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What is climate change?

Climate change will increase the frequency of drought.Climate change, sometimes called “global warming”, is the most serious and most complex environmental issue ever to confront the international community.

The earth’s climate is changing – air and sea temperatures are rising, and droughts, floods and storms are becoming more frequent. The earth’s climate has changed before, of course, but gradually over thousands and even millions of years, long enough for most plants and animals to adjust through evolution or migration.

During the last century, however, the climate has been changing more rapidly. Average surface temperatures have risen by about 0.6oC during the last century. Projections of these trends suggest much more dramatic warming could occur in future. The potential consequences for plants, animals and humans are enormous.

Hydro power may become less reliable.In New Zealand, global warming is expected to accentuate the effects of events like El Niño, which have brought damaging droughts to farmlands, and costly water shortages to our hydroelectric dams. It will increase a wide range of risks such as forest fires, biosecurity threats, and endangerment of New Zealand’s native species.

The enhanced greenhouse effect

Global warming is caused, at least in part, by what is sometimes called the “enhanced greenhouse effect”. The earth’s atmosphere traps heat from the sun and makes our planet habitable – that is the “greenhouse effect”. But when we burn fossil fuels or release carbon dioxide (CO2) in other ways, such as deforestation, we increase CO2 concentrations in the atmosphere. This enhances the greenhouse effect, trapping more heat and causing average temperatures on the earth to rise.

Sheep wearing devices to measure their methane emissions.CO2 isn’t the only greenhouse gas (GHG). Other gases, including methane (CH4) and nitrous oxide (N2O), also contribute to climate change by enhancing the greenhouse effect.  Measurements by scientists have shown that methane released by inefficient digestion in ruminant animals (sheep and cows) is the largest source of greenhouse gas from New Zealand.

Methane is also produced from landfills, wastewater treatment and rice paddies. Sources of nitrous oxide include biological processes in soils that are increased by fertiliser application and animal urine, and wastewater treatment. There are a handful of other, mainly industrial, chemicals that also contribute to climate change.

To slow down the global warming process and eventually bring it under control, a global effort is being mounted to reduce emissions of all GHGs. Governments, socially responsible businesses and individuals all have roles to play in this effort.

The scientific evidence

A vast amount of research has been done on climate change, its causes and implications. There is virtually no argument that the climate is changing, and there is a growing international consensus that emissions from human activities are largely responsible.

Global temperature trends from the last one thousand years have been reconstructed from fossil evidence and, over the last century or so, from instrument readings. Recent readings show a steep increase.

IPPC data show how temperatures are rising since 1900 (far right).

Atmospheric concentrations of carbon dioxide and methane over the last few hundred years have been measured from air bubbles trapped deep in polar ice, and more recently, from instrument readings. The pattern is of steep increases of both these GHGs since pre-industrial times.

IPPC data - GHG concentrations are rising.

If global warming is allowed to proceed too far, there are feedback mechanisms such as increasing forest fires and loss of soil carbon which could create dangerous increases in GHG emissions in the second half of this century.

Hadlye Centre graph shows how soil carbon stocks might decline.

More detailed information regarding the current state of evidence of climate change is available from the website of the Intergovernmental Panel on Climate Change (IPCC) at You may also want to visit, a commentary site on climate science by working climate scientists for the interested public and journalists.

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