And imagine no country on the planet has an army. And then someone comes up with the idea. I hope you like the guy.

It has the same problem so many other idealisms have : it's not a Nash-equilibrium and thus certainly not Pareto-efficient. Good ideas in politics are things that are Nash equilibria, or very close to them, and don't exist yet. Great ideas are Pareto-efficient, but don't exist yet.

Any idea that doesn't have the Nash-equilibrium property should be fought tooth and nail because it will, by necessity, blow up really really really badly for everyone. They cannot exist for any length of time since it only takes one non-cooperative person to blow them up.

Like stopping climate change by law, for example, which is an idea that's obviously not a Nash-equilibrium for the economies involved (it is, however, Pareto-efficient for the politicians involved, which is why it's getting pushed through). One country can blow up the whole arrangement, and profit from the fact. Hell, one large company could do it. And they can profit by doing just that, which means that one of two things will happen :
1) that country/company grows until it, by itself, is so big it blows up the balance by itself (will never happen)
2) every other company is adopts it's practices.

And exactly what you'd expect happens : politicians get better from pushing through those laws, but the economies behind them spend untold amounts of money to get around those laws, and they do it successfully (for example by threatening those politicians in giving them the carbon tax back as subsidies or leave regions of production, which will leave an economic mess and lots of unemployed). The net-effect of the effort, you might think, is good : it slows down the progress, or at least, doesn't speed it up. Well, not quite. Once a company finds such a loophole, it has a limited window to exploit it. And because the company cannot just use the direct approach, they generally have to make use of less efficient methods to unload their byproducts, which means more energy is needed, which means more byproduct, and because the loopholes will get closed, it means they have to use them as fast as possible, so they have to dump it quicker.

But of course, nobody wants to hear the anti-idealists message. It's not bad news per se, but it's also not good news. Pushing through a 99% drop in fossil fuel usage is simple, but it cannot be done in Washington or at the UN. It requires finding an energy production scheme that blows fossil fuel out of the water. Fossil fuels have 3 big advantages :
1) energy density is liquid fossil fuels is bigger than any technology we have. Sure, it doesn't beat currently-in-the-lab technologies by that much anymore, but it still beats production technologies by a factor 2 easily.

A Tesla electric car has to carry a battery pack of 450 kg, for a range of roughly half what you'd get with 55kg of fossil fuels. Not the most efficient battery technology available, but within 20% of what you can achieve in the lab.

Needless to say, this gives fossil fuels a massive advantage. Lugging around Tesla batteries for power would not be economical. But you can transport fossil fuels once around the globe by burning ~ 0.2% of their weight (by ship). The Tesla battery equivalent would be ~30%.

2) Fossil fuels do not require infrastructure. For example, one energy source that beats the living hell out of fossil fuels for density, is uranium-235. There is just no contest. If you took a graph listing the energy density of a Tesla car at 1cm from the axis. You'd see fossil fuels at about 20cm, and you'd see uranium at about a kilometer.

Problem is that the smallest nuclear reactor (just like a car is a fossil-fuel reactor), is about the size of a 2-bedroom apartment. You can make them smaller, but not without accepting serious disadvantages.

So fossil fuels have a massive advantage in infrastructure required. I once saw a 35% efficient kerosine engine core, for a model plane, that was 5x4x2 cm in dimensions. The fuel tank was double the size of that engine, weighed a few hundred grams, and kept the propellor going for ~1.5 hours with more power than you'd ever need for the plane.

Of course electricity can be transported through cables, which are much simpler than pipelines, but that requires infrastructure. We would not be able to operate mines in inland Indonesia, or many parts of Africa with electricity, for example. Just can't be done.

3) Efficiency of production. Even with the above things, there's still a few substances that could potentially beat fossil fuels : liquid hydrogen is one, there's a few others. I doubt you would like the accidents that can occur with them any better than you like nuclear meltdowns (although that's true of pretty much any energy-dense material for obvious reasons). But there's another, more important economic consideration : making them has negative ROI (in fact less than 0.25, and practical methods, less than 0.1).

Making liquid hydrogen costs a LOT more energy than you're ever going to get back out of them. Of course, technically that's true of fossil fuels as well, but not in practice. In practice producing oil had, until 2000, an ROI of about 9. For every kw of energy invested you'd get 9kw burnable fuel. Right now it's about 3, and dropping fast (which is why the oil rig count everywhere is going up to such insane levels. It takes ~4 pumps today to get the amount of oil that 1 pump would provide just 10 years ago. That number goes up exponentially, and so far we've followed the exponential curve. That's going to stop of course, and soon).

So stopping global warming is easy. You have to find a fuel that's energy dense (so not batteries), easy to collect from somewhere, and can be transported, without infrastructure, around the globe for a tiny percentage of the energy it contains. If you also want to not find out what happens to our world when available energy drops of a cliff, you have ~6-7 years left (best guess, could be 15 years, could be 1), to find it.

Please keep in mind that keeping people alive during winter at any latitude above halfway between florida and newyork requires heating them up, so please do not say "we'll just wait for oil to run out", because you'll have to relocate all of Canada to Texas in 15-20 years to go with that option, and of course, lots and lots of people will not move but die. Without using to much energy for building new buildlings, of course.

That would be the Pareto-efficient way to do it. And if you do it that way, there's very good news : it can't fail. There's also the Nash-equilibrium way to do it : conquer the middle east, not like before, but actually destroying it's population and really stealing the oil. The Nash-equilibrium way can be seen as the "default" option. Barring a Pareto-efficient solution, one way or another, it will happen, just the details of when and how are in doubt (I'm not saying exactly this will happen, but something along those lines must happen).