What credit for carbon credits?

Carbon-neutral, 100% offset or even "zero-carbon" travel, tour operators and airlines, and more broadly companies with high greenhouse gas emissions, are making massive use of "carbon offset credits". But behind the promise of mitigating, or even erasing, the impact of our activities by planting trees, to take the most popular offset with the general public, lies a poorly regulated market, prone to abuses and whose effectiveness is disputed.