On several occasions, Citizen Editor Neil Godbout and I have discussed writing longer articles - "feature pieces" - on specific scientific topics as the length of these columns sometimes limits the depth of discussion.
Climate change might be one of those features except it would run to thousands of words in length. The topic is vast with literally thousands of articles being published annually. And our understanding of climate change continues to evolve.
This week, Gerry Lundquist wrote an interesting letter outlining some of the issues, but perhaps it was a little incomplete. I will try here to fill in some of the details I missed in my previous column.
Is carbon dioxide the only greenhouse gas? No. Not by a long shot.
Water vapour or gaseous H2O is a big contributor but so are methane, nitrous oxide, chlorofluorocarbons, hydrofluorocarbons, ozone, nitric oxide, sulfur dioxide, sulfur hexafluoride, ethane, and the list goes on and on.
It is actually easier to talk about the constituents of the atmosphere that are not greenhouse gases. Essentially, only mono-atomic gases (e.g. helium, neon, etc.) or simple homonuclear diatomic gases (e.g. oxygen, nitrogen, etc.) are not greenhouse gases. Every other gas absorbs infrared radiation with varying levels of effectiveness.
This is a big and important point. The relative ability of a gas to interfere with the flow of heat through the atmosphere is a function of two principle variables - its concentration and its ability to interact with the infrared region of the spectrum.
Water, for example, is both abundant and has powerful interactions with infrared radiation. It is a major contributor to the overall surface temperature of the Earth. It is largely responsible for the Earth's mean surface temperature being a balmy 15 C rather than a chilling -80 C or so. But because the water content of the atmosphere is so variable and pervasive, it tends to get missed in public discussions of air temperatures and climate change.
However, in the models developed by climate scientists, it is a significant player. Tracking water vapour concentrations in the atmosphere is a major undertaking and requires massive amounts of computing power. But most climate scientists would say we have a reasonably good handle on what water does.
Carbon dioxide might have a relatively low concentration compared to water (water is typically between 100 and 50,000 ppm whereas carbon dioxide is 400 ppm). But carbon dioxide interacts with infrared radiation in a very strong fashion. Furthermore, it has a long persistence or half-life in the atmosphere.
This is one of the other important variables - how long will a gas hang around? It varies considerably and depends on a number of factors. For example, the present half-life for carbon dioxide is estimated to be somewhere between 30 and 100 years but as the world warms and vegetation increases that might drop to say 25 to 75 years. Carbon dioxide is consumed through photosynthesis and more plants means more photosynthesis.
However, and there is almost always a however in talking about such things, as desertification is a result of increasing surface temperatures, it is race between increasing vegetation in the northern latitudes versus dying vegetation around the equator and in the oceans. Which one wins will have a major impact on the planet's future atmosphere.
Climate science is also not only interested in the gases in the atmosphere - their concentrations and greenhouse potential - but also the physical phenomena associated with the atmosphere. Modeling needs to take into account the Earth's albedo, ocean currents, temperature gradients, wind speeds, precipitation, snowfall, snow load, vegetation, and a host of other variables which impact surface temperatures.
Indeed, the temperature profile of the atmosphere itself is a variable. Imagine trying to use data on air temperature as a function of altitude as both an input variable and an output variable in any simulation. It results in a feedback loop and can quickly get out of control if not modeled correctly.
Soot, aerosols, ocean acidification, gas exchanges rates between water and air and a host of other variables come into play as well.
For example, changing ocean pH changes the rate and extent to which carbon dioxide can be absorbed by the oceans. The oceans are the major sink for carbon dioxide - far outweighing terrestrial vegetation - and altering the ability of the oceans to absorb carbon dioxide and other greenhouse gases greatly alters the ability of the ocean to absorb carbon dioxide and other greenhouse gases. Another feedback loop.
Climate science is complex. It is far more than just worrying about carbon dioxide. Maybe I have failed in my efforts to simplify the science to show proper respect for the complexity. But there is one thing that I am sure of - atmospheric carbon dioxide is a major contributor to the increasing atmospheric surface temperature of the Earth.
