Climate change, global warming and greenhouse gases
Climate data and common terms explaining the causes and effects of climate change.
On this page
- Has the climate shifted?
- Has the climate changed in the past?
- How do we determine past climate?
- How is the climate expected to change in the future?
- What is the IPCC?
NIWA identified in August 1998 that a significant shift in the New Zealand climate has occurred during the past 20 years. The changes have resulted from a strengthening of highs to the north of New Zealand, squeezing stronger westerly winds over southern and central New Zealand. Since 1977:
- the north and east of the North Island has become 10 percent drier and five percent sunnier, with more droughts.
- the west and south of the South Island has become 10 percent wetter and five percent cloudier, with more damaging floods.
- night temperatures continue to rise.
- fewer frosts are occurring nation-wide.
- major South Island glaciers have retreated. The volume of ice in the Southern Alps has reduced by 11% in the past 30 years.
The Earth's climate has exhibited marked "natural" climate changes, with time scales varying from many millions of years down to a few years. For example:
- Changes in land and ocean floor topography have had major influences on global climate at time scales of 50 million to 150 million years. (These changes influenced the patterns of absorption of incoming radiation from the sun, and affected circulation patterns in the atmosphere and oceans).
- Over the last two million years the onset and recession of the great Ice Ages were probably influenced by changes in the earth's orbit and the tilt of its axis, which caused systematic variations in the amount and distribution of solar radiation. Global average temperatures varied by about 5 - 7°C.
- Since the end of the last ice age (14,000 - 10,000 years ago) globally averaged surface temperatures have fluctuated over a range of up to 2°C on time scales of centuries or more. Factors influencing these changes probably included fluctuations in the radiation output from the sun, and changes in circulation and overturning in the oceans.
- Over periods of a few years, fluctuations in global surface temperatures of a few tenths of a degree are common. Some of these are related to the El Niño - Southern Oscillation phenomenon, and major volcanic eruptions have also had some effects.
View the images on the right for New Zealand's estimated mean yearly temperatures since the last ice age and Mean annual temperatures over New Zealand, from 1864 to 1998 inclusive.
Information about past climate is obtained from piecing evidence together from various sources, including:
- Iie cores: The ratio of oxygen isotopes in ice indicates the temperature at the time ice was deposited as snow. Also, air bubbles can be analysed to measure carbon dioxide and methane concentrations at the time the bubbles were trapped in the ice.
- fossil Pollen: Different classes of plants produce pollen grains with different distinctive shapes. Such pollen grains are often found preserved in sediment cores from ponds, lakes and oceans. They provide information on the type of plants that grew nearby when the sediments were formed.
- lake sediments: Composition and sedimentation rates change in response to environmental conditions. Pollen in the sediments can indicate the type of vegetation present, and plankton biota indicate physical and chemical conditions in the lake water.
- ocean sediment cores contain primitive shelled animals (foraminifera) whose abundance in the surface layers of the ocean depends on surface water temperature and other conditions.
- loess deposits on land of wind borne material. Its accumulation at a particular location can provide information on past windiness and dryness.
- glaciers: Variations in the past size of glaciers can be inferred from the location of moraines (rocks and debris deposited by glaciers) and buried soils, and in the presence of glacial features in the landscape. In New Zealand, cool summer temperatures are only one factor in promoting ice accumulation on glaciers, and snow accumulation rates also respond to changes in the strength and direction of the westerly wind flow and sea level pressure in summer.
- speleothems: Glacial deposits embedded within speleothems (stalactites and stalagmites) can be used to indicate periods of glacial advance (the speleothems can be dated using uranium isotope techniques). A cave in Fiordland New Zealand, which has been repeatedly overrun by glaciers, provides information going back 230,000 years.
- tree ring width depends on the soil moisture, temperature and other growing conditions. Annual rings of trees in temperate forests can be used to reconstruct past climates.
- boreholes: It is sometimes possible to deduce past surface temperatures going back several hundred years by measuring the way temperature varies with depth in a borehole several hundred metres deep (at a suitable site not disturbed by groundwater flow). This is because fluctuations in ground surface temperatures propagate slowly downwards into the earth as a "temperature wave".
- instrumental Measurements and written or oral records: In New Zealand, quantitative records of temperature and other meteorological records are available only for the past 150 years. Such records must be analysed carefully, to identify the influence of any non-climate factors (such as changes in observing site or method, or encroaching urban development).
Projected Future Changes based on the IPCC 2007 Assessment
There are likely to be more severe droughts and/or floods in some places, and less severe droughts and/or floods in others. Some models suggest a possibility of more extreme (i.e. high) rainfall events. It is not yet possible to predict whether changes are likely in the occurrence or geographical distribution of severe storms, such as tropical cyclones.
Model projections for increases in global sea level by 2100, for the same scenarios, range from about 18 to 59 cm. Due to limited understanding of some important drivers of sea level rise, these projections likely underestimate future sea level rise. In addition, regional sea level changes may differ from the global mean value.
Model projections, based on a range of emission scenarios and climate sensitivities, suggest an increase in global mean surface temperature of between about 1.8°C and 4°C by 2100. Regional temperatures could vary significantly from the global mean value, and actual annual to decadal changes would include considerable natural variability.
Predictions are still subject to uncertainties, and there is less confidence in predictions for regions the size of New Zealand than for the entire planet. Because of limits in understanding of aspects of the climate system some future changes may come as a surprise.
In the videos below one of New Zealand's leading climate scientists, Dr. David Wratt, discusses changes in weather due to climate change and how rising levels are likley to affect us in the future.
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). It is charged with assessing the most up to date scientific, technical and socio-economic research in climate change.
The IPCC produced major assessment reports in 1990, 1995, 2000, and 2007. Their Fifth Assessment Report is scheduled for completion in 2014.
- What are greenhouse gases?
- Are they changing in the atmosphere?
- Do we know why?
- Why do we expect climate to change?
- What can we do about it?
- Have greenhouse gas emissions caused global temperatures to rise?
- What is the greenhouse effect?
- What is the enhanced greenhouse effect?
- How does methane gas from cows damage our environment?
Greenhouse gases warm the atmosphere by absorbing some of the thermal radiation emitted from the Earth's surface. Incoming solar radiation is transmitted through the atmosphere to the Earth's surface. The energy is retransmitted by the Earth's surface as thermal radiation. Some of the thermal radiation is absorbed by the greenhouse gases instead of being retransmitted out to space, and so there is a warming of the atmosphere. The important greenhouse gases which are directly influenced by human activities are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone. Water vapour is also an important greenhouse gas.
The concentrations of greenhouse gases in the atmosphere are increasing due to human activities. The Industrial Revolution has resulted in an increase in the concentration of CO2 in the atmosphere of about 30%, from 280 ppmv around the year 1700 to a value of over 360 ppmv at the present day. Accurate measurements of CO2 have been made in New Zealand at Baring Head since 1971. Data from ice cores show that during the 17th and 18th centuries the CH4 concentration in the atmosphere was about 0.7 ppmv. The current CH4 concentration is more than double that value, approximately 1770 ppb.
The increasing concentration of CO2 is caused by the burning of fossil fuels (such as oil, gas and coal), and the destruction of forests. These activities release large amounts of CO2 into the atmosphere. The main natural source of CH4 is from wetlands. A variety of other sources of CH4 result directly or indirectly from human activities, for example from ruminant animals, rice paddies, leakage from natural gas pipelines, and from the decay of rubbish in landfill sites. However, CH4 growth rates have declined since the 1990s, potentially due to reduced gas pipeline leaks and the drying of wetlands.
Increased amounts of greenhouse gases in the atmosphere will absorb more thermal radiation, and the Earth's surface and the lower atmosphere will warm. This extra warming is called the enhanced greenhouse effect.
The United Nations Framework Convention on Climate Change (UNFCCC) came into force on 21 March 1994. The ultimate objective of the Convention is:
... stabilization of greenhouse gas concentrations in the atmosphere at a level that would
prevent dangerous anthropogenic interference with the climate system...
New Zealand is a party to the UNFCCC and has signed the Kyoto Protocol. The Kyoto Protocol commits us to returning our emissions of greenhouse gases back to 1990 levels, on average, over 2008-2012. Important contributions that can be made by ordinary individuals are:
- Ensure maximum energy efficiency at home;
- Support where possible the provision of energy from renewable sources;
- Drive a fuel-efficient car and choose means of transport which tend to minimise overall energy use;
- When buying wood products, check that they originate from a renewable source.
- Greenhouse gas concentrations have continued to increase in the atmosphere. This is due largely to human activities, mostly fossil fuel use, land-use change, and agriculture. About 47% of the warming effect of greenhouse gas increases over the last 100 years is due to carbon dioxide.
- The second most important greenhouse gas produced by human activities is methane, which accounts for about 35% of the increased warming over the past 100 years (this is an important aspect of New Zealand's greenhouse gas emissions since sheep and cows produce methane).
- Warming by greenhouse gases is offset in some regions by cooling due to small airborne particles generated by burning fuel. These are concentrated around areas of industrial activity in the Northern Hemisphere and in developing countries. (The cooling effect of aerosols over the New Zealand region is expected to be small).
- Global mean surface temperature increased by 0.74°C between 1906 and 2005, a change which is unlikely to be entirely natural in origin. The balance of evidence suggests a discernible human influence on global climate. Much of the 1.8±0.5 mm yr-1 average global sea level rise between 1961 and 2003 may be related to the rise in global temperature.
Energy emitted from the sun ("solar radiation") is concentrated in a region of short wavelengths including visible light. Much of the short wave solar radiation travels down through the Earth's atmosphere to the surface virtually unimpeded. Some of the solar radiation is reflected straight back into space by clouds and by the earth's surface. Much of the solar radiation is absorbed at the earth's surface, causing the surface and the lower parts of the atmosphere to warm.
The warmed Earth emits radiation upwards, just as a hot stove or bar heater radiates energy. In the absence of any atmosphere, the upward radiation from the Earth would balance the incoming energy absorbed from the Sun, with a mean surface temperature of around -18°C.
The presence of greenhouse gases in the atmosphere, however, changes the radiation balance. Heat radiation (infra-red) emitted by the Earth is concentrated at long wavelengths and is strongly absorbed by greenhouse gases in the atmosphere, such as water vapour, carbon dioxide and methane. As a result, the surface temperature of the globe is around 15°C on average, 33°C warmer than it would be if there was no atmosphere. This is called the natural greenhouse effect.
If extra amounts of greenhouse gases are added to the atmosphere, such as from human activities, then they will absorb more of the infra-red radiation. The Earth's surface and the lower atmosphere will warm further until a balance of incoming and outgoing radiation is reached again (the emission of infra-red radiation increases as the temperature of the emitting body rises). This extra warming is called the enhanced greenhouse effect.
Methane is a so-called greenhouse gas. Greenhouse gases impact the environment through warming the atmosphere. The concentration of greenhouse gases has risen significantly in the past 200 years, in part due to human activities. One of the biggest contributors to the atmospheric methane concentration is farmed livestock, especially cattle and sheep. These animals produce methane naturally as part of their digestive process, and belch it, especially while 'chewing the cud'. As the human population has grown, the number of farmed animals has increased markedly to meet the human demand for food through meat and dairy products.
The links below provide graphs of recent trends in the most significant atmospheric greenhouse gases, namely carbon dioxide, methane and nitrous oxide.
- Amount of carbon dioxide in the air at Baring Head since 1971
- Amount of carbon dioxide in the air at Baring Head since 1999
- Growth rate of carbon dioxide in the air at baring head since 1971
- Amount of radioactive carbon 14 in atmospheric carbon dioxide in the air at Baring Head since 1955
- Amount of methane in the air at Baring Head since 1990
- Growth rate of methane in the air at Baring Head since 1990
- Measurement of carbon 13 in atmospheric methane at Baring Head since 1988
- Amount of nitrous oxide in the air at Baring Head since 1996
- Amount of oxygen (measured as a ratio of oxygen to nitrogen) in the air at Baring Head since 1999
The 'Carbon dioxide exercise' provides questions for students relating to graphs showing levels of carbon dioxide in the atmosphere.
The above is an overview of the causes and results of the Greenhouse Effect, describing the major greenhouse gases, New Zealand's greenhouse gas emissions, atmospheric chemistry, clouds and climate change, and options for the future.
Links to other websites on climate change and global warming
- NASA Goddard Institute of Space Studies on climate research
- United States National Oceanic and Atmospheric Administration (NOAA) on climate research
- United Kingdom Meteorological Office pages on climate change
- United Kingdom Department of Environment, Food and Rural Affairs pages on climate change