On what authority are carbon credits issued?
Carbon credit accreditation is a process done by independent accrediting agencies. It is comparable to the way that universities are accredited by various agencies saying that their degrees are legit. You sign up to do a project to reduce carbon output, and then the accrediting agency confirms that you did just that.

Carbon offsetting isn't a future's market, it is a (financial) commitment to a project right now that is dedicated to lowering greenhouse gasses. Here is a list of potential projects, and here is a map showing projects in progress with a certain company. 

I'm actually really glad you asked this question, because I just assumed that carbon offsets were effective proven means, like planting trees. But looking at some of these projects is bringing up some doubt in my mind. I don't see how burning methane into CO$_2$ is doing anything except turning a larger short-term problem into a smaller long-term problem. Or how to measure the exact number of tons of carbon you are saving by changing domestic behavior.