The Scottish Government is coming under mounting pressure to make new cuts in pollution as it faces failing to meet its statutory targets for reducing carbon emissions for the third year running.

An unprecedented alliance of all the opposition parties in Holyrood is privately urging SNP ministers to agree moves that would cut carbon pollution from vehicles, farms, houses and government.

At the same time a Philippines diplomat, Yeb Sano, famed for weeping and fasting at the world's last major climate summit, has made a passionate plea for Scotland to keep its promises and abide by its climate law.

On Tuesday, ministers are due to announce long-awaited figures on Scotland's carbon emissions for 2012. They are widely expected to show that, as in 2010 and 2011, emissions will not have been reduced in line with the levels required under the Climate Change (Scotland) Act.

This was predicted by the UK's expert Committee on Climate Change, which warned in March that emissions in Scotland were likely to have increased in 2012.

A combination of a cold winter, burning more coal in power stations and a small increase in economic activity "would suggest a level of emissions higher than the target in this year", it said.

Part of the problem was that Scotland's overall greenhouse gas inventory had been revised, making it "more difficult" for Scotland to meet its legal targets, the committee pointed out. There were two options, it argued: adjust the targets, or find "additional opportunities to reduce emissions that go beyond current and proposed policies".

Leading politicians from four opposition parties are now urging ministers to do the latter. Labour's Claire Baker, LibDem Tavish Scott, the Conservative's Jamie McGrigor and the Green's Patrick Harvie have this weekend written to the Minister for Environment and Climate Change, Paul Wheelhouse, proposing five ways of cutting pollution.

The Sunday Herald understands that the joint letter, which has not been released, suggests action on transport, energy efficiency, housing, agriculture and governance. The MSPs say they are making a series of modest, constructive proposals, on which they would back ministers to help meet future climate targets.

"There are real concerns that the news from the Scottish Government on Tuesday won't be good and, as the independent Committee on Climate Change predicts, Scotland will miss the targets," said Baker, Labour's environment spokesperson.

"The Parliament passed an ambitious climate change act but in each year the targets have been missed we have only heard excuses rather than positive action. This cannot go on and we must take steps to get Scotland back on track."

Harvie, the Scottish Green Party's co-convener, argued that Holyrood had managed to achieve consensus five years ago on the goal of cutting climate emissions but not on the action needed. "The first few years have been deeply concerning, but I think there's a growing mood of support for bolder action, and I hope the government sees the opportunity that's before it," he said.

The 60-strong umbrella group, Stop Climate Chaos Scotland, which organised the joint letter from the MSPs, pointed out that the most vulnerable people around the world were suffering from climate change. "That's why it's so important that countries such as Scotland, with strong climate legislation, ensure the promises made to reduce carbon emissions are kept," said the group's co-ordinator, Gail Wilson. "As we move towards the climate talks in Paris in 2015, we need strong examples of countries stepping up and putting climate justice into action.

"If we find out, as expected, that this third target is missed, we hope it will be a signal for renewed efforts and commitment from across government to get us on track to a low-carbon economy."

Writing exclusively for the Sunday Herald, Yeb Sano, climate change commissioner for the Philippines, has made an impassioned plea to Scottish ministers.

Sano hit the headlines worldwide last November with an emotional address to delegates at the UN climate talks in Warsaw about the super-typhoon that was laying waste his country.

According to environmental group WWF Scotland, Sano made it clear that Scotland could be a catalyst for change and a leader on climate. "We know we have the tools to be part of the solution, to cut air pollution, to insulate our homes from cold winters, to protect ourselves from fossil fuel price shocks," said director Lang Banks.

"Faced with the destruction experienced by Yeb Sano and millions of others across the world, we mustn't waiver in our commitment to turn ambition into action. We hope that news on Scotland's third climate target will trigger a concerted effort from the Scottish Government."

The Scottish Government pointed out that carbon emissions had been reduced by almost 10% between 2010 and 2011. "Our target would have been met in carbon terms if the baseline had not been substantially increased, retrospectively," said a Government spokeswoman.

"We have a moral duty to act, which is why the Scottish Government established the world's first climate justice fund - now worth £6 million to support climate vulnerable communities in Sub-Saharan Africa - and are working hard to help Scotland achieve our ambitious greenhouse gas emissions reduction targets."

She added: "We will show international leadership in continued commitment to our stretching targets to encourage other countries to pledge the tough actions that will be required to limit global warming to 2C and this message will be repeated when the minister gives his statement on the 2012 inventory on Tuesday."