Abolition of national boundaries and functioning of human society as one country.
Without that human race will never cooperate with each other to find the optimum solutions. The problem is not just we damaging our environment, the problem is also we are doing it in such a way that it is going to take a lot longer for nature to recover itself.
The problem is just not one particular process or industry or material but the problem is any organism functioning out of nature's system which supports itself. Basically anything you do that is outside prehistoric monkey days of humans is going to damage the environment. Take an example, the simple act of raising cow herds for food and farming needs produces so much methane, but we haven't even figured it out how to control it, in fact, we never even talk about it.
People talk about recycling as a "great solution", but it's not, often recycling will produce more pollution. Plastic by just getting exposed to sunlight produces pollution that traps multiple times heat than CO2, but we never even talk about it.
Problem is that less than 20% of the human population lifestyle is such a waste-producing machine that it is impossible for other 80% to even fulfill basic life needs.
Just establishing green tax, carbon footprint levels and what not is not going to change that, you need to first establish the equilibrium between that 20% wasteful population and 80% others. You have to normalize the distribution of resources based on their efficiency considering everything and not just -who has more money or more weapons, or even who can offer cheap labor so someone seating across the sea can make more bucks. Establishing organizations for namesake doesn't work, WHO or UN or Trade unions are all examples of the failure of humanity, they are all even if established with good intentions are right now just tool for someone else to prove a point or pull a political feather from someone else's hat.
No country will listen to the country that is most wasteful and no leader will look past his own nations (at best) and beyond his benefit (at worst), we will end up spending all our time debating about "Who is more responsible" and "who should follow more restrictions".
But there will never be any solution, till its too late. Take for example, there have been fair warnings for the last 15 years about Chinese wet markets, but did anyone listen? In fact those markets flourished, now see what it has done? And FYI, they are still not completely closed and waiting for re-opening again as millions of dollars of trade is lost with each passing week.
What does that tell you? When it comes down to it, humans don't think beyond their immediate benefits, in good case most certainly not beyond their own immediate circle and if they are extremely loyal and morally right, at most of their own country. Nobody gives two shits about what happens at some other place. And nope, shutting down job producing activities just because it produces pollution is also not right, that's something overlay passionate people for cause ignore and end up causing more damage to the cause.
Just my 2c