In a previous blog, I used a lake surrounded by houses to illustrate the Tragedy of the Commons, a situation where the actions of individuals, motivated by their own self-interest, results in the destruction or damage of common assets, much to the detriment of the collective group. The pollutant used in the example was sewage, which is bio-degradable. As long as the emission rate is less than the lake’s ability to absorb or process the pollutant, it does not cause permanent damage to the environment. This type of pollutant is known as a fund pollutant.

A stock pollutant, by contrast, is one for which the environment has little or no absorptive capacity. Common examples are persistent synthetic chemicals, non-biodegradable plastics, and heavy metals. Stock pollutants accumulate in the environment over time. For stock pollutants, the emission rate is less important than the total amount that has accumulated in the environment. Since stock pollutants persist for long periods of time, they create a burden for future generations. 

Using our previous example to show the nature of a stock pollutant, suppose that one of the homeowners used a certain type of pesticide on their lawns and gardens that eventually ends up in the lake due to rain and normal storm water runoff. Also suppose that unlike the sewage, the pesticide contains a chemical such as cadmium, a heavy metal that does not degrade through natural processes and persists indefinitely. Once the cadmium enters the lake, it essentially does not go away, at least naturally.

Stock pollutants present a different set of problems from fund pollutants and require different methods of control. As long as the rate of emission for fund pollutants does not exceed the rate at which the environment can absorb the pollutant, the situation can be controlled and stabilized. With stock pollutants, what matters is the total amount released.

Clearly, the environment has the ability to absorb CO2. Nature provides us with two principal carbon sinks. The first is photosynthesis whereby plants absorb CO2 and release oxygen as they grow. The second carbon sink is the oceans which can absorb CO2 through both biological and physicochemical processes. However, the ability of our forests and oceans to absorb CO2 is limited and when emissions are greater than the maximum absorption rate, CO2 accumulates in the atmosphere following the pattern of a stock pollutant. Visualize a bathtub. The rate at which the water leaves the drain is like the absorption rate. If we add water at a rate greater than the rate the water is leaving the tub, then the water level will rise and eventually the tub will run over.

In pre-industrial times, CO2 concentrations in the atmosphere were stable at about 280 parts per million (ppm). In the last hundred and fifty years or so, concentrations have increased to over 400 ppm and are continuing to climb. Before the industrial revolution the carbon cycle was in balance, with CO2 emissions being absorbed by the natural carbon sinks. Since that time, we have both increased the rate of CO2 emissions and reduced the absorptive capacity of the environment through deforestation as well as acidification and warming of the oceans. As result, CO2 concentrations have increased 40%. In bathtub terms, we have both increased the rate of water that is entering the tub, and at the same time, restricted the drainage.

So the issue with CO2, as well as many other classes of pollutants, is the ratio of emissions to absorption capacity. For the last 150 years, and especially the last 50 years, CO2 emissions have far exceeded the absorption rate of our forests and oceans and the increased concentrations are creating an economic and environmental burden for future generations. Until emission rates are smaller than the capacity of the environment to absorb and process the emissions, CO2 functions like a stock pollutant. Even if/when emissions are less than the absorptive capacity, it will take a long time for the natural carbon sinks to reverse the direction and reduce concentrations.

Another issue with CO2 is that it causes no immediate health or environmental danger. It does not cause cancer or asthma. Except in extremely high, oxygen-depriving concentrations, it is not harmful to human health. But as nearly 100 percent of technical papers produced by climate scientists attest, CO2 along with other greenhouse gases such as methane, are causing global warming[1]. CO2 emissions have a delayed and indirect impact on our health and wellbeing. In the long-term, it is a threat to all of us. Previous generations did not understand the link between their carbon emitting life styles and increasing global temperatures, rising sea level and extreme weather events. Today, the connection is clear to all but the most staunch deniers and delayers.

Our policy makers and governmental leaders have demonstrated little ability to respond to future threats. Only a crisis seems to precipitate action, and most politicians can’t see beyond the next election. The problem with global warming is that by the time we reach the point of a crisis it may be too late. Our CO2 producing global economy has so much momentum that it is impossible to make quick changes in direction. Think of a huge aircraft carrier trying to stop or change direction quickly. Global warming is also being accelerated by positive feedbacks, such as the reduction of snow and ice cover, the release of previously frozen methane gas, and the warming of the oceans which reduces its ability to absorb CO2.

The Intergovernmental Panel on Climate Change (IPCC), recommends a stabilization goal of less than 550 ppm, but many scientists believe that this is far too high and have recommended goals of 350 ppm[2], a 10% reduction from the current concentration. Whatever the safe level is, stock pollutants like CO2 present special challenges for public policy. If we reduce CO2 emissions below the capacity of the environment to absorb them, then gradually, concentrations in the atmosphere would be reduced to safe levels, but this will take a long time.

CO2 concentrations and stock pollutants in general raise the question of fairness. Is it fair to control emissions for all when only a few parties are responsible for the majority of existing concentrations? Back to our lake example: suppose that one homeowner had been using the cadmium laced pesticide for decades and concentrations in the lake were mainly due to releases from his property. Fairness would dictate that this homeowner take more responsibility to clean-up the lake, as opposed to spreading the costs equally among all homeowners, many of whom had not contributed at all to the problem.

This fairness issue has been a major stumbling block in the development of international policy on controlling greenhouse gases. The Kyoto Protocol, adopted in 1997 (effective in 2005), exempts developing nations such as China and India from the tough reduction target that applies to developed countries. The protocol has been ratified by 191 countries, with the United States and Canada[3] being the only significant holdouts. Developing countries argue that this is fair, since the developed countries are responsible for most of the problem. The United States has argued that it will not accept the targets unless they apply uniformly to all countries.

Greenhouse gases represent a global tragedy of the commons. Each country, if acting in its own self-interest, will do nothing, since addressing CO2 unilaterally would be economically disruptive and costly. Without some mechanism such as cap-and-trade to assign a cost to CO2 emissions, electric utilities, cement producers, and transportation operators have no economic motivation to operate their facilities or equipment more efficiently or switch to technologies that have lower carbon emissions. There is no free lunch, so society will eventually pay the cost of our shortsightedness, as future generations have to cope with desertification, extreme weather, and rising sea level.  

[1]           “Scientists Respond to Dani the Denier”,
[2]           See
[3]           Canada originally ratified the protocol, but later withdrew.