On the reduced lifetime of nitrous oxide due to climate change induced acceleration of the Brewer-Dobson circulation as simulated by the MPI Earth System Model

Abstract:

Greenhouse gas induced climate change will modify the physical conditions of the atmosphere. One of the projected changes is an acceleration of the Brewer-Dobson circulation in the stratosphere, as it has been shown in many model studies. This change in the stratospheric circulation consequently bears an effect on the transport and distribution of atmospheric components such as N₂O. Since N₂O is involved in ozone destruction, a modified distribution of N₂O can be of importance for ozone chemistry. N₂O is inert in the troposphere and decays only in the stratosphere. Thus, changes in the exchange between troposphere and stratosphere can also affect the stratospheric sink of N₂O, and consequently its atmospheric lifetime. N₂O is a potent greenhouse gas with a global warming potential of currently approximately 300 CO₂-equivalents in a 100-year perspective. A faster decay in atmospheric N₂O mixing ratios, i.e. a decreased atmospheric lifetime of N₂O, will also reduce its global warming potential.

In order to assess the impact of climate change on atmospheric circulation and implied effects on the distribution and lifetime of atmospheric N₂O, we apply the Max Planck Institute Earth System Model, MPI-ESM. MPI-ESM consists of the atmospheric general circulation model ECHAM, the land surface model JSBACH, and MPIOM/HAMOCC representing ocean circulation and ocean biogeochemistry. Prognostic atmospheric N₂O concentrations in MPI-ESM are determined by land N₂O emissions, ocean-atmosphere N₂O exchange and atmospheric tracer transport. As stratospheric chemistry is not explicitly represented in MPI-ESM, stratospheric decay rates of N₂O are prescribed from a MACC MOZART simulation.

Increasing surface temperatures and CO₂ concentrations in the stratosphere impact atmospheric circulation differently. Thus, we conduct a series of transient runs with the atmospheric model of MPI-ESM to isolate different factors governing a shift in atmospheric circulation. From those transient simulations we diagnose decreasing tropospheric N₂O concentrations, increased transport of N₂O from the troposphere to the stratosphere, and increasing stratospheric decay of N₂O leading to a reduction in atmospheric lifetime of N₂O, in dependency to climate change evolution.