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During a heat wave, this might be a good time to warn that Scotland's homes will have to get a bit chillier.

That's if new targets are to be met for reducing Scotland's carbon emissions.

A renewable action plan released this week by the Scottish Government means that only the most verdant of greens could accuse it of a lack of ambition - even though it is under attack from opponents today for a lack of action on household insulation grants.

The implications of the action plan, set out in hard figures, indicate the extent to which the economy, and business and household energy consumption, are on track for a revolution.

Part of the plan is to develop technologies that aren't just untested: some remain theoretical.

Hydrogen fuel cells will have to come a long way to meet the expectations of storing offshore wind energy.

And whereas biofuels have been blamed for taking millions of hectares out of food production, pushing up world grain prices, Britain and Ireland are sharing in a £4.5m research project to figure out the potential for farming, or at least harvesting, marine algae as a source of bio-energy.

The most striking figure is in home energy use and heating in particular. This is what's left over to reach the overall targets on energy use once Scottish Government's officials have figured out what's achievable in renewable electricity generation.

They want that to reach an ambitious 50% by 2020.

Then there's the transport sector, which they hope can be pushed up to 11% renewable energy sources within 11 years - up from around 1% now. Much of that would be from electric cars and biofuels.

So heating, which represents roughly 50% of Scotland's and Britain's energy use, is going to have to play a big part in the change.

It's been seen as the sleeping giant of the carbon emissions challenge.

And if these targets are to be met, that household energy demand is going to have to fall by a whopping 32% between 2005 and 2020.

With domestic space heating takes up nearly 40% of Scottish heat energy demands, and household hot water responsible for another 12%, the implications are that a lot more effort will have to go into energy conservation measures in 2.4 million existing Scottish homes as well as placing demanding requirements on new-build.

In rural homes, away from the gas grid, biomass, solar thermal and air or ground heat pumps, are going to play a prominent role.

We'll hear a lot more about renewable heating in towns and cities too, including district heating schemes, to burn biomass and draw hot water from industries or other sources, for pumping through local homes and businesses.

The first large-scale such projects are soon to be commissioned in Irvine and Invergordon.

And it looks like the thermostat will have to be turned down a bit. How might that be achieved?

Well, the Scottish Government's Renewables Action Plan backs the idea of a "renewable heat incentive which involves a levy on fossil fuel suppliers" - though it also says the power to impose that lies in the domain of the UK Government.

Just to be clear, that means your bills going up again.

The UK Government legislated last year for such incentives for renewable heat, and is scheduled to be telling us more about its plans in the next few weeks. It's what Whitehall ministers like to call "the Great British Refurb".