What is a tonne of CO2?

About as much as

• 100 trees can absorb in a year 
• Driving across America in a car 
• A 10,000km long haul flight, per passenger
• £175 of gas and electricity at current UK prices

The Carbon Trust publish some useful conversion ratios for this kind of thing. We use a figure of about 10kg per tree which is widely quoted on the internet; the United Nations Environment Programme puts it at about 12kg a year, but 10 kg / tree (or 100 trees per tonne) is a pretty good rule of thumb.