Dear friends of ours are confident that their 10-acre woodlot, which they manage sustainably, compensates for all the greenhouse gases they produce living an average rural Maine lifestyle. Trees absorb carbon dioxide and produce oxygen, right? Meanwhile, they drive around some, but their house is modest and they don’t have a yacht or a Florida condo, so they aren’t producing a lot of CO2, right? They’re very proud of this equation and believe they are doing their part to slow the dangerous warming of the earth.

Bless their hearts, but could it be so? If, after all, you divide up the forested acreage of Maine by the number of residents, there is a 10-acre lot for each of us – and it might look like the state is inherently carbon neutral. Yes, we are all in favor of relying on numbers – but sometimes it’s not simple. Particularly in anything to do with biology/ecology: unlike physics, there are few situations where it’s a yes-or-no, black-or-white answer! (probably why we prefer physics)

How much CO2 does a forest absorb? All plants use carbon dioxide as they grow.  In biology class we learned about photosynthesis—how the leaves absorb light energy from the sun and CO2 from the atmosphere to produce oxygen and grow. CO2 absorption rates vary hugely around the world, and depend on things like climate, tree species, and age of the forest. Wet, tropical areas growing eucalyptus trees have about the highest rate, up to around 15 tons of CO2 per acre per year.  A typical acre of mixed woods in Maine, if left alone, will absorb around 4 tons of CO2 per year.

So, if left to grow without harvest, 10 average acres in Maine would sequester about 40 tons of CO2 per year.  Based on the national average, a typical Mainer has a CO2 footprint of around 16 tons per year. If that were all there is to it, it would look like about 4 acres of forestland per person would be more than sufficient to absorb Maine’s annual CO2 budget, and there would be 6 more acres doing the job of reducing the CO2 in the atmosphere! Woohooo!

But: There are two problems with this reasoning: first it assumes that the forest is left to grow undisturbed, and, more importantly, it does not consider the natural carbon cycle. Those 10 acres also have to absorb all the CO2 from natural sources.

The same acre of woods also produces CO2 and methane, through the decay of dead material and the respiration of all of the animal species. In fact, there are some tropical forests that now emit more CO2 than they absorb—mostly due to improper forest practices.

There is a natural, complex process called the Carbon Cycle, which involves carbon absorption and emission from the oceans, plants, animals, and even rocks. Since the beginning of the industrial revolution in the late 1700s, humans have added to the flow of carbon in this cycle by digging up and burning fossil fuels (coal, oil, and gas). These fuels came into being over a period of about four hundred million years as plants and plankton decayed and through various processes became the coal, oil, and gas that we use for most of our energy.  In a mere 250 years we have burned a substantial fraction of what took hundreds of millions of years to form—most of it in the last 50 years—piling this super-concentrated carbon on top of the normal carbon cycle in nature.

At this point in time, from burning fossil fuels, making concrete, and deforestation, we are adding CO2 to the atmosphere at a rate that is around 4% higher than the “natural” rate.  While this sounds small, the result is a steady increase in the CO2 concentration in the atmosphere, which has caused the oceans and forests to increase the rate at which they take up CO2. However, this increase hasn’t been sufficient to keep the atmospheric CO2 concentration in check. The consequence of this increase is a rising average global temperature (greenhouse effect) and pronounced acidification of the oceans.

So back to the question of how much CO2 these 10 acres of forestland can absorb of the average Mainer’s carbon footprint. The details are a bit much to work out in a column like this, but the bottom line is that forests in the US absorb about 14% of the excess CO2 we produce. If one works this out, 10 acres of forest can absorb about 50% to 60% of your carbon footprint, as long as it is left to grow without harvesting.

So should you just forget about the trees, cut them all down and put up solar panels for your energy? No, we still need the forests to do their jobs—jobs which are infinitely complex and important to all life. But if you could use just 1% of that land for photovoltaics, we can say with some certainty that your 1/10th of an acre of solar energy would more than cover your carbon footprint. That’s physics.

Paul Stancioff, PhD., is a professor of Physics at the University of Maine Farmington who studies energy economics on the side.  He can be reached at pauls@maine.edu.  Cynthia Stancioff is a nature-lover who is obsessed with re-wording things. Previous columns can be found at https://paulandcynthiaenergymatters.blogspot.com/.

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