Sustainability is usually defined as ensuring that our actions today do not limit the ability of future generations to meet their own needs. The use of renewable energy sources is considered to be sustainable because they can be used forever without being depleted. The sustainability of solar, wind and hydro electric power is easy to understand because as long as the sun shines, the wind blows, and water flows down hill, these sources can continue to supply our energy needs. Wood is also considered to be a renewable energy resource, but on the question of sustainability, the picture is not quite as simple.

Wood is a renewable energy source in the sense that a tree cut for fuel will naturally be replaced by a young tree that springs up in its place. This is certainly true, but there are conditions attached. The use of wood as a fuel is not sustainable if the trees are harvested in a way that damages the site. For example, if a stand of mature hardwood trees were clearcut the site could be damaged by erosion and the elimination of shade to such an extent that high value hardwoods would not re-grow there for many generations, if ever. Sustainable forest management usually means that the site is maintained with a variety of tree species of various ages and that harvesting practices select only those trees that can be removed without damaging the forest ecosystem. The best and most obvious examples of sustainable forest management are the wood lots in farm country that have yielded firewood and lumber for generations of farm families and today still look healthy and productive.

The use of wood fuel is also sustainable on the condition that it is converted to heat with reasonable efficiency. Wood that is burned in fireplaces at very low efficiency is wasted for just the brief pleasure of watching the flames. On the other hand, if wood is burned in a modern EPA certified stove or fireplace, its use can immediately reduce the consumption of one of the other heating fuels like oil, natural gas or propane. This type of displacement is important because it is one of the ways we can reduce the carbon dioxide emissions that are linked to the problem of global climate change.

Oil, natural gas, propane and coal are fossil fuels, meaning that their formation has taken millions of years. They are not renewable fuels and their use is not sustainable because they will run out some time in the future. Of more immediate concern is the fact that the burning of fossil fuels results in the emission of carbon dioxide (CO2), the main greenhouse gas. When wood is burned CO2 is also emitted but this CO2 is just one part of a short-term natural process called the carbon cycle.

As trees grow, their leaves absorb carbon dioxide from the air in a process called photosynthesis. This carbon dioxide is converted to the carbon that is used in the tree to build its structure. Chemically, about half the dry weight of wood is carbon. The ability of trees to absorb carbon dioxide is the reason environmentalists promote tree-planting as a way to reduce net greenhouse gas emissions from human activities. In one sense it could be argued that burning wood for heating is a problem because it means fewer live trees are available to absorb CO2 from the atmosphere. However that argument does not take account of the second part of the natural carbon cycle.

After a tree reaches maturity it dies because of disease, rot and insect infestation and falls to the forest floor. There, it decomposes, a process of slow oxidation which emits CO2. In fact, whether a tree is processed into firewood and burned, or whether it dies and decomposes on the forest floor, the same amount of CO2 is emitted. In either case, the space the tree had occupied in the forest is now opened up to sunlight that spurs the growth of young trees on that site, increasing their absorption of CO2 from the atmosphere.

The process of CO2 absorption by trees and its emission back to the atmosphere during decomposition or burning of wood is the natural, renewable carbon cycle. The carbon cycle of the forest and the use of wood as fuel is sustainable provided the trees are harvested so the site is not damaged and the fuel is converted efficiently to heat energy.

Your family can encourage and participate in the sustainable use of wood energy in three important ways.

1. Let your firewood supplier know you care about the source of the wood being offered for sale. Does it come from sustainably managed forests? If more customers asked their suppliers this question, all of the participants in the firewood business would know that the public is watching and interested in the way they conduct their business.

2. Processed firewood made up of perfectly regular wedges of straight grain wood is less likely to be from a sustainable source than rougher looking firewood that has crooked pieces, various sizes including unsplit small diameter pieces, and some pieces with sections of rot. Firewood that looks a little rough is more likely to come from over-mature trees of the kind that can be removed without affecting the health of the forest and are not of much use for other purposes like lumber, furniture production, or even CO2 absorption. Also, rougher looking firewood results when more of the tree is processed, meaning there is less waste.

3. Use an efficient appliance so the maximum heat energy is extracted from each piece of firewood. This objective is best achieved by using an EPA certified advanced combustion wood stove or fireplace. These units are around 70 per cent efficient, which is up to 30 per cent more efficient than older airtight stoves. These stoves and fireplaces also have the advantage of burning much cleaner – producing less air pollution – than conventional equipment.