Lilies are grown in a greenhouse in Almere, the Netherlands. Just as this structure allows tropical plants to grow in a cold climate, the natural greenhouse effect created by Earth’s atmosphere warms our planet (iStock).
How did oxygen form on early Earth?
The early atmosphere was composed mainly of water vapor, carbon dioxide and monoxide, nitrogen, hydrogen, and other gases released by volcanoes. By about 4.3 billion years ago, the atmosphere contained no oxygen and about 54 percent carbon dioxide. About 2.2 billion years ago, plants in the oceans began to produce oxygen by photosynthesis, which involved taking in carbon dioxide. By two billion years ago, there was one percent oxygen in the atmosphere, and plants and carbonate rocks caused carbon dioxide levels to decline to only four percent. By about 600 million years ago, atmospheric oxygen continued to increase as volcanoes and climate changes buried a great deal of plant material—plants that would have absorbed oxygen from the atmosphere if they had decomposed in the open. Today, our planet’s atmosphere levels measure 21 percent oxygen, 78 percent nitrogen, and only 0.036 percent carbon dioxide.
What is the greenhouse effect?
The greenhouse effect, as its name implies, describes a warming phenomenon. In a greenhouse structure, closed glass windows cause heat to become trapped inside. The greenhouse effect functions in a similar manner, but on a planetary scale. In general, it occurs when the planet’s atmosphere allows heat from the Sun to enter but refuses to let it leave.
Without this greenhouse effect on Earth, life as we know it would not exist. On our planet, solar radiation passes through the atmosphere and strikes the surface. As it is reflected back toward space, some solar radiation is trapped by atmospheric gases such as carbon dioxide, methane, chlorofluorocarbons, and water vapor, resulting in the gradual increase of Earth’s temperatures. The rest of the radiation escapes back into space. Without this heat, life as we know it would be impossible, Earth would be about 100 degrees cooler, and the oceans would freeze.
Why is global warming important to humans?
The scientific consensus is that global average temperatures are rising, a phenomenon often referred to as global warming. Many scientists believe human activity has greatly contributed to the buildup of greenhouse gases in Earth’s atmosphere in the past century or so, and hence Earth’s gradual warming—around 1 degree Fahrenheit (0.5° Celsius). One recent study by an international panel of scientists predicted that the global average temperature could increase between 2.5 and 10.4° Fahrenheit (1.4 and 5.8° Celsius) by the year 2100 and that sea levels could rise by up to 2 feet (just over a half meter).
What is the biggest culprit? Although there are other gases, such as methane and chlorofluorocarbons, that increase global warming, most experts point to carbon dioxide as the worst pollutant in this case. This gas is released into the atmosphere mainly through burning of fossil fuels, such as coal, gasoline, and diesel. The gas also forms from the destruction of natural vegetation, such as the burning of forests to turn into grazing meadows for livestock. In this case, the carbon dioxide releases in two ways. First, the destruction of plant life through human actions causes less carbon dioxide to be absorbed out of the atmosphere; and secondly, rotting vegetation in clear-cut forests releases carbon dioxide.
What is ozone and how did it benefit the early Earth?
Ozone (O3)—compared to the oxygen (O2) we breathe—usually refers to a blanket of gas found between 9 and 25 miles (15 and 40 kilometers) in the layer of Earth’s atmosphere called the stratosphere. The so-called “ozone layer” is produced by the interaction of the Sun’s radiation with certain air molecules. The blue-tinged ozone gas is also found in the lower atmosphere. While beneficial in the stratosphere, ozone forms photochemical smog at ground level. This smog is a secondary pollutant produced by the photochemical reactions of certain air pollutants, usually from industrial activities and cars.
The stratosphere’s ozone layer is important to all life on the planet because it protects organisms from the Sun’s damaging ultraviolet radiation. Scientists believe that about two billion years ago, oxygen was being produced by shallow water marine plants. This sudden—geologically speaking—outpouring of oxygen helped to build up the ozone layer. As the oxygen levels increased, ocean animals began to evolve. Once the protective ozone layer was in place in the atmosphere, it allowed the marine plants and animals to spread onto land, safe from the Sun’s radiation.
The formation of the ozone layer in the upper atmosphere early in Earth’s history created a radiation boundary that protects life on the planet. Today, scientists are concerned about the hole in the ozone that has appeared over the South Pole, as seen in this 1987 satellite image (National Oceanographic and Atmospheric Administration).
BEGINNINGS OF LIFE
When did life first begin on Earth?
No one knows the precise time that life began on Earth. One reason is that early life consisted of single-celled organisms. Because the soft parts of an organism are the first to decay and disappear after death, it is almost impossible to find the remains. In addition, because the organisms were so small, they are now difficult to detect in ancient rocks. Some modern viruses are only about 18 nanometers (18 billionths of a meter) across and modern bacteria typically measure 1,000 nanometers across, which is much larger than the early organisms.
In addition, because scientists have found so little fossil evidence, it is difficult to know all the true shapes of the earliest life. Scientists believe that early life was composed of primitive single-cells and started in the oceans. The reason is simple: life needed a filter to protect it from the incoming ultraviolet energy from the Sun—and the ocean waters gave life that protection.
Could life have arrived from outer space?
There is another theory of how the precursors of life were brought to Earth—known as panspermia. Scientists theorize that comets and asteroids bombarded the early Earth, bringing complex organic materials, many of which survived the fall to our planet.
Scientists know there are such organic materials in space. In the late 1960s, radio astronomers discovered organic molecules in dark nebulae. Since that time, other sources have been found, including organic molecules existing in space bodies such as asteroids, comets, and meteorites. In 1969, analysis of a meteorite showed at least 74 amino acids within the chunk of rock. Scientists began to speculate that the organic molecules could have traveled to Earth via meteorites, cometary dust, or, during the early years of Earth, by way of comets and asteroids.
Although many scientists argue that the heat from the impact of a giant asteroid or comet would destroy any organic passengers, many other scientists disagree. They propose that only the outer layers of a large body would be affected, or that the fine, unheated dust of comets could have brought the necessary amino acids to Earth. If this theory is true, we are apparently all—from dinosaurs to humans—made of “star stuff.”
Despite such gaps in knowledge, scientists estimate that the first life began about four billion years ago. These organisms did not survive on oxygen, but carbon dioxide.
What were the conditions on the early Earth that scientists believe may have led to life?
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