“Have you ever wondered if [carbon offsets] are actually legit?” is the opening line in a recent video from Deutche Welle’s YouTube channel Planet A.

The video is meant for anyone trying to understand just what it means to offset that (longed for) flight to see family—and just how far to trust a mechanism that allows fossil fuel companies, no less, to declare themselves zero-carbon.

The answer: it’s complicated.

Using the example of a non-profit project in northern Germany that uses offset dollars to restore local degraded peatlands to their natural state as extremely rich carbon storehouses, the video makes clear that offsets can be “legit” if several criteria are all met: a project actually reduces carbon emissions, more or less permanently; the carbon emissions produced would not otherwise have occurred; and “bragging rights” (who gets to claim the offset: in this case, the German airline that directs consumer dollars to the peatland restoration project) are clear and traceable to one beneficiary. That’s easy enough in the case of a single German airline underwriting a peatland project in its home nation; not easy at all when corporate offsetting goals begin to overlap with national ones, as they do when the same German airline seeks further offsets via tree planting projects in Nicaragua.

Trouble is, these conditions are very rarely met, especially in the case of “voluntary” offsets that tend to be poorly regulated.

The first problem is that far too many offset projects that promise to reduce emissions do nothing of the sort. Citing a “landmark” 2016 study, Planet A reports that “85% of mandatory offsets the Kyoto Protocol had put in place were not decreasing CO2 in the atmosphere.”

A more recent study by a carbon offset broker found that more than 90% of 100 voluntary offset projects—mostly popular afforestation and conservation projects—didn’t deliver on their promises.

Sometimes, projects reduce emissions for a while, but are set up to fail over the longer haul—as an example, Planet A cites tree planting projects that 10 years on have been clearcut for timber. The video also red-flags the problem of banking on tree planting offsets in an era of rapidly increasing wildfires.

The second category of pitfalls applies to emission reductions that aren’t “additional”, since they would have occurred even without the offset project. One study found that more than 50% of wind farms funded with carbon offset dollars would have been built anyway, Plant A says.

The video also warns of “sweet bookkeeping magic” that allows more than one entity (a company here, a country there) to take credit for the offset.

If any of these factors are in play, and they all too often are, the offset is useless, Planet A warns. As well, many offset projects, especially the cheaper ones involving afforestation, “have a history of disrespecting land rights of Indigenous and local communities.”

For individual consumers, without the time or the training to analyze deeply, the question is how to know at a glance whether a particular offset project is legitimate, Planet A says. A good rule of thumb seems to be that you get what you pay for: The higher the price (it costs C$91.50 to offset one tonne of CO2 in that legit German peatland project), the better the odds that emissions reductions are actually taking place.

As for whether or not the programs are actually needed, it will be “extremely hard to reach our climate targets without some form of offsets,” Planet A says, citing the Intergovernmental Panel on Climate Change. What remains urgently needed, however—alongside absolute reductions in emissions—is a regulatory environment to prevent carbon offset projects from being just another kind of greenwashing.