A hitchhiker's guide to a warming earth

So, all this talk about global climate change getting you confused? Here’s a basic guide to the facts.

The earth’s current temperature (averaged, merged) is about 15°C, fluctuating from about 8 to 20°C, depending on the time of year. Now if you do a simple 1D radiation balance calculation using the Stephan-Boltzmann equation (considering the blackbody radiation of both the earth and the sun), you will come up with a temperature of -18°C, which is obviously much lower. So what explains the difference (thank goodness, or we’ll be freezing our buns off)? 

The answer is that the earth’s atmosphere traps some of the sun’s heat. To understand this, we have to discuss radiant flux and irradiance. Radiant flux is the energy per unit time associated with the emission of photons (in watts). Irradiance is the radiant flux per unit area. We can use Wien’s Law to calculate the peak monochromatic irradiance of the sun and the earth. The sun’s rays, centering around 0.5 µm wavelength, are hitting the earth, and the earth is re-radiating in the infrared at around 11 µm. Most of the infrared re-radiation from the earth escapes, but some amount is trapped by gases that absorb at wavelengths between 1 and 10 µm. This is the greenhouse effect — the trapping of infrared radiation by gases, and this phenomenon keeps the earth warmer than predicted by blackbody calculations.

Among these greenhouse gases (GHGs) are ground-level ozone, methane, water vapor, nitrous oxide, and carbon dioxide. Methane is a potent GHG (about 20 times more potent than CO₂, considered over 100 years). From a policy standpoint, we also have to worry about anthropogenic sources of methane, such as landfills and anaerobic digesters. For example, we have to make sure methane produced is converted to CO₂ by burning the methane. Researchers are also now looking at the methane released by cows (don’t laugh — it’s a big source!), by changing cows’ diets, for example. Water vapor is another potent GHG, but the residence time of water vapor in the atmosphere is very short (in the order of days), and so its impact is not as much as that of CO₂. Nitrous oxide (N₂O) is also a potent greenhouse gas that is about 298 times more potent than CO₂, considered over 100 years. Another effect of nitrous oxide is its reactions with stratospheric ozone, which acts to shield the earth from the sun’s UV rays. So, more nitrous oxide depletes the ozone layer, which then makes the sun’s rays more effective in heating up the earth. 

Carbon dioxide is a concern because of its very long residence time in the atmosphere. Impulse response function models show that roughly 40 percent of CO₂ emitted in a pulse would stay in the atmosphere for a hundred years. So the CO₂ we emit now will stay in the atmosphere for more than a century, and keep on trapping infrared radiation from the earth. Conversely, to maintain CO₂ concentrations in the 450 ppm range (a level which some find marginally acceptable), we need to go back to 1950 emissions, which is a serious challenge. 

The hooplah over CO₂ has to do with short-term and long-term carbon cycles. Carbon dioxide is a product of respiration, so for example, humans and other organisms that eat carbon end up exhaling CO₂. However, these direct emissions are part of the short-term cycle. Think of it this way: the carbon we humans exhale come from food that contained that carbon. That food came (ultimately) from plants and other primary producers, which captured the carbon from CO₂ in the atmosphere. So that carbon can be thought of as simply cycling through the atmosphere. The problem is the carbon that has been trapped for millions of years that is now being unreleased to the atmosphere. These are the fossil fuels (oil, coal) that we humans use to power up human society and develop national and global economies. So the emissions of concern are those from the energy used to produce food, power up our homes and buildings, and allow us to operate our cars and other machinery. These carbon emissions end up increasing the CO₂ concentration in the earth’s atmosphere: a ballpark number is about 0.47 ppmv of CO₂ per billion ton of C emitted. 

Now the reality is that global CO₂ emissions have increased dramatically: a 145-fold increase in annual energy CO₂ emissions from 200 million metric tons in 1850 to 30 billion tons today. Where did all these CO₂ come from? The obvious answer is industrialization — the internal combustion engine, and the fossil fuel-based economies that have powered human society. As a result, CO₂ concentrations in the atmosphere have increased in recent decades. The Mauna Loa, Hawaii measurements (the longest continuous record of actual measurements) show an increase of CO₂ from around 316 ppmv in 1959 to 386 ppmv in 2008. The Mauna Loa site is considered the best site for CO₂measurements, as impacts of vegetation, human activity, and volcanic activity can be excluded.

We can also consider evidence from paleoclimatology, over longer (millions of years) periods. This body of evidence includes ratios of oxygen and hydrogen isotopes from ice core data, taking into account Milankovitch cycles (changes in the earth’s orbit around the sun), and other data. The evidence shows that 65 million years ago, during the Cenozoic era, the earth was about 10°C warmer. It was also an earth that had no sea ice, and sea levels were 75 m higher. So in fact, considered over millions of years, the earth has actually undergone periods of glaciation and warming, and the trend from 50 million years ago is a global cooling. However, the important thing to know is that this cooling is not explained by the sun’s brightness, the earth’s orbit, the location of the continents, and other factors. The cooling is best explained by the decrease in concentration of CO₂, which has gone down from 1500 to 180 ppmv during periods of glaciation. In other words, over the millennia, when CO₂ goes down, the earth cools. When CO₂ goes up, the earth warms up. Obviously, we do not want a return to an earth that is 10 degrees warmer.

For us humans, the relevant thing to consider is that the present CO₂ concentrations are HIGHER than the natural variability of CO₂ concentrations over the last 650,000 years. And when you do the calculations, the anthropogenic CO₂ forcing rate is about 20,000 times higher than the natural CO₂ forcing rate. The reconstructed data from 90-plus studies show that the temperature anomaly (with respect to 1961-1990) is a plus 0.5 degrees C. The inevitable conclusion is that: (1) the earth is warming, beyond the natural variability; and (2) humans are in the driver’s seat with respect to present-day climate. Given where and how people live, the warming due to CO₂ increase will have tremendous impacts on humans all over the world, and it behooves us to think about how to prevent and prepare for these changes.

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Francis L. de los Reyes III is an associate professor of Environmental Engineering at North Carolina State University. He conducts research and teaches classes in environmental biotechnology, biological waste treatment, and molecular microbial ecology. He is on the editorial board of Water Research, was a 2008 Balik-Scientist, and was a 2009 TED Fellow. He has received Outstanding Alumni Awards from UPLB and Iowa State University. He is a member of the Philippine-American Academy of Scientists and Engineers. E-mail at [email protected].