Speed remains the one facet of global warming that almost nobody talks about.
Climate deniers point out, correctly, that there were comparatively few extinctions during the massive climate upheavals of the ice ages (until the megafauna extinctions at the end, which were likely due to over-hunting by humans, not climate). Nobody calls these people on the fact that the retreat of the glaciers took twelve thousand years.
Nine degrees of warming over twelve thousand years is radically different from two or three degrees of warming in a single human lifetime.
Speed matters. The faster a change happens, the more destructive it is because everyone else has less time to adapt. Speed is the only difference between a bullet thrown by hand and one shot out of a gun, but the latter can kill you and the former cannot. The measure of our current climate crisis is not simply how many degrees the planet warms, but how quickly the warming happens.
Take, for example, the purple martin. As discussed earlier, they have a scheduling problem. Like many other birds, they depend on a population explosion of insects in the spring in order to feed their young. A progressively earlier spring means a progressively earlier spike in insects, but purple martins (and at least some other birds) cannot adjust their migration schedules to adjust. They are long-distance migrants who have no idea what the weather is like on their breeding territories until they get there. They must rely on instinct to tell them when to head north. As spring changes, their inner calendars don’t, putting their chicks at risk.
Now, obviously, even purple martins can and do adjust themselves to different seasons, because the climate has never been completely static and purple martins still exist. What they cannot do is choose, as individuals, to migrate earlier when spring comes early. Changing an instinct takes a long time. It takes evolutionary time.
Evolution is less a process of learning and more a process of editing. An odd bird with an instinct to migrate early might be better able to feed its chicks when spring comes early. More of its chicks (some of them odd) will survive, while starvation edits out the chicks of more traditional birds. From one generation to the next, the proportion of odd birds in the population will increase. This editing process only happens between one generation and the next, so real change takes multiple generations–maybe hundreds of years. If the proportion of odd birds does not get high enough to sustain the species before spring moves completely, we’ll lose purple martins.
And climate change is moving so quickly at the moment that by the time the birds have adapted to the new conditions, even newer conditions will have come in and changed everything again.
Some bird species can adapt their schedules, at least to some extent, while others are less threatened by scheduling problems to begin with, but the point is that functional ecosystems depend on a careful choreography among the life cycles of different organisms. These carefully timed interactions cannot be changed on a dime. They can change, but they cannot change as quickly as the climate is changing.
Rapid environmental change causes extinctions, but the number of species lost is dwarfed by the number of ecological relationships lost. What goes extinct first is always the specialist species, the ones dependent on conditions being just so, the ones that have special relationships with very specific ecological partners. These are the ones that make living systems rich and complex. What is left behind are the generalists that can adapt to any conditions and any group of other species. So it has been in major extinction events in the past, and so it is now.
And we know these generalist species because they are the ones that do very well in urban areas where rapid change is common; pigeons and rats come to mind as examples. Given time—ten million years, perhaps, rich, complex ecosystems will evolve again.
We have a choice, here between ecological complexity–what many humans consider beautiful and awe-inspiring- a world of creatures that can tolerate almost constant change, the pigeons and rats. The next few centuries will likely be something in the middle of those extremes, but where it the middle the future will be depends on what we do now.
Speed is the issue, not absolute temperature. A world that warms two degrees in the next hundred years could be very different than a world that warms the same two degrees over the next thousand years. The hour is late, and it may sometimes seem as if there isn’t much left that can be done to avert disaster, but this is not true.
Even if digging in our heels only slows the rate of catastrophe, a slower catastrophe could give many species the one thing they need to survive; time.