WHEN you have changed all the light bulbs, turned off all the appliances, sold the car, bought a bicycle and transferred to GreenPower, it may seem hard to reduce that carbon footprint even further.

This is where carbon offsets come in. Offset schemes - paying for an emission reduction that happens elsewhere - are becoming increasingly popular in Australia but, with more than 50 schemes on the market, it is also becoming increasingly difficult to establish their credentials.

Enter Carbon Offset Watch, a joint effort by the Total Environment Centre, the Institute for Sustainable Futures (at the University of Technology, Sydney) and the consumer advocate group Choice.

COW, to use its rather unfortunate acronym, has ranked 20 of Australia's carbon offset retailers. It measured for quality and reliability, how it calculated carbon footprints, whether it encouraged customers to reduce their footprints, and the type of offset projects it supported: renewable energy, energy efficiency, reduced landfill or forestry. Five were rated outstanding: Climate Friendly; Cleaner Climate; Climate Positive; the Southern Metropolitan Regional Council; and the Carbon Reduction Institute.

"We found that there are real differences in the quality and reliability of carbon offsets provided by different retailers, so we are taking the confusion out of the process so that people can make an informed decision," said Chris Riedy, a research director at the Institute for Sustainable Futures.

For rankings and tips see www.carbonoffsetwatch.org.au.