By Richard Black
Environment correspondent, BBC News website
Official greenhouse gas figures hugely underestimate Britain's contribution to climate change, a report concludes.
Firms must count emissions from factories abroad, the report argues
Christian Aid says adding in emissions from UK-funded operations in other countries would raise the UK's share of the global total from 2% to about 15%.
British companies wanted globalisation, it says, and must take responsibility for the associated emissions.
The charity is calling on the government to ensure that companies measure their emissions thoroughly.
"Our research reveals a truly staggering quantity of unreported carbon dioxide is emitted around the world by the top 100 companies on the London Stock Exchange," said Christian Aid's senior climate change analyst, Andrew Pendleton.
"The government should now oblige companies to report their emissions properly," he told the BBC News website. "In our view, this is a litmus test of how serious they are about climate change."
Working with the environmental research company Trucost, Christian Aid attempted to calculate emissions associated with FTSE-100 companies.
"While only 2.13% of the world's CO2 emissions emanate from the UK's domestic economy," says the report, entitled Coming Clean, "through the process of globalisation, CO2 is emitted around the world on Britain's behalf, in China, India, Africa and elsewhere.
"Britain's apparently light carbon footprint rapidly begins to assume a much greater profile when worldwide investments made with British money, through the mighty City of London, are taken into account."
Not everyone would agree with the charity's conclusion, but its argument is that Britain benefits from those investments, either by bringing cheap goods to the UK or by creating profits which flow back into the British economy, so the emissions ought at least to be accounted for within Britain.
Using this methodology, the UK would account for between 12% and 15% of the global total.
Researchers also found that few companies are fully aware of their own emissions, even those relating directly to activities such as heating their buildings and running vehicles.
The Carbon Disclosure Project (CDP), which aims to persuade companies to release data on emissions, has recorded a gradual rise in the numbers prepared to make their figures public.
But Trucost found that only 16 of the FTSE 100 report emissions according to the internationally recognised Greenhouse Gas Protocol (GHGP).
"Some of the largest companies are beginning to get it, and you have companies like Marks and Spencer and Tesco and BSkyB all beginning to seek leadership positions," said Trucost's head of corporate services Neil McIndoe.
"Almost all of them have environmental policies, and sometimes they're very similar to each other, basically because they copy and paste the wording from the website of the one next door.
"But across the FTSE 100, you're lucky if you can get 20% of companies to tell you anything in numbers about the environment."
Trucost and Christian Aid argue that full disclosure is essential to the proper functioning of a global carbon market, which according to the forum of global legislators meeting in Washington last week is essential if greenhouse emissions are to be constrained.
The Department for Environment, Food and Rural Affairs (Defra) says it has been actively involved in supporting the work of the Carbon Disclosure Project.
A spokesman said: "We facilitated the distribution of the CDP questionnaire to FTSE companies in 2006, so as to assist investors in attaining more climate-related information on UK plc than before.
"The last CDP round generated the highest-ever response rate in 2006, with 72%, or 360, of the FT500 companies responding, up from 47% of companies that responded in 2003."