Why CO2 and methane accelerate the greenhouse effect ?
[part of this text was published, Athens News 28.11.2008]
Yannis Zabetakis
Lecturer of Food Chemistry and Lead Auditor (HACCP – ISO22000, ISO9001), Chemistry Dept, University of Athens
Venus is too hot (450o C), Mars is too cold (-53o C), and Earth is just right (13o C). Earth has a surface temperature comfortably between the boiling point and freezing point of water, and thus favors humans whose body temperature is 36.6o C. The temperature of our planet cannot be explained by simply suggesting that Earth orbits at just the right distance from the sun to absorb just the right amount of solar radiation. Our moderate temperatures are also the result of having just the right kind of atmosphere. Parts of our atmosphere act as an insulating blanket of just the right thickness, trapping sufficient solar energy to keep the global average temperature in a range that life can develop.
Greenhouse Gases
Four gases, mainly water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), all act as effective global insulators or as we call them “greenhouse gases”. They all are molecules composed of more than two atoms, bound loosely enough together to be able to vibrate with the absorption of heat. The major components of the atmosphere (N2 and O2) are two-atom molecules too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect.
Carbon dioxide (CO2) consists of one carbon atom with an oxygen atom bonded to each side. When its atoms are bonded tightly together, the carbon dioxide molecule can absorb infrared (IR) radiation and the molecule starts to vibrate. Eventually, the vibrating molecule will emit the radiation again, and it will likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the surface from the cold of space.
Global warming potential (GWP) is a measure of how much a greenhouse gas contributes to global warming. It compares the gas in question to that of the same mass of carbon dioxide (by definition, CO2 has a GWP of 1). A GWP is calculated over a specific time interval and the value of this must be stated whenever a GWP is quoted or else the value is meaningless. The substances subject to restrictions in the Kyoto protocol either are rapidly increasing their concentrations in Earth’s atmosphere or have a large GWP.
Carbon dioxide equivalent
Carbon dioxide equivalency is a quantity that describes, for a given mixture and amount of greenhouse gas, the amount of CO2 that would have the same GWP, when measured over a specified timescale (generally, 100 years). The carbon dioxide equivalency for a gas is obtained by multiplying the mass and the GWP of the gas. For example, the GWP for methane over 100 years is 25 and for nitrous oxide 298. This means that emissions of 1 million metric tonnes of methane and nitrous oxide respectively are equivalent to emissions of 25 and 298 million metric tonnes of carbon dioxide.
This explains why we should be really careful in reading the latest figures on the increase of the levels of CO2 and CH4 in the atmosphere. According to the US government’s Earth System Research Laboratory in Colorado, atmospheric CO2, the primary driver of global climate change, rose by 0.6 per cent, or 19 billion tonnes last year whereas methane’s levels increased by 0.5 per cent, or 27 million tonnes, after nearly a decade of little or no change. Are we approaching faster than originally anticipated the tipping point?
Links
1. http://www.sciencedaily.com/releases/2008/04/080423181652.htm
2. http://blogs.edf.org/climate411/2008/05/01/ghg_rise_accelerating/