A trillion trees by 2030, each fixating 30 kg of CO₂ per year [1], with a global CO₂ emission of about 40 trillion kg by 2030 - roughly linearly extrapolated from [2] - that would capture about 75 % of global emissions, much more than I would have guessed. A billion trees on the other hand are just 0.075 % of global or 0.6 % of US emissions [3]. Also according to [1] a billion trees will need 20,000 km², a trillion 20,000,000 km², that are 0.2 % and 200 % of the US land area. And a trillion trees till the end of 2030, that are 369 million trees per day every day, that seems quite ambitious. The numbers used are the first thing a search turned up, I hope they are at least good enough for the orders of magnitude.
[1] https://www.encon.eu/en/calculation-co2-offsetting-trees
[2] https://www.statista.com/statistics/276629/global-co2-emissi...
[3] https://www.statista.com/statistics/183943/us-carbon-dioxide...