Hursty I read a bit about it a few years ago and it was pretty depressing, dunno that anything has changed.
Essentially it was 50 / 50 if your offsetting happened at all. Companies outsourcing offsetting to other companies who outsourced and so on and so on, and then the ones who did physically do something might not be up to the task, trees planted found dying and not being replaced etc, plastic collections being redumped or having a giant travel footprint. Even the good tree planting ones are fraught, most plant mono culture plantation style trees, which, yes, collect the carbon, but create habitat deserts that fuck all wants to live in, you may as well grow palm oil as plant a conifer or pine in Britain.
For a flight to the states I had a look at paying for individual trees to be planted in Scottish rewilding programme, think it was around £15 a tree, and they guaranteed slow growing, native trees that supported wildlife, and that would be replaced if they didn’t reach maturity etc. Think it was about £100 to offset the flight, and will take about 300 years to do the required offsetting.
