annual occurrence by the 0.5 end of the century. The implications for dryland agriculture are clear. Demand for irrigation water is going to increase, and pressure on groundwater and rivers will be consider- -0.5 able. Water availability is already a contentious issue in many parts of -1.0 the country, and climate change will only make matters worse. -1.5
A direct consequence of warmer—and shorter—winters will be a -2.0 reduction in snow cover. The permanent snow line in the mountains will rise, while snow cover below this will be shorter-lived. The amount of snow that falls may actually increase, however, owing to the intensification of precipitation. Ski-field base stations may eventually have to be moved upwards to be within reach of the new snowline, but there could still be plenty of the white stuff up there.
There will also be a marked impact on New Zealand’s glaciers. Over the last 100 years, the surface area of the glaciers has reduced by 35 per cent, although since 1978 increased snowfall has offset the effect of warming, keeping things roughly in balance. As warming increases, however, that balance will be upset, and further reductions in ice mass are likely. NIWA’s latest studies in this area suggest that, by the end of the century, warming in the Southern Alps could be significantly greater than over the rest of the country. In one scenario, the Mt Cook region could experience an average summer-temperature increase of 6°C (from 14°C to 20°C), which would raise the freezing level by 1000 m, shrink glaciers, lessen snow cover and possibly cause large rock falls and landslides.
Sea level round New Zealand has risen by 25 cm since the middle of the 19th century and by 7 cm since 1950. The IPCC projects a further increase of 18–59 cm over the next 100 years, plus an unknown rise from the melting of the ice sheets in Greenland and Antarctica. However, sea level at any given time is affected by many different factors. Onshore winds and ocean currents push water up against the land, and intense storms can cause a phenomenon called storm surge.
At the centre of a severe storm, atmospheric pressure is low and the sea bulges. When a storm coincides with a high tide along a low-lying coast, this bulge raises the tide higher than normal, and flooding may extend a long way inland. Not only does a rise in sea level increase the potential for this sort of damage, it also has less immediate impacts. It can accelerate coastal erosion and flood groundwater systems with seawater, spoiling them for agricultural use. Low-lying coasts previously liable to only occasional flooding by the most extreme high tides can be flooded by every tide. Estuaries may be enlarged as tidal influence reaches further upstream. Changes in sediment supply from rivers and in wave direction can have a big impact. In coastal Canterbury, a reduction in southerly waves could reduce the movement of river sand up the Pegasus Bay coastline, resulting in the erosion of up to 50 m of shoreline near the mouth of the Waipara River and up to 80 m near the Waimakariri River.
The impacts these changes in climate and sea level have on life in New Zealand will depend on how rapidly they take place and just how much warming occurs. Human systems are better able to adapt to change than natural ecosystems because humans are able to see a problem coming and plan a response. Farmers and horticulturalists can change the crops they grow to suit the new conditions. Grape growing might take off in Southland, and banana plantations become established in the far north. Farms on the east coast of both islands will certainly face problems of increased risk of drought, and access to good supplies of water will become ever more critical. Although plant breeders can produce new varieties better adapted to drought, considerable ingenuity will be required to overcome the increasingly acute water shortage.
For natural ecosystems the rate of change is crucial. If it is low, plants and animals will be able to “keep up”; if it is high, only the most adaptable species—those that can survive in the widest range of ecological niches—are likely to survive. Species adapted to only a narrow range of conditions or food sources will find adaptation much more difficult. Take tuatara, for instance. Their sex is determined by the temperature at which their eggs are incubated. Too warm (above 22ºC), and they become predominantly male. Males already outnumber females by nearly two to one in some island refuges. In the mountains, as the permanent snow line moves upwards, the tolerance zones of some alpine plants and animals may simply disappear.
The changes that global warming is going to bring to New Zealand during the 21st century are going to be significant, but where the country is likely to be most vulnerable is with respect to climate change elsewhere. New Zealand may warm more slowly than most places, but if its major export markets undergo damaging change, the economic impact will be severe.
It is also vulnerable to what the rest of the world does to try to limit climate change. Its greenhouse gas emissions are tiny—around 0.5 per cent of the global total. Even if it were to achieve the government’s target of carbon neutrality—no net emissions of greenhouse gases—this would have no discernible impact on the course of global climate change. On the other hand, if the rest of the world decides to limit air travel because of its impact on global warming, this will hit New Zealand’s tourism trade. Kiwi exporters are already having to work out how to counter the perception overseas that buying New Zealand produce is bad for the planet because they are so many “food miles” away from their markets.
If New Zealanders fail to take serious steps to limit their emissions, they run the risk of having their exports taxed or shunned. They will be affected both by climate change itself and by the wider world’s response to it. How New Zealand meets these challenges will shape many aspects of its human and natural systems over the coming decades.