Ministers are accused of “grossly underestimating” the impact of cutting air taxes after they admitted it could cause twice the increase in aircraft emissions previously forecast.
The admission was revealed in a Scottish Government consultation into its new air departure tax (ADT), which would be half the current air passenger duty (APD).
However, the figures were not published until nearly a week after MSPs voted to approve the tax from next year.
APD is £13 per passenger on shorter flights and £78 on those over 2,000 miles.
The document said emissions could increase by the equivalent of between 0.087 and 0.101 million tonnes of carbon dioxide compared to 0.05-0.06 in previous estimates in 2014.
Green MSP Patrick Harvie said: “This is a damning report and Scottish ministers must explain why it was published after they asked MSPs to vote on the creation of a new aviation tax rather than before.
“I first asked the SNP to show the environmental impact of their policy in 2012 and it took them two years to do so. Now we learn they grossly underestimated and have tried to sneak out the true figures.
“The doubling of the environmental impact of this policy underlines what Greens have been saying for years: growing aviation makes no sense while the buses, trains and cycle routes we all rely on every day are ignored.
“This policy is a giveaway for businesses and the rich, will damage public finances and more clearly than ever harms Scotland’s reputation as a leader on climate change.”
Labour transport spokesman Neil Bibby said: “Cutting taxes for the frequent flying few won’t make Scotland any greener or fairer.
“It will take almost £190 million out of the Scottish budget – and now we learn emissions may be two-thirds higher than the SNP originally predicted.
“Its plans to slash air passenger duty are rapidly becoming an uncosted, environmentally damaging shambles.”
But finance secretary Derek Mackay said: “Our plans for ADT are a key part of our economic strategy for sustainable growth.
“However, we do recognise improving air connectivity may lead to an increase in aviation emissions.
“The final climate change plan will accommodate any projected changes in aviation emissions, as part of a balanced approach to meeting Scotland’s climate targets by driving down overall emissions.
“Aviation currently accounts for less than 4 per cent of total emissions in Scotland. The latest figures show our policy to reduce the overall ADT burden by 50 per cent may only lead to a very small increase of less than 0.3 per cent.”