The government has been criticised for slashing a target to decarbonise home heating in its final climate change plan.
Climate Change Secretary Roseanna Cunningham defended the decision to reduce a target to have 80% of domestic buildings' heat supplied using low carbon technologies by 2032 to just 35%.
Ms Cunningham told MSPs at Holyrood the decision had been taken in response to independent advice that the original target was not credible.
But she faced accusations that ambition had been scaled back in many areas between a draft of the plan released last year and the final version unveiled last week.
Labour's Claudia Beamish called for an explanation of why sectoral targets had changed "so dramatically for the worse" between the two plans.
She said: "The stark difference between the plan and the government's earlier draft is puzzling, in some areas swinging quite dramatically from unrealistic to unambitious or, in the cabinet secretary's own words, more achievable."
She welcomed the government's drive to cut transport emissions by phasing out fossil fuelled vehicles, but added it was "so disappointing to see that used, in my understanding of the final plan, to reduce effort in other sectors."
Green MSP Mark Ruskell said: "It certainly does appear that this plan is is very different to the draft because if you add up all the Scottish Government policies in the final plan it results in emissions reductions one million tonnes less than in the draft."
Lib Dem Liam McArthur: "It's right that the cabinet secretary takes on board points made about deliverability of targets but can she really justify a collapse in the target for low carbon domestic heat from 80% in the draft to 35% in the final plan?"
Tory MSP John Scott said the final plan "lacks specificity and ambition across most sectors" and had ignored many suggestions made by MSPs and Holyrood committees for improvement.
Responding to the criticism Ms Cunningham said it was "utterly absurd" and "quite ridiculous" to suggest a lack of ambition, adding that Scotland was "one of the most ambitious governments in the world when it comes to tackling climate change".
She said changes had come about between the two documents as a result of factors including a new set of greenhouse gas emissions statistics.
On heat she added: "The independent adviser the Committee on Climate Change has already advised that the transition to near zero emissions buildings is likely to take decades and we should be realistic about the contribution this sector can make to targets in 2032 and they criticised the original ambition in the draft plan."
She added: "We have to look as a government as to what is actually doable."
Ms Cunningham acknowledged that the ambitions of the plan would be "difficult to achieve" with "bumps on the road ahead".
She said: "We are not taking any easy options, because this government believes that we have a moral obligation to act, and we are confident that Scotland's unique gifts - plentiful renewable energy resources, a strong legacy of innovation, and the ingenuity of the people of Scotland - will enable us to realise the opportunities that lie ahead."