The Greens have been discussing with the government a proposal for a two-year carbon tax as an interim alternative to the blocked trading scheme, but Senator Brown says those talks have stalled.

The Greens senator Christine Milne and the Climate Change Minister, Penny Wong, who held several meetings last month on the tax proposal, have not met for two weeks.

A Senate vote on the amended version of the emissions trading scheme negotiated with the former opposition leader Malcolm Turnbull has been delayed until May.

The government believes it has a double dissolution election trigger with an earlier version of the scheme, after it was rejected twice last year.

Senator Brown said yesterday he thought the government was keeping its options open on how to break the climate deadlock - including a double dissolution election - and hoped the Prime Minister, Kevin Rudd, would make a decision soon.

''They are keeping their options open. Therefore the Greens proposal of a carbon levy is very much on Mr Rudd's books and he hasn't made up his mind on it, and he should,'' Senator Brown said.

''This is more than an option, it is very a serious legislative alternative and the government hasn't rejected it some months afterwards.''

A spokeswoman for Senator Wong said the minister would continue discussions with Senator Milne. "Unlike Senator Brown, the government will not comment on negotiations in the media,'' she said.

The Herald revealed last month that Senator Wong and Senator Milne have talked to the independent senator, Nick Xenophon, and the dissident Liberal senators, Sue Boyce and Judith Troeth - who crossed the floor to support the emissions trading scheme - about a short-term carbon tax.

The Greens proposed the two-year tax of $23 per tonne of carbon in January as a ''circuit breaker'' to the climate change deadlock.