'Greenhouse gas' complicated, but must be understood


We hear about the importance of reducing our carbon footprint. This primarily refers to limiting our generation of carbon dioxide gas by burning fossil fuels. Carbon dioxide is the principle "greenhouse gas" contributing to global warming.

The story of how CO2 affects atmospheric temperatures is not simple. One must understand the mechanism of that warming and how sensitive it is to CO2 concentration.

Also, it is important to know where carbon exists in large quantities and how it is exchanged between those reservoirs.

Atmospheric CO2 comprises only a very small fraction of the Earth's carbon. Huge amounts of carbon are dissolved in the oceans, buried in ocean sediments, sequestered in some varieties of rock and buried under sedimentary rock. The rate at which carbon passes between these large reservoirs and the atmosphere is complex and crucial to temperature stability on the planet.

As a percentage, CO2 is a relatively small portion of the atmospheric gases. Neglecting water vapor, the atmosphere is 78 percent nitrogen, 21 percent oxygen, about 1 percent argon, less than 0.04 percent carbon dioxide, and smaller traces of other gases.

Most molecules of carbon are harmless. Importantly, carbon plays the central role in the chemistry of all living things. In concentrations usually encountered, CO2 is not dangerous to life, but is required by photosynthesizing plants.

Carbon dioxide is problematic in the atmosphere at elevated concentrations because of how it interacts with "light." Most of the sun's energy falling on the Earth's surface is in the visible part of the electromagnetic spectrum. The Earth's atmosphere is largely transparent to these wavelengths.

Reflective surfaces such as clouds and snow send sunlight directly back into space. Light that penetrates the atmosphere warms surfaces. Those warmed surfaces emit radiation in a lower frequency range called the infrared.

Nitrogen and oxygen are transparent to infrared radiation. They allow this energy to be lost back into space. Carbon dioxide absorbs infrared radiation and warms the air. This effect is analogous to warming of air in a nursery greenhouse, hence the name greenhouse gas.

Too much carbon dioxide in the atmosphere enhances the blanketing effect of the atmosphere, causing temperatures to rise. Of course, low concentrations have the opposite effect.

Some natural processes remove CO2 from the atmosphere quickly and for short times. They have relatively small impacts on concentrations. Others, take a very long time, but put carbon in storage for hundreds of millions of years.

We must understand these sequestration times to grasp the impact of human-caused imbalances to atmospheric concentrations.

There are exchanges of carbon between surface rocks and the underling crust and mantle of the Earth. It begins when atmospheric CO2 dissolves in surface waters forming carbonic acid.

Silicate rocks are weathered by carbonic acid. The results are bicarbonate ions and minerals that precipitate out of the water to form clays.

Once washed into the oceans, the bicarbonate ions along with calcium (also a product of weathering) are utilized by marine organisms like coral to form calcium carbonate. This is the stuff of their shells.

When they die, they fall to the ocean floor and the carbon becomes stored as limestone.

Finally, limestone is subducted beneath the Earth's crust into the mantle by plate tectonics. This long cycle is completed when volcanic eruptions spew carbon dioxide from the transformed limestone back into the atmosphere. The full cycle takes hundreds of millions of years.

Carbon is also cycled between the biosphere and atmosphere. The biosphere is that part of the Earth's surface where life exists. Exchanges between these two reservoirs occur over time scales ranging from a few days to decades or a few centuries.

Photosynthetic plants use sunlight to convert atmospheric CO2 into organic matter. The energy stored in the resulting starches and sugars is vital to life.

Animals eat plants or other animals. That consumed organic matter is oxidized, or converted back to CO2, by a metabolic process called respiration.

Fungi and bacteria cause decay of plant and animal matter. This decay converts organic tissue to CO2 if oxygen is present or methane (another greenhouse gas) if it is not.

It takes an average of about nine years for the complete cycling of carbon between the biosphere and atmosphere.

Deforestation has had a large impact on carbon stored in the biosphere. Forest floors store 73 percent of the carbon that exists in all soils. Forests themselves have 86 percent of all the carbon stored in above ground plants and animals.

Oceans are among the largest reservoirs of dissolved CO2. Exchanges of CO2 occur between the top layers of the ocean and the atmosphere. This is especially so in regions of oceanic upwelling and downwelling.

The amount of CO2 dissolved in the ocean is dependent on its concentration in the atmosphere, temperature of the water and pressure at depth in the ocean.

In shallower depths, dissolved CO2 reacts to form bicarbonate, which is used by marine organisms as described above. However, by far the largest carbon reservoir lies in deep waters, where high pressure sequesters CO2 for many thousands of years.

The new player in this picture is the burning of fossil fuels by humans. Particularly since the beginning of the industrial revolution, people have burned large quantities of organic matter for energy needs.

The carbon released by burning fossil fuels was sequestered long ago. Most coal was laid down during the Carboniferous period between about 360 and 300 million years ago.

Early in this time period sea levels were low and gradually tropics extended very far north. Forest with the first bark-bearing trees and extensive swamps covered vast expanses.

When inundated by acidic swamp water, the organic matter was prevented from decaying and peat bogs developed. Subsequently, the bogs were buried by mud deposits and sediments. Over time and under great pressure the bogs were transformed into coal.

Oil and natural gas have similar natural histories. Fossil fuels are said to be nonrenewable because their creation occurred long ago over many millions of years under unique conditions.

They constitute a huge reservoir of carbon removed long ago from the atmosphere. What took many millions of years to store away, humans are releasing back into the atmosphere over a couple centuries.

In 2006 about 37 percent of our energy came from oil, 27 percent from coal, and 23 percent from natural gas. Only about 13 percent came from non-fossil fuel sources such as hydroelectric and nuclear.

Nature has tendencies to dampen out extremes and restore equilibrium. The physical and biological cycles I have described above are interactive. That is, carbon passes back and forth between them.

In some instances, feedback loops cause processes that sequester carbon to speed up when CO2 concentrations increase. Unfortunately, from the human perspective, many of these tendencies for stabilization take too long to occur.

Nature's restorative process takes many millennia or millions of years while complex life can be devastated over the course of decades or a few centuries. If we want to be masters of our fate, we must learn how we push processes out of equilibrium and how we can mitigate our disruptive activities.

Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at steveluckstead@charter.net.


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