The thin layer that supports life
Our life-sustaining atmosphere
Earth’s atmosphere is the only one that we know of that can sustain life. Its chemical composition of gases that are non-condensible at normal atmospheric conditions is about 78% nitrogen gas, 21% oxygen gas, 1% argon, and many other trace gases, including carbon dioxide. The amount of water vapour varies from 0 – 4%, depending on location, seasons, and time of day. Atmospheric gases help sustain life by regulating our planet’s temperature, protecting us from getting too much of the sun’s damaging UV rays, and facilitating many natural cycles.
Moving up from the surface of the Earth, our atmosphere is divided into four layers: the troposphere, the stratosphere, the mesosphere and the thermosphere. However, most atmospheric gases are found in the troposphere, which stretches about 15 km above the surface of the earth. While a height of 15 km may seem quite large, compared to the size of earth, it is incredibly thin. To put this distance in perspective, the distance from the centre of Earth to its surface is 6400 km, while the troposphere extends only about 15 km from its surface. If the Earth was the size of a globe, the atmosphere would be about as thin as the paint surrounding the globe.
Earth’s thin atmosphere is fragile, and the pollutants we release can compromise some of the ways in which our atmosphere supports life, even though they only make up a tiny fraction of the atmospheric gases.
1. Temperature regulation
Our atmosphere helps sustain life by regulating Earth’s temperature, which is predominantly due to the effects of greenhouse gases. A few naturally occurring greenhouse gases are carbon dioxide, methane, and water vapour. All of Earth’s greenhouse gases compose much less than 1% of the atmosphere. Despite being present at part per million or lower levels, these gases have a profound effect on warming our temperature. Greenhouse gases absorb energy in a way that warms the surface of Earth. Without any greenhouse gases, the average surface temperature of our earth would be near -18 ˚C! Naturally occurring levels of greenhouse gases play an important role in maintaining an average global temperature of about +15 ˚C, supporting the flourishing of life on Earth.
2. Protection from UV radiation
Our atmosphere also prevents too much damaging ultraviolet sunlight from reaching the earth. Although exposure to small amounts of certain types of UV radiation can be beneficial by stimulating Vitamin D synthesis in the skin, excessive exposure to UV radiation can have damaging effects on life on Earth. For example, excessive UV exposure in humans leads to an increased risk of developing skin cancer, retinal damage, and cataracts.
Thankfully, our natural atmosphere protects us from getting too much UV radiation from the sun. Specifically, our atmosphere contains higher concentrations of ozone in the lower stratosphere, sometimes called “the ozone layer.” Oxygen molecules and stratospheric ozone absorb some of the most damaging types of UV radiation from the sun before it reaches Earth.
However, because they were designed to be unreactive, after being released into the air, CFCs don’t break down in the troposphere or come back down to Earth’s surface when it rains. Instead, the only way CFCs are removed from the troposphere is by moving slowly up to the stratosphere. There, CFCs take part in cycles of chemical reactions that rapidly break down ozone.
However, because of CFCs were designed to be unreactive, they don’t readily break down. As a result, they don’t break down in the troposphere or come back down to Earth’s surface when it rains. Instead, the only way CFCs are removed from the troposphere is by moving slowly up to the stratosphere. There, CFCs take part in cycles of chemical reactions that rapidly break down ozone. Due to topography and local atmospheric conditions over the Antarctic during the winter and spring, annual events have occurred in the Antarctic spring for the past several decades, where the ozone depletion is so great that we refer to the loss of ozone as a “hole” in the ozone layer. This very substantial loss of ozone over a period of several months each year has serious effects on organisms, especially simple marine life. Although, CFCs and other ozone depleted substances have gradually been phased out since the late 1980s, the long-term effects of their past usage continue today.
3. Involvement in natural cycles
Our atmosphere also plays a crucial role in a variety of natural cycles.
An important natural cycle for humans and many other living organisms involves the complementary processes of cellular respiration and photosynthesis. In cellular respiration, our cells use oxygen and the sugar, glucose, producing energy, water, and carbon dioxide. Oxygen comes from the air we breathe in, and carbon dioxide is a “waste product” that we breathe out. The main goal of this process is to use glucose (a molecule we can get from the food we eat) to make the energy our cells need to function.
It may be helpful to think of photosynthesis as the opposite chemical reaction of cellular respiration. Photosynthesis uses energy (from the sun) to make glucose (which many plants need to grow). In photosynthesis, plant cells use carbon dioxide, water, and energy from the sun and make glucose and oxygen.
To summarize, carbon dioxide is taken up in photosynthesis, and produced during cellular respiration. By contrast, oxygen is taken up in cellular respiration and produced during photosynthesis. Each of these processes allows certain plant and animal cells to obtain what they need in order to live and grow. The atmosphere contains the oxygen and carbon dioxide molecules necessary for this exchange.