With carbon dioxide and other pollutants being piped into the air all over the world, many scientists believe human activity is causing a slow but inexorable warming of the entire globe. Are humans really to blame, or is the earth just going through another of its natural stages?
Climate change has become a media obsession, but the earth’s climate is variable by nature, and it has gone through drastic changes in its history. In fact just 18,000 years ago (a mere blip in geological time), the North American continent and Europe were in the grip of an ice age, with glaciers extending south as far as New York. So much water was stored in the earth’s ice caps that the sea level was up to 400 feet lower than it is now.
The earth’s climate reflects the balance of energy between the planet and its atmosphere. You’ve seen how precipitation, condensation, and wind tend to distribute heat from solar radiation around the globe. Evidence is mounting that the earth goes through regular cycles that disrupt its energy balance and produce extremes of heat and cold.
Of course, at that time the nomadic people who inhabited the land that would become America could migrate south to more temperate climates, but if the glaciers began moving in today, the consequences would be catastrophic. Why does the earth experience such dramatic changes in its climate from time to time? And what—or who—is responsible?
You may have come to think of the greenhouse effect as a bad thing, but were it not for greenhouse gases, the planet’s average temperature would be about 61°F colder than it is now. It’s important that a certain amount of heat be trapped near the surface, or summer would turn to winter, and winter would be—well, you get the idea.
During the twentieth century, the earth’s average temperature increased by about 1°F, with half of that rise taking place between 1975 and 2000. The last few decades have seen the warmest summers in the Northern Hemisphere since about A.D. 1000 or perhaps even earlier. In addition, the sea level is rising at the rate of up to a foot per century.
All these symptoms indicate a warming trend, just as a high thermometer reading indicates that a patient has a fever. But the fact that someone has a high temperature doesn’t tell the doctor what’s causing it, and the same is true of global warming. There’s little doubt the earth is getting warmer, but the debates over the cause are hotter still.
A Serbian astronomer named Milutin Milankovitch proposed one theory back in the 1930s that involves three components of the earth’s position in space. The planet’s orbit isn’t a perfect circle, but more of an oval, or ellipse. Over a period of 100,000 years, the orbit becomes more and then less elongated, changing the distance the Sun’s energy must travel to reach Earth. This process, called eccentricity, reduces and then increases the effect of solar radiation on the surface over time.
Precession, the slow wobble the earth makes as it spins on its axis, is the second part of the theory. Over a period of about 26,000 years, the north polar axis describes a narrow circle in the sky, first pointing to Polaris (the North Star), and then Vega. This changes the seasons in which the earth is closest to the Sun, and the theory predicts that a significant seasonal and climatic alteration results.
The third component of Milankovitch’s theory is the earth’s tilt on its axis, known as obliquity. This degree of tilt changes as well, from 21.5 to 24.5 degrees over a period of 41,000 years. The planet is now in the middle of the cycle, but as the tilt increases or decreases, the difference in warmth between polar and equatorial regions changes, affecting the severity of the seasons.
The three components are collectively known as Milankovitch cycles. The scientist theorized that when parts of the three variables occur at the same time, their combined effects are responsible for major climate changes. In the 1970s, NOAA’s CLIMAP (Climate: Long-Range Investigation, Mapping, and Prediction) project, which aimed to develop a detailed climatological map of the ancient world using computer modeling, found evidence in deep-sea core samples that substantiated Milankovitch’s theory. The samples showed a strong correlation between long-term climate variations and Milankovitch cycles.
Further studies have shown that ice ages reach their peaks every 100,000 years or so—the same amount of time it takes for the earth’s orbit to stretch and contract. Examination of ice sheets in Earth’s colder regions also indicates that there was about 30 percent less carbon dioxide in the air during ice ages, adding a cooling effect to the atmosphere that could have reinforced and lengthened the colder period.
Because recurring ice ages are one of Earth’s regular cycles, it’s not only possible there’ll be another one, it’s inevitable. They only happen once every several thousand years, though, so don’t break out the parkas just yet.
Once the earth enters a warming phase, certain atmospheric effects can help sustain the change. If the surface temperature is slowly rising, more and more water will evaporate from the oceans, saturating the atmosphere with tons of extra vapor. With more water vapor in the air, the absorption of infrared radiation speeds up, heating the atmosphere and increasing the rate of evaporation even more. Self-sustaining processes like this are known as positive feedback mechanisms.
One way that climate changes can take place is through the redistribution of the earth’s landmasses. According to the theory of plate tectonics, the planet’s continents ride on a layer of molten magma, and although the annual rate of motion is only a few inches, over millions of years continental plates can move considerable distances.
Before the continents broke apart and began drifting, the theory states, they were grouped together in a huge landmass called Pangaea. Some scientists think this large area gathered so much snow in colder periods that it reduced average temperatures over the entire globe. Another factor could be continental uplift, the process that formed the Tibetan Plateau. Newly raised mountains and plateaus could have disrupted prevailing winds and caused other drastic changes in the atmosphere. And as continents broke apart, ocean currents would have shifted, creating deserts in some places and rain forests in others.
Continental drift causes monumental stresses to build up underground, resulting in earthquakes. Many observers have reported strange “earthquake lights” during these events. Although there is no definitive cause for these events, some researchers think that seismic stresses may generate high voltages that are then released into the atmosphere.
Another way that creeping continents can disrupt the atmosphere is through volcanic degassing, which happens when one continental plate meets another. This often takes place deep in the ocean at boundaries called ridges, where molten material from deep in Earth’s mantle rises to the surface, spreading out as the plates retreat from each other. If the plates collide, the heavier one will be pushed downward, where it begins to melt. Either way, large quantities of carbon dioxide are produced, enhancing the atmosphere’s heat-retaining ability over the long term.
Nature has been dumping CO2 and other gases into the atmosphere for eons, but until the beginning of the Industrial Revolution, humanity’s primary contribution was ash and smoke from wood fires and methane emissions from farm animals. In the 1700s that began to change with the invention of the coal-fired steam engine, steamboats, and locomotives. Suddenly, transportation was faster and more economical, and with the advent of coal-fueled factories, goods could be produced much more efficiently, at lower cost, and in greater quantities.
The downside, of course, was pollution. But there was a seemingly limitless supply of air, and with mankind now the master of nature (or so it was believed), few gave atmospheric contamination a second thought. As a result, much damage was done before the problem was identified.
A rise in the sea level increases the likelihood and severity of coastal flooding, and has harmful effects on marshes and wetlands along the shore. It can destroy animal habitats and cause saltwater intrusion into aquifers, thus polluting water supplies. The economic effects can be truly disastrous, making beaches and businesses in tourist areas literally disappear.
Within the past several decades the subject of global warming and climate change has become a priority in many countries. In 1988 the United Nations and the WMO formed the Intergovernmental Panel on Climate Change (IPCC) to monitor the earth’s atmosphere and study the effects of humanity’s influence. The IPCC is made up of scientists from all over the world; their job is to assess the possible impacts of climate change and inform national leaders of their findings.
In 2013 the IPCC released a report that affirmed its previous position: global warming is real and is changing the earth’s climate in ways that natural processes cannot. According to the agency, mankind is now in the midst of a long-term environmental experiment that could have serious negative consequences in the years ahead.
Proponents of the global warming theory predict that as the earth continues to heat up and more greenhouse gases are released into the atmosphere, the rate of temperature increase will accelerate. They predict a rise in the average global surface temperature on the order of 1°F to 4.5°F over the next fifty years, as compared with the 1°F increase during the entire twentieth century. During that time, the sea level rose 4–8 inches, and the EPA estimates it could rise another 12–18 inches by 2100. The IPCC’s prediction range is 3.5–34.6 inches. Some scientists even think that with enough sustained warming the West Antarctic Ice Sheet could slide into the sea, raising ocean levels up to 20 feet.
As temperatures increase, more frequent and intense heat waves could cause heat-related deaths to soar. The heat would also aggravate pollution problems, causing more respiratory distress and other illnesses. Forests and farmland could be swallowed up by expanding deserts, reducing crop output and causing an even greater temperature spike.
The loss of plant life would reduce the earth’s ability to remove carbon dioxide from the air, amplifying the greenhouse effect. Higher temperatures and rainfall amounts would also increase deaths from insect-borne diseases such as malaria and the West Nile virus, which first appeared in the United States in 1999. And with vegetation drying up in the hotter regions, wildfires could run amok across the globe.
The amount of heavy precipitation over land has also increased during the past century, a trend that some scientists say will speed up as the earth warms. Their theory: increased warmth will result in more evaporation, directly generating more rain and snow, causing increased flooding in some areas. With higher sea surface temperatures would come more frequent and much stronger hurricanes, since much more warm water vapor would be available to feed the giant storms.