Soil carbon accounted for 60 per cent of the proposed emission reductions in Mr Abbott's climate policy, or about 85 million tonnes of carbon a year by 2020.

But according to the analysis, only 27 million tonnes a year is possible and only 18 million tonnes at the low price the Coalition has budgeted to pay for soil carbon from its multibillion-dollar ''direct action'' emissions reduction fund.

Last month the Coalition said its scheme would match the government's promise to cut emissions by at least 5 per cent by 2020, but would do this by buying abatement directly from farmers and industry - not by putting any price on carbon.

But the analysis, by ClimateWorks - a partnership between the Myer Foundation and Monash University - and McKinsey management consultants, suggests the scheme would either deliver far smaller emission cuts than the Coalition claims or would cost far more than the $3.2 billion budgeted over the first four years.

Even if the Coalition achieved the 27 million tonnes a year ClimateWorks thinks is possible, and all its other planned reductions, the overall impact would be a 6 per cent rise in emissions in 2020 compared with 1990, rather than a 5 per cent cut.

The executive director of ClimateWorks, Anna Skarbek, said her analysis had used ''realistic assumptions'' about how much carbon could be stored through new ways of planting crops or managing pastures and grasslands, and about how much of Australia would be subject to the new methods.

The analysis says Australia could cut its emissions by 25 per cent - five times as much as either main party is promising - at the relatively low cost of about $185 a year a household. It says the cheapest way to make cuts would be through a carbon price and direct government action.

Ms Skarbek briefed advisers to the Prime Minister, Kevin Rudd, and several ministers yesterday and former opposition leader Malcolm Turnbull. Advisers to Mr Abbott and opposition climate spokesman Greg Hunt were unavailable.

The Greens criticised ClimateWorks for not modelling much tougher cuts than 25 per cent.

Mr Turnbull told ABC's Four Corners last night Mr Abbott's decision to reverse the Coalition's previous support for an emissions trading scheme was ''entirely political''.