Personally, I have three issues when thinking about contributing to charities like Wren:

1) Will my money go where they say it'll go? How can I know that? 2) How do I know that I'm actually helping the environment? How do I know the money doesn't end up in some corrupt individual's pocket at the target country? I know first-hand how some people end up profiting handsomely from subsidies with various tricks. 3) Am I actually offsetting X tons of carbon? How does preservation even offset any carbon, since it's preventing more carbon from being released, instead of offsetting what I did release.

Thanks for the effort, it seems like a very worthwhile cause!