What is the atmosphere?
The atmosphere is the thin veil of gas molecules that separates the Earth from the cold void of space. Its heat trapping ability helps to keep the Earth warm enough for life, and it also protects the Earth from harmful shortwave solar radiation and cosmic rays. This protective layer was initially formed by gases vented during the geologic tumult of the young Earth but was later altered by the work of photosynthesizing organisms of the early biosphere providing the oxygen we depend upon. The atmosphere is a dynamic body that interacts with all the “spheres” of Earth. The dynamism of the atmosphere can be witnessed everyday as it transfers solar heat from the equator toward the poles creating regular wind patterns such as the trade winds. Locally, we experience this mass movement of air molecules as a gentle breeze or the rare extreme of a tornado. The atmosphere in motion also transfers water evaporated from the oceans to the continents, providing precipitation critical to sustain terrestrial ecosystems. Without the atmosphere, the Earth would be like a bigger version of the moon – cold and lifeless.
What makes up the atmosphere?
The atmosphere’s principal constituents of nitrogen (78%) and oxygen (21%) have varied little for millions of years. Within the remaining 1 percent, there are trace gases such as argon and a group of gases known collectively as the greenhouse gases (GHGs), which serve the important role of trapping heat the Earth receives from the sun and make the planet hospitable for life.
(composition chart) Water vapor, carbon dioxide, methane, nitrous oxide, and ozone -- the major GHGs -- have the ability to absorb longwave radiation, warming the Earth 33 ˚C from otherwise subzero conditions. Air bubbles trapped in Antarctic ice reveal how for the last 800,000 years the concentration of carbon dioxide fluctuates with the ice ages. Over the last 450,000 years, the fluctuation ranges from about 180 parts per million by volume (ppmv) during the depth of an ice age to about 280 ppmv during the naturally occurring warm interglacials.
The proportion of major gases in the atmosphere have remained about the same for millions of years. The trace gases and in particularly the radiatively important greenhouse gases are more variable. Note that the figure represents gases in the dry atmosphere. Water vapor varies from about 1 to 4 %.
The figure above shows an ice-core record taken from the Vostok Station in Antarctica. Antarctica is so cold that annual accumulation of snow doesn't melt. Previous years of snow and ice are preserved in annual layers with the oldest ice at the bottom of the core. Air bubbles trapped within the ice can be sampled for the concentration of each gas. The peaks of the temperature graph shown in blue represent the interglacials -- the periods between ice ages. The interglacials coincide with the elevated concentration of carbon dioxide shown in green. Generally the atmosphere contains more dust shown in red during the ice ages.
Source: Petit et al., Nature, 1999.
The atmosphere extends out about 120 km from the surface of the Earth and is comprised of four major layers—troposphere, stratosphere, mesosphere, and thermosphere. The lowermost layer, the troposphere, contains the majority of the mass of the entire atmosphere (about 80 percent). As you ascend in altitude from sea level, atmospheric pressure drops exponentially. At 8 km, the height of Mount Everest, there is about one-third the amount of oxygen, and at 80 km, there is virtually no oxygen at all.
How do humans affect the atmosphere?
Ever since humans began to significantly alter the landscape through the development of agriculture after the last ice age about 10,000 years ago, our species has increasingly become an important agent of global-scale change. Extensive combustion of fossil fuels and deforestation are the key factors in the changing concentration of carbon dioxide. Since the beginning of the industrial revolution in the late 18th century the combustion of coal, followed by oil and natural gas, has increased the concentration of carbon dioxide in the atmosphere by 38%, to over 380 ppmv.
This change in CO2 alters the energy balance of the Earth (the balance of incoming solar energy and outgoing energy radiated to space), and consequently the Earth’s temperature – now about 0.8˚C above pre-industrial temperatures and rising. This human alteration of the atmosphere in turn affects agriculture, human health, coastal communities, and the terrestrial and marine ecosystems of the biosphere.
The Aurora Borealis (Aurora Australis in southern hemisphere) is a fantastic light display of the polar regions revealing the usually invisible collision of particles high in the Earth's atmosphere. Particles of the solar wind originating from the sun are colliding with atoms and molecules of the upper atmosphere mediated by the Earth's magnetosphere. The collisions excite electrons which produce visible light seen as auroras.
The Keeling Curve. While it may be difficult to imagine that humans could have a measurable impact on a body so expansive as the atmosphere, measurements recorded over the past century indicate a significant human influence on the atmosphere and the concentrations of its constituent gasses, carbon dioxide in particular. This graph, compiled from regular measurements taken atop Mauna Loa in Hawaii was one of the first such quantifiable indicators of significant human influence on the atmosphere.
Earth's Energy Balance. This figure from Trenberth, Fasullo, and Kiehl (BAMS 2009) represents the Earth’s global mean energy budget for period of March 2000 to May 2004 in watts per square meter. The yellow incoming arrows show shortwave radiation from the sun reflected to space and absorbed in the atmosphere and at the Earth’s surface. The tan outgoing arrows, show longwave radiation. Note that the incoming solar radiation minus reflected solar radiation almost equals the outgoing longwave radiation with a net downward 0.9 watts per square meter as the estimated annual mean energy imbalance.
Source: Trenberth, K. E., J. T. Fasullo, and J. Kiehl, 2009: Earth's global energy budget. Bull. Amer. Meteor. Soc. March 2009.
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