A Discussion About Climate Change – Part 2: Atmosphere

It is more than just the air we breathe. It is the shield that protects and encompasses our planet. Should we battle the sun with a damaged shield?

In the first installment of this series, we took a look at cycles of planetary motion that have influenced the climate of Earth for billions of years. Be sure to read the first part for a more comprehensive explanation!

Milankovitch Summary

As a brief summary, the Milankovitch cycles collectively describe a constant fluctuation in the orbital position of Earth relative to the sun which directly affects the amount of solar radiation that Earth receives. These slight differences have encouraged significant changes in the temperature of our planet since it first formed and will continue to do so indefinitely. The cycles are natural and extremely powerful, but they occur gradually over vast time scales. They are also not the only forces that influence the climate of our planet.

Earth's Atmosphere

^{'Normal' atmospheric conditions where the ozone layer (white) filters out harmful UV radiation while some sunlight (orange) is absorbed by the planet and the rest is reflected back to space. Scales grossly exaggerated!}

Earth's Atmosphere is comprised of approximately 78% nitrogen, 21% oxygen, 1% argon, and a handful of trace gases. The atmosphere has not always been rich in oxygen; over the course of billions of years it has slowly evolved from an inhabitable sea of toxic gases to the air that we breathe today. One major source of oxygen is the respiration of carbon dioxide by photosynthetic organisms like algae, cyanobacteria, and plants. Oxygen and nitrogen may make up 99% of the atmosphere, but neither gas interacts with solar radiation. The trace components of Earth's atmosphere that absorb solar energy are known as 'greenhouse gases' due to the way they trap heat, similar to the glass of a greenhouse or automobile.

Trapped Heat

Imagine parking a car in the sun on a hot summer day with the windows rolled up. The air in the car rapidly warms up to deadly temperatures that far exceed that of the outside air. The windows of the car allow sunlight and heat to pass into the car, but most of that energy is unable to escape. In a similar way, greenhouse gases in the atmosphere can trap heat and enhance the amount of solar radiation that the planet absorbs.

Greenhouse Gases

^{An atmosphere with increased greenhouse gas (grey dots) absorbs more incoming solar radiation and causes the planet to warm more rapidly. Scales grossly exaggerated!}

The 0.1% of trace gases in the atmosphere include carbon dioxide (CO₂), neon (Ne), helium (He), methane (CH₄), nitrous oxide (N₂O), ozone (O₃), and various complex fluorinated gases. Other than the noble gases (Ne, He), all of the trace components are greenhouse gases. These gases are critically important and without them the planet would be about 60 degrees (Fahrenheit) colder than it is at present.

Water Vapor

Water vapor is a greenhouse gas that may account for as much as 60% of the greenhouse gas warming effect observed today. The amount of water vapor that can be suspended by air is directly proportional to the air temperature. As the global temperatures increase, the capacity of water vapor that can be suspended by the air also increases. This factor makes the concentrations of other greenhouse gases even more influential, as any net increase in greenhouse gas is multiplied by the increased water vapor carrying capacity.

Natural Sources

The concentration of CO₂ and other greenhouse gases are influenced by several natural processes. One of the largest sources is volcanic eruptions, which is generally a mixture of carbon dioxide, methane, water vapor, and various other gases. Active tectonic plate boundaries and volcanoes worldwide are constantly emitting massive amounts of gas and dust into the atmosphere. Additionally, the process of lava and magma cooling into rock can release gases. Animals, swamps, peat bogs, and decaying organisms are natural sources of methane and carbon dioxide. Greenhouse gas sources are even found throughout the oceans.

Anthropogenic Sources

It is time to address the elephant in this global room. Human activity has altered our planet and increased the concentrations of greenhouse gases. In countries like the United States, the extraction, processing, and burning of fossil fuels such as oil, natural gas, and coal has increased (often exponentially) since the industrial revolution. This has unquestionably led to an increase in the amount of CO₂ in the atmosphere since the gas is released as a byproduct of fossil fuel combustion. This growing demand for electricity, the heating and cooling of buildings, and rapid transportation does not appear to be slowing any time soon.

Other human activities such as livestock grazing, deforestation, chemical and aerosol production, industry, agriculture, and mining also contribute to the increasing concentrations of greenhouse gases like methane, CO₂, and chlorofluorocarbon (CFCs). Fortunately, some progress has been made in recent years toward reducing the production of CFCs in the United States and other countries after it was shown that they can lead to ozone layer depletion.

Natural Carbon Sinks

Nearly three-quarters of the surface of our planet is covered by water. The ocean is the single largest CO₂ sink that there is. In addition to carbon being dissolved directly by the water, there are photosynthetic organisms (algae, cyanobacteria) that respire carbon and other life that use carbon to form rigid carbonate shells. On land, any type of photosynthetic organism has the ability to fix carbon and turn CO₂ into O₂. This includes trees, grasses, crops, algae, shrubs, and other plants.

Effects of Warming

Short-Term

Over a time scale of decades to centuries, the effects of a continued warming trend are two-fold. An increase in temperature and atmospheric water vapor will lead to increased crop yield in many places, though cycles of drought and extreme weather conditions like hurricanes will also become more common. The melting of ice in Antarctica and Greenland will cause a rise in sea level and a loss of some coastal land, but the warming trend will cause inhospitable land in the polar regions to become more temperate.

Long-Term

When greater time scales are considered, the consequences of increasing greenhouse gas concentrations may be counter-intuitive. As more CO₂ is dissolved by the warming oceans, the ocean waters become more acidic and less hospitable for most forms of life. In fact, the diversity of life will decrease as species struggle to adapt to the changing climate and go extinct. On time scales this great, the Milankovitch cycles discussed in part 1 start to have their own influence on global climate and muddle the blame. Ultimately, it seems that the long-term consequence of warming might be extreme cooling...

Negative Feedbacks

^{Increased cloud cover reflects much of the incoming solar radiation and causes the planet to cool faster. Scales grossly exaggerated!}

The vast clouds of ash and dust propelled into the atmosphere during volcanic eruptions and other celestial events (such as meteor impacts) can help to counteract the increase in greenhouse gases by reflecting solar radiation before it reaches earth. This can cause prolonged periods of cooling that encourage the onset of ice ages.

Similarly, as global temperatures increase and as water vapor accumulates in the atmosphere, the vapor forms into clouds. An increase in cloud cover will reflect most sunlight away from the earth before it has a chance to reach the surface and contribute heat. This will lead to a cooling of the planet and in turn reduce the amount of water vapor that can remain suspended in the air.

Ocean Currents and Ice Caps

^{A disruption of the flow of ocean currents and seasonal extremes can cause the formation of ice caps. The increased albedo of snow on the Earth's surface reflects solar energy. Scales grossly exaggerated!}

The melting of ice caps during warming trends may alter the ocean currents that carry tropical waters from the equator to the polar regions and presently make northern Europe temperate. The currents are driven by density differences associated with water temperature and salinity, and they may decline or stop entirely if too much fresh water is introduced into polar regions from melting ice. This would cause more extreme cold at the poles and extreme heat at the equatorial regions. As the poles gradually become covered with ice caps, the white snow reflects most sunlight in another negative feedback loop.

Coming Up in Part 3

In the next installment of this discussion we will take a closer look at current data and historical proxies used to identify concentrations of greenhouse gases and Milankovitch cycles. We will consider all of the positive and negative feedback factors together and examine the models and simulations that are attempting to predict the future fate of our planet. Finally, we will take a closer look at the most important molecule on our planet... H₂O