>> well, when mr. bain started doing this, it was very difficult.we had a l of trouble in florida getting courts t allow dna testing. we had onease that went up and down through the syste will ton dedge. finally florida has create a post conviction a state and now 48 states have such statutes othe books. and the federal governme has one as wl. t when we started this work, there were no stes that had post nviction dna states. and in fact, are trying to t into court was se diicult because there were statutes of limitations where courts wouldn't even lo at newly liss covered evidence, even edence as powerful as dn identification. >> reporter: i wonr how f sent true perpetrator found through dntesting. how often does that cur? >> a lot. right now the are 105 individuals who ha been identified as the real assailants in these cas. out of t 200 -- 248 post conviction dna exoneration what's rlly important to emphasizhere is that dna is only present in about 10% of cases. in oth words, biology that can be tested, that n be determinative of w really committet

that will help in the case of mr. bain.but i gather it's a rather haphazard system around the country. >> yes, and there are laws in some jurisdictions, not in others. there are restrictions that shouldn't be there. and frankly, it's not enough money. you know, just think about it. when these cases get to trial and federal civil rights actions, when the small needle can be threaded by climb ants, -- claimant its, jurors routinely give them a million dollars a year as well they should given what really somebody is entitled to who has been incarcerated in a maximum security prison as an innocent person, for even a year. >> reporter: already, thank you very much. >> thank you. >> lehrer: now a closing conversation about climate change to cap this week's stories. newshour correspondent paul solman talks to a nobel prize winner about his perspective on the links between climate change and biodiversity. >> there are natural -- way before humans show should up. but it is clear the extinction rate now is a hundred to a thousand and