I don't think plants are a viable solution for quite a number of reasons.

The most fundamental is that they are inherently temporary storage. Plants absorb various gases as they live, and then emit it as they die. So you're just kicking the can, and not even especially far. For instance a tree can absorb up to around 50 pounds of CO2 per year. To put that into contrast an iPhone 7 results in about 123 pounds of emissions, so you need 2.5 trees per iPhone per year. Alternatively a gallon of gasoline releases about 19 pounds of CO2. So for every 10 gallons of gas a person uses you'd need about 4 full grown trees. And again that is per year, per person. Look around your house and imagine how many trees you'd need just to temporarily mask just your own personal consumption. It'd be a small forest, per person.

That leads to the second problem - scale is limited. With an unbelievably massive planting effort we could probably work to temporarily mask our emissions today. And while our emissions today are a problem, you can't ignore the future. Right now the vast majority of the world doesn't emit much per capita, but that's because most of the world lives in less developed areas. As China, India, Africa, and other areas continue to develop their emissions per capita are going to continue to increase. We should expect worldwide emissions to sharply increase over the coming decades, and so we need solutions that can scale accordingly.

And the final issue is that these solutions can, at times, go up in flames. For instance a recent study on wildfires has indicated that wildfires in places such as California can end up contributing even more to their CO2 output than the vehicles in those areas. [1] Mass planting of greenery would work as temporary storage, but it's not an especially stable form and so the net effect would be very difficult to accurately predict.

[1] - https://www.livescience.com/1981-wildfires-release-cars.html

Sure, these reports are talking about the point of no return in terms of human activity having an effect. In the grand scale of climate humans play an extremely small role. For instance with CO2 we account for less than 4% of all emissions in the worldwide CO2 system. The reason that our effect is meaningful is because it's a system that's in equilibrium. CO2 was being absorbed and released in a fairly balanced system, though one that does tend to have its own cycles over time. So even small changes to the system can have a disproportionate effect.

In other words imagine 2000 calories is what you need to sustain your weight at your current activity levels. And you increase the caloric intake just 5%. Even though you've added almost nothing to what you regularly consume, you're going to go from zero weight gain to some very non-zero amount of gain. You eat thousands of calories and then adding a hundred more suddenly makes you go from a-okay to weight gain? It's a logical but counter-intuitive problem. The exact same thing is happening with worldwide CO2 levels and the increase in atmospheric greenhouse gases.

But now enter feedback mechanisms. As the Earth heats up natural processes begin to release more and more CO2. For instance through the melting of permafrost or otherwise frozen gases (the likely culprit here). You reach a point that the Earth is producing enough additional CO2 that even if you instantly cut human emissions to 0 it would be unlikely to have any meaningful effect. So when the UN speaks of things such as irreversible change, they are speaking exclusively of solving the problem by reducing our emissions.

Scrubbing does not have this problem. Instead of trying to change our role in the equilibrium (for instance trying to decrease those 100 calories) it works on the system as a whole. And you can scrub arbitrarily large amounts. You could even make places like Venus habitable again with scrubbing technology - though to emphasize - Venus is many many many orders of magnitude away from anything we would need to solve on Earth. There are also other grand motivations for developing it. For instance, atmospheric scrubbing, once well developed, could even begin to work as contingencies against life destroying natural or unnatural disasters - nuclear fallout, mega-volcano eruption, meteorite impact, etc. And the opposite (atmospheric injection) could help resolve natural disasters like gamma ray blasts which are one plausible explanation for the Ordovician mass extinction event. We're still just as vulnerable to many of these dangers today. So these are all very important technologies, but don't have a private economic incentive, so it's one area I think would be wise to consider increasing funding for. And again, there's no better time than now.