Climate change

This article was first published in Growing Today in 2004. It was written in February and March of that year, and probably deserves to be up-dated with the latest research.

Since we bought our little piece of North Canterbury seven years ago, we’ve experienced an El Niño drought, a La Niña drought, an in-between drought, a once in 100 year flood, a once in 50 year frost and enough wind to make any tree grow with a lean. I’m looking forward to finding out what an average year’s like. Is our climate changing? I don’t know, I haven’t been here long enough to say, but the weather has certainly kept me interested.

This year we’ve had a blistering Christmas and New Year with near record heat, then a cold and wet February, bringing catastrophic floods to the south of the North Island. April brought snow to low levels, at least a month or two early. The National Institute of Weather and Atmosphere (NIWA) reports that 2003 was notable for the number of extreme weather events, and overall it was warmer than the long term average. Some interpret this as a sign that climate change is happening, others insist that it’s all part of the natural variation in our weather. In fact, it’s probably both at the same time.

The world is warming up. The evidence is now very strong indeed. 16 of the 17 hottest years since records began have occurred since 1980. The 1990s were the hottest decade ever. 2003 tied with 2002 as the second hottest year on record, slightly behind 1998. In New Zealand, the annual average temperature has increased by 0.7C over the last 100 years. Every season except spring is showing signs of increased warmth, and there’s been a significant reduction in the frequency of frost. In Alaska, winter temperatures have warmed by more than 4C in the last 30 years. Studies in Britain have shown that autumn lasts longer and spring arrives earlier now than 100 years ago. The cause, according to the Intergovernmental Panel on Climate Change (IPCC), the body of scientists convened to study the issue, is increased levels of greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapour and others) in the atmosphere. Large quantities of those gases are produced by human activities — burning fossil fuels, animals belching methane, and chopping down (and burning) forests. In other words, human activity is causing the world to warm up.

Greenhouse gases warm the earth by trapping the sun’s heat, like the glass in a greenhouse. If the Earth had no atmosphere, the average surface temperature would be a rather brisk -18C, as opposed to about +15C. Globally, carbon dioxide is the most significant of the greenhouse gases, but in New Zealand, methane produced by sheep and cows is also important, not least because it’s 30 times better than carbon dioxide at absorbing energy. NIWA estimates that our animals produce around 1.5 million tonnes of methane a year (mainly by belching, not f*rting). A typical dairy cow produces 370 litres of methane per day, a sheep about 60. You could drive a car for 4 km on one cow’s daily gas production — which might open up interesting prospects for dairy farmers with large herds.

It’s carbon dioxide that’s grabbed the headlines though, because the amount in the atmosphere has been steadily increasing. Before the industrial revolution, CO2 made up about 280 parts per million by volume of the atmosphere. In 2003, the Mauna Loa observatory in Hawaii recorded 373 parts per million, a 33 percent increase. Last year’s figure was 3 ppm higher than in 2002, far exceeding the average annual increase of 1.8 ppm over the last decade. There are suggestions that this large leap has been caused, at least in part, by Asian countries that have been industrialising rapidly, with China in particular building large numbers of coal-fired power stations to keep its rapidly expanding economy going. Felling and burning the rainforests in SE Asia and Brazil doesn’t help, either.

Working out what an increasing greenhouse effect means in terms of the weather and climate we can expect over the next 50 to 100 years is not straightforward. Forecasting New Zealand’s weather from day to day is a notoriously difficult process — trying to look a week or so into the future is about the best we can get with modern techniques. Climate is what you get when you add up and average out all our weather (something plants are very good at), so a climate forecast is like a weather forecast that tries to look years ahead. Doing it on a global scale doesn’t make it any easier — there are a huge number of variables that have to be factored in. However, advances in the models that meteorologists use for weather forecasts (which form the basis of the global climate models, or GCMs), and rapid improvement in the power of the computers used to run these models, has allowed climate scientists to steadily improve their projections of what our future climate might be like. If the amount of greenhouse gases carries on increasing, the world is going to get warmer. Depending on how much CO2 and methane we pump into the air, the IPCC’s last forecast (in 2001) estimated that the world could warm by anywhere between +1.5C and +5.8C by the end of this century.

This doesn’t mean that everywhere will be 1.5C warmer all the time. The GCMs suggest that warming will be greatest at the poles, which fits rather neatly with recent observations of thinner Arctic ice, ice sheets breaking up in Antarctica, and warming in Alaska. Down here in the southern hemisphere, the oceans around us will tend to moderate the rate at which we warm up. Dr David Wratt, NIWA’s lead scientist for climate change, and a member of the IPCC’s advisory bureau, says that New Zealand will probably warm up at about two thirds of the rate of the rest of the world. “The models show increased westerlies” he told me, “and that will mean more rain in the west and on the mountains, and drier conditions on the east coast.” A tendency for the east coast to become drier is likely to increase the frequency and severity of droughts. “Irrigation issues will become very important” says Dr Wratt. Rivers that are fed from the Main Divide such as Canterbury’s Waimakariri or Otago’s Clutha could carry more water, but rivers that rise to the east of the Alps could have problems with low flows.

The increase in average temperatures will not mean the end of cold weather, just that it will become less likely. The difference between an ordinary summer and a warm summer can be as little as 1C in average temperature, so if we get an increase of that order, then what we now regard as warm summers will become “normal”. Very hot weather will also be more likely, which will add to the severity of any water shortage problems. The number of degree days will increase, and therefore the growing season in most regions will be longer. On the other hand, the winter snowline will rise, which could cause problems for ski-fields.

Beyond the increasing warmth, there is also the probability that extreme weather events will become more common, and possibly also more severe. “Extremes have important effects” says Dr Wratt, “frost, drought and floods can all have a dramatic impact. There have already been statistically significant increases in the frequency of heavy rainfall in parts of the northern hemisphere, and there’s some evidence of changes here.” By the second half of this century, a flood currently regarded as a once in 40 year event could become a once in 20 year event. Frosts already seem to be declining in frequency and severity, and average night temperatures have risen. This doesn’t mean that unseasonal frosts won’t happen — frost-prone vineyards will still have to be careful — but it does suggest that they should become less common. However warmer winters aren’t always good news. A 2001 report on the likely impacts of climate change on New Zealand agriculture by Dr Gavin Kenny suggested that in 50 years time Bay of Plenty kiwifruit growers could be having problems with a lack of winter chilling. Some crops will tend to shift southwards, and new crops may become economically viable. Bananas could thrive in Northland, olives in Dunedin. Some recent research suggests that parts of South Canterbury and Otago will become suitable for commercial vineyards. Perhaps even Southland could join the wine boom.

Extra heat will bring with it changes in the distribution of pests. Sub-tropical grass species could spread south through the North Island threatening a reduction in pasture productivity. Damaging insect pests could extend their range, or new pests arriving from overseas could find our warmer climate more to their liking. It isn’t all bad news, however. Increased CO2 in the atmosphere will tend to encourage faster plant growth, at least in the short term, but this will be limited by water and nutrient availability. In other words, crop plants may grow faster, but will require more fertiliser inputs and greater irrigation. Pasture production could be boosted, at least in part because of the longer growing season. Fewer damaging frosts will be good news for growers of many crops. And people who have been climatically rather optimistic in the choice of crops they’ve planted may strike it lucky.

To explore how farmers and growers might respond to climate change, a two year study is being carried out in east coast areas, under the aegis of the Hawke’s Bay Climate Change Adaptation Group and Dr Kenny. Workshops were held last year in the Bay of Plenty, Gisborne, Hawke's Bay, Nelson, Marlborough and North and South Canterbury, and farmers were asked what issues they might face and how they might respond if the climate had warmed 1C and rainfall dropped by ten percent by 2050. The positive benefits participants foresaw included opportunities for diversification and land use change, a longer growing season with less winter feed needed for stock, a “kinder” climate for humans, and farmers being able to respond to change by adapting their farming systems. The challenges included managing water supplies for irrigation, the changes to pasture, crops and livestock, possible negative effects on animal health, changes in weed, pest and disease prevalence, and the potential for social problems arising from more stress, some from the impacts of land use changes. The report on the workshops, called The View From The Ground — Adapting To Climate Change in Eastern New Zealand, makes interesting reading, and gives a sense of the resilience of the farming community in the face of this latest challenge.

If climate change in New Zealand turns out to be relatively gentle and gradual, farmers and growers should be able to respond and adapt. “There are issues to be concerned about” says NIWA’s Dr Wratt. “Adaptation is the key, and the best way to do that is to think on a local level.” Sensible strategies will vary from region to region and property to property, but people in high rainfall areas should be thinking about coping with more, while dry properties will need to plan for less rain and more drought. Things will be tougher if temperatures increase faster than the bottom of IPCC prediction range, and the potential for significant damage caused by extreme weather events will certainly be important on a regional level.

There is a danger, however, that the global climate could do something other than warm gradually. There is evidence from ice cores in Greenland that in the past the northern hemisphere climate has made rapid flips from warm interglacial to ice age. In some cases it could have happened in as little as ten years. This change is believed to triggered by a change in ocean currents, normally driven by water cooling and sinking in the far North Atlantic. If this sinking stops, the warm current that keeps Europe’s winters relatively mild could switch off, leading to a rapid cooling of the European climate — especially in winter. The last time it happened, 12,700 years ago, it ushered in a 1,300 year cold spell. Summer temperatures in Britain averaged only 10C and winter temperatures plunged to -20C. Until recently, scientists believed this was a relatively small risk, and any cooling effect might be relatively minor — just enough to offset the continued warming. However, new data from the Atlantic is suggesting that we may be nearer than expected to a flip from warm to cold. The risk of sudden, as opposed to gradual change is certainly there, but how big that risk is remains to be seen.

This sort of rapid change would clearly be catastrophic for the northern hemisphere, devastating European agriculture and causing unimaginable economic and social problems. What would happen to the New Zealand climate in those circumstances is not clear. Perhaps our oceans would buffer us against the worst excesses, but we would not escape the economic consequences of global climate dislocation.

For the time being, the odds seem to favour gradual change, and in those circumstances New Zealand is probably as well placed as any country in the world to cope with and adapt to a warmer climate. On a local level, I am already hatching schemes for more water storage to give my trees and vines a buffer against drought, and if we’re going to get more Norwesters, I’m going to plant more shelter. If there’s a risk of more extreme weather, perhaps I should do a bit more to stabilise some of the gullies on the property. Just good land management, and sensible insurance against a warming future.

Science, controversy, and politics

Most climate scientists agree that the world is warming up, and that man’s activities are causing it. Some scientists, and many lobbyists (who are often funded by oil and coal companies), disagree. Despite the weight of evidence and the effort that’s been put into the study of the global climate in the last 20 years, a few still try to pick holes in the data and rubbish the predictions. Climate change “sceptics” claim that climate change may not be real, or that if it is, perhaps the sun’s causing it, not us. New Zealand has its share of sceptics, and no shortage of people who would like to believe them. This where science and politics mix, and the results are often messy.

The argument runs like this. If the world is warming up, and we’re causing it, we ought to do something about it. That “something” is the Kyoto protocol, which - oversimplifying considerably - is an international agreement to restrict greenhouse gas emissions. But if the world isn’t warming, or human activities have nothing to do with any warming that is happening, then we have a good excuse for doing nothing - certainly nothing inconvenient or expensive. Many countries, including New Zealand and the European Community, have decided to apply the Kyoto rules, while others, notably the US and Australia, are hanging back. Kyoto, they say, is bad for business. The domestic New Zealand debate is polarised along roughly left/right lines, with anti-Kyoto voices coming from some farming and business lobbies, while support is to be found amongst environmentalists and the Labour government. Complicating the issue is the fact that Kyoto on its own will not bring down CO2 emissions enough to reduce warming by much, and from a New Zealand perspective, our contribution to the world’s greenhouse gas emissions is tiny. But can we afford to do nothing? The science suggests no, but any answer is inevitably political.

Web Resources

There is an immense amount of information (and disinformation) on global warming and climate change available on the web. The New Zealand government’s climate change website is: On it you can find all the official publications relating to climate change in NZ, including papers on possible impacts by region. The report “The View From The Ground” covering the east coast farmer workshops is available as a pdf download from the “Impacts” page.

The NIWA website also has lots of useful information.

The full text of the latest (2001) report from the Intergovernmental Panel on Climate Change is available at

If you want to find out what the US climate change sceptics are saying, check out The George C Marshall Institute website at, or the Greening Earth Society at The latter organisation is funded by “municipal electric utilities [and] their fuel suppliers”.

To get an overview from the other end of the spectrum, Greenpeace has a lot of useful information at The World Wide Fund for Nature also has lots of info at

You can even help to run the computer models that are used to predict future warming. The Oxford-based project ( allows you to download a programme that runs in the background on your computer, using up spare processor cycles to calculate future climate scenarios. The site is also a good source of information about climate modelling in general. So far, they have received 20,000 completed forecasting runs, simulating a total of 1.5 million years of weather.

[Added May 2006] A good site to learn about current climate science is Real Climate. It's run by working climate scientists.