Counting carbon emissions is a very, very messy business. That point is rammed home by the first fully public ranking of the UK’s top 100 companies by their carbon emissions, released today.

The rankings have been produced by the Environment Investment Organization (EIO), a non-profit research group that wants to bring commercial emissions data into the open. Similar rankings for global companies are to follow, the group says: ultimately it hopes to help investors reward greener companies. But, as it states in its report [pdf]: “Carbon reporting is, with few exceptions, extremely inconsistent”.

The first problem is that while 35% of the companies offered public, complete data verified by auditors, others provide incomplete or unverified data, or nothing at all. The list divides companies into separate groups based on disclosure.

Then, it’s not clear that self-reported emissions are accurate. For example, Intercontinental Hotels ended up at the bottom of companies in the ‘Public, complete and unverified’ category, reporting huge emissions. But, says EIO’s operational director Sam Gill, “If you ask me, those looked fairly reasonable, and the chances are that other companies didn’t report accurately.”

Finally, the EIO’s rankings had to forgo the concept of counting a firm’s indirect greenhouse gas emissions, created as a result of its supply chain. This includes emissions produced by waste disposal, by extraction and production of raw materials and fuels purchased by the firm, and much more (which all comes under ‘Scope 3’ of the Greenhouse Gas Protocol, the internationally recognised standard for reporting emissions.) Only direct emissions and those caused by consumption of purchased electricity have been included.

The result is that it’s of little value at the moment to see that insurers Amlin and financial services firm Aviva are best in carbon intensity (emissions per million dollars of turnover) – since nothing about these companies’ supply chains is included. Gold-mining company Randgold is shamed at the bottom for its total non-disclosure.

For more accurate assessments of companies’ environmental impact right now, turn to analysts such as London’s Trucost, who produce their own environmental rankings. Meanwhile, organizations like the Carbon Disclosure Project get firms to reply to questionnaires about their environmental performance. But neither of these are readily available to the general public. EIO wants data on carbon emissions to be publicly available and freely accessible. The name and shame list is a start.

Assuming that the rankings improve, EIO has a plan to help investors put more cash into the most efficient firms. The idea is to create environmental investment baskets of top companies – much like the FTSE 100 or Dow Jones indexes, but whose contents are weighted based on the rankings. If big pension funds chose to invest in these indexes, they would end up subtly shifting their money to carbon-efficient firms.

The Carbon Disclosure Project (tracking the UK’s FTSE index) and Trucost already produce such indexes. Trucost’s Richard Mattison says around £0.5 billion is invested using his firm’s environmentally-weighted stock-tracker bundles. This is peanuts compared to the money invested using standard trackers, but at least allows investors to put their assets into something that hedges against the risk of poor performance because of climate change, Mattison says.

If anything, the main driver right now for big companies to cut their own and their supply chain’s emissions is not investor pressure or fear of forthcoming regulation. Instead, companies like Walmart want to save money and are worried by the higher costs of raw materials prices (such as oil, wheat and cotton), Mattison points out.

Still, EIO hopes that investor sway through indexes like these, or through activist investment movements (such as the United Nations ‘Principles for Responsible Investment’), might send a clear message to a company’s share price. An index for guiding investment in emerging markets could be especially valuable, since these are often skewed towards carbon-intensive companies, such as those that mine ore or cut down forests.

And the public list makes it clearer which companies deserve to call themselves ‘green’ – something that doesn’t necessarily mesh with public perception, as New Scientist showed in an investigation published in February 2010.