By Richard Black
BBC environment correspondent
Tough negotiations with developing nations are expected on CO2
The commitment by G8 nations to reduce greenhouse gas emissions by 80% by 2050 suggests that the leaders of those nations are serious about starting a fundamental revolution in the way society meets its energy needs.
Nothing else than a top-to-bottom refit can do the job.
Virtually all electricity generation will have to come from renewables, nuclear power or so-called "clean" coal - if that technology can be made to work on a commercial scale.
The amount of electricity generated in Western countries will have to rise significantly - doubling or even trebling - as transport and the heating systems for homes and businesses switch away from fossil fuels.
Alongside a re-fuelling revolution would go a frugality revolution, as societies put an end to energy wastage.
All of that is implied by the 80% target.
But so far, leaders have not talked collectively of how to jump-start this revolution.
The journey has a low-carbon destination, but no roadmap.
And that is what makes some suspicious that the big targets are more about dressing windows than about re-writing the world's energy rulebook.
The G8 target is just that, so far - a target. Expect it to be heavily shot at
Another generator of suspicion is that the target is dim in the future - a time when, as the UK's climate and energy secretary Ed Miliband admits, none of the leaders setting the target are likely to be in office and able to be held accountable.
Do governments set targets on health or the prison population or the number of people in higher education 41 year in advance?
Generally speaking, they do not; and that is why developing countries and environment campaigners want targets for 2020 or even 2015, to oblige Western nations to start along the transformation path now.
The EU and Japan have set 2020 targets domestically, and legislation going through the US Congress should lead to a commitment from Washington too.
But with the exception of the EU, no-one is setting the kind of 2020 targets that are anything like what developing countries demand - and many EU nations are in reality struggling to meet even their far more modest Kyoto Protocol commitments.
So now, G8 leaders will try to persuade some major developing countries to halve global emissions by 2050.
Governments such as China's that are concerned about climate impacts may find the figure desirable.
But they are unlikely to be impressed with what they see as fine words from the West unmatched by real short-term commitments.
Unmatched, too, by money. Of the major developing nations, India especially has long argued that it will not compromise its economic growth by agreeing to climate curbs.
So if the West wants the developing world to go through its own energy revolution, it is going to have to fund a lot of it.
Sums totalling hundreds of billions of dollars per year are mentioned - and, burned by decades of broken pledges on aid, this time developing country governments are likely to want to see the money itself rather than some amorphous pledge.
Another issue is that many of the poorest nations are not involved in the current talks.
And there is no guarantee that a deal which suits countries whose economies really are developing, such as Mexico or Brazil, will find favour with those that bear the label "developing" but in reality are anything but.
The G8 target is just that, so far - a target. Expect it to be heavily shot at.