The Supreme Court will rule, this term, on whether Massachusetts and other states have standing to challenge the EPA's failure to regulate greenhouse gas emissions from motor vehicles. By all accounts, the justices seem divided on the standing question. Here is an effort to make a little progress on the underlying questions.
Apparently motor vehicle emissions from the US account for about 6% of worldwide greenhouse gas emissions. That sounds like a lot, but if the EPA did what the plaintiffs want it to do, the reduction in climate change would be exceedingly small. No EPA regulation would reduce that 6% to 0%. And because climate change is a product of the "stock" of greenhouse gas emissions, and because such emissions are rapidly growing in China and India (among other places), it is reasonable to say that even if the US did what Massachusetts et al. seek, the increase in global temperatures by 2100 would be essentially unaffected.
Suppose that global temperature would otherwise be expected to increase by 3 degrees C by that date. If the EPA acted to regulate motor vehicle emissions, the increase would almost certainly still be 3 degrees C (falling only by a small fraction of 1 degree C).
If so, how has Massachusetts been injured? The question is reasonable in light of the fact that existing doctrine requires (1) injury in fact that (2) can be traced to the defendant's conduct and that (3) is likely to be redressed by a degree in the plaintiff's favor.
There are two possible answers to this question. Under existing doctrine, the Court does not give standing to those whose injuries are purely speculative. But suppose that you suffer a 1/10,000 chance of dying from exposure to a certain level of particulates. Do you have standing? Many environmental cases suggest that you do. You can challenge a regulation even if your injury is purely probabilistic. This makes perfect sense. Many environmental statutes are designed to reduce risks. In any case a 1/10,000 chance of death has an expected value, which is positive and not "entirely speculative." (Studies suggets that people would pay $600 to avoid that risk, and losing $600 is certainly an injury.)
The upshot is that if Massachusetts can show that it faces a 1/X chance of suffering an injury of Y, and if Y is large, then Massachusetts should have standing. On the affidavits in the case, it is reasonable to conclude that Massachusetts has met this burden, at least at the motion to dismiss and summary judgment stages. (The Court's rejection of "purely speculative harms" is reasonable, but the Court has yet to grapple with the concept of expected value. A 1/10,000 risk of death can be considered speculative, but it is certainly an injury in fact.)
A second possible ground for standing is this. If the government deprives me of $10, I have standing to challenge its decision. To count as an injury, a plaintiff need not show a large harm; a real harm will do. Suppose that Massachusetts can allege that on the best projection, or a reasonable projection, the regulation of greenhouse gas emissions from motor vehicles will prevent at least $1000, or $500, in aggregate harm to the state. If so, Massachusetts should have standing. It need not show that action by the EPA will have a hugely beneficial effect. And on the affadavits in the case, it is again reasonable to conclude that Massachusetts has met its burden, at least at the motion to dismiss and summary judgment stages.
Standing doctrine is a true mess, in part because the Court has not adequately grappled with harms that consist of "risks" or "opportunities." Environmental statutes are typically designed to reduce risks, and people have standing even if they are not 51% likely to die, or get sick, as a result of pollution.
The climate change case in the Supreme Court is tricky, on standing grounds, because action by the EPA would have so little effect on climate change. Nonetheless, Massachusetts et al. should probably be given standing on one of the two grounds I have sketched. Even if the Court rejects this conclusion, it should not rule in a way that would disable people from challenging probabilistic harms. Such a ruling would increase the current mess -- and badly misunderstand the regulatory regimes under which challenges have long been brought. (I have not explored the merits of the case, which have their own complexities.)