For a nerd like me, this case is really interesting. Let me see if I can bring you along.
If you want to sue somebody, you have to have some kind of injury. The legal term is "standing," as in "you have to have standing to sue." The federal courts usually apply a test something like "a plaintiff must allege [an actual] personal injury fairly traceable to the defendant's allegedly unlawful conduct and likely to be redressed by the requested relief." There are several elements to this test for standing--actual personal injury, traceability, redressibility. Personal injury is the most tricky of the elements because courts usually require that injuries be somehow unique or special to the plaintiffs- i.e., injuries shared by everyone or a huge class of people don't give rise to standing. Injuries also have to be relatively certain and not purely speculative.
In EPA, the Court apparently had some new standing ideas they wanted to share with us. Massachusetts claimed that the EPA's refusal to regulate carbon dioxide as an air pollutant caused global warming, which in turned caused the sea to rise, which then caused Massachusetts to lose coast lands. The state estimated that it waters could rise as much as 70 centimeters by 2100, causing an injury to the state. The Supreme Court accepted that the state had standing.
Chief Justice Roberts took the court to task in his dissent, pointing out that global warming doesn't harm Massachusetts in any particular way, that the models predicting rising ocean levels are both incomplete on their own terms and fail to take account of all relevant factors, that the relief sought by Massachusetts (having the EPA regulate new cars and engines only) is so narrow that no scientist believes that any regulation could possibly change the presence or pace of global warming, and that the opinion ignores the greenhouse emissions of the rest of the world. Needless to say, the Court's standing analysis is novel and presents serious problems to those of us who try to assimilate these opinions into cohesive doctrines.
If anything mitigates the flaws of the standing analysis, it is the fact that Court relied heavily on the fact that Massachusetts, as a state, can have "special solicitude" not available to ordinary citizens. States can have "quasi-sovereign" interests, meaning that they can have interests that underlie the interests of the citizens of those states, i.e., a state has a quasi-sovereign interest in its lands, even if the state does not own the land, because the state has a kind of title in the land. Alabama therefore has quasi-sovereign interests in Alabama land against out-of-state polluters or trespassers.
It is important to note that the Court apparently changed the quasi-soveriegnty standing analysis as well as the normal standing analysis because quasi-soveriegnty standing has traditionally allowed states to have standing only where that state's citizens have standing--the state stands in the shoes of its citizens. But, as noted above, traditional standing analysis would suggest that nobody has standing. Furthermore, the Chief Justice noted that states have never before been allowed to claim their quai-sovereign interests against the federal government because the federal government presumably has the same interests as the state.
Quasi-sovereignty does not come up all that often (the Court went back to 1907 to find the exemplar case) and therefore I hope that this special solicitude analysis will not be often applied.
Should the Court choose to loosen standing generally, everything I have learned in my federal courts class will be instantly obsolete.
Standing is important. Courts exist to solve disputes (short for "cases or controversies") between parties. "Disputes" comprise a subset of "problems." If too many "problems" become "disputes" because of weakened standing requirements, courts get too busy and become involved in addressing issues for which they are less and less compotent. Legislatures are much better at solving problems.