ONLY a third of Scots believe that ambitious government targets to tackle dangerous climate change can be achieved, according to a new survey.  The SNP administration wants to cut greenhouse gas emissions - mainly made up of carbon dioxide (CO2) - by 42 per cent by 2020, as set out in the Climate Change act.  But the survey has revealed that only one in three Scots think the carbon reduction pledge is attainable, while almost half of the population do not know what the targets are.  The opinions of 1,000 people surveyed in the quarterly Scottish Nature Omnibus will come as a blow to the SNP, which needs the support of the wider population to reach what are some of the most ambitious climate change targets in the world.