• Choose a category
• All
• Classic-Fiction

By Root 765 0

The Palaeocene–Eocene Thermal Maximum (PETM) saw a rate of carbon release that was likely ten times slower than we are currently managing,40 but overall a hefty 2–7 trillion tonnes of carbon were vented by a combination of volcanic activity, methane hydrates, and peat oxidation, leading to a global warming of five or more degrees.41 (Humans have so far released about half a trillion tonnes of carbon.42) Although in terms of its overall impact the PETM is a minnow among mass extinctions, it did lead to one of the top three worst reef-gap events in the entire Phanerozoic (the last half-billion years). There is clear evidence of global ocean acidification, with calcifying corals extinguished and green algae proliferating.43 Indeed, the extinction of corals was close in magnitude to that of the much more dramatic late Cretaceous extinction that killed off the dinosaurs.

So the geological record provides a clear and unambiguous warning that intervals of very high CO2 release and global warming can cause dramatic ocean acidification and disaster for coral reefs. But this is not always the case. A hundred million years ago, during the mid-Cretaceous, chalky plankton thrived—despite atmospheric CO2 reaching 2,000 ppm, and the pH of the oceans dropping as low as 0.8 units below today’s levels.44 These represent far more extreme “hothouse” conditions even than the worst-case scenarios projected for 2100, and yet the thick chalk beds of southern England and elsewhere testify to the fact that these more acidic (but still alkali) oceans were extremely friendly to calcium carbonate-fixing organisms. There are other “hothouse” episodes of extreme greenhouse conditions at different times in the Jurassic, Triassic, and other eras that similarly left reefs seemingly undamaged.

The explanation for the apparent contradiction, however, is simple: time. The oceans are well buffered: As relatively acidic surface waters mix with those deeper down, sediments on the seafloor neutralize the acid—but only over tens of thousands of years.45 Evolution can also work its magic over many millennia, allowing life-forms to adapt to a changed environment—and new species to emerge—through the ever present pressure of natural selection. But in today’s looming acidification crisis, time is not on our side. Having examined the published geological evidence of major carbon releases over the last 500 million years, I cannot find any episode—even during the darkest days of the worst mass extinction—that comes close to the current rate of human carbon emissions. In other words, all of our power-station chimneys, car exhausts, jet engines, and so on across the whole world are releasing carbon dioxide an order of magnitude more rapidly even than the greatest super-volcanic eruption of the last half-billion years. I know this seems like an unlikely assertion, but then humans are an unlikely species. To get a sense of the scale of the enterprise, visit the website http://trillionthtonne.org, which constantly counts down the tonnes as they are released. The digits speed by so rapidly as to be unreadable; today’s rate of emissions is about two hundred tonnes every second.

THE OCEANS OF THE FUTURE

The path forward is rather colorfully illustrated by one of the models. This projection of the future shows a map of the global oceans, assigning a color gradient to the seawater saturation state of aragonite, the form of calcium carbonate used by coral polyps. Where the water is well saturated (blue), corals can flourish, but once acidification makes it less saturated (yellow and then red), reefs will not only stop forming but eventually begin to dissolve. The illustration shows six maps, each representing a different concentration of CO2 in the atmosphere. Under preindustrial levels of around 280 ppm, everything looks good for corals, with large areas of healthy blue throughout the tropical seas. On today’s map (380 ppm), the blues are fading somewhat, with yellows creeping in from the edges as aragonite saturation drops. At 450 ppm the situation deteriorates further, while