The costs of reserving half the Earth's land and oceans will easily be in the trillions in terms of economic opportunity costs.
In addition, it will need to be defended. My guess is that much of the critical areas needed for this effort are inhabited or exploited by some of the poorest people in the world and they will desperately fight anyone who tries to take away their livelihood (witness for example the backlash of people in coal country to any attempt to curtail coal)
Given how humanity is not really doing much about climate change which in the next few decades will significantly affect humans directly and costs a fraction of what is being suggested here to mitigate, I have no hope that anything even resembling this will come to pass other than by someone forcibly conquering the world and then issuing a decree.