The scoop on Carbon Dioxide in the air

Climate change is a contentious issue framed by questions across social, economic, cultural, political, scientific and ethical realms. Was the early ripening of apples in Vermont this year a product of global warming? Maybe, maybe not. Will it happen again next year? How much will sea level rise in the 21st century? How long until no glaciers remain in Glacier National Park? When they are gone, will it still be known as “Glacier National Park?” Can we correlate events like Hurricane Katrina or the recent floods in Pakistan to warming oceans? It makes sense intuitively that the warmer Indian Ocean produced more evaporation than usual, and this caused a stronger-than-normal monsoon, which led to tragic events in Pakistan, but can we be certain of the cause? We can also question potential sources of error in climate proxies used to provide temperature records from the recent geological past, or the role that policy should play in responding to carbon emissions, or how the average citizen should respond.

The myriad questions and uncertainties surrounding climate change and its impacts on humans and ecosystems are pretty well known, and the uncertainties embedded in these questions form much of the fodder for debate and global warming skepticism; however, in the complicated world of climate change and global warming, there is one certainty and that is the effect of carbon dioxide on the temperature of the atmosphere.

The molecular structure of CO2 causes heat to be “absorbed” and “trapped” in the atmosphere, and there can be no doubt that higher concentrations of CO2 in the atmosphere will trap more heat. The question, “Will increasing CO2 in the atmosphere cause the Earth’s atmosphere to warm?” is not a difficult one. The answer is based on fundamental laws of chemistry and physics, and trying to argue against the role of CO2 in Earth’s atmospheric heat balance would be comparable to trying to disprove the Law of Gravity.

The great 19th century chemist Arrhenius knew that increasing CO2 in air would cause warming, and here is why. The sun sends energy to the Earth in the form of radiation across the electromagnetic spectrum, much of which occurs as ultraviolet (UV) radiation, visible light and infrared (IR) radiation. We can feel the effect of UV rays when they burn our skin, and we can see the visible part of the spectrum.

When UV rays strike the surface of the Earth, their energy is transformed to IR radiation, which carries heat away from the surface, upward into the air, in the direction of outer space. IR radiation is something we can all feel when we sense the heat emitted from hot pavement or bedrock surfaces. As IR rays travel away from Earth’s surface towards outer space, they encounter gases in the atmosphere.

Nothing really happens when this radiation encounters nitrogen or oxygen molecules, but when IR radiation encounters CO2, it causes bonds in the symmetrical CO2 molecules to stretch and bend. (Other molecules also behave in this way, including water, ozone and methane.) Heat is re-radiated outward in all directions, some back towards the surface of the Earth.

This re-radiated energy represents heat that would have otherwise left the atmosphere for outer space. The name for this phenomenon is the Greenhouse Effect — without it, Earth’s average surface temperature would be about -17 oC, and life as we know it would not exist.

However, too much of a good thing means Earth’s average temperature is rising to levels not seen for hundreds of thousands, if not millions, of years. If we seek to maintain the climate and ecosystems in which humans and other species have evolved and prospered, we must do something to control the CO2 content of the atmosphere.

Pete Ryan is a Professor of Geology