"The most recent scientific evidence indicates that the worst-case projections of the Intergovernmental Panel on Climate Change are being realised. There is  significant risk many trends will accelerate, leading to an increasing possibility of abrupt or irreversible climatic shifts," they say.

The current draft of the bill sets the target for cutting emissions at 34% by 2020, with the aim of making 80% cuts by 2050. But climate change minister, Stewart Stevenson, said the 2020 target will rise to 42% "should as we hope the European Union agrees to reduce its greenhouse gas emissions by 30% by 2020".

"The 34% emissions reduction target by 2020 target proposed in the Scottish climate change bill falls far short of what is required to prevent catastrophic climate change. Any delay in delivering effective emissions reduction significantly increases the long-term social and economic costs."

The latest view from the world-leading Tyndall Centre for Climate Change Research based in six universities shows a target of at least 42% is needed to give the best chance of avoiding "dangerous climate change", the letter continues. Inaction, it warns, is "inexcusable".

"The current proposal to reduce emissions by just 34% by 2020 is wholly inadequate if this bill is to provide leadership during this important year for climate change," the letter says. "The bill must be amended to set a target from day one of at least 42% reductions by 2020. We urge you to follow scientific recommendations in order to deliver what is required of the Scottish climate change bill."

The scientists signing the letter include Dr Hugh Montgomery from University College London, Iain Stewart from the University of Plymouth, United Nations adviser Dr Andrew Dlugolecki and the co- chairs of the Climate and Health Council, professor Mike Gill and Dr Robin Stott. Other signatories are (right) professor Peter Smith from the University of Aberdeen, professor Alan Werrity (left) from Dundee University, Dr David Reay (left) from Edinburgh University and (right) Dr Steven Gillespie from the University of Glasgow.

Scientists join a huge, growing chorus of voices from all sectors of society and all parts of the globe demanding tougher action including both celebrities and business leaders like Brendan Dick from BT Scotland and Ian Marchant from Scottish and Southern Energy to call for a 42% target.

They are backed by an unprecedented coalition of environmentalists, politicians, trade unions, students, faith groups, charities and community organisations, representing 40% of Scotland's population, and egged on by countries set to suffer most, like Malawi, Ethiopia and the Maldives.

Although ministers say they want to follow the science, their promises are not reflected in the wording of the bill, Stop Climate Chaos Scotland coalition chairman (left) Mike Robinson argued. "The overwhelming scientific consensus is 42% by 2020."

The tropics have spread 277km since 1980. Arctic sea ice is expected to disappear by 2015. A study last week said Scottish winters could be up to 24% wetter, summers up to 20% drier by 2080, with more floods, storms and heatwaves.

"If a country like Scotland cannot set targets which are the very minimum required, how on earth can we expect countries such as China, India and Brazil to do so?" Robinson said. "The world is casting about for leadership. Scotland is well placed to take that lead, to show intent ahead of the negotiations in Copenhagen."

The current draft of the bill sets the target for cutting emissions at 34% by 2020, with the aim of making 80% cuts by 2050. But climate change minister, Stewart Stevenson, said the 2020 target will rise to 42% "should as we hope the European Union agrees to reduce its greenhouse gas emissions by 30% by 2020".

Full feature Sunday Herald