Tree planting is not a viable strategy for decarbonizing the atmosphere on human time scales.
“Planting a billion hectares of trees won’t be easy,” he said. “It would require a massive undertaking. If we follow the paper’s recommendations, reforesting an area the size of the United States and Canada combined (1 to 2 billion hectares) could take between one and two thousand years, assuming we plant a million hectares a year and that each hectare contains at least 50 to 100 trees to create an appropriate treetop canopy cover.” (NASA)
This is not to say that we shouldn’t plant trees. We should, but the idea that tree planting will result in reductions of greenhouse gases over the course of a single human life time on the order of the ~teratonnes of anthropogenic CO₂ is fantasy. If we want to re-establish a stable climate sooner than 1,000 years, we will have to pump the carbon back to the place where it came from: underground. Thus, CCS.
The magnitude of the problem can be challenging to comprehend. There is about 1 Ttonne CO₂ to mitigate, which, at $100 per tonne, would cost $100 trillion USD to fully sequester. Throwing billions of dollars at it would not even start to make the smallest measurable dent in the problem at any scale whatsoever.
However, if the current rate of annual solar panel adoption continues at 26% for the next 18 years, then the global production of energy will be sufficient to pretty rapidly decarbonize the atmosphere at low cost, as the amount of solar energy will be triple that being produced globally from all sources at present.
Now, if that doesn’t happen, then another way to pay for decarbonizing is to bring about world peace and disarmament. The US annual defense budget is ~$800B. If the dividend from world peace was directed to climate mitigation, we could get rapid, dramatic reductions in CO₂ over the course of a century. And we would no longer have the threat of nuclear war looming over human civilization. I find that this idea is generally met with scepticism, but, unlike other government expenditures like healthcare and welfare, war is a highly discretionary expenditure that can be rendered unnecessary by some important people signing some papers.
This is a serious, expensive problem and the solutions, unfortunately, are going to need to be proportionally serious and expensive.