Global warming and climate change

The temperature of the earth’s surface has risen over the past 100 years. A small part of this increase has probably been caused by natural climate variations, but there is strong evidence that most of the warming over the past 50 years is a result of greenhouse gas emissions caused by human activity.

Climate models predict that greenhouse gas emissions will continue to increase atmospheric temperatures. The rise projected for the next 100 years is likely to be more rapid than any natural variations over the past 10,000 years. Because rising temperatures cause changes (often drastic) in the climate, the effect of global warming is often referred to by the more general term, “climate change”.

Greenhouse gases

Greenhouse gases that naturally occur in the atmosphere make life on earth possible. Without them, too much heat would escape from the atmosphere and the earth’s surface would freeze. In too high a concentration, greenhouse gases hold in excessive heat and cause the earth’s climate to become more and more unstable.

The main greenhouse gases are:

  • carbon dioxide from burning of fossil fuels and from deforestation;
  • methane from farm animals and waste (as a greenhouse gas, methane is 21 times more powerful than carbon dioxide);
  • nitrous oxide from soil (nitrous oxide is 310 times more powerful as a greenhouse gas than carbon dioxide);
  • synthetic gases like sulphur hexafluoride, perfluorocarbons and hydrofluorocarbons.

We call these gases “greenhouse gases” because they act like the outside covering of a greenhouse, letting the sun’s warmth through to heat the ground, but preventing it from escaping back into space. Greenhouse gases absorb heat radiated or reflected from the ground, increasing the temperature of the atmosphere.

Almost 50 percent of New Zealand’s greenhouse gas emissions are made up of methane and nitrous oxide, the two gases most closely associated with farming.

The effects of climate change

The effects of climate change are already measurable – the world’s temperatures and sea levels are rising, and most glaciers are retreating. Changes in regional rainfall patterns have already been observed and are expected to alter more strongly as climate change continues. The frequency of some extreme weather and climate events such as heatwaves, droughts and floods is also expected to increase.

These changes are likely to influence native ecosystems, agriculture, coastlines, and our economy, infrastructure, health and security. For example, changing weather patterns could cause increases in the number of refugees seeking international support, as repeated droughts and floods drive people from their traditional homes.

Not all impacts will necessarily be negative and the severity of impacts will vary across the globe. But it is almost certain that, overall, more people will be harmed by climate change than will benefit from the changes. Adverse impacts will become ever more predominant, and beneficial effects are expected to diminish because of larger cumulative emissions of greenhouse gases and associated changes in the earth’s climate.

Because of the long life-time of some greenhouse gases in the atmosphere, there will be time lags of decades to centuries between reductions in emissions and a corresponding halt to temperature increases.

In his report on the economics of climate change to the UK Treasury in October 2006, former World Bank economist Sir Nicholas Stern said climate change risks for the global economy were as great as those “associated with the great wars and the economic depression of the first half of the 20th century”.

Evidence for climate change

There is now clear evidence that  the earth’s climate system has demonstrably changed since pre-industrial time, and that most of the warming over the last 50 years has been caused by emissions of greenhouse gases created by humans.

The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) reports that warming is expected to continue, with an increase in globally averaged temperatures of between 1.4 and 5.8° C. This is two to ten times larger than observed warming in the 20th century.

How high the temperatures go, how soon and whether changes can be reversed depend on human action. The greater the reduction in greenhouse gas emissions, and the earlier they are made, the smaller and slower the projected warming and rise in sea levels.

Contact for Enquiries

Sustainable Land Management and Climate Change
MAF
Pastoral House
25 The Terrace
PO Box 2526, Wellington
Tel: 0800 CLIMATE (254 628)
Contact this person