Climate change is the biggest and most controversial environmental issue of our times. Or rather, the cause of climate change is.

The fact that the Earth's climate has changed over its history - sometimes with cataclysmic consequences, called mass extinctions, for many of the planet's inhabitants - is not disputed. However, what has been the cause of fierce debate is whether or not human activity is currently causing a warming of the world.

What climate change, man-made or not, is not - is short term weather. These trends are much bigger and much longer term than a hot summer or a cold winter, we're thinking more of ice ages than cold snaps when we talk about climate change.

There are a number of reasons why the Earth's climate has changed historically. As the continents have moved through the process of plate tectonics they see changes in their climate, both as a result of the influence of the changing oceans and the size of landmass.

The Sun also plays a role: as the main source of heat and light for the planet, its activity is a major player in our climate and it is not a constant; fluctuating both cyclically and as it goes through its lifespan as a star.

The Earth's position relative to the sun is also not as constant as you might like to think, we're not in a circular orbit and the tilt of the planet also changes, causing changes in how all that heat and light from the Sun hits the planet's surface. Volcanic activity too can change climate by putting large amounts of material into the Earth's atmosphere and thus reflecting heat away from the surface.

Such large eruptions are however rare, in fact, the phrase ''once in a blue moon'' probably comes from the change in the atmosphere caused by ash plumes from the eruption of Krakatoa in 1883. It's also been theorised that asteroid strikes on the planet have a similar effect, throwing material into the sky, and some scientists believe that the end of the age of the dinosaurs may have been caused by a giant asteroid hit.

The final reason why climates change - and this is where the controversy comes in - relates to human activity, or anthropogenic global warming, which is what is meant when you read a news story about climate change.

Primarily, this has referred to the misleadingly named greenhouse effect. While a greenhouse warms the air by allowing in and retaining heat and not allowing in cooling air, greenhouse gases warm the planet by absorbing the Sun's heat and then reemitting it into the atmosphere.

The main greenhouse gases are: water vapour, carbon dioxide (CO2),methane, water vapour, ozone, nitrous oxide and CFC-12, a chlorofluorocarbon the use of which in many countries as an aerosol propellant and refrigerant has been banned. With the exception of CFC-12, which is man-made, these gases have historically existed in the atmosphere and there have been natural fluctuations (for example volcanoes emit CO2) in their levels.

The most common of these gases and thought to be the most significant greenhouse gas is water vapour but it's one on which human activity has little effect. As air warms it can hold more water, the increase in water vapour is said to be responsible for a possible amplification of global warming as the temperature warms.

Plants, which rely on CO2 to survive and which use and store it as they photosynthesise are said to be natural carbon sinks and over history natural variations in the levels of CO2 in the atmosphere are thought to have been balanced by their action.

However, since around the middle of the 18^th Century, human activity affecting the levels of carbon dioxide in the atmosphere has rapidly increased. Since the industrial revolution took hold we not only burned more CO2-emitting fuels, from wood to coal to oil, but we have also massively reduced the amount of vegetation on the planet.

Is the Climate Changing

In July 2010 the British Government's Meteorological Office and the United States National Oceanic and Atmospheric Administration issued findings that they said showed unequivocally the world was warming. Using 10 indicators, seven temperature measures and three ice or snow cover measures, they said that each of the last three decades has been warmer than the last and successively broken temperature records.

Action on Climate Change

The reason why climate change has become so controversial is because people are being asked to make massive lifestyle changes in their lifestyle to help mitigate the effects of man made global warming. If action on climate change amounted to legislation to outlaw, say, wooden pencils then, while scientists may debate the rights and wrongs of the issue, you can almost be sure that our media would not be filled with the dispute.

The roots of world-wide action on climate change date back to the 1988 foundation of the Intergovernmental Panel on Climate Change (IPCC) by the World Meteorological Organisation, a department of the United Nations in 1988.

Since its foundation it has reported regularly on the state of climate change, with its 1990 report inspiring the United Nations Framework Convention on Climate Change (UNFCCC), the first international treaty that aimed to reduce global warming, which was signed at the so-called Earth Summit in Rio de Janeiro in 1992.

One of the key moments in the growth of concern about global warming was the release in 2006 of the film, An Inconvenient Truth. The documentary followed former US Vice President Al Gore as he tried to convince audiences about the seriousness of climate change. Gore won a Nobel peace prize as a result, but, like everything else to do with climate change the film has been the subject of much debate, particularly when schools have tried to show it to pupils.

The countries that signed the treaty have met since, with much fanfare, but often to little effect. The most recent major meeting was at Copenhagen in 2009 and was widely criticised by environmentalists.

Much of what has been agreed is also controversial, particularly so-called carbon trading arrangements which aim to set a marketplace for carbon credits sold by those who live with a small carbon footprintor contribute to carbon reduction by, for example, planting trees, to those who pollute.

Most countries have set targets for the reduction in carbon emissions. For example, the British Government's Climate Change Act of 2008 set legally-binding targets of a 34% reduction by 2020 and at least 80% by 2050.

Consequences

Again, the possible consequences of climate change are the subject of much to-ing and fro-ing with accusations of irresponsible scare-mongering and reprehensible complacency flying between the parties.

However, the IPCC has produced estimates - and the sheer complexity of climate systems and thus the difficulty of predicting how they will react using computer models makes them open to criticism - of what may happen as temperatures rise. Broadly speaking, most are catastrophic to both human life and to many other species on the planet.

Controversy

Unless you've already moved under a rock in preparation for climate chaos, you will have noticed that the issue of global warming is a controversial one.

There has been criticism of the IPCC and its work, the so-called climategate scandal involving leaked emails from the University of East Anglia's climate studies centre and doubt has been cast on the very idea that humans could be causing warming of the globe.

Even the controversy is controversial. Environmentalists often refer to climate sceptics as climate deniers, claimed by their opponents to be a deliberate attempt to ally them in the public mind with far right wing holocaust deniers. Many who criticise the science that claims to show that human activity is causing global warming are accused of being funded by the oil industry and free market think tanks who oppose the sort of government regulation that it seems will be necessary to implement large reductions in greenhouse gases, especially CO2

Personal Action

One of the strongest ideas of the green movement has been 'think global act local', which empowers people to believe that their own actions can have an effect on problems that are as big as the planet.

This applies to climate change arguably more than any other issue. What can I possibly do? Has even become a plaintive refrain of whole western nations shrugging their shoulders as they watch the rapid and dirty industrialisation of new economic giants like China and India.

However, once you accept the idea of climate change, then doing nothing doesn't really seem an option. It's possible to join any number of groups which campaign for environmental issues and almost all of which make global warming a major part of their efforts. Lobbying your elected representatives as an individual or as part of a group is your right as a voter.

The good news is that changing your lifestyle to reduce your carbon footprint might not just be cheap; it might even save you money, because broadly speaking, the less you consume, the less damage you are likely to do. You can, of course, speed a good deal of money on advice and carbon trading too.

Reduce your car use and try and drive more fuel efficiently, if you can, buy a car powered by alternative, greener means. Try not to use products made from oil, looking for green and vegetable-based alternatives is a good idea. Cut down on your power use - while efforts are being made to introduce renewable energy (and you can opt to pay a little more to use them with some providers) to western societies, the vast majority of our heating, light and power comes from carbon emitting production methods

And, grow! Plants are carbon sinks, if you have land and can plant trees then you're making a difference. Find more on how to reduce your carbon footprint.