Climate change may not be forever, but it’ll be for a long, long time. Who — or what — will be around thousands or millions of years hence, when the consequences of our casually massive carbon emissions are still playing out? And do we owe them anything?

According to philosopher William Grove-Fanning, currently at the Environmental Studies Program at Trinity University in San Antonio, the phrase “future generations” first started showing up in the late 1960s, in discussions of bioengineering and nuclear waste. These days, it shows up constantly in discussions of climate policy (and on “Seventh Generation” household products marketed to the eco-conscious — but no longer bought by our household since we noticed that they dye their diapers brown to make them look more ‘natural’ or ‘recycled’). As the climate changes, it won’t just — or even mostly — affect those alive today. We may bite the big one before things get truly strange and/or horrendous. But people toss off the phrase “future generations” so glibly, without really specifying whom they are talking about.

Grove-Fanning figured that most people probably imagine their grandchildren or great grandchildren. And most people are right; the next two or three or four generations may well suffer a great deal thanks to our actions. But, by sheer numbers, there will be more people in the many, many future generations after that. So even if the worst harm will be in the “short” term of the next few hundred years, the vast majority of the people who will suffer at least some harm are in the far future. To figure out how far, in both time and genetics, he did some research on two questions:

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1. How long will the effects of climate change last?

2. Who will be around at the end of that period?

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Direct climate effects of CO2 releases to the atmosphere “will persist for tens, if not hundreds, of thousands of years into the future,” according to at least one study. But if climate change drives many species extinct, it could take roughly 10 million years for Earth to become as diverse as it is now again. For Grove-Fanning, forcing future generations to live in a biologically impoverished world is clearly harming them, since biodiversity provides many ecological services and provides psychological benefits.

In answer to No. 2, Grove-Fanning cites a recent paper from the journal Evolution on the surprisingly brisk turnover in hominin species, which estimates the average lifespan of hominin species as about 2.3 million years.

The unavoidable conclusion, if these time estimates are in the right ballpark, is that some of the future generations that may be harmed by climate change won’t even be human.

There are two key implications to this. One is that — wow — we could be on track to mess things up so badly that not only our children and our children’s children will be mad at us, but an entire new species may be shaking whatever kind of appendage their fists have evolved into and cursing our names with new languages and new religions. That should give us pause. The other is that, hey, if these guys won’t be human, maybe we don’t owe them the same moral obligations we owe our own species. This is the general view of Grove-Fanning. “The proper metaphor for obligations isn’t like a light switch. It is more sliding scale. Yeah, I have obligations to people in the deep future, but they are probably much weaker than the ones I have for my great grandchildren.”

This research came out of Grove-Fanning’s work on what motivates people to act to help people or prevent harm. In general, he finds, evolution has endowed us with a moral radar that responds very well to the “particular and the concrete.” If a town is wiped out by a hurricane, people will flock to the scene, eager to help. But suggest that their actions will hurt an abstract concept like “biodiversity” or a shadowy and temporally distant group called “future generations” and they do nothing. The fact that many of the future generations won’t even be human probably ain’t going to help, even if National Geographic commissions an illustration of a sad and sweaty looking post-human hominin.

But all this talk of far future generations does make one think. Sure, human brains may be incapable of caring about distant and abstract people, thanks to evolution. But before another thousand years are out, humanity may be able to choose to change that with biotechnology. In Unfit for the Future, philosophers Ingmar Persson and Julian Savulescu propose just that. They call it “moral bioenhancement.”

If we take them up on it, I wonder if the end of the human species might come sooner than we expect. For if we became that forward thinking, thoughtful, and morally rational, could we really still call ourselves human?