We all contribute to global warming every day. The carbon dioxide we produce by driving our car and leaving the lights on adds up quickly. You may be surprised by how much Co2 you are emitting each year. A growing number of so-called 'carbon footprint calculators' or 'carbon calculators' have been designed by both governments and private enterprise to help you calculate your personal or business impact and learn how you can take action to reduce or even eliminate your emissions of carbon dioxide. There are currently over 150 such calculators on the web alone.

A growing number of experts are claiming that inaccurate calculation of greenhouse gas emissions in particular carbon dioxide, using so-called carbon footprint calculators is undermining the method of using carbon offsetting to combat climate change.

As a result, critics of the system say that those who rely on them may be deceiving themselves that their resulting offset purchases are having their intended effect. And nowhere is the criticism more vociferous than with those calculators that focus primarily on air travel. Carbon offset supporters maintain the system allow individuals to fight climate change without altering their behavior through compensating others to reduce emissions of greenhouse gases on their behalf.

Online travel company Expedia Inc. threw its support behind carbon offsetting on Thursday, allowing its customers to continue traveling exotic destinations with a clearer conscience. But the airlines use a different method to calculate carbon emissions, casting doubt about the credibility of current offset calculators. In June 2008 , The United Nation's International Civil Aviation Organization (ICAO) unveiled a carbon calculator aimed at standardizing emission calculations by various airlines. However, the model can nevertheless provide misleading results, according to at least one expert, who firmly believes that producing a single number is crude.

A number of examples cited appear to confirm this:

1) If you go from Heathrow to Athens, ICAO gives 217 kilograms (kg) of CO2. That hides huge variations - fly in a full (Boeing) B767 and it's nearer to 160 kg per person, or fly in a half-empty (Airbus) A340 and it's more like 360 kg."

2) A carbon calculator on the British Airways website found 314kg per person of CO2 emissions for a trip from London Heathrow to Athens, whereas a Lufthansa calculation found 260kg per person of CO2 from London Stansted.

Critics of carbon offsetting say the system creates a false impression of fighting climate change, focusing on insignificant efforts such as building wind turbines or planting trees instead of addressing the underlying problem by flying less or burning less coal. Expedia published a survey that revealed the public's ignorance of carbon offsetting. Ten percent of the 2,000 British survey participants believed offsetting meant walking to work instead of driving, with one percent of men believing it meant properly putting out a barbeque. Just one in three understood the true meaning of the term.