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ESTIMATES OF EMISSIONS

Uncertainties in current estimates

Current estimates of greenhouse gas emission are not without uncertainty. We face many possible pitfalls in calculating emissions for ecosystems as extensive and diverse as Canada's farmlands. We still do not even understand all the processes that affect emissions. And so we admit that each estimate is subject to potential error. Of the three gases, N2O has the highest degree of uncertainty (Fig. 24). Estimates for this gas could be off by 50% or more. Despite their uncertainty, these values are the first comprehensive estimates of greenhouse gas emission from Canadian agriculture and provide a reference point for showing trends.

Though valuable as a first approximation, the estimates will likely change as we learn more. Ongoing research will teach us more about the processes leading to emission and allow us to build better models. As well, new techniques that simultaneously measure all three gases over large areas will allow us to evaluate better the models' reliability. We can therefore expect more definitive estimates in the future, but we need not wait for their arrival before trying to reduce actual emissions.

Image: Figure 24: Estimates of CO2, CH4, and N2O emissions in CO2 equivalents from Canadian agriculture, showing relative uncertainty for each gas.

Figure 24: Estimates of CO2, CH4, and N2O emissions in CO2 equivalents from Canadian agriculture, showing relative uncertainty for each gas.

Global warming potential

Global warming potentials (GWPs) are a simple way to compare the potency of various greenhouse gases. They help policy makers compare the effects of reducing CO2 emissions relative to another greenhouse gas for a specific time horizon. For example, a small reduction in N2O can be just as if not more effective than a larger reduction in CO2 emissions.

The heat-trapping potential of a gas depends not only on its capacity to absorb and re-emit radiation but also on how long the effect lasts. Gas molecules gradually dissociate or react with other atmospheric compounds to form new molecules, with different radiative properties. For example, CH4 has an average residence time of about 12 years, N2O 120 years, and CO2 200 years. Over a 20-year period, CH4 has 56 times the radiative impact of CO2. However, as time proceeds some of the CH4 molecules are broken down into CO2 and H2O. Therefore, over a 100-year period, CH4 has a global warming potential of 21 times that of CO2.

Global warming potentials are presented for 20-, 100-, and 500-year time horizons. In The Health of Our Air, we use the 100-year GWPs recommended by IPCC. Calculations of warming potential are continually refined, so these numbers are subject to revision as understanding improves.

Relative global warming potential
(CO2 equivalents per unit mass of gas)
  Time horizon
Gas 20y 100y 500y
CO2 1 1 1
CH4 56 21 6.5
N2O 280 310 170

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Date Modified: 2003-08-27
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