Why is it that everyone talks about global warming these days? And what actually causes climate change?
The main thing affecting our climate is the amount of sunlight absorbed at the Earth's surface, which is more in the tropics, in summer and when the sun is near its zenith. There are other things that influence how much sunlight is absorbed, like the amount of clouds, snow or ice, which reflect sunlight back out to space cooling the climate.
But the other important factor is the atmosphere. If the Earth had no atmosphere, the average temperature of its surface would be about 30 degrees lower than it is today. It would be too cold for most life to survive.
There are important gases in the atmosphere, sometimes called "greenhouse gases" that warm the climate by preventing the loss of heat radiation from the Earth's surface out to space. The most important greenhouse gas is water vapour. But adding more water vapour to the atmosphere doesn't warm the climate; it falls out as rain, usually in less than a week.
There are other greenhouse gases, such as carbon dioxide and methane, that stay around in the atmosphere much longer. When extra carbon dioxide is added to the atmosphere, about half of it stays there for more than 50 years, warming the planet.
Climate has changed a lot in the past, like during ice ages when global temperatures were about 6 degrees lower than today. Ice ages are started by changes in the Earth's orbit around the sun, which allow snow and ice to build up in polar regions, reflecting more sunlight and cooling the climate system. As the climate cools, the amount of water vapour in the atmosphere drops and more carbon dioxide is soaked up by the ocean, so there are less greenhouse gases around to warm the climate.
Why are we hearing so much about climate change recently? There has been a 0.8-degree jump in global average temperatures over the past 100 years, which is larger than anything we've experienced for at least the past 1000 years. Signs of this warming are found in rising ocean temperatures, melting of mountain glaciers, shrinking snow packs, earlier flowering of plants in spring and less Arctic sea ice in summer.
Carbon dioxide and methane concentrations have increased dramatically too: their levels in the atmosphere are much higher than at any time over the past million years.
These increases in greenhouse gases are not due to natural cycles but are caused by human activity. Burning fossil fuels like oil, coal and natural gas, deforestation and agriculture all release greenhouse gases faster than the planet can recycle them back into the environment.
MAN-MADE OR NATURAL?
But how do we know if greenhouse gases have caused most of the global warming we've seen over the past century? Studies have shown that natural climate variations, like changes in the intensity of sunlight or volcanic eruptions, are too small to explain the observed warming.
There is a very good match between what we expect to see with increasing greenhouse gases and what has actually been measured by scientists.
This includes greater warming at high latitudes, in winter and at night, together with cooling in the upper atmosphere.
The evidence for climate change can't be explained by natural causes alone - greenhouse gas increases are warming our world.
Emissions trading: One way of reducing greenhouse gas emissions. An authority (national or international) sets a limit on emissions and then creates emission permits up to that cap. Companies or countries are allocated permits or have to buy permits to pollute. Companies with high emissions may have to buy permits from other companies that have extra permits because they reduce their emissions.
Carbon dioxide equivalent (CO2) concentration: The amount of carbon dioxide in the atmosphere that causes the same change in global radiation as a mixture of other greenhouse gases like methane and nitrous oxide.
Global warming potential (GWP): An index measuring the effect on global radiation of a long-lived greenhouse gas over 100 years, relative to that of carbon dioxide. The GWP represents the combined effect of the differing times these gases remain in the atmosphere and their relative effectiveness in absorbing heat radiation.
IPCC: Intergovernmental Panel on Climate Change, an organisation established under the United Nations to assess all aspects of climate change. Its latest report was released in 2007 and is available at www.ipcc.ch.