Right now, probably about 5-6 of the top sources of CO2.

You'd also need 1-3 nations responsible for deforestation.

However, the problem is that you'd need the goals to be met.

So far, that's not happened.

I think that the reality is that we (the world) needs a better source of non-fossil power than we have today.

Our problem is in finding what the CO2 production is in every country or region. It is difficult enough to find coal production statistics for one state in Australia where I live, which incidentally is a major coal supplier to China, Korea and Japan. While I might, after considerable research, find the coal, oil and gas production & consumption in each of the nations listed in your question, there is essentially no way that I can estimate the net production of CO2 or CH4 by land use changes. I doubt that such emissions are even measured except sporadically, anywhere. So there is no way that I can make a realistic or even ballpark estimate. That's why large research organisation exist. They have the resources to do these estimates, I do not.

Which reminds me, for some reason, of a problem that most scientists in the west do not face. A chemist from India was complaining on the R & S pages about superstition in India, even among relatively educated people. His lab had set up an instrument to monitor methane production, possibly by termites, I forget the exact details. One of their own clerical staff had gone out afterwards and coated the thing with vermilion powder for good luck, which completely wrecked several thousand rupees worth of instrumentation. That was not the only similar experience that they had, they found another field instrument draped in flower garlands, evidence that someone had been worshipping it.

Given all the thumbs up for the roulette, I thought I'd place a warning.

The old double-down method. Problem is that say you start with $100 on say red, while you only have a 0.16% chance of not hitting red for 10 tries, you have $102,300 and have to put down $102,400 and hope for red. That is a serious bankroll.

The reason the casinos don't mind is that eventually you are going to hit a string of "bad luck". That 1 in 613 or 0.16% chance may seem small, but it will likely come around before you get your bankroll up enough to push past it.

This method only works assuming you already have an infinite bank roll BECAUSE your rate of hitting the string of "bad luck" (whether that be 10 bad rolls or 12 or 14, whichever busts you) is faster than your rate of increasing your money.

Lets say you hit that string of 10 misses on your 613th set of tries. You will have earned 61200 which will not be enough to bankroll that 613th set.

Of course the casinos also already have stopped this by placing both a minimum and maximum bet, ensuring this method will not work. So don't throw away your money doing this.

To answer the question thiough,

China, India, Most of Europe, the US and Russia would have to sign to have a prayer of success.

There are a lot of other developing countries that will eventually go through a large growth in CO2 production, but they cane be brought onboard after they have atained a measure of economic growth, at least enough to sustain their population.

Ideally, all of the nations of the world should sign on. But it is wrong to assume that there would be no benefit is only some nations were to sign. First of all, why would the nations that don't sign have any competitive advantage over those that do sign? How does wasting energy and driving SUVs or even the 2 cent per kilowatt hour difference between coal and nuclear power make a nation competitive. It would seem to me that the nations that develop new energy sources would be the ones to have an advantage as easy oil becomes more scarce. There is also the idea of leading by example.

As far as the Kyoto Accord goes, the biggest flaw was not the lack of stiff penalties, but the lack of short term targets. Canada, for example, did not fail to meet its Kyoto targets because of Stephen Harper deciding to withdraw from Kyoto. Canada failed to meet its Kyoto targets because the previous Liberal government did nothing to reduce the consumption of fossil fuels when they were in office from 1992, the year of the Earth Summit, to 2006, when Stephen Harper first won a minority government.

US EU China India Russia Japan Canada Australia South Korea, but going forward the rest of the world also accounts for a large share over 30%.

Placing a tariff on imports would put a dent in emissions, but the large internal markets would also make it not effective. More likely the signatories like China would cheat.

EU has some of the most energy efficient countries already. If only we take example from germany and iceland. But I don't think restrictions is the right way to go, you're just gonna piss people off. Set up incentives for renewable energy and people will follow suit. Also, reforest the amazon. That rainforest is responsible for almost a quarter of the worlds oxygen, and trees love CO2.

If you treat the EU as one country then probably six but Brazil is currently playing catch-up and will appear on the graph soon.

However, politics is also a factor here. Note how China has overtaken the USA in the graph below. As China has roughly 4 times the population their graph could be 4 times higher than the US. Similarly for India. Their graph could reach 3 times that of the US. How is it going to be explained to these people that they need to remain poor because of a suspected problem that, anyway, might not amount to much?

Conversely, who will explain to Obama that the US needs to be as poor as India if it wants to make a substantial CO2 emissions reduction?

In the second reference below, it claims that the US has about 46,100 turbines and that will supply 15.5 million homes. Assuming 4 people to a home worldwide that means we need 117 times as many turbines. This is over 5.3 million turbines. Taking your 50 year timeline then we need to build over 18,000 per year or nearly 300 per day.

If we factor in that the US electricity usage will remain constant per person and that the rest of the world wants to use the same amount then we will need four times the energy. Also factor a population growth of 50%, the need to replace oil in cars (factor of 2). The combined factor is 12. So 3,600 new wind turbines would reduce emissions from fossil fuels to nearly zero. but we only need 70% so we will only need about 2500 new turbines, manufactured, installed, commissioned and connected each day.

We still need something to provide the power when the wind is not blowing, of course.

And in 50 years, the old ones will be wearing out and need replacement.

Getting countries on board enough to set a target is only a small part of the problem.