A recent peer-reviewed study makes the outrageous claim that production and consumption of food could contribute as much as 0.9 degrees Celsius (1.6 degrees Fahrenheit) to global warming by 2100, from emissions of the greenhouse gases methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂).

Such a preposterous notion is blatantly wrong, even if it were true that global warming largely comes from human CO₂ emissions. Since agriculture is considered responsible for an estimated 15-20% of current warming, a 0.9 degrees Celsius (1.6 degrees Fahrenheit) agricultural contribution in 2100 implies a total warming (since 1850-1900) at that time of 0.9 / (0.15–0.2), or 4.5 to 6.0 degrees Celsius (8.1 to 10.8 degrees Fahrenheit).

As I discussed in a previous post, only the highest, unrealistic CO₂ emissions scenarios project such a hot planet by the end of the century. A group of prominent climate scientists has estimated the much lower range of likely 2100 warming, of 2.6-3.9 degrees Celsius (4.7-7.0 degrees Fahrenheit). And climate writer Roger Pielke Jr. has pegged the likely warming range at 2-3 degrees Celsius (3.6-5.4 degrees Fahrenheit), based on the most plausible emissions scenarios.

Using the same 15-20% estimate for the agricultural portion of global warming, a projected 2100 warming of say 3 degrees Celsius (5.4 degrees Fahrenheit) would mean a contribution from food production of only 0.45-0.6 degrees Celsius (0.8-1.1 degrees Fahrenheit) – about half of what the new study’s authors calculate.

That even this estimate of future warming from agriculture is too high can be seen by examining the following figure from their study. The figure illustrates the purported temperature rise by 2100 attributable to each of the three greenhouse gases generated by the agricultural industry: CH₄, N₂O and CO₂. CH₄ is responsible for nearly 60% of the temperature increase, while N₂O and CO₂ each contribute about 20%.

To properly compare the two figures, we need to know what percentages of total CH₄, N₂O and CO₂ emissions in the Happer and van Wijngaarden figure come from the agricultural sector; these are approximately 50%, 67% and 3%, respectively, according to the authors of the food production study.

Using these percentages and extrapolating the Happer and van Wijngaarden graph to 78 years (from 2022), the total additional forcing from the three gases in 2100 can be shown to be about 0.52 watts per square meter. This forcing value corresponds to a temperature increase due to food production and consumption of only around 0.1 degrees Celsius (0.18 degrees Fahrenheit).

The excessively high estimate of 0.9 degrees Celsius (1.6 degrees Fahrenheit) in the study may be due in part to the study’s dependence on a climate model: many climate models greatly exaggerate future warming.

While on the topic of CH₄ and N₂O emissions, let me draw your attention to a fallacy widely propagated in the climate science literature; the fallacy appears on the websites of both the U.S. EPA (Environmental Protection Agency) and NOAA (the U.S. National Oceanic and Atmospheric Administration), and even in the IPCC’s Sixth Assessment Report (Table 7.15).

The fallacy conflates the so-called “global warming potential” for greenhouse gas emissions, which measures the warming potential per molecule (or unit mass) of various gases, with their warming potential weighted by their rate of concentration increase relative to CO₂. Because the abundances of CH₄ and N₂O in the atmosphere are much lower than that of CO₂, and are increasing even more slowly, there is a big difference between their global warming potentials and their weighted warming potentials.

The difference is illustrated in the table below. The conventional global warming potential (GWP) is a dimensionless metric, in which the GWP of a particular greenhouse gas is normalized to that of CO₂; the GWP takes into account the atmospheric lifetime of the gas. The table shows values of GWP-100, the warming potential calculated over a 100-year time horizon.

The final column shows the value of the weighted GWP-100, which is not dimensionless like the conventional GWP-100 but measured in units of watts per square meter, the same as radiative forcing. The weighted GWP-100 is calculated by multiplying the conventional GWP-100 by the ratio of the rate of concentration increase for that gas to that of CO₂.

As you can see, the actual anticipated warming in 100 years from either CH₄ or N₂O agricultural emissions will be only 10% of that from CO₂ – in contrast to the conventional GWP-100 values extensively cited in the literature. What a waste of time and effort in trying to rein in CH₄ and N₂O emissions!