Had GM crops not been grown in 2020, for example, an additional 23.6 billion kilograms of carbon dioxide would have been emitted into the atmosphere, which is the equivalent of adding 15.6 million cars on the roads.
The peer reviewed paper examined changes in farming practices arising from growing GM crops between 1996 and 2020 on global GHG emissions. The three main GHGs of relevance to agriculture are carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). The scope for GM crops contributing to lowering levels of GHG comes from two principal sources:
a. Reduced fuel use from fewer herbicide or insecticide applications;
b. The use of ‘no-till’ (NT) and ‘reduced-till’ (RT) farming systems collectively referred to as conservation tillage, have increased significantly with the adoption of genetically modified herbicide-tolerant (GM HT crops). The GM HT technology has improved farmers’ ability to control weeds, reducing the need to rely on soil cultivation and seed-bed preparation as means to getting good levels of weed control. The advantages of conservation tillage include lower fuel costs, reduced labour requirements associated with soil preparation, enhanced soil quality and reduced levels of soil erosion, resulting in more carbon from crop residues remaining in soil, which leads to lower GHG emissions, improved levels of soil moisture conserving and reduced soil temperature fluctuations from the insulating properties of crop residues.
Measuring the magnitude of fuel savings
The fuel savings arise from making fewer insecticide applications with the use of GM insect resistant (GM IR) crop technology in maize, cotton and soybeans (in countries where farmers previously used mechanical forms of application) and the switch from conventional tillage to reduced tillage or no tillage systems facilitated by GM HT crops. These have delivered permanent savings in carbon dioxide emissions. In 2020, the fuel related savings were 2,330 million kg of carbon dioxide, arising from reduced fuel use of 948 million litres. These savings are equivalent to taking 1.68 million cars off the road for one year.
Over the period 1996 to 2020, the cumulative permanent reduction in fuel use has been about 39,147 million kg of carbon dioxide, arising from reduced fuel use of 14,662 million litres. In terms of car equivalents, this is equal to taking 25.9 million cars off the road for a year.
The largest fuel use-related reductions in carbon dioxide emissions have come from the adoption of GM herbicide-tolerant HT technology and how it has facilitated a switch to reduced and no-till production systems with their reduced soil cultivation practices. This accounted for 92% of the fuel and carbon dioxide savings in the period 1996-2020, within which GM HT soybeans accounted for the largest contribution (68% of the total savings). These savings have been greatest in South America.
As well as the tractor fuel use for tillage having been reduced, soil quality has been enhanced and levels of soil erosion cut. In turn, more carbon has remained stored in the soil leading to lower emissions of carbon dioxide.
Soil carbon savings
Based on the areas of GM HT crops using RT/NT production systems in North and South America in 2020, an extra 5,750 million kg of soil carbon was sequestered. This is equivalent to 21,101 million kg of carbon dioxide that has not been released into the global atmosphere. In terms of removing vehicles from the road, these savings are equivalent to taking 14 million cars off the road for one year.
Applying the same approach to estimating the annual soil carbon sequestration levels for each year between 1996 and 2020 and aggregating the findings suggests that the additional amount of soil carbon sequestered since 1996 has been equivalent to 93,745 million kg of carbon or 344,044 million kg of carbon dioxide that has not been released into the global atmosphere, equivalent to taking about 228 million cars off the road over this period.
A note of caution in interpreting this data. This estimate is likely to significantly overstate the true soil carbon sequestration benefits from the adoption of RT/NT systems over this 25-year period because some of the additional soil carbon sequestration gains from RT/NT systems will have been lost from some subsequent plowing of land in these crops and production systems.
Estimating these possible losses that may arise from subsequent lowing would be complex and difficult to undertake. One US study (Claassen R et al, 20182) estimated that approximately 20% of the combined area of corn, soybeans, cotton and wheat in the US were in continuous NT/RT production systems during the period 2012-2016. If this estimate was used as the basis for estimating the cumulative reductions soil carbon associated with GM HT crop adoption-facilitated NT/RT farming in North and South American agriculture, it would equate to a saving of 18,750 million kg of soil carbon and 68,809 million kg of carbon dioxide.
However, in South America, the most recent data relating to the share of NT/RT in GM HT soybean production continues to show very high levels of retention of NT/RT (90% plus) compared to some fall back having taken place in the US. This suggests that using the US-specific research as the basis for estimating cumulative soil carbon savings in South America would understate the real savings. The total cumulative soil carbon savings from the use of RT/NT practices facilitated by GM HT crop technology in North and South America is more likely to be in the range of 68,809 million kg of carbon dioxide to 275,230 million kg of carbon dioxide.
Aggregating these soil carbon savings with the fuel related savings referred to earlier puts the cumulative carbon dioxide savings for the period 1996-2020 to be between 107,959 million kg and 314,372 million kg, or equal to removing between 72 million and 209 million cars off the road for a year.
