Sea Change

One of the challenges in communicating the urgency of climate change is that our yardsticks aren’t grabby enough. Atmospheric carbon dioxide concentrations higher than 500 parts per million are impalpable to even humans with super-sensitive respiratory systems. Conceptualizing a two-degree increase in global average mean temperature is only slightly less challenging.

This is not one of those yardsticks: If we continue on the current emissions path, the oceans will experience greater changes in biodiversity in the next 85 years than in the previous 3 million years. According to a study released today in the journal Nature Climate Change, we are changing the state of life in our oceans—not just extinctions but also species invasions and relocations—at a rate unknown to science.

The authors of the study, an international team of geoscientists and oceanographers based in France, the United Kingdom, and Monaco, built a computer model of the ocean and populated it with pseudo species that created their own pseudo communities. It sounds eggheaded and theoretical, but the technique was designed to avoid a major obstacle to this kind of research—most of the oceans in the real world remain mysterious to scientists. We have a very poor understanding of what species occupy which ocean zones and at what levels of abundance. A model allowed the scientists to observe the effects of temperature changes without being thwarted by the limitations of existing data.

Since the study deals primarily with imaginary sea creatures, the results do not speak directly to the fate of whales, dolphins, or tuna. Instead, it tells us about net changes in biodiversity. Even in a future of modest climate change, things are going to change substantially. In the equatorial regions, where things are already on the warmer side, a temperature increase of two degrees Celsius will produce species losses of 10 percent to 12 percent. Note that two degrees is our goal for mitigating climate change—the minimum expected warming—and only the most optimistic scholars think we’ll achieve it.

The effect in polar regions would be even more pronounced. As waters warm, species living near the equator will flee north and south, probably elbowing out the current locals. (I realize fish don’t have elbows. It’s an expression.) Overall, the researchers predict a 20 percent change in the average abundance of species near the poles, including both the invaders and the vanquished.

Those numbers are troubling, but these maps forecast a downright apocalyptic maritime future.

The first one illustrates the net change in biodiversity that the researchers expect in the oceans under what the Intergovernmental Panel on Climate Change calls “Representative Concentration Pathway 8.5.” Put simply, this is the scenario in which humanity’s carbon emissions continue to rise through the end of this century. Under these circumstances, biodiversity will change more than 20 percent in the vast majority of the world’s oceans, with changes closer to 30 percent in some areas. (Again, this number refers to the average change in species abundance—how many more or fewer clownfish or eel would live in that stretch of sea.)

The second and third maps put the magnitude of those changes into perspective. The second one illustrates the marine biodiversity change between the last glacial maximum (which ended about 20,000 years ago) and today. The third map depicts the marine biodiversity change between the mid-Pliocene, a warm period that ended three million years ago, and today.

You see how much more red is on the first map than on the second and third? It indicates that we could change the ocean’s biodiversity more in 85 years than nature did in 3 million.

Stop and think about that for a second. What was the world like three million years ago? Lucy, the hominid of species Australopithecus afarensis that revolutionized our understanding of human ancestry, was still alive. North America was connected to Asia via a land bridge, allowing giant camels to drift between the continents. There were 10-foot-tall sloths living in Ohio.

Obviously, global warming isn’t going to cause the exact same changes in the next 85 years—evolution doesn’t work like that. The point is that three million years is a very long time in ecological terms. To collapse the same degree of biodiversity changes into a single century would be unprecedented and likely catastrophic.