Is Nitrogen Deposition a First-Order Climate Forcing?

The answer appears to be yes.

The 2005 National Research Council report on page 96 specifically stated the following,

“Several nonradiative forcings involve the biological components of the climate system”

with one type of summarized as,

“Biogeochemical forcing involves changes in vegetation biomass and soils. For example, increased nitrogen deposition caused by greater anthropogenic emissions of ammonia (NH3), nitric oxide (NO), and nitrogen dioxide (NO2) is a biogeochemical forcing of the climate system (Holland et al., 2005; Nitrogen deposition onto the United States and Western Europe: A synthesis of observations and models. Ecological Applications, 15, 38-57). This deposition has altered the functioning of soil, terrestrial vegetation, and aquatic ecosystems worldwide. Galloway et al. (2004; Nitrogen cycles: Past, present and future. Biogeochemistry 70:153-226). document that human activities increasingly dominate the nitrogen budget at the global scale and that fixed forms of nitrogen are accumulating in most environmental reservoirs.”

This biogeochemical forcing results in significant alterations in the physical components of the climate system such as the surface albedo, and the partioning of atmospheric turbulence into sensible and latent heat components, which subsequently affects all other aspects of the climate system

A seminar was presented October 6, 2005 at Colorado State University that summarized the current knowledge of nitrogen deposition, The informative abstract is,

“Using Observations and Models to Understand Biosphere-Atmosphere
Nitrogen and Sulfur Cycles

by Elisabeth A. Holland
NCAR Atmospheric Chemistry Division

The global nitrogen cycle has been profoundly perturbed during the industrial era through fossil fuel combustion and agricultural intensification. The atmospheric deposition networks designed to address the impact of acid rain deposition onto rural and remote areas in the some most industrialized regions of the world provides a key data set with adequate temporal and spatial coverage to understand the changing global nitrogen cycle. The measurements were made by the National Atmospheric Deposition Program and National Trends Network (NADP/NTN) and European Monitoring for the Environment Programme (EMEP).

To construct continental scale N budgets, we produced maps of N deposition fluxes from site-network observations for the US and Western Europe. The maps and analyses are necessarily restricted to the network measured quantities and consist of statistically interpolated fields of aqueous NO3- and NH4+, gaseous HNO3 and NO2 (in Europe), and particulate NO3- and NH4+. Western Europe receives five times more N in precipitation than the conterminous US. Estimated N emissions exceed measured deposition in the US by 5.3-7.81 Tg N. In Europe, estimated emissions better balance measured deposition, with an imbalance of between -0.63 to 2.88 Tg N suggesting that much of the N emitted in Europe is deposited there. Taking the imbalances in the two regions, more 50% of the N emitted in the US is exported from the continent, and some of the exported N may fall on Western Europe.

We examined the 25 year record of precipitation removal of a atmospheric nitrogen, ammonium and nitrate, using a seasonal trend LOESS statistical approach . On a continental scale for both the US and Western Europe, there was little overall trend in ammonium or nitrate wet deposition. The lack of a clear trend suggests that the emissions of ammonia and nitrogen oxides over the time period between 1978 and 2003 have been relatively constant. By contrast, sulfur dioxide deposition has decreased by as much as 50% over the same time period. There has been a clear difference in the effectiveness of a series of Transboundary Air Pollution Agreements and Clean Air Acts targeting the emissions of sulfur and nitrogen oxides. The Clean Air Acts have been successful at reducing sulfur dioxide emissions but have not been successful at reducing nitrogen oxide emissions.

These spatially and temporally explicit N deposition budgets and N and S trend analyses provide key tools for verifying regional and global models of atmospheric chemistry and transport, and represent critical inputs into terrestrial models of biogeochemistry.”

The modeling of the influence on long-term weather due to nitrogen deposition is in its early infancy. However, even now it needs to be recognized that this non-radiative biogeochemical climate forcing must be accounted for in any assessment of the relative and absolute role of the diversity of different human climate forcings.

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