CARBON emissions would rise under the Coalition's climate change policies, costing the Federal Budget at least $4 billion more than calculated, modelling released on Thursday revealed.
The modelling, completed by Sinclair Knight Merz and Monash University for the Climate Institute, showed it would cost at least $4 billion extra to achieve the Coalition's promised cuts to emissions.
Under the policy, the Opposition has promised to reduce carbon emissions between 5% and 25% of 2000 levels by 2020.
But Climate Institute chief executive John Connor said the modelling showed the Coalition would need to spend between $4 billion and $15 billion more than estimated to achieve the cuts.
"The $4 billion is a third of Government estimates, but the emission shortfall is a substantial risk to the Budget that could be substantially reduced if the Coalition relaxed its restrictions on international offsets," he said.
But Opposition Leader Tony Abbott on Thursday said the Coalition could achieve its targets, saying buying extra international offsets was "shirking your duty".
"We are confident that the money we have set aside will purchase the emissions reductions we need," he said.
However, the SKM modelling showed the Labor Government's existing laws would reduce emissions by 40% more than the Coalition's plans, by 2020.
It also found the Coalition plan would subsidise a "business-as-usual" scenario at a cost of up to $50 billion to 2020, and not reduce emissions.
However, Mr Connor said while the current framework outperformed the Opposition proposals, "both major parties have work to do" when it comes to reducing emissions.
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