"Carbon credits" can be quite fake.

A Financial Times investigation found. It was always pretty obvious -- wasn't it? -- that these things seem fraudulent, but we accepted it -- didn't we? -- because Al Gore presented it as true and put his credibility on the line. This has to undercut his vouching for the science in "An Inconvenient Truth." But that's okay, not because I don't want to take the science seriously, but because it's dangerous to have a politician who purports to embody truth and gets people to buy it.

Jonathan Adler says:

The bottom line is that if Al Gore and Leo DiCaprio truly want to be sure they are reducing their carbon footprint, they are going to have to reduce their own energy consumption, rather than paying others to do it for them.

I've never seen why it was enough for these characters to buy their way out of an environmentally damaging lifestyle. If they have money to spend on making the world greener, why don't they contribute it as an act of philanthropy and then also reduce their carbon footprint? Why would having the money to spend make the damage you do acceptable, especially if you're preaching to people who don't have the money that they, unlike you, will have to change the way they live? That never made sense.