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Key facts about climate change

Earth is getting warmer

There is lots of evidence that tells us the average temperature of the world's atmosphere and oceans have increased - not just direct temperature measurements on land and at sea, but also changes in the dates when lakes and rivers freeze and their ice melts, the extent of snow cover, reduction in glaciers, extension of growing season, and change in the heat stored in the ocean. These indicators provide unequivocal evidence that the climate is changing.

It is very likely that most of the global warming over the past 50 years is due to the increase of greenhouse gases from human activities.

Many natural systems are being affected

Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes, particularly temperature increases.

Levels of carbon dioxide and methane in the atmosphere have increased as the result of human activities and are now higher than they have been in 650,000 years

We know this from a number of ice core studies. Snow traps tiny bubbles of air as it falls and is compressed into ice. Over the years, more and more ice layers stack up on top of each other. Drilling into ice sheets in Antarctica and Greenland provides a record of what the atmosphere was like back in time. These analyses provide very clear and consistent results that today's greenhouse gas concentrations are now far higher than they were at any time during the past 650,000 years. Other measurements show how our global greenhouse gas emissions have grown. They increased by 70% between 1970 and 2004.

If we don’t reduce global greenhouse gas emissions, carbon dioxide concentrations in the atmosphere will continue to rise during the 21st century

The Intergovernmental Panel on Climate Change (IPCC) has used a range of scenarios to determine what might happen to the world’s atmosphere if we don’t take action to reduce global greenhouse gas emissions.

The scenarios indicate it is very unlikely that carbon dioxide concentrations in the atmosphere will stabilise before 2100, unless global action is taken to reduce emissions.

Without action, the world's average temperature is very likely to change more rapidly during the 21st century than during any natural variations over the past 10,000 years

Climate changes have occurred naturally in the past, and some regional changes have been significant. But globally, our climate has been relatively stable for the past 10,000 years.

The rate of global temperature change over the next 100 years will almost certainly be unlike anything human civilisation has experienced before, if the world does not take action to reduce greenhouse gas emissions.

Climate change is not just rising temperatures

New Zealand might expect to see changes in wind patterns, storm tracks, occurrence of frosts, and the frequency of heavy rainfall events. These effects will become more pronounced as time goes on.

The effects of climate change will continue beyond 2100. Sea-levels will continue to rise for centuries after greenhouse gas concentrations have stabilised

The climate system takes time to change. In particular the deep oceans take centuries to heat up when the atmosphere above them warms. This means that oceans will continue to heat up, and therefore expand causing sea levels to rise, even if greenhouse gas concentrations in the atmosphere are no longer increasing.

The climate system is very complex and there are still uncertainties about future climate changes

Future climate change depends on how much greenhouse gases are emitted in future (and hence on the development pathway of human society) and also how the Earth responds to the increased heating. So we cannot be precise about how much the climate will change in the future. We are generally sure of the direction of change (for example, the world will become warmer, and global average sea-levels will rise), and we can give plausible ranges for those changes (for example, the range of emissions looked at by the IPCC is expected to increase the world's average temperature by between 1.1 and 6.4 degrees Celsius by 2100).

To manage the wide-ranging risks resulting from climate change, we need to account for the full range of those possible outcomes. We cannot rely on only the best case scenario coming true - we need to be prepared for the other possible scenarios as well.

The Intergovernmental Panel on Climate Change (IPCC) is the best source of information on global climate change. Its reports undergo a painstaking multiple review process and cover all relevant viewpoints.

Because of the complex and interdisciplinary nature of climate science, it is relatively easy for individual scientists and even individual organisations to have an unbalanced assessment of the risks of climate change.

The IPCC brings leading experts in the various fields together, including those with unorthodox or sceptical views, to assess the current state of knowledge across international scientific literature. Their reports are peer-reviewed in an open and international process. This ensures that a range of possible futures is captured, but no "special interest" skews the balance of the reports. This inclusive review process sets the IPCC apart from most other organisations and individual opinion pieces. This is the most reliable way of achieving a balanced assessment of the scientific knowledge in such a complex subject area.

Last updated: 20 March 2008