I am about to do some work on this, but do not know at the moment other than through anecdotes. So I may change my mind on this, and appreciate comments or refutations.
The theory of offsets, in general, is obvious. If you produce, say, a tonne or so of greenhouse gas then some people estimate buying the planting of 4 or so trees will absorb about 1 tonne of CO2 for a period of 100 years depending on the trees. This clears you of the guilt of greenhouse gas production and supposedly balances it all out.
However, at best, there is always a lag. A business emits a tonne of CO2 probably in couple of hours or years depending on what they are doing, and it takes over 20 years or so (wild guess, but it is not instantaneous) for the trees to pull it down. So the gas stays in the air for quite a while – all else being equal. As implied above, the trees could die and release the CO2, if it is not done properly. People could also harvest them, or burn them, if the offsetting was done really badly (I believe this has happened overtly in Brazil, but again I may be wrong).
Sometimes people spend offset money supporting forests in Indonesia, or somewhere. What often happens is that local forest people get chucked off their land, and get forced to move out of the area, so they stop being a secure, largely self supporting ‘community’ and have to engage in wage labour without support or connections. This dispossession can also provoke ecological problems as the people may have lived in the forest for thousands of years, looking after the forest and protecting it, or changing it in some way. When they go, diseases can spread with greater ease, pests get out of control, fire becomes more deadly as there is less clearance of fallen timber or undergrowth (for firewood or grazing) and so on. So the process may not only destroy ways of life, but make the forest vulnerable.
I’ve also heard of people being allowed to preserve land somewhere else to offset the destruction produced by the mine. Of course the land elsewhere is not the same as the land being mined, and frequently does not have the creatures who were endangered by the mine – and sometimes people claim the preserved terrain is not of the same rarity, or even with similar properties to that being destroyed etc.. In any case this is simply not destroying more land/ecosystem for the moment, rather than ‘making’ new replacement land or ecosystems. If such a process continues, then new land keeps being destroyed until we run out of mining land. This is not quite the same idea as the drawdown offset, but its often lumped together.
Farmers in Narrabri told me, and the people I was with, that one of the mining companies in the local area did offsets by planting trees (not sure what this was about), however they planted trees in areas in which any farmer could have told them the trees would not grow. Indeed the farmers showed us dying and stunted trees planted in rows, with absolutely no effort made to replicate the local scrub. And they told us the company just left them after planting, making no efforts to water or protect them – this, of course, may be mistaken. But there was little chance of mistaking the parlous condition of the young trees or the nature of the drought which had been going on for years.
I have also seen tree planting in the Hunter Valley, but this seemed pretty clearly to have the function of screening the dead mines from roadside observation. I don’t know if they got offsets from this.
There are people who argue offsetting is all marketing. It allows people to claim they are carbon neutral or pretend to themselves they are not contributing to climate change when they are. It also takes money and motivation away from investments in better technology. Plans for moving into low emissions technologies can get shelved because of the offsets.
Having said that, I think the evidence is that we need to stop emissions, and we need to drawdown CO2 or methane. There is some suggestion we can do this through technology. However, reducing emissions is the priority.
Planting trees, or seaweed, is not a bad idea as drawdown, but it is probably not a great idea if used as offsets, and it may only defer the problems as eventually organic life dies and releases gases – sometimes fewer gases if the forest is a real functioning forest, because insects and other creatures consume the dead trees and effectively bury them… But that does not seem to count for much when people are discussing offsets.
I tend to agree with the cynics. It is a method of trying to put a voluntary price on destruction, when it is better to stop the destruction itself.