- Global CO2 emissions from fossil fuels are ~35 billion tonnes (35 trillion kgs) 
- Over one year, on average, a mature tree will absorb about 22 kg of CO2 from the atmosphere 
Following from 1 & 2, we need to plant 1 trillion trees to offset global CO2 emissions from fossil fuels.
This doesn’t mean that we stop the other initiatives, it just means that with a simple (not easy) solution such as planting mini forests, we can offset our CO2 emissions and reap additional benefits.
The picture below is from Afforestt, a company following the Miyawaki Technique for planting dense mini forests. More than 3,000 forests have been created globally so far. One of the main benefits of this technique is that after 3 years the forests become self-sustainable and require no maintenance. 
Turns out that there is already a Trillion Tree Campaign. Worth checking out.