Why Carbon Dioxide Takes All the Blame, But Should It?
Why is carbon dioxide in the spotlight?
The reason carbon dioxide takes all the heat and is so focused upon in the great battle to stop climate change is because it is used as a standard of measurement for other greenhouse gases. In order to normalize measurements of each greenhouse gas and compare them based on their weighted impact, a metric known as global warming potential (GWP) is used to make comparisons. The Intergovernmental Panel on Climate Change (IPCC), among other organizations, publishes tables of figures like these and one can be found here.
So, given this common practice, and that when talking about emissions of various potentially harmful gases the unit used is kilograms of CO2 equivalent, it is no surprise why everyone blames carbon dioxide for the environmental woes of today.
So what are the worst pollutants?
The worst pollutants, or those that should be targeted for reduction as much or more than CO2, are methane, chlorofluorocarbons/hydrochlorofluorocarbons, nitrogen dioxide. Here are the 100 year global warming potentials for these gases:
Nitrogen Dioxide: 296
The 100 year potential means that this is an integrated calculation of the irradiative forcing potential over a 100 year time period. This is the most commonly used although there are also 20 year and 500 year potentials as well.
So from this we can see that all of these gases have a much greater potential to impact the environment and contribute to climate change than CO2, which naturally has a global warming potential of 1.
Atmospheric persistence is another factor that determines how large of an impact a gas will have. Carbon dioxide is rather difficult to determine precisely and may remain in the atmosphere for 25 years and it may remain for 200 years. However, there are many mechanisms by which carbon dioxide may be removed. Several 'sinks' exist such as forests and oceans. The remaining gases persist for:
Methane: 12 years
Nitrogen Dioxide: 114 years
Emissions of CFC's have largely been reduced or eliminated since the Montreal Protocol. They do still persist in the stratosphere and may continue to damage the ozone layer and contribute to global warming until they are gone completely. HCFC's, chosen because of their higher stability, less damage potential, and lower persistence are still being emitted, but have relatively low impacts. The stratosphere has even started to recover since the transition from CFC's to HCFC's.
That leaves nitrogen dioxide and methane.
Methane is one of the major candidates for the most harmful greenhouse gas. Primarily emitted from agricultural practices (livestock especially), industry, and waste decomposition, something the world has a lot of, puts methane in the running for most abundant greenhouse gas as well. Lesser emission sources include combustion of natural gas and naturally emitting wetlands.
With the rapidly growing global population there is increasing need for food, energy, and more waste to deal with. All of these factors contribute to increasing emissions of methane and will into the future until population growth levels out. For all of these reasons, methane is dangerous to our environment and needs the same attention as CO2.
The drastically increased global warming potential and decade long residence time are not small features. It is abundant as well, with global totals of approximately 20% those of CO2.
Nitrous oxide, like methane, has much more potential for harm than most people think. Agriculture, transportation, and industry all contribute to emissions of N2O. Global totals are much less than both CO2 and methane, but emissions will rise with population growth as well. As more people inhabit the earth and more and more countries industrialize and attain a westernized status, each of these sources will rise contributing to a rise in N2O levels globally.
The main harm in N2O, besides its contribution to global warming (300 times more than CO2 per unit) is its ability to destroy stratospheric ozone. The ozone layer is vital for reflecting harmful radiation coming to earth. As it is depleted, there is higher potential for global warming and increased incidences of human health impacts,such as skin cancer.
The way N2O affects the ozone layer may be shown in the following chemical equations:
N2O + hv --> N2 + O
N2O + O --> 2NO
N2O + O --> N2 + O2
The hv above stands for the presence of light and the fact that it photolyzes, or breaks apart, the molecule in question. What this is primarily showing is that N2O has a high propensity to react with free oxygen molecules and form stable compounds. This depletes the supply of free oxygen molecules for their reaction with diatomic oxygen (O2) which is the mechanism by which ozone (O3) is created in the stratosphere.
So what now?
So what does this all mean? Well not everything is clear, even to climate scientists and atmospheric chemists. The experts that run models to predict the future can not know what will happen with any great degree of certainty.
Which emissions will increase the most, what innovations in technology will reduce or remove pollutants and in what direction the world is heading in general.
However, it is true that CO2 has taken the brunt of the criticism in the global warming blame game. There are valid reasons for it, but I hope I have made the case that it is not the only greenhouse gas that should be worried about, and there are many others with a lot greater potential for harm.
Thoughts are welcome in the comments below. This article is by nature opinionated. I personally feel that CO2 isn't the worst thing out there affecting our environment and that efforts could be better spent focusing on other pollutants, other emissions sources, or other environmental issue altogether.
What do you think is the worst greenhouse gas?
I have left out water vapor for a few reasons. It is not a greenhouse gas like the rest of them. I did not discuss it here is because there are not a great deal of large anthropogenic sources of it and it is in a positive feedback loop. This means that when temperature rises, it rises as well in a big loop.
Thus, because water vapor is quite abundant in the atmosphere does not necessarily mean it is the largest problem. There are a lot of factors weighing on water vapor and its effects. It is rather more complicated than the rest.