That a plague would be good for turtles made sense to me. As a sea turtle biologist, I’ve long studied how pernicious human foes—including organized crime, drug cartels, and high-seas piracy—have been godsends for sea turtles. In the absence of deliberate programs to protect sea turtles, which prefer solitude, anything that keeps humans away can be beneficial.

Sea turtles evolved from a common ancestor that dates back over 150 million years, making them one of the oldest groups of reptiles that inhabit the earth. There are seven species of sea turtles, six with a hard shell and bony plates, and one with soft leathery tissue—hence its name, the leatherback.

All turtles begin life in eggs bound in a nest. They can live to nearly 100 years old and thus humans often describe them as symbols of wisdom, longevity, endurance, protection, patience, and creation itself.

Despite their stature in myth, turtles now face a range of threats—wildlife trading, incidental capture in fishing gear, being killed for their meat or shells or eggs, and loss of habitat due to development on nesting beaches—all or some of which can drive certain populations close to extinction.

When I was in high school in the 1980s I had the opportunity to study leatherback turtles alongside researchers and volunteers on an island off of Puerto Rico. We counted the eggs laid by females that came ashore at night, and we recorded the daily temperature of the incubating nest. Such data helps predict the sex ratio of turtles in a nest because, in turtles, sex is determined after fertilization by the temperature in which they incubate. If the temperature of the nest is a certain “threshold” temperature, the turtles will be female; below threshold, they will be male. In this way, and over the course of the nesting season, sex ratios can be somewhat regulated and optimized.

This work fascinates me. Studies like these have also been instrumental in improving scientists’ ability to predict how climate change will impact the world. Warming waters and warming beaches, for example, could change the sex ration of turtles.

I’ve spent my career as a biologist seeking to understand and minimize threats to sea turtles. Over the years, I’ve collected stories from my fellow sea turtle biologists about who or what could reduce declines in sea turtle populations. It turns out there is a common element to all of their stories: fear.

Fear has made Italy especially hospitable for sea turtles. Of course, Italy has the expansive beaches and crystal-clear blue water that are suitable for sea turtles to forage and lay their eggs. But the Italian places most likely to host relatively healthy populations of nesting loggerhead sea turtles are the islands of Calabria and Sicily, where fear of criminal organizations is greatest.

That fear reduces the potential for development and tourism, thereby leaving plenty of wide-open beaches and near-shore waters for turtles to roam freely. Turtles also have benefited from a particular void in Italy’s culture: Italians have never made a practice of hunting turtles for food.

A similar dynamic has supported the critically endangered East Pacific leatherback turtle in Central America. The waters off Costa Rica are some of the most productive marine biotic zones in the world, with an abundance of everything from tiny phytoplankton to large predators such as sharks and whales. Those waters are also a “drug highway” for cartels, which transport mostly cocaine from South America to the U.S. via various sea-going vessels. The presence of these drug boats, and the occasional drug bundles washing up on shore, frighten coastal communities and have been known to keep local people out the waters. That allows leatherback turtles, once commonly poached on their nesting grounds, to flourish under the unintended protection of some of the world’s largest international syndicates.

Another unlikely partner in the conservation of sea turtles are the high-seas pirates that have invaded the western Indian Ocean. Those waters used to be centers of commercial tuna fishing, and sea turtles were often incidentally captured in fishing gear. But the pirates’ presence has diminished the fishing industry—and thus saved turtles’ lives.

But of course, pirates, criminals, and plagues are not long-term solutions to the sea turtle’s problems with human activity. The Endangered Species Act, the 1973 federal law, has also offered crucial protections for sea turtles in U.S. waters. The Act essentially regulates U.S. commercial fishing to account for the recovery needs of threatened and endangered species, such as all sea turtles. The ESA process involves creating a recovery plan for every endangered turtle population—each with an identified critical habitat and specific interventions, such as protecting turtle nesting grounds or minimizing the number of fishing boats in certain times and places. A recent evaluation found that most sea turtle and marine mammal populations significantly increased once an ESA recovery plan was in place.

But it’s unclear if such progress will continue. The ESA has been weakened, most recently to allow more fishing in marine conservation areas. So, in the meantime, fear of drug runners, pirates, and mobsters—as well as COVID-19—are providing a lifeline to some of our most vulnerable sea creatures.

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This question is a difficult one. In fact, it is considerably easier to start with the opposite question: What can the deep history of the oceans not tell us about the future? Understanding what ocean history is unable to tell us—and then what it can reveal—establishes the limits of our current knowledge and provides a window onto what may lie ahead.

The deep history of the oceans definitely cannot tell us about the consequences of human-caused pollution with long-lived materials, such as non-biodegradable plastics or radioactive waste. Also, it cannot explain the consequences of pollution from other harmful substances, such as highly toxic synthetic chemicals that had never existed before we manufactured them, or heavy metals that only recently appeared in such concentrated forms—thanks to human processing. The impact of such products only can be understood by careful study and monitoring of the effects they have going forward.

Common sense plays a role as well. For example, we intuitively know that animals, large or small, will not survive having their innards stuffed full of plastic, or being severely entangled in plastic debris. Yet, we first allowed ocean plastic pollution to grow to frightening proportions before we began to think about tackling the problem. But at last things are beginning to change, following several years of graphic news coverage about large-sized plastic pollution and microscopic particles.

But for some subjects only serious study will do. For example, the impacts of radiation on the marine ecosystem resulting from nuclear testing, spillages from shipwrecks, and shore-based accidents, are surprising. These effects seem limited, both at Bikini Atoll, which endured 23 nuclear test explosions, and around the site of the Fukushima reactor accident. It turns out that the main problem for ocean animals lies not with passing through radiation in the water, but with direct ingestion of radioactive materials. This is because radioactive substances are quickly diluted in the vast volumes of the oceans, and because water itself is a good radiation absorber. Radioactive pollution in the ocean therefore is a major concern only close to its release points, as it enters the food web. It’s less of a concern for animals that occasionally pass through—which is good news, given that humanity enthusiastically dumped radioactive waste in the oceans from 1946 until well into the 1990s.

It turns out that the real killers are invisible chemicals—especially polychlorinated biphenyls (PCBs) previously used in paints, coolant, electronics components, and fluorescent lights. These substances have been released in vast quantities and can be traced through the food web, from the smallest plankton at the beginning to sharks and whales at the top. Similar to other long-lived pollutants, PCBs accumulate to higher and higher concentrations from bottom to top in the food web.

The scale of the PCB problem is enormous: Up to 10 percent of all PCBs produced have now made it into the oceans. That’s a large amount, but it also means that up to 90 percent of PCB pollution in oceans is yet to come, since oceans are the final station for everything carried by water. We need to prevent the PCBs from entering the environment, which requires a massive clean-up operation focused on waste-collection sites (especially landfills). This is progressing, but some 83 percent of PCBs remain to be eliminated.

We know from estuaries and coastal regions that these dangerous pollutants have already become widespread in marine organisms. And they’ve even been found in creatures on the sea floor at the greatest depths of the Pacific, our largest ocean. In whales, typical marine apex predators, the impacts include devastating infant mortality when females pass lethal amounts of PCBs to unborn or suckling calves.

Having addressed human impacts that are completely new to the oceans and the wider Earth system, we can now return to the original question of what deep ocean history can tell us about the issues we’ll face in the future. The three main ones are global warming and ocean acidification, oxygen loss in the oceans, and mass extinction.

Deep ocean history gives us particularly good information about the impact and ultimate fate of our large and quickly rising levels of carbon emissions. It is obvious from Earth’s history that this will, in the short term—over decades to centuries—lead to global warming as well as ocean acidification. Because of the speed at which these two processes are developing under our current emissions trajectory, their combination jeopardizes many ecosystems, both in the oceans (including our all-important coral reefs, home to one-third of all marine biodiversity) and on land.

The oceans’ deep history also reveals that nature has no fast mechanisms to cope with these levels of carbon emissions. Over thousands of years, a process called carbonate compensation—where waters affected by ocean acidification interact with carbonate in sea-floor sediments—will start to reduce the atmospheric carbon dioxide levels somewhat. But the geological record clearly demonstrates that a full clean-up of our current levels of carbon emissions will take a few hundred thousand years.

Nature has provided us with a beautiful illustration of this, especially around 56 million years ago, after there was a natural pulse of carbon dioxide and/or methane into the climate system. About 6 C global warming occurred, and carbonate dissolution developed over vast tracts of the deep-sea floor, while oxygen concentrations in large tracts of the ocean fell dangerously low. A wave of extinction swept through the deep sea, with some groups of organisms losing half of their species count. The system needed about 200,000 years to recover.

So nature can, and will, surely cope with our emissions, but not on timescales relevant to society, and not necessarily in ways that humans will appreciate. Deep ocean history shows us that all manner of ecosystem mayhem—including a large-scale wave of death and extinction in the deep seas—is likely to occur while Mother Nature is going about this clean-up.

Another area where deep ocean history gives us a critical warning about the future is de-oxygenation of the oceans. Oxygenation refers to the amount of dissolved oxygen in the water, and oxygen is vitally important for almost all life on Earth, except for a few specialist microbes. Warming of the oceans, combined with river inflow that is too full of fertilizers and other nutrients, has changed environmental chemistry so that oxygenation of open oceans and coastal seas has been steadily declining during the past half-century. The volume of ocean with no oxygen at all has quadrupled; the volume of ocean where oxygen levels are falling dangerously low has increased even more.

Deep ocean history is full of episodes lasting several thousands to hundreds of thousands of years, during which deep water in complete ocean basins became totally oxygen-deprived. In those cases, it was often related to natural cycles of runoff from land that washed nutrient-rich soils from land into the sea, and processes that severely inhibited deep-ocean circulation (which brings oxygen into the deep sea).

Today, the problem is mostly related to an enormous amount of artificial and human and animal waste-derived nutrients being dumped into the oceans, and to global warming, which is occurring 10 to 100 times faster than in recent geological history. Given that de-oxygenation is widespread and quickly expanding in the modern ocean, the specter of widespread oxygen deficiency is looming large on a global scale. We see it in many of the world’s major lakes as well. It is a vastly destructive process for ecosystems, and in the deep history of the oceans it has been a player in virtually all major mass extinction events. For smaller events in ocean history, where only a single basin was affected, ecosystem recovery took several thousands of years. For the biggest events, where (almost) the entire world ocean was affected, recovery times ran into the hundreds of thousands of years. After true mass extinctions, recovery times extended to millions of years.

The third main process for which history offers important background information is mass extinctions. Extinctions are always underway: Estimates are that “background” extinction rates are of the order of two species per year for every 1 million species on Earth. Given that there are roughly 10 million species on the planet today, a “normal background” extinction rate would be 20 species per year. Yet recent studies put the actual number of extinctions happening today at more than 1,000 times that “normal background” value. This implies that more than 20,000 species per year are becoming extinct, out of the 10 million species on Earth!

How does this compare to the greatest of the five major mass extinctions of the past half-billion years? Well, reconstructions indicate that Earth’s greatest mass extinction, the End-Permian event 252 million years ago, saw a rate of extinction of roughly 150 species per year, though we cannot exclude brief intervals with higher peak rates. Still, it is clear that modern extinction rates are right up there with the worst nature has gone through in a half-billion years.

The insight that we are living through extinction rates unsurpassed in the last 500,000,000 years puts a fine point on the argument that we are indeed in the beginning of the “sixth major mass extinction.” All five previous mass extinctions were exceptionally hard on complex, specialized organisms, and left more resilient, simpler forms to inherit the Earth. Humans are among the most complex, specialized organisms around today, with a deep dependence on other complex, specialized organisms. If we don’t turn the tide on extinction rates urgently, both in the oceans and on land, then all bets are off for our survival. For the first time in Earth’s history, one species has the power to decide the fate of all others.

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