 THE Scottish Conservatives yesterday said they would back an increase in speed limits on major roads to 80 miles an hour - despite calls from environmental groups for a reduction.  Jackson Carlaw, the party's climate change spokesman, said that the Scottish Conservatives would oppose using new powers framed in the Scotland Bill to reduce the speed limit.  Environmental campaigners have called for the speed limit to be dropped to 50mph on main roads in a bid to cut down on carbon emissions.  Mr Carlaw added: "I can confirm today that Scottish Conservatives will not support this and that it will not feature in our manifesto. Indeed we may well seek - were such powers delegated to this parliament - to increase the speed limit to 80mph on certain highways."  His contribution came as the transport committee delivered its response to the SNP administration's plans to achieve climate change targets.  Green MSP Patrick Harvie, convener of the transport, climate change and infrastructure committee, welcomed the government's aims.  The Scottish Government has set ambitious targets including a 42 per cent reduction in carbon emissions by 2020, and an 80 per cent reduction by 2050.  Mr Harvie said: "There is little benefit in setting challenging objectives and making ambitious estimates of the reductions we can achieve, if government budgets don't contain sufficient financial provision to allow delivery."