That was an interesting read and a fun puzzle to play around with too.
One idea I came up with that could be a posable solution would be to use genome therapy, or the likes, on certain tree species from all the forests on the planet in order to increase their CO2 absorption rate in order to reduce cardon gas in the air. Of course, this would take a hundred years or so to do but here is the basics of my thinking.
You would probably need to start with a few hundred or so young saplings from a set forest ecosystem and change/train the cells that absorb carbon from the air to be more efficient in doing so. They would also need to be able to sense a prolonged increase of CO2 in the air over a year or two and be able to start seed production a little more quickly than their non-modified species in order to propagate and establish themselves sooner in order to take advantage of the increased CO2 gathering around that eco-system. But the absorbed cardon needs to be stored somehow. Maybe in the tree itself by increasing the trees density or into the ground thought the root system?
One very important thing is that the modified trees would need to be, aside from increased CO2 absorption and propagation, indistinguishable in the system. They would need to have the same life span, growth rate, shape/size, seed germination time, drought tolerant and disease/insect issues as the non-modified species in order to maintain a seamless and balanced ecosystem and thus, increase the intelligence of the planet.
This also brings up the question of cross pollination by non-modified species. Will the modified gene code be past stably on to the next generation and if so for how long? Will mutations occur? If so how, when and what will be the outcome to the system? Will the fourth stage of the intelligence of the planet be maintained and continual to be self-sustaining? I still see about a hundred years for the trees to be establish around the globe and another two or three hundred years before we could answer any of the last questions to see if we did reach the fourth stage.
Just a thought.