wiped out individual species, now we threaten whole biota.
THE AGE OF HUMAN-INDUCED GLOBAL CHANGES
The decline of sea turtle populations is but one example of the changes to global ecosystems that have been caused by human activity. Since the end of the last ice age about 12,000 years ago, humans have lived in the geological epoch called the Holocene. But according to the 2016 panel of geologists convened to examine the issue, in their report to the Geological Congress, we have now entered a new geological epoch: the Anthropocene, which is dominated by the activities of a single species. Scientists have drawn this conclusion from an analysis of the long-term impacts of human activities on the biosphere: climate change from fossil fuel combustion that increases carbon dioxide concentrations in the atmosphere and causes ocean acidification, plastic pollution, the disruption of the natural cycles of nitrogen and phosphorus by modern methods of agriculture, the widespread introduction of toxins into the environment, and the irradiation of the atmosphere from nuclear weapons testing.
One way of viewing these huge changes has been put forward by an international group of scientists who proposed nine planetary boundaries “within which humanity can continue to develop and thrive for generations to come.”10 We have already crossed or are close to crossing four of these nine boundaries—climate change, biosphere integrity, land-system change, and the nitrogen and phosphorus cycles. As lead researcher Will Steffen notes, “Transgressing a boundary increases the risk that human activities could inadvertently drive the Earth System into a much less hospitable state, damaging efforts to reduce poverty and leading to a deterioration of human well-being in many parts of the world, including wealthy countries.”11 One of the proposed boundaries is biodiversity, or “biosphere integrity.” As Colin Waters and his colleagues note in Science magazine:
Although Earth still retains most of the species that were present at the start of the Holocene, even conservative estimates of extinction rates since 1500 CE are far above mean per-million-year background rates, with a notable increase from the 19th century onward. Current trends of habitat loss and overexploitation, if maintained, would push Earth into the sixth mass extinction event (with ~75 percent of species extinct) in the next few centuries, a process that is probably already underway.12
The article goes on to note that the most significant reason for mass extinction is due to land-use changes and the restriction of “wild” nature to smaller and smaller areas. “The terrestrial biosphere has undergone a dramatic modification from 1700 CE, when almost 50% of the global ice-free land area was wild and only ~5% was intensively used by humans, to 2000 CE, when the respective percentages were 25% and 55%.”13
Species evolve in interaction with one another and depend on the presence of others. Thus, when one species becomes extinct or shifts its range, detrimental effects may occur to the stability and survival prospects of other species and the healthy functioning of the ecosystem as a whole.
The Warming Planet
The Paris Climate Agreement, signed in December 2015 by world leaders from 194 countries and the European Union, states that human-caused climate change represents “an urgent and potentially irreversible threat to human societies and the planet” that will require “deep reductions in global emissions.” The agreement notes “with serious concern” the “significant gap between the aggregate effect of Parties’ mitigation pledges in terms of global annual emissions of greenhouse gases by 2020 and aggregate emission pathways consistent with holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.”14
The agreement goes on: “Parties should, when taking action to address climate change, respect, promote and consider their respective obligations on human rights, the right to health, the rights of indigenous peoples, local communities, migrants, children, persons with disabilities and people in vulnerable situations and the right to development, as well as gender equality, empowerment of women and intergenerational equity.”15 Currently there is no mechanism in place to achieve these lofty—and urgently necessary—objectives.
Based on temperature records dating back to the nineteenth century, 2016 was the third year in a row to set a global temperature record. Compared to the base period of 1880-1920, the earth was warmer in 2016 by an average of 2.27°F (1.26°C), a level last seen over 100,000 years ago.16 Even if we stopped all production of fossil fuels right now, today, we are already locked in to at least another 1.8°F (1°C) of average warming. The amount of warming guaranteed if the Parties to the 2015 Paris Agreement do everything they say they’re going to, will put the world on track for a truly catastrophic warming of up to 7.2°F (4°C), warmer than the planet has ever been during our existence as a species. Scientists have estimated that the extra energy we have been putting into the atmosphere since 1998, by trapping more greenhouse gases, is equivalent to exploding four atomic bombs every second—over two billion nuclear detonations.17
The last time that global temperatures dropped below the twentieth-century average was February 1986. And the record heat of 2016 brought the world within touching distance of the 1.5°C maximal limit declared dangerous at the Paris meeting the previous year. As of September 2016, eleven of the twelve previous months had set monthly high temperature records. July 2016 set a record for the warmest month ever recorded and then August tied the record.
Such rapid warming of the land and sea is devastating, particularly as it combines with other impacts and leads to further instability and detrimental cumulative effects. The significance of turtles to humans and to the integrity of the ecosystem goes beyond their contribution to human culture and how they are simultaneously threatened by that culture. They are also natural predators of jellyfish, helping to keep their populations in check. As drift animals, jellyfish are swept into every ocean, and the gigantic leatherbacks migrate over vast oceanic distances to chase their prey. Since jellyfish are almost all water and not much protein, leatherbacks must eat huge quantities of jellyfish to stay alive. One study reports that in a single day leatherbacks eat 73 percent of their body mass in jellyfish, an amount that equates to several hundred lion’s mane jellyfish per turtle.18 With leatherbacks driven to near extinction, jellyfish populations have been proliferating, leading to another, even greater problem. Jellyfish subsist by eating huge quantities of fish eggs and fry. If jellyfish populations are not kept in check by their natural predators they will help to undermine the base of oceanic ecosystems.
But the ultimate threat to life on earth, particularly leatherbacks and other nesting reptiles that bury their eggs in sand, is rapid climate change prompted by warming of the atmosphere and oceans—a fact recognized as early as 1953. In The Edge of the Sea, Rachel Carson explained the importance of a stable temperature to the oceans:
Life in the aggregate is lived within a relatively narrow range of temperature. The fact that our planet Earth has a fairly stable temperature helps make it hospitable to life. In the sea, especially, temperature changes are gradual and moderate and many animals are so delicately adjusted that they cannot tolerate an abrupt or extensive change in temperature of the surrounding water. If such occurs they must migrate or die…. Now our climate is changing and we are moving into a warm cycle of unknown duration.19
Turtles lack sex chromosomes. Their genes do not directly determine whether a baby turtle is male or female. Instead, buried eggs take their gender cue from the ambient temperature of the sand. For leatherbacks, temperatures below 85°F (29.4°C) produce a clutch that is mostly male; above that, it’s mostly female. With a relatively tiny 3.6°F (2°C) increase, a nest will produce all females, which is already beginning to happen.20 A few degrees higher yet, and the “boiled” eggs don’t hatch at all.
Unless we do something about limiting climate change to less than 2°C, these majestic
