Atmosphere

views updated

Atmosphere

Introduction

The mixture of gases surrounding Earth to a height ofaround 300 mi (482 km) is known as the atmosphere. It is composed of two main gases and around 20 trace gases. The main gases are nitrogen and oxygen, while the trace gases include carbon dioxide, water vapor, ozone, and argon. Although the main gases have remained at a constant concentration for millions of years, the concentrations of the trace gases are liable to change.

The atmosphere is composed of four layers. The troposphere, which is the layer closest to Earth, contains most of its mass. The density of the atmosphere decreases with distance from Earth’s surface. The four layers do not mix because their temperatures are different. The atmosphere plays an important role in life on Earth. However, an increase in the levels of greenhouse gases threatens to cause widespread climate change through global warming.

Historical Background and Scientific Foundations

Earth is surrounded by a layered mixture of gases known as the atmosphere. Nitrogen makes up 78% of the atmosphere and oxygen 21%. These proportions have remained unchanged throughout human history. The remaining 1% is made up of a number of trace gases including carbon dioxide, nitrogen oxides, rare gases like argon, and ozone, an allotrope of oxygen. The composition of Earth’s atmosphere was once very different, however. At the time of Earth’s origins, around 4.5 billion years ago, the atmosphere was mainly hydrogen and helium. Over time, these light gases diffused into space. Volcanic emissions would have added carbon, nitrogen, oxygen, and sulfur to the atmosphere. Most of the atmospheric oxygen appeared later, being generated by the photosynthetic activity of blue-green bacteria and green plants.

The atmosphere extends to around 60 mi (97 km) above the surface of Earth and its different layers are known as the troposphere, the stratosphere, the mesosphere, and the thermosphere. Each one has a different composition of gases. Also, the temperature of the atmosphere changes with altitude. There are no firm boundaries between the layers, but they tend not to mix because of the temperature differences.

The troposphere is the atmospheric layer closest to the surface of Earth. It contains 80-90% of the total gas in the atmosphere, with the force of gravity accounting for its relatively high density. The temperature of the troposphere falls uniformly with altitude, reaching about -58°F (-50°C) at around 9 mi (14.5 km) above the ground in tropical regions, and at around 5 mi (8 km) in polar regions. Thereafter the temperature remains constant to around 12 mi (19 km), which is the upper limit of the troposphere. This region of constant temperature is known as the tropopause. Without its existence, water vapor would rise until the energy of the sun split it into oxygen and hydrogen, which would then escape into space. Therefore, the tropopause is essential to life. Water vapor in the tropopause falls back to the surface of Earth as rain and snow. The air in the troposphere moves ceaselessly in vertical and horizontal convection currents, distributing heat and moisture around the planet and this layer is therefore the source of weather and also climate, which is the long-term pattern of the weather.

The troposphere gives way to the stratosphere, which extends to 30 mi (48 km) above Earth’s surface. Temperatures rise with altitude in the stratosphere to around 32°F (0°C) at its upper limit. The warming occurs because of chemical reactions involving ozone, which convert solar energy to heat. Around 99% of the atmosphere’s total gas is contained in the troposphere and stratosphere together. However, the stratosphere is

WORDS TO KNOW

MESOSPHERE: The third layer of the atmosphere, extending from the stratosphere to about 50 mi (80 km) above Earth.

STRATOSPHERE: The region of Earth’s atmosphere ranging between about 9-30 mi (15-50 km) above Earth’s surface.

TROPOSPHERE: The lowest layer of Earth’s atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth’s surface.

far less dense than the troposphere. It is of similar chemical composition, except for containing about 1,000 times more ozone and very little water vapor. Ozone, which contains three oxygen atoms per molecule compared to oxygen’s two, absorbs ultraviolet B radiation from the sun and thereby protects life on Earth from its harmful effects.

Above the stratosphere, temperatures decline once more, reaching around -148°F (-100°C) at an altitude of 50 mi (80 km). This layer is called the mesosphere or middle layer, and it is where meteors falling to Earth burn up because of friction with atmospheric gases. Ice clouds form from water vapor in the mesosphere and these are sometimes visible from Earth’s surface. The temperature remains constant again to an altitude of 56 mi (90 km) in an atmospheric region called the mesopause. This gives way to the outermost layer of the atmosphere that is known as the thermosphere. Here, solar energy splits molecular oxygen and nitrogen into single atoms. The thermosphere extends to around 300 mi (482 km), although there is no strict boundary dividing it from the near vacuum of outer space. Temperatur begins to rise sharply in the thermosphere to 2,192°F (1,200°C) and beyond. In the lower layer of the thermosphere, pulses of high-energy radiation can cause ionized gas to emit light. The resulting glow is known as the aurora borealis and aurora australis, or Northern or Southern Lights, which are visible from certain high latitude areas, such as northern Norway.

Impacts and Issues

The atmosphere plays a vital role in controlling the temperature of Earth’s surface. Energy from the sun is partly reflected by clouds and atmospheric gases, but about a quarter of it is absorbed by the so-called greenhouse gases, including carbon dioxide, nitrogen oxides, methane, and ozone, which warms the atmosphere. Around half of the incoming solar radiation reaches Earth’s surface, where it is either absorbed or reflected. Some of this energy is re-emitted, but at a longer wavelength that is readily absorbed by the greenhouse gases. The phenomenon is known as the greenhouse effect, because the atmosphere resembles the glass of a greenhouse that traps heat inside while transmitting sunlight. If the atmosphere was devoid of greenhouse gases, then these longer infrared wavelengths would escape, leaving Earth much colder than it is now and incapable of supporting life.

Human activities such as intensive agriculture and the burning of fossil fuels like coal have increased the concentrations of greenhouse gases in the atmosphere over the last hundred years or so. This has led to a small, but significant, increase in the temperature of Earth known as global warming. It is hard to predict what the consequences of global warming might be, but climate change and change in species’ habitats seem likely to occur. The Kyoto Protocol is an initiative to secure global co-operation in limiting emissions of greenhouse

gases into the atmosphere in an attempt to limit the damage that might be caused by global warming.

Meanwhile, the chemical action of chlorofluorocarbons (CFCs), chemicals once widely used in aerosol products, has led to a depletion in stratospheric ozone which means that more ultraviolet B radiation from the sun is reaching Earth, with potentially damaging biological effects. The Montreal Protocol of 1987 aims to protect the ozone layer in the stratosphere by limiting or banning CFC use. However, it will take many years before the burden of CFCs in the atmosphere and ozone destruction is halted.