A Quota for Agricultural GHG Emissions (Methane and Nitrous Oxide)

Abstract

Excessive human emissions of greenhouse gases (GHGs) are one of the “forcing” factors that has contributed to a change in global average temperatures by changing the balance of radiation at the top of Earth’s atmosphere. Other anthropogenic forcing factors include change in albedo (Earth’s reflectivity) due to land-use change and the emissions of aerosols and aerosol precursors. There is a Planetary Boundary for the change in radiative forcing since preindustrial times of ≤±1 W/m².

Carbon dioxide, methane, nitrous oxide, and halocarbons are called “long-lived” or “well-mixed” gases. This means that they remain in the atmosphere long enough that the location of the source of the emissions is irrelevant. The impacts are experienced on a global scale. The warming effects of long-lived gases can all be expressed in terms of equivalent emissions of carbon dioxide (CO₂e). This chapter is about methane and nitrous oxide. The basis of the limit proposed for these gases is that remaining emissions would come almost entirely from agriculture and hence we have called them agricultural GHG emissions or Ag-GHGs.

To operate within the Planetary Boundaries will require a net withdrawal of carbon dioxide from the atmosphere and a phase out of halocarbons to zero emissions. In contrast, it is possible to continue to emit a small amount of methane and nitrous oxide without exceeding the Planetary Boundaries. Moreover, based on current prediction, it is anticipated that small amounts of these emissions will remain necessary in order to feed the global population. If carbon dioxide, halocarbons, methane, and nitrous oxide emissions were considered collectively, it would be conceivable that the Planetary Quota could be met without withdrawal of carbon dioxide or a complete phase out of halocarbons. As such, these pressures are addressed separately, and only methane and nitrous oxide are considered collectively.

The limit for methane and nitrous oxide emissions is based on the 2100 targets for these gases under the most stringent emission reduction pathway proposed by the Intergovernmental Panel on Climate Change (IPCC) in their fifth Assessment Report and was reinforced in the IPCC Special Report on 1.5 °C. The IPCC figures are based on almost eliminating nonagricultural emissions of both gases and optimizing the conflicting goals of minimizing emissions while maximizing agricultural output per land area in order to meet global food demands. As such, we have collectively termed these emissions agricultural GHGs, or Ag-GHGs.

The PQ for Ag-GHGs is net emissions ≤5 GtCO₂e/year. This limit can be compared to the net emissions of Ag-GHGs associated with any scale of human activity—the “Ag-GHG footprint”.

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