By Richard Black
Environment correspondent, BBC News website
At first sight, the G8 agreement on climate change promises much.
Leaders are "committed to avoiding the most serious consequences of climate change", and determined to stabilise greenhouse gases in the atmosphere at levels that would avoid "dangerous climate change".
In fact, this is exactly what leaders of nearly 200 countries signed up to in the original UN Framework Convention on Climate Change (UNFCCC), agreed at the 1992 Earth Summit. So if re-stating a 16-year-old commitment is progress, then this is clearly a success.
The question ever since Rio has been what to do about it. But the reality of negotiations within groups such as the G8 is that every party needs to emerge with bits of language that they can point to and say "I won".
So here is the key sentence in all its diplomatic finery: "We seek to share with all parties to the UNFCCC the vision of, and together with them to consider and adopt in the UNFCCC negotiations, the goal of achieving at least 50% reduction of global emissions by 2050, recognising that this global challenge can only be met by a global response, in particular, by the contributions from all major economies, consistent with the principle of common but differentiated responsibilities and respective capabilities."
The G8 are crawling forward on emissions cuts at a time when giant leaps and bounds are needed
Peter Grant, Tearfund
So the EU emerges with an apparent commitment to cut emissions by at least 50%.
The US and Canadian administrations can say that it is only a commitment if the major developing countries play ball, and that the 50% figure concerns global emissions, not necessarily their own.
And the major developing countries, involved on the sidelines of the G8 summit, can point to inclusion of the UNFCCC phrase "common but differentiated responsibilities" as continued acknowledgement that far less would be required of them than of developed economies.
The host nation Japan appears to have won two key concessions.
One is that different industrial sectors could be set different targets with the aim of preserving competitiveness.
The second, which is more important, concerns the baseline year against which carbon savings would be measured.
With very few exceptions, the UN process has always used 1990 as the baseline.
But Japan argues this is unfair. The significant gains in energy efficiency it made before 1990 are effectively penalised, it says, while the gains made in Europe after 1990 through the clean-up of Soviet-era industry and the switch to natural gas are rewarded.
Campaigners say the G8 has shown far less ambition than is needed
The G8 document does not specify a baseline year, but asked by reporters, Japanese Prime Minister Yasuo Fukuda said it was "current levels".
This would be significant in at least three ways.
From a practical standpoint, emissions have risen by more than a quarter since 1990; so a 50% cut from now is worth far less than a 50% cut from 1990 levels.
On the diplomatic front, it would raise a big question for the EU, which has taken 1990 as the baseline for its own target of cutting emissions by 20% by 2020. The UK's domestic targets also use 1990.
And from a philosophical point of view, it would again amount to turning the clock back 16 years, and saying "we're going to ignore what we said then and start again from here".
The EU will continue to insist that 1990 stays as the baseline in UN talks; and as the G8 document does not specify any date, any party can select whatever it feels is more politically acceptable when reporting back to its electorate.
But just by raising the issue, Mr Fukuda has thrown up yet another thing for parties to argue about.
So it is perhaps not surprising that campaign groups have lined up to criticise the deal.
WWF said it confirmed the recent trend of industrialised countries showing less, rather than more, of the leadership required.
"The G8 are responsible for 62% of the carbon dioxide accumulated in the Earth's atmosphere, which makes them the main culprit of climate change and the biggest part of the problem", said the director of the group's global climate initiative, Kim Carstensen.
"WWF finds it pathetic that they still duck their historic responsibility, and refuse to turn from the main driver of the problem into the main driver of the solution."
Tearfund, which campaigns on issues of developing world poverty rather than the environment per se, added that using a 1990 baseline was crucial.
"Concrete commitments on climate change are the acid test of success at this summit," said international director Peter Grant.
"The G8 are crawling forward on emissions cuts at a time when giant leaps and bounds are needed."
The other main gripe of these organisations is that 2050 is too distant. They have been urging parties to commit to shorter timescales for achieving cuts, as the EU has done with its own 2020 target, arguing that this removes the option of delaying action until it is too late.
Instead, the statement merely acknowledges that "a long-term global goal will require mid-term goals and national plans to achieve them" - without specifying what these goals should look like.
Elsewhere, there is acknowledgement that the poorest countries are going to need help to adapt to climate impacts, and that clean energy technologies need to be developed and rolled out rapidly.
There is support for the rapid development of "clean coal" demonstration plants, in particular, and recognition that some countries will seek to lower carbon emissions through investing in nuclear.
Place in the world
It is important to recognise what the G8 could not achieve.
The UNFCCC is the sovereign body for making global agreements, and the two-year road leading from last December's UN climate summit in Bali to next year's in Copenhagen is still the most important route to a low-carbon future.
Nothing that the G8 or G8+5 or G20 or any other expansion of the core group could agree would change that.
What this week's gathering could have done was to point the way and ease the path, by agreeing a common front to take into the UN process.
On Wednesday, G8 countries meet the large group of "big emitters" or "major economies", the latest stage in a process formulated by the Bush administration.
The group includes major developing countries such as China, India and Mexico. And they have already set out their stall, responding to the G8 declaration with a statement calling on rich countries to go further and faster, committing to cuts of 25-40% by 2020 and 80-90% by 2050.
So far, then, this G8 summit has confused the issue rather than clarifying it.
Governments are as divided as ever on what they are prepared to pledge and what they want to achieve; and re-opening the baseline year question is potentially hugely destructive.