There is a lot of evidence that climate change leads to extinctions. Over the past century, humans have witnessed changes in climate that correlate with species dying out. And geologists have known for quite some time that the Earth's biggest extinction events — where over 75 percent of species die out in "mass extinctions" — have all occurred during periods of climate change. The question is whether there's a causal relationship between climate change and death. Are life forms really that sensitive to temperature changes that a few degrees up or down can destroy us?
The answer to that question is no, but not for the reason you might think. Climate change isn't causing extinctions directly — but it's definitely a catalyst.
In a new research paper published in the Proceedings of the Royal Society B, a group of evolutionary biologists examined 136 examples of extinctions that appeared to be related to climate change. Of those, only seven turned out to be clearly connected with anthropogenic, or human-created, alterations to the climate. In these cases, temperature changes weren't the issue. Instead, climate change led to knock-on effects that caused extinction. In fact, the biggest extinction threat came from ways that climate change wound up altering the way different species interact.
In a news article, the Royal Society sums up five ways climate change set off extinction events:
1. Changes in temperature can cause species to dramatically increase or decrease their activity. Possible outcomes include starvation for creatures whose activity levels go down. Creatures like algae sometimes go into hyper-growth mode when there is climate change, however. When algae levels go up, they tend to suck more oxygen out of the water. And that kills fish and other sea creatures who depend on that oxygen to breathe. So one species' gain can be another one's doom.
2. Climate change can alter rainfall. This means freshwater species may lose their habitats and die. Other species may lose their sources of drinking water and die of thirst.
3. Multiple extinctions can result when, for reason 1 or 2, a "beneficial species" goes extinct. These beneficial species include prey species, pollinators, and host species. These are species that other species depend upon to survive. Without them, more species will go extinct.
4. The climate can also affect the life cycle of "harmful species" like predators, competitors, and pathogens. If there are too many of any of these harmful species, they can eat all the food in an area — or fell entire populations with an epidemic.
5. Climate change can affect the way species keep track of time by gauging temperature and sunlight. For example, insects and birds often time their migrations and movements to keep up with maturing plants. When climates change, plants may start to mature at different times — and so the insects and birds may arrive at the wrong times and starve.
In other words, climate change causes extinction indirectly. But when it does this, it almost always causes multiple extinctions at once. One species dying out can affect many others. Or, as in the algae and pathogen examples, one species living large can wipe out many species.
Read the full study in the Proceedings of the Royal Society B.