Geological formations across the US have the potential to hold up to three trillion metric tonnes of captured carbon dioxide, representing enough capacity to store all the carbon emissions from point sources across North America over the next 1,000 years.

That is the assessment of the US Department of Energy (DOE), which this week announced plans to invest $126.6m over the next decade in two projects to test the ability of two potential sinks to permanently store approximately two million tons of carbon dioxide.

The tests will see one million tonnes of CO2 from an ethanol production facility pumped into the 3,000 feet deep Mount Simon Sandstone in Ohio and a further one million tonnes injected into the San Joaquin Basin in Central California from a local power plant.

The DOE said the projects would demonstrate the entire CO2 injection and storage process, including pre-injection assessment of the sites and post-injection monitoring to ensure CO2 does not escape.

The funding, which will be accompanied by $56.6m in investment from the private sector, follows awards of over $300m late last year to four similar projects in the Mid West and Southern States.

Acting Deputy Secretary of Energy Jeffrey Kupfer said that collectively the six projects would assess the suitability as carbon sinks of the "most promising of the major geologic basins". He added that these geological formations alone had the potential to store more than 100 years of the CO2 emissions from all major point sources in the US.

"Tests like these will help provide the confidence and build the infrastructure necessary to commercialise these technologies, and will enable the US to continue using its vast resources of coal while protecting the earth for future generations," he said.

Carbon capture and storage has emerged as a key plank in the Bush administrations strategy for tackling climate change and has been widely touted as a means of continuing to exploit fossil fuels while still curbing carbon emissions.

However, environmentalists remain deeply sceptical about the effectiveness of the technology, arguing that it remains largely untested and will take too long to deploy. A report published this week by Greenpeace claimed that large scale deployments of carbon capture systems were unlikely prior to 2030 and also warned that the process was energy intensive and inherently risky, because it is impossible to guarantee that stored carbon would not leak back into the atmosphere at some point in the future.