The Scottish government will face a key test of its environmental commitment this week when it launches a public consultation on plans to cut Scotland's climate pollution by 80% by 2050.

The long-awaited report on proposed climate change legislation is due to be unveiled by Scottish ministers at Edinburgh's Botanic Gardens on Tuesday. Although it is not expected to contain any major surprises, it will be closely scrutinised to check that the government is serious about reducing emissions.

The most hotly contested issue will be how the government proposes to ensure progress towards the 80% target year-on-year. Environmental groups are demanding statutory reduction targets of 3% a year, but this is being resisted by ministers.

"The levels of reduction required in each year have not yet been determined," said a spokeswoman for the Scottish government. "We believe that independent, expert advice is needed on what the trajectory of emission reductions should be.

"The Scottish government believes that reporting an annual measure of progress within a multi-year budget process would be the best means of achieving this," she said.

The government spokeswoman pointed out that emissions from planes and ships whose journeys started and ended in the UK would count toward legislative targets. International aviation and shipping emissions, however, would be best addressed at an international level, she argued.

"The Scottish government is pleased that at the European Union Environment Council meeting in December 2007 it was agreed that aviation would be included in the EU Emissions Trading Scheme from 2012."

Dr Dan Barlow, acting director of WWF Scotland, described the proposed climate bill as "the most important piece of legislation we will see in our lifetime".

"WWF Scotland wants to see this commitment enshrined in statute with a robust bill containing measures that will put us on a steady path to reducing greenhouse gas emissions," Barlow stated.

"Annual emission reduction targets of at least 3% should be put into statute, in line with the Scottish National Party manifesto commitment, along with emissions from aviation and shipping. These measures will help safeguard the futures of millions of people and species around the world."