Science to Live By: Here Comes the Sun


by J. Dirk Nies, Ph.D.

We’re in the throes of the sweltering days of August. All spring and summer the northern hemisphere has been inclined toward the sun, bathing lawns and gardens, fields and forest in and around Crozet with abundant sunshine. This month, let’s take a look at this source of energy which makes life on earth possible. As we do, we’ll discover patterns and principles that support and underpin the natural economy of green plants.

How much energy does the sun emit? Try imagining a 386 trillion trillion watt incandescent light bulb shining at the center of the solar system; that’s our sun. The sun generates this immense power by converting matter into energy at a mass-to-energy conversion rate of 4.7 million tons per second following Einstein’s iconic equation e = mc2, where e = energy, m = mass, and c = the speed of light. For perspective, Americans generate about 250 million tons of trash each year. The sun converts this much mass into energy every 53 seconds. Imagine solving our solid waste disposal problem in less than one minute, while simultaneously generating enough energy to run the US economy for 200 million years!

Not only is the flow of solar energy huge, but it is also remarkably steady and extraordinarily reliable. Earth has never experienced a solar power outage in more than 4.5 billion years. Wouldn’t it be nice if our power grid were as dependable?

Nuclear fusion takes place deep within the sun’s core. Over and over again, the masses of four hydrogen atoms are combined together under extreme pressures and temperatures to make one slightly less weighty helium atom. Energy released through this continuous obliteration of mass makes its way up from the interior to the sun’s surface over a period of tens of thousands of years. Today’s sunlight represents energy generated when mammoths, mastodons and saber-tooth tigers roamed North America at the end of the Pleistocene Epoch.

How much solar energy does the earth receive? From one vantage point, the amount is miniscule, less than one half of one billionth of the sun’s output. Earth’s meager apportionment of the sun’s largesse is owing to the fact that the earth is 93 million miles away and so very small compared to the sun. (If the weight of the entire solar system were set to one ton, super heavyweight sun would weigh in at 1,997 pounds while featherweight earth would barely tip the scales at one tenth of an ounce!)

Fortunately the sun’s output is so immense that the small portion earth receives is large, averaging 1,361 watts per square meter. Scientists call this number the solar constant. The entire earth is steeped in a continuous flow of 174 quadrillion watts of solar energy. Using the light bulb analogy again, that’s equivalent to 1,740 trillion 100-watt light bulbs continually shining down on earth from the top of the atmosphere. Using a nominal electric utility rate of ten cents per kilowatt-hour, the earth is the recipient of $17.4 trillion worth of solar energy every hour, free of charge.

Added up over a year, I estimate that the earth receives enough solar energy to raise the temperature of the atmosphere nearly 2,000 degrees Fahrenheit! Obviously, this catastrophic warming is not occurring, but why not? The reason is that the earth re-radiates this solar energy back into space as infrared heat. This means that the profound cold of outer space is essential for maintaining a temperate climate on earth, as essential as coolant to prevent a car’s engine from overheating on a hot summer’s day (which my daughter recently discovered first-hand when she came for a visit).

Currently the outward flow of energy from earth into space, which I define as the “terrestrial constant,” is of similar magnitude to the amount coming in from the sun. If, however, the finely-calibrated balance between in-coming and out-going energy were to drift persistently in one direction or the other, the climate will become too hot or too cold to support life.

As it turns out, earth radiates energy so efficiently that greenhouse gases are necessary to keep atmospheric temperatures sufficiently warm for life to thrive. Without heat-trapping gases in the air, our earth would be like a home with all its doors and windows wide open on a frosty winter’s day. Temperatures would be about 60 degrees cooler, average highs in Crozet would stay below freezing year round, and the world would be in a perpetual ice age.

Four naturally occurring gases give rise to most of the observed greenhouse effect in the earth’s atmosphere. In order of their importance, they are water vapor (H2O), carbon dioxide (CO2), methane (CH4) and ozone (O3). Photosynthetic plants play, either directly or indirectly, a significant role in contributing to and maintaining these greenhouse gases at their present levels. Here’s how.

Green plants transpire water, discharging moisture through their stems, leaves and flowers into the air. A single large tree loses several hundred gallons of water through its leaves on a hot, dry summer’s day. A forest is a living waterfall that flows in reverse, moving water up from the ground through the trees and into the air. Transpiration accounts for about three-quarters of the water that is vaporized over land and one-eighth of that vaporized over the entire globe.

Green plants remove and sequester carbon dioxide from the atmosphere, transforming this gas into solid biomass. Photosynthesis, which worldwide converts roughly 0.1 percent of the solar energy striking the earth to chemical energy, takes hundreds of billions of tons of CO2 out of the air each year. This process helps counterbalance emissions of CO2 that occur during respiration, decomposition of biomass, and burning of wood and fossil fuels.

In the digestive tract of ruminants (cattle, sheep, goats, deer) and in swamps, bogs, wetlands or any similarly moist places where there is limited oxygen, certain types of microbes convert biomass to methane. For example, a cow emits into the air somewhere around 250 to 500 liters of methane per day from digestion carried on by microorganisms that live in its gut. Given a worldwide ruminant population greater than 3.5 billion, that adds up to a lot of natural gas.

And finally, green plants emit oxygen to the air as a by-product of photosynthesis. A small proportion of this oxygen is converted to ozone by photochemical reactions in the troposphere and stratosphere.

So, in closing this three-part topic, what we have learned? The most important concept to understand and remember is that photosynthesis converts abundant, reliable and renewable solar energy to chemical energy. Secondly, using this chemical energy, plants make their food and tissues from locally available materials, principally atmospheric CO2 absorbed through leaves, along with water and minerals drawn up through the roots. As a rule, green plants make the environment more hospitable over time, enriching the soil with humus and helping maintain the climate. Photosynthesis supplies oxygen to the atmosphere and provisions the front end of the food chain upon which all life depends. Our human economy, at its foundation, is utterly dependent upon photosynthesis. From the food we eat to the air we breathe, from our clothing made of cotton to our homes framed with wood and warmed in winter by logs on the fire, we are relying upon and reaping the benefits of photosynthesis. For all this and for the beauty they add to our world, we have much to be grateful for and much we can emulate in the ingenious and sustainable economy of green plants.