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# 5.22 Environmental Change on Earth

The mass extinctions at the end of the Paleozoic and Mesozoic eras show that biological evolution can be disrupted if changes are too large or too abrupt. That is why there is such concern today about environmental changes caused by worldwide industrial growth. The time scale of such changes (decades), is much faster than the time scale for noticeable evolutionary change (roughly 100,000 to a million years). Rapid change of the environment is a threat because evolution cannot keep up. That is the basis for environmental concern today. Note an important point: the problem is not just change (which occurs all the time), but rather the rate of change. Current estimates suggest we are losing about 5% of the planet’s plant and animal species per decade, implying that within a few centuries, Earth will have experienced a mass extinction comparable to the biggest ones in the history of the planet. To put it another way, if someone looks at the fossil record of the Earth a million years from now, our period may stand out as another Great Dying. The same economic growth that supports a large human population is also killing our planet.

One example of a human-caused environmental problem is the damage to the ozone layer. At 20 to 30 kilometers above the ground, ultraviolet rays from the Sun are absorbed by the ozone layer. The fact that most of these rays don’t reach the surface is good for us, because ultraviolet rays damage organic molecules. Energetic ultraviolet photons that hit organic molecules can actually break them apart and cause skin cancer. Certain chemicals called chlorofluorocarbons (or CFCs, such as Freon) are widely used in air conditioners and other devices. As is now well known, these chemicals slowly seep into the atmosphere where they eventually break down the ozone molecules. This destroys the ozone layer, lets the ultraviolet light through, and increases the risk of skin cancer and random genetic mutation for everyone. Continued CFC use could create great dangers in the next century. Therefore, international agreements have been reached to phase out CFC production.

The discovery of the CFC threat to the ozone layer is a good example of spin-off from one area of science to another. The recognition of the damage from CFCs to the ozone layer came in part from geochemists and planetary astronomers studying chemical processes in the atmospheres of Venus and Mars. They predicted damage to the ozone layer from CFC chemicals, and this was first detected over Antarctica, where a natural winter "hole" in the ozone layer broadened dramatically through the 1980s. Satellite data in the early 1990s showed that the winter breakdown of ozone had begun at high northern latitudes as well. Record winter ozone depletion over Antarctica occurred in 1993; by 1997 the depletion seemed to have stabilized. Subsequent years saw oscillating levels of ozone in the atmosphere. Most scientists attribute the widening of the ozone hole to CFCs, and its stabilization to the phase-out of CFCs. If the phase-out continues, experts in this field predict a decline in ozone damage by the mid-2010s, and there are indeed signs that the ozone level has stabilized. The ozone issue shows that scientific data and international cooperation can be used to deal with an environmental problem in the nick of time.

Another example of rapid, human-caused change is the increase in carbon dioxide (CO2) in our atmosphere. The use of fossil fuels burning, the clear-cutting of millions of acres of forests, and the release of pollutants from factories all lead to the buildup of carbon dioxide in the atmosphere. A carbon dioxide increase of more than 100% has been observed in data taken since the beginning of the Industrial Revolution in the 1700s. The rate of increase is accelerating. In the past 30 years, the level has increased by 20%, and the current rate of increase is 0.5% per year. The effects of this increase on our planet are liable to be complex. Earth scientists are using computer models of the atmosphere to understand this important issue.

Carbon dioxide, though a minor constituent of the air, is one of the most effective of the so-called greenhouse gases. In both a greenhouse and the Earth's biosphere, incoming sunlight warms the ground, and the ground re-radiates thermal infrared radiation. Thus, the greenhouse effect is a warming of the air that occurs because the thermal infrared radiation can't get back out of the system easily. In a greenhouse, glass blocks it and seals in the warm air. In the Earth's atmosphere, carbon dioxide molecules absorb the infrared radiation, heating the air itself. Certain other gases, including water vapor and methane (CH4), also absorb some of the thermal infrared radiation, adding to the effect. Water vapor's effect explains why a cloudy, humid night does not cool down as fast as a clear, dry night. Methane is also increasing; its concentration in the air has doubled since the early 1800s.

There is a lively, sometimes acrimonious, debate over global warming. Science, economics and politics are all mixed up in this debate. It seems difficult to answer the simple question: is the Earth actually warming up? Even though current computer models of the global climate are very sophisticated, they are less complex than the real world. For example, significant additional climate variations arise from eruptions of volcanic dust, and subtle variations in solar radiation affect the ozone content and structure of the stratosphere. Another problem is the incomplete atmospheric data; computer models are only as reliable as the data that are fed into them. Perhaps the most worrying issue is the influence of the oceans. The oceans store nearly ten times as much of the Sun’s energy as the atmosphere, and largely drive the Earth’s climate. Yet temperature data for the oceans is even sparser than for the atmosphere. We are trying to measure small changes in large and complex system, and extrapolate those changes into the future. Prediction from past data is always tricky.

The debate over global warming perfectly reflects the theme of how science works. We are dealing with a situation where data is limited and models of the Earth’s atmosphere are incomplete. Induction means that we cannot be certain of our conclusions. While there is a clear and undeniable increase in carbon dioxide, the increase in global temperature is less certain. In part, this is because the steady increase is superimposed on large year-to-year fluctuations. The complexity of the climate system means we are unsure how carbon dioxide changes translate into global temperature changes. Our certainty will increase with more data and better models.

Most climate experts expect that the increasing carbon dioxide will cause an average warming of the climate by the next century, and many believe it has already started. The exact amount of climate change during the next few decades, and its economic effect, is hard to predict. Nonetheless, virtually all studies predict climate changes as a result of projected increases in carbon dioxide. Most models predict net warming by 1°C to 3.5°C by the year 2100. Most models also predict increasing extremes in rainfall and temperature. One degree doesn’t sound like much, but even changes in average temperature of a degree or less can have big effects on agriculture by changing growing seasons and increasing the spread of tropical diseases. Some idea of the effects of a 3°C change comes from the finding that the ice ages involved average temperature drops of only about 5°C!

Sometimes science impacts public policy. Politicians would like a clear and unanimous statement from scientists about global warming, but the nature of science means that such certainty is elusive. Our understanding of the birth of the universe is hampered by just the same things as our understanding of climate change — limited data and imperfect models. But most people are clearly more concerned about global warming than they are about the big bang. With good reason! When the potential consequences of inaction are momentous, can we really afford to wait before acting?

The problems with the ozone layer and possible global climate change illustrate the difficulties of using recent scientific discoveries to make good public policy in a democratic society. The ozone issue is a success story. Faced with clear evidence and a broad scientific consensus that CFCs were affecting the ozone hole, politicians decided to act. The international action to phase out CFCs, and the apparent reduction in the ozone loss rate, seems like a textbook case of applying new findings to benefit society. It perhaps helped that the economic costs of phasing out CFCs were not too high since replacement chemicals were available.

Carbon dioxide is the cause of global warming and nations still argue about what steps to take in order to reduce carbon dioxide production. The United States and Western Europe produce most of the world’s carbon dioxide: 23% and 14% respectively. The technology exists to move away from the use of fossil fuels, but the economic impact would be enormous. On the other hand, the highest per capita production is by newly industrialized countries. These countries are projected to become the largest producers of carbon dioxide, and they claim that curbs on carbon dioxide would hamper their industrial growth. In the interests of short-term economic prosperity, and because the data on global climate are still poor, several countries (including the United States) are calling for a delay in action until more studies can be made. In this political environment, it is difficult to transform scientific observation into policy. Even if the predicted temperature changes are detected, researchers will continue testing whether they are due to the greenhouse gases or some other cause. Observations of the whole Earth from space, over a period of years, will help clarify the issue.

What are the broad implications of the relationship between science and society? Science itself is value-neutral. In other words, the general conduct of scientific research is not good or evil, and the scientific enterprise has no political agenda. However, the consequences of scientific knowledge can be both good and bad. The energy stored inside every atom can be used to create clean power, as in a fusion reactor, or it can be used to make weapons of mass destruction. Understanding of genetic structure can be used to help fight disease, or it can be used for dubious forms of genetic engineering. It is up to societies to decide how they use the fruits of scientific knowledge.

Science is an objective enterprise, but scientists are people who have opinions too. The trick is to distinguish scientific issues — which can be decided by the use of evidence and logic — from the matters of opinion. Scientists will often disagree, because of competing theories or incomplete evidence. But if you ever hear a scientist expressing a personal or political opinion, you should be suspicious. It is the duty of good scientists to keep their scientific knowledge and their opinions separate. The best answer is for citizens to stay well informed. If people are aware of the basic scientific issues, then they can participate in these important debates.

The biggest shift in our thinking about our planet occurred in 1968, when the astronauts of Apollo 8 were the first humans to ever stare back at the Earth from deep space. It is a striking vision, a world with delicate oceans and a thin sheath of atmosphere. It is not a coincidence that the environmental movement dates back to this time; it was shaped by the awareness created by our trips to the Moon. Now 7 billion people jostle on the planet, and we have the ability to alter our global environment in many ways. Our biggest challenge is to use this power wisely.