Why is there so much concern over small rises in carbon dioxide (CO2) concentrations in the atmosphere?
In Brief: Small is a relative term. In other complex systems, like the human body, changes in chemical concentrations that make the recent rise in atmospheric carbon levels look enormous can mean the difference between life and death. The pre-industrial to industrial age difference in CO2 concentrations (280 to 390 ppm) is about the same as the difference in CO2 concentrations between the height of the last ice age (180 ppm), when a massive ice sheet extended from the Arctic to the Ohio River, and the current interglacial period (280 ppm) when human civilizations have existed.
“Great changes can be brought about by very slight forces.” — Julius Caesar
In science, whenever you hear the terms big or small, fast or slow, light or dark, it’s important to ask relative to what? Are the current changes in carbon dioxide (CO2) levels today really that small? In absolute terms, CO2 takes up 0.04 percent of the atmosphere. So what if the levels double to 0.08 percent? Can a change on that scale really have noticeable effects?
Some substances are very powerful, a tiny amount of hot pepper, for example, can completely transform a recipe. Also consider the effects changes in chemical concentrations have on the human body, which is, like Earth’s climate, a complex system with feedbacks and thresholds. A doubling of CO2 levels in the atmosphere from 0.04 percent to 0.08 percent is about the same magnitude as the blood alcohol level change when an average-sized person takes a shot of 80-proof liquor. For some people, ingesting this much alcohol can cause major changes in perception and it causes at least noticeable effects for just about everyone else. Much smaller changes can have even more dramatic consequences. A dosage that brings cyanide levels to fifteen-millionths (0.0000015) of one’s body mass is considered lethal. The fear you feel from a life-threatening situation is driven by only five ten-millionths of a gram (0.0000005 grams) of adrenaline per liter of blood.
Compared to what is required to poison someone or create an adrenaline surge, recent changes in carbon dioxide concentrations are substantial. Save the last two centuries, all of recorded human history (10,000 CE to present) has unfolded with carbon dioxide concentrations hovering around 280 out of every million molecules in a given volume of air (or about 0.028 percent of the atmosphere). Only slight variations occurred, with some CO2 added by volcanic processes, a relatively small amount breathed out by animals, and some CO2 absorbed by oceans, plants and soil, all resulting in a natural balance over time (decades to centuries). This natural balance was upset by the Industrial Revolution, when carbon that had taken tens of millions of years to accumulate was taken out of Earth’s depths and rapidly added to the atmosphere as humans began burning coal and then oil. Even though Earth’s plants, soils and oceans are now absorbing almost half of the CO2 we emit through fossil fuels, the atmospheric concentration continues to rise year after year. It’s now close to 390 parts per million (ppm) and still climbing by around 1.5 to 2.5 ppm per year.
This pre-industrial to industrial age difference in CO2 concentrations (280 to 390 ppm) is about the same as the difference in CO2 concentrations between the height of the last ice age (180 ppm), when a massive ice sheet extended from the Arctic to the Ohio River, and the current interglacial period (280 ppm) when human civilizations have existed.
Most (99 percent) of the atmosphere does not absorb energy from the sun. After the sun’s rays hit Earth’s surface, the energy radiates back toward space. Well mixed greenhouse gases trap some of this outgoing radiation. Despite the fact that they account for less than 1 percent of the atmosphere, they do the work involved in keeping Earth above freezing. Note that the well mixed greenhouse gases do not include water vapor; Earth’s atmosphere would not be warm enough to hold water without the more permanent presence of CO2, methane, NO2, etc. The extra CO2 that has been added means that more outgoing radiation is being absorbed, most of which is quickly transferred from the atmosphere and stored in the oceans. The excess energy that has been absorbed and stored in the upper 2,300 feet of the world’s oceans since 1950 is about the same amount of potential energy stored in 2.5 million one-megaton hydrogen bombs.
View an animation of the shrinking of the Laurentide (North American) Ice Sheet over the past 18,000 years.
Keep track of changes in Earth’s atmospheric carbon emissions at NOAA’s Earth Systems Research Laboratory.
View and animated graphic of Earth’s greenhouse effect.
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