1) Unethical carbon offset companies sell you trees or a stake in a renewable energy project. 

The problem is that the renewable energy projects already exist and that the trees in most cases would have been planted anyway by the carbon offset companies' planting partners. Neither the trees nor the renewable energy projects appear to remove any additional carbon dioxide (CO2) from the atmosphere. Those buying offsets are simply subsidising existing project.
If the carbon offset companies created their own projects or planted their own trees, then their marketing claims would carry more weight.
 
2) All trees will eventually die, root and release the carbon dioxide stored in each tree back into the atmosphere. While planting trees is a good thing, it is therefore not a permanent solution. And over the long term this will invalidate the carbon neutral claim.

 

3) It is a scientific fact that If you plant 100 trees, an average of 5%-10% of the trees will die within the first 10 years. This is due to diseases, pest and animal attacks, handling errors, climate and so on. It is therefore very likely that the volume of trees sold will be unable to absorb the expected volume of CO2 and hence, make an activity carbon neutral.

 

4) It was thought that rising concentrations of CO2 would boost plant growth and naturally increase the amount of CO2 plants absorb. But a team at the University of California discovered that if insufficient amounts of nitrogen gas exist in the environment, or other nutrients in short supply, this will limit plant growth regardless of how much extra carbon dioxide is available.
Hence, investing in trees is unlikely to result in these trees absorbing the predicted volume of carbon throughout the life cycle of the tree, as claimed by the carbon offset companies.

 

5) In early 2006, Frank Keppler from the Max-Planck Institute in German discovered that trees produce 33% of the world's methane. So while the trees may absorb CO2, they also release methane, a greenhouse gas that is eight times more potent than carbon dioxide. This finding appears not to be included in the calculation made by the unethical carbon offset companies.

This again means that the trees sold are unlikely to absorb the predicted volume of carbon throughout the life cycle of the tree, as claimed by the carbon offset companies.