Summer holidays are on the way. You’ve booked a trip to Australia for all the family, but now your green conscience is kicking in. Flying a family of four from Kuala Lumpur to Melbourne and back produces more carbon dioxide emissions than the average American family car does in one year.

Can carbon credits ease your pain? Opinion is divided, but many scientists now believe carbon credit schemes (where carbon dioxide emissions are offset using techniques such as planting trees) do more damage than good.

“Personal carbon offsets are akin to indulgences sold by the Church in the Middle Ages,” said Roger Pielke Jr, an expert on science policy from the University of Colorado in Boulder. He argues that carbon credits merely assuage our guilt while we carry on polluting and emissions continue to rise.

Worse still, it isn’t certain that all carbon offset schemes actually mop up the carbon dioxide they claim to. “The science behind some of these schemes is still not clear,” said Wouter Buytaert, an environmental scientist at Imperial College in London, U.K.

For example, planting fast growing pine trees on grassland will help to lock up carbon in the tree, but it may also disrupt the soil and release carbon already stored in the grassland.

And in some cases there can be severe negative side effects too. Extensive areas of grassland in the upper tropical Andes, South America, have already been planted with pine trees, paid for by carbon credit schemes. The pine trees guzzle water much faster than the native grass, reducing stream-flow by around 70 percent and drying up the water supply for towns and cities downstream such as Cuenca and Quito in Ecuador, noted Buytaert. “It is just a case of substituting one problem for another,” he said.

Carbon Offsets Are a Luxury Item

Other offset schemes based around technology transfer (such as providing solar panels for people in developing countries) may be based on more solid science, but Pielke says even these are not enough.

“The only form of offset that I think would make sense is if someone wanted to pay to have a certain amount of carbon dioxide directly removed from the air and permanently sequestered (unlike planting a tree, which is temporary). Of course at costs of up to $500 a ton [for carbon air capture], I think we'd see that even as indulgences, offsetting has its limits,” he added.

One fundamental problem with carbon offset schemes is that they can only ever work for the privileged few. “It is impossible to offset the carbon emissions for everyone in the world,” said Imperial College's Buytaert.

When Offsets Are Valuable

However, some scientists believe that carbon credits can still be a valuable tool. “They are a worthwhile option, when you have done everything else possible to reduce your emissions,” said Dave Reay, an environmental scientist at Edinburgh University in the U.K.

Reay concurs that not all offset schemes are equal, and says that some form of benchmarking is required. However, he thinks that many of the worst schemes have now been weeded out, and that carbon offsets are necessary in order to tackle climate change. “It is better not to fly at all, but if you really have to then offsets are a good last resort,” he said.

Erik Blachhford, CEO of TerraPass, a U.S.-based company that sells offsets and invests in carbon reduction projects such as agricultural methane and landfill gas capture says people generally come to TerraPass as part of an overall strategy to reduce their footprint.

"People don’t buy carbon offsets as an excuse to keep on polluting, they buy them after they have taken steps to cut their carbon footprints as much as they practically can," said Blachford.

By Kate Ravilious, May 19, 2009, Green Guide , National Geographic