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Earth's atmosphere has changed markedly in 4.6 billion years, but, apart from asteroid and volcano impacts, no changes have been as rapid as the recent growth of greenhouse gases and the depletion of the ozone layer through human activities. In fact, for the past few million years, as humans and most of our fellow species were still evolving, the atmosphere's composition has stayed relatively stable.

During this time, small fluctuations in greenhouse gas concentrations have been associated with significant climatic changes, particularly the recurrent pattern of alternating ice ages and warm interglacials. Now, as greenhouse gases rapidly accumulate in the atmosphere, the current Holocene interglacial may turn out to be warmer than any of the previous ones, with unpredictable impacts on weather, ecosystems and economies.

Conclusive evidence that the human-induced greenhouse gas build-up may change the climate by boosting the natural greenhouse effect (greenhouse forcing) is still being sought. However, the Scientific Working Group of the United Nations Intergovernmental Panel on Climate Change (IPCC) concluded in 1995 that "the balance of evidence suggests that there is a discernible human influence on global climate."

New Zealand is as exposed to the impacts of climate change as any other country. Distance from other large land masses may offer protection from short-lived atmospheric contaminants, but long-lived contaminants affect New Zealanders as much as anyone else. In fact, our average annual temperatures have risen more than the global average this century. While some of the potential impacts of climate warming may be positive for New Zealand (e.g. extended growing seasons and new crops), others are likely to be negative.

The projected negative impacts include greater weather variability, more floods and droughts, increased pressures on water supply and disposal systems, increased risk of being invaded by infectious diseases, pests and weeds, and a reduction in alpine and cold-adapted plant and animal species. These impacts may be further compounded by repeated unwelcome visitations of the El Niño-Southern Oscillation (ENSO) climate pattern which some scientists suspect of being intensified by global warming.

A feature of most climate change scenarios is their tendency to focus only on a fixed point in time (e.g. 2050) as if the impacts will stabilise at that point. However, on current trends, greenhouse gases will continue to accumulate for several centuries, meaning that the scenarios envisaged for 2050 are just the start of what lies ahead. Aware of the potential for climate change to cause severe disruption both locally and globally, New Zealand as a Party to the United Nations Framework Convention on Climate Change (FCCC) undertook to return net carbon dioxide emissions to 1990 levels by 2000 and keep them at that level from then on.

However, with just three years to go, New Zealand's 'net' emissions of carbon dioxide are projected to be well above 1990 levels in the absence of further policy measures. On the other hand, much of the global warming potential of the carbon dioxide increase has been offset by small declines in methane and nitrous oxide emissions. These are already below 1990 levels and are expected to remain below them by the end of the decade. Perfluorocarbon emissions have also declined and are expected to be stable. However, other greenhouse gases have increased. Along with all other developed country parties, New Zealand is now in the process of negotiating new objectives for all greenhouse gases for the period beyond 2000.

New Zealanders have also become more aware of their vulnerability to ozone depletion. Rising skin cancer rates have reinforced this awareness, even though most cases are attributable to sunburning that occured in the decades before ozone depletion. However, the risk is increasing. Solar radiation is now 6-9 percent more intense in New Zealand than it was three decades ago and New Zealand summers coincide with the season of lowest ozone density. While many people now take active measures to avoid sunburn, it will be half a century or more before the ozone layer recovers and the risk of sun damage to people, property and other species declines.

That recovery depends very much on worldwide efforts to limit the use of ozone-depleting substances under the international agreement known as the Montreal Protocol. So far, the Protocol and its amendments have been hailed as successful, though ozone depletion will continue to occur for some decades because of the longevity of the ozone-depleting chemicals and because many of the world's poorer nations have yet to begin limiting their use of these.

New Zealand's phase-out of ozone-depleting substances is meeting the timetable set out in the Montreal Protocol. Imports of most synthetic ozone-depleting substances were stopped on 1 January 1996. However, manufactured products containing chlorofluorocarbons (CFCs) can still be imported, and imports of methyl bromide for quarantine use and fumigation of export products have increased. As a result, methyl bromide now appears to be the major ozone-depleting gas emitted in New Zealand.

The idea that human actions could have measurable impacts on something as vast as the atmosphere was almost unheard of two decades ago, and is still resisted by some today. Given this, the speed of the international response to both ozone depletion and the risk of climate change has been quite remarkable. However, the response has not been uniformly successful. The ozone issue has been relatively easy to deal with because ozone-depleting substances play a relatively small role in the world economy and alternatives have readily been found.

The climate change issue has been harder to deal with because fossil and wood fuels play such a vital role in most of the world's economies, as do rice paddies and ruminant farm animals. New Zealand is therefore not alone in finding it hard to contain the rise of greenhouse gases. Fortunately, we are also not alone in the search for solutions and are participating fully in international efforts to develop an effective response, not only for New Zealand, but for the world.