by Alok Jha
Fertilizers are another major source of pollution. These contain copious amounts of nitrogen and phosphorus to help crops grow, but on a typical farm, a significant fraction of the material applied to fields is not used by the growing crops. Instead, it trickles out into rivers and the sea, where the sudden bounty of nutrients can cause algae to bloom, creating a thick blanket that can quickly choke the water and create a “dead zone,” where oxygen is depleted in the water, killing other plants and driving away any fish or marine mammals.
Effects on wildlife
If pesticides and pollutants manage to collect in high enough concentrations in animals, they can kill them outright. Smaller amounts can reduce fertility or stunt growth.
Spraying fine particles or liquid chemicals over a large area is relatively simple to do. Farmers dust their crops with pesticides using small planes; determined terrorists could do the same over cities with poisons.
From a physiological point of view, an animal’s biological systems cannot tell the difference between a pollutant molecule and, say, one of its own hormone molecules. As a result, its natural mechanisms for dealing with the invading molecules go into overdrive, and may break down more than just pollutants. The result is an imbalance, and since hormones regulate things like growth and physical development, abnormalities are almost inevitable.
“The more polluted a bear is, the less of an immune response it can generate to an immune challenge,” says Derocher. He blames the problem on polychlorinated biphenyls (PCBs), which persist in the environment decades after they were banned. Dutch scientists carried out research on seals in the 1990s and found that animals that were fed contaminated herring caught off the Netherlands, where European rivers run into the sea, had only half the breeding success of those fed fish caught in the open North Atlantic. The former group also had suppressed immune responses.
In many species of carnivorous birds, such as hawks and eagles, pesticides have accumulated to catastrophic levels. Pesticide use has also been implicated in the collapse of bee populations around the world, in itself a doomsday scenario that could cripple the production of food and crops forever (see p.80).
The destruction of food webs by pollution makes ecosystem problems even worse. If the plants in a body of polluted water are left uneaten (because the animal population has dropped), they will eventually die and fall to the bottom, decaying and releasing hydrogen sulfide and other gases, thus causing further environmental degradation.
On the ground, or rather in it, pesticides and pollutants have more invisible effects. The structure, fertility and formation of soil depends on the huge numbers of invertebrates (from the tiniest protozoa through nematodes to familiar earthworms) that live in the few inches below the surface of the ground. A single square meter of soil can contain as many as a million arthropods, such as springtails, beetles, millipedes, spiders and ants.
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No organism has evolved to deal with these human-produced chemicals.
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“In places where only a few of these animals are present, the soil is usually of poor structure and contains distinct layers of undecomposed organic matter near the surface,” says Clive Edwards of the Rothamsted Experimental Station in Hertfordshire. “Soil that contains few invertebrate animals or none still produces crops if it is well tilled and artificially fertilized (although in fact the process of cultivation tends to reduce the number of soil animals). If there were no invertebrate populations in woodland soils, however, the process of soil formation would be very slow or would stop altogether, with drastic ultimate effects on the soil’s fertility.”
And it is into this delicate area of soil that modern agriculture deposits its most potent chemical pesticides.
Effects on people
In this global chemical dance, humans are just another animal, albeit at the top of the food chain. “If we were exposing the population to harmful levels of chemicals that can mimic estrogen and chemicals that can block the action of hormones, the effect you’d see is an increase in hormone-related cancers—cancer of the breast, the prostate, the testes,” says Gwynne Lyons, a director of Chem Trust, a charity that works to protect humans and wildlife from harmful chemicals. “And you would expect to see an increase in birth defects, [problems with] the reproductive tract, undescended testes. You might also expect girls to be coming to puberty earlier. All those effects do seem to be happening.”
A large number of the 300 or so man-made chemicals found in humans are those that have been banned for decades—PCBs and other pesticides such as DDT—but Lyons points out that it is not possible to turn off the exposure tap overnight, and new effects on health might be discovered far into the future. Scientists have already found evidence, for example, of a link between Parkinson’s disease and long-term and heavy exposure to pesticides—the strongest associations were in people with Parkinson’s who had been exposed to herbicide and insecticide chemicals such as organochlorides and organophosphates.
What can we do about it?
Chemicals have given us the world we live in today, and there is no sense in regretting their use. As with so many things after the profligate 20th century, we just have to be more intelligent and sustainable about deciding which ones we should use. Farmers, for example, could get by with less fertilizer without much reduction in their crop yields. Using fewer long-lived compounds is another step, so that any that do escape into the environment get broken down into harmless products quickly.
As far back as 1967, environmental scientist George M. Woodwell wrote about the need to pay more attention to the accumulation of persistent toxic substances in the ecological cycles of the Earth, pointing out that the size of our planet will not let us hide the problem forever. “It affects many elements of society, not only in the necessity for concern about the disposal of wastes but also in the need for a revolution in pest control,” he said. “What has been learned about the dangers in polluting ecological cycles is ample proof that there is no longer safety in the vastness of the earth.”
Ozone Destruction
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The demise of the ozone layer is the environmental catastrophe that everyone was concerned about before climate change came along to steal its thunder. Their reason for worrying about this delicate band of gas? Less ozone in the atmosphere means less life on Earth, plain and simple.
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Concern about the ozone layer was at its most fervent in the late 1980s and early 1990s. Thanks to years of industrial-scale refrigeration, this part of the Earth’s atmosphere, a shield against the ravages of radiation from space, was thinning. For some, there was even a faraway but apocalyptic thought: if the ozone layer was to disappear completely, what chance would we have of survival?
What is the ozone layer?
Ozone is not pleasant stuff. A type of oxygen molecule that contains three rather than the usual two atoms of the element, it is an irritating and corrosive gas found throughout our atmosphere. The part we are interested in, the ozone layer, starts between 10 and 16 km (6 and 10 miles) above the surface of the Earth and extends up to 48 km (30 miles). Here, in the stratosphere, ozone is more concentrated than anywhere else, accounting for around three molecules in every 10 million molecules of air. It doesn’t sound much, but this layer contains 90 percent of the atmosphere’s ozone and it keeps life going on Earth.
This stratospheric ozone is crucial because it absorbs most (up to 99 percent) of the harmful ultraviolet radiation coming from the Sun. Without this filter, plants and animals would be damaged by high-energy UV rays. According to NASA, every one percent reduction in the Earth’s ozone shield would increase the amount of UV light reaching the lower parts of the atmosphere by two percent.
UV radiation, a high-frequency form of light emitted by our Sun, can be divided into three broad categories based on how energetic it is—labeled A, B and C in ascending order of energy.
UV-C is the most dangerous to life, but it is all screened out by the ozone la
yer and does not reach the Earth’s surface. Artificial UV-C radiation has antimicrobial properties, and is used for sterilization (more on this later). Most UV-B is also absorbed by the ozone layer, but some does reach the surface of the Earth. It can be harmful to human skin and is the main cause of sunburn. This is radiation we actually need, in small doses, because it helps our skin to produce vitamin D, a crucial ingredient in healthy bones and nervous systems. Excessive amounts, though, can lead to genetic damage or skin cancers.
The human body’s natural defense against UV-B is to increase the level of brown pigment, called melanin, in the skin. This chemical can absorb UV radiation and dissipate it as heat, reducing its potential for harm. It is also therefore the pigment that produces a suntan.
The lowest-energy radiation from the Sun, UV-A, mostly passes through the ozone layer and reaches the surface of the Earth. Though this radiation can be harmful in very large doses, it is significantly less worrying than the other types of UV.
All three types of UV radiation can damage collagen in the skin, which makes it less supple and look older. But the most dangerous result of the rays is their potential to damage genetic material. The radiation can be absorbed by the DNA molecules in the center of cells, which can cause breaks in the long chains. If the body’s repair mechanisms do not spot the breaks and repair them, they can lead to the death of the cell or even mistakes in the copying process when it reproduces. In some cases, this can lead to aggressive cancers.
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The most dangerous result of the rays is their potential to damage genetic material ... In some cases, this can lead to aggressive cancers.
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UV light is dangerous to organisms other than humans too: UV lamps producing high-energy rays are routinely used to sterilize air and water of bacteria and viruses. The radiation breaks molecular bonds in the DNA of the microorganisms, rendering them unable to reproduce.
The ozone hole
Scientists first became worried about a hole forming in the Earth’s protective ozone shield in the 1970s. Decades of evidence pointed to an accumulation of human-produced chemicals that seemed able to break down ozone molecules. These compounds—combinations of chlorine, fluorine, bromine, carbon and hydrogen known as halocarbons, and combinations of chlorine, fluorine and carbon called chlorofluorocarbons (CFCs)—had been widely used for decades in fire extinguishers, refrigerators, air-conditioners and the manufacture of electronics.
These compounds are very stable, and whenever they escape into the atmosphere, they float to the stratosphere in one piece. When they reach the ozone layer, the molecules are split by energetic UV light and the chlorine atoms begin a chain reaction that ends up tearing apart hundreds of thousands of ozone molecules. This means that lots of incoming UV-B radiation is left unabsorbed and is therefore able to reach the Earth’s surface.
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TIME UNTIL OZONE RECOVERY
50–100 years
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Satellite measurements of the ozone in the atmosphere show that these gases are collecting largely at the poles, and that over Antarctica the ozone layer has depleted to such an extent that there is essentially a “hole” there for several months every year during springtime. Measurements earlier this decade showed that over the South Pole, up to 60 percent of the total overhead ozone is gone between September and November. The atmosphere above the Arctic also experiences ozone depletion of around 20–25 percent for short periods between January and April every year. All these reductions in ozone are associated with local increases in UV radiation reaching the Earth’s surface.
The size of the ozone hole above Antarctica has changed over the decades depending on the use of CFCs and other ozone-destroying chemicals. Despite being banned in 1986, CFCs still linger and it will take some years before the hole is entirely repaired.
Is the ozone layer likely to go?
In 1986, after it had been established that CFCs and halocarbons were causing so much damage, international governments got together to sign the Montreal Protocol. This required the 195 signatories to stop production of ozone-depleting gases and to develop ozone-friendly versions instead. It meant that CFCs were replaced with hydrochlorofluorocarbons (HCFCs), which in turn will eventually be replaced with compounds that cause no ozone depletion at all, such as hydrofluorocarbons (HFCs).
So far it seems to have worked, with scientists noting that the removal of ozone-depleting gases has slowed ozone loss in the stratosphere in the past decade. According to the US National Oceanographic and Atmospheric Administration (NOAA), the Montreal Protocol means that the ozone layer has a chance of recovering and is expected to get back to normal in the next 50–100 years.
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Over Antarctica the ozone layer has depleted to such an extent that there is essentially a “hole” there for several months every year.
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A report by the Stockholm Resilience Centre in 2009 assessed various environmental threats to civilization and gave the stratospheric ozone layer a relatively clean bill of health. “The appearance of the Antarctic ozone hole was proof that increased concentrations of anthropogenic ozone-depleting substances, combined with polar stratospheric clouds, had moved the Antarctic stratosphere into a new regime,” it declared. “Fortunately, because of the actions taken as a result of the Montreal Protocol, we appear to be on the path that will allow us to stay within this boundary.”
But ongoing success depends on several factors, according to David W. Fahey, a physicist at NOAA. Governments will need to continue observing the ozone layer to promptly notice unexpected changes, he argues, and they will also have to ensure that nations adhere to regulations—the phase-out of HCFCs will not be complete until 2030. Scientists also need to keep an eye on new industrial chemicals, in case they have ozone-destroying potential.
There is one other wild card, however. CFCs might have been banned, but these are not the only gases that can deplete ozone—others include the oxides of nitrogen, and also hydroxyl ions (produced when water molecules are split high in the atmosphere). In 2009, scientists at NOAA’s Earth System Research Laboratory calculated that nitrous oxide (N2O) was now the biggest ozone-depleting gas emitted by humans. This gas is a by-product of farming and other industrial processes, and is also used by dentists as an anesthetic (so-called laughing gas).
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CFCs might have been banned, but these are not the only gases that can deplete ozone.
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NOAA scientists calculated that since it is not going to be phased out any time soon, nitrous oxide will continue to erode the ozone layer for the remainder of the 21st century at the very least.
The ozone layer might be recovering for now, but you can bet it will still face some serious risks in the years to come.
Asteroid Impact
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It’s a common theme of disaster stories: an object is hurtling toward the Earth from outer space, its path inexorable, the potential damage absolute. If life survives after the impact, it will undoubtedly be drastically diminished.
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It has happened before. Geological records show that the Earth has been impacted in the past by some very large objects. Sixty-five million years ago, the dinosaurs (along with more than half the other species on the Earth) were wiped out by a six-mile-wide asteroid that smashed into the area around Mexico. In 1908, 5,000 Km2 (2,000 sq miles) of forest in the Tunguska region of Siberia was flattened by a 300-foot-wide object from space, which disintegrated six kilometers (four miles) from the ground and released the energy of 1,000 Hiroshima-sized atom bombs.
And in 1994 we got our first chance to watch what happens when two heavenly bodies collide, when fragments of the comet Shoemaker-Levy 9 collided with Jupiter—some of the resulting scars on Jupiter’s surface were bigger than the Earth itself.
It will happen again. In late 2004, scientists became worried about a 400 m (1,300 foot) wide asteroid called Apophis, named after an an
cient Egyptian spirit of evil and destruction, that seemed to be on course to swing very close to the Earth in 2036. NASA estimated that if it hit the Earth, Apophis would release more than 100,000 times the energy generated in the nuclear blast over Hiroshima. Thousands of square miles would be directly affected by the blast, but the whole of the Earth would feel the effects of the dust released into the atmosphere.
Watching the skies
There are two types of object that could cause trouble for Earth. Comets are balls of ice and dust left over from the formation of planets. They normally lurk on the edges of the solar system but can be dislodged by the gravity of the Sun and get into the paths of planets.
Asteroids are hard lumps of rock thought to be the beginnings of a planet that never quite managed to form between Mars and Jupiter. There are more than a million known asteroids, and an object around half a mile wide hits the Earth every 100,000 years. Objects larger than six kilometer (four miles) wide, which could cause mass extinction, will collide with Earth every 100 million years. Experts agree that we are overdue for a big one.