What is climate change?
Earth’s atmosphere is made up of oxygen, a large amount of nitrogen and a small percentage of greenhouse gases, such as carbon dioxide and methane.
Greenhouse gases act like a blanket around the Earth. They trap warmth from the sun and make life on Earth possible. Without them, too much heat would escape and the surface of the planet would freeze. However, increasing the concentration of greenhouse gases in the atmosphere causes the Earth to heat more and the climate to change.
This process is often called global warming, but it is better to think of it as climate change. This is because it is likely to change other aspects of climate as well as temperature, and also bring about more extreme climate events such as floods, storms, cyclones and droughts.
Multiple lines of evidence show climate change is happening
There is lots of evidence that tells us the average temperatures of the world's atmosphere and oceans have increased over the past 150 years.
The evidence includes:
- direct temperature measurements on land
- changes in the dates when lakes and rivers freeze and their ice melts
- a reduction in the extent of snow cover in the Northern Hemisphere
- a reduction in glaciers
- extended growing seasons of plants
- changes in the heat stored in the ocean
- changes in rainfall patterns resulting in more floods, droughts and intense rain.
A number of biological changes have also been observed.
- shifts in the ranges of some plant and animal species
- earlier timing of spring events such as leaf-unfolding, bird migration and egg-laying for some species.
Together these indicators provide clear evidence that the climate is changing.
It is extremely likely that humans are the dominant cause of recent warming
It is true that climate change has been driven by natural causes in the past. Our climate has changed over millions of years — from ice ages to tropical heat and back again. Natural changes over the past 10,000 years have generally been gradual. This has enabled people, plants and animals to adapt or migrate. , However, some prehistoric climate changes may have been abrupt and are likely to have led to mass extinction of species.
Over the past 150 years there has been a marked and growing increase in greenhouse gas producing activities such as industry, agriculture and transportation. These human-induced activities are increasing the level of greenhouse gases in our atmosphere and causing the Earth to heat up at an unprecedented rate. This recent warming can only be explained by the influence of humans.
The levels of carbon dioxide and methane in the atmosphere are increasing
Human activities have caused carbon dioxide and methane in the atmosphere to increase to the highest levels in at least 800,000 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.
Direct measurements of atmospheric concentrations of greenhouse gases show how our global greenhouse gas emissions have grown in past decades.
These analyses prove that today's greenhouse gas concentrations are far higher than they were at any time during the past 800,000 years
The Earth’s temperature is changing at a rate unprecedented in recent history
Globally, our climate has been relatively stable for the past 10,000 years. If the world does not take action to reduce greenhouse gas emissions, the global average temperature is very likely to change more rapidly during the 21st century than as a result of any natural variations over the past 10,000 years. This will make it difficult for plants and animals to adapt to climate change.
The effects of climate change will continue even after emissions are reduced
The climate system is very complex, and takes a long time to change. As a result, the effects of climate change will continue even if we reduce emissions now. For example, 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. Although we cannot avoid climate change entirely, reducing our emissions can limit its impact.
Future climate change
How the climate will change in the future largely depends on the total sum of greenhouse gases emitted since the start of the industrial revolution. Greenhouse gas emissions have continued to increase over past decades and limiting climate change will mean reversing this trend. Future climate change also depends on how the Earth responds to the increased heating. So we cannot be precise about future climate change. But we are generally sure of the direction of change (eg, the world will become warmer and global average sea-levels will rise).
It is predicted that without additional efforts to reduce greenhouse gas emissions, the Earth’s surface temperatures will increase between 3.7°C and 4.8°C by 2100, relative to the average temperature from 1850-1900. However, if we reduce greenhouse gas emissions by 40 to 70 per cent by 2050, the average warming is likely to stay below 2°C.
Climate change will affect New Zealand
Climate change is already affecting our climate. It is likely to impact our agriculture and other climate-sensitive industries, our native ecosystems, infrastructure, health and biosecurity, as well as having broader social and economic impacts.
New Zealand can expect to see changes in wind and sea current patterns, storm tracks, the occurrence of droughts and frosts and the frequency of heavy rainfall events, as well as rising temperatures. The impacts of climate change in New Zealand will become more pronounced as time goes on.
Watch a video on the science of climate change
The Intergovernmental Panel on Climate Change (IPCC) provides comprehensive assessments of climate change science covering the physical science basis, impacts, adaptation and vulnerability, and mitigation.
The following video funded by the UN Foundation summarises information from IPCC’s 2013 report on the physical science of climate change.
Climate Change — The state of the science [Vimeo website]