A few hundred meters below the surface, there is a region in the Eastern Tropical Pacific where oxygen runs out. Not entirely, not to the point of total anoxia, but low enough that the microorganisms carrying out their regular tasks in the water column switch to alternative metabolic pathways, using nitrate rather than oxygen to decompose organic matter. Nitrous oxide is a result of that process. Over time, it builds up in these oxygen minimum zones, accumulating in the cold, dark water where the gas has nowhere to go until it is forced toward the surface.
There’s an upwelling. The gas-rich water from the oxygen minimum zones rises with everything else along the Pacific coast and in open ocean areas where deep, cold water rises toward the surface due to wind patterns and thermohaline circulation. The nitrous oxide equilibrates with the atmosphere above as it reaches the surface, and this exchange can be significant in areas where deep-water upwelling is active. Gas that has been produced in the dark for months or years is being ventilated by the ocean and released into an atmosphere that already contains far more of it than it did prior to civilization.
Although nitrous oxide does not receive as much public attention as carbon dioxide or methane, the data describing its climate impact are substantial enough to demand more. It has about 300 times the warming effect of CO2 per unit of mass during a 100-year period. It lasts in the atmosphere for about 114 years, which is longer than most methane and shorter than some CO2, but long enough that current emissions will continue to warm the climate well into the twenty-second century. The Montreal Protocol has essentially phased out industrial chlorofluorocarbons, but N2O is not subject to similar international limits, making it the primary ozone-depleting chemical still building up in the stratosphere.
Agricultural sources, such as soil emissions from nitrogen fertilizers and livestock waste management systems that produce N2O during decomposition, have received the majority of attention when it comes to nitrous oxide as a climate problem. These sources are serious and authentic. The marine contribution, and particularly the significance of expanding oxygen minimum zones in driving that contribution upward, has received less attention in the models that guide climate projections. Researchers studying agriculture and the atmosphere are starting to pay more attention to the feedback loop that is created by the relationship between land-based nitrogen loading, OMZ expansion, and N2O generation.
The process is as follows: surplus nitrogen from agricultural runoff finds its way into coastal waters and, ultimately, the open ocean as nitrate. After blooming on the nutrients, phytoplankton die and sink, taking organic carbon with them. Bacteria need oxygen as they break down this organic materials in the subsurface layers. An oxygen minimum zone develops or grows when the oxygen input from surface mixing is insufficient to replenish what is being used, as is the situation across wide areas of the Eastern Tropical and subarctic Pacific. As a metabolic consequence of their nitrogen chemistry, the specialized microorganisms that flourish in these low-oxygen settings generate N2O. More microbial activity, bigger OMZs, and more N2O production result from more organic matter reaching deep. This is made worse by climate change, which increases the OMZ limits by lowering the amount of oxygen that surface water can contain.
An additional layer is added by the acidification connection. In Pacific waters, dissolved CO2 is reducing pH in ways that seem to increase microbial N2O production rates over their baseline levels, especially in the subarctic where cooler water absorbs atmospheric CO2 more readily. The direction of the effect is consistent with what the models of microbial metabolism in low-pH conditions predict, but the chemistry isn’t fully understood because ocean biogeochemistry at these depths involves complicated interacting processes.
The growth of marine N2O emissions under warming and fertilizer loading scenarios is not fully included in current climate estimates, which makes this a modeling problem as much as a monitoring concern. When oxygen minimum zones were smaller and better defined, the models used to estimate temperature trajectories and establish emissions objectives treated the ocean’s N2O budget with plausible assumptions. The discrepancy between measured and estimated N2O flow from ocean surfaces in upwelling regions has been increasing as those zones have grown over the past few decades. Although the precise amount of the correction is yet unknown, the direction is clear: the ocean is contributing more than the conventional forecasts account for, and the factors influencing that contribution are becoming more prominent rather than less.
