Atmospheric Input of Chemicals to the Ocean

Lead agency: World Meteorological Organization

History of WG 38, 2008-2013

Working Group 38 was formed in 2008 because of growing concern about the impact of atmospheric deposition of both natural and anthropogenic substances on ocean chemistry, biology, and biogeochemistry as well as climate. It has held meetings at the University of Arizona, Tucson, AZ, in 2008, at IMO in London in 2010, in Malta in 2011, and at the University of East Anglia, Norwich, UK in 2013. Sponsors of those WG 38 efforts have included WMO, IMO, SCOR, SIDA, the European Commission Joint Research Centre, the University of Arizona, the International Environment Institute at the University of Malta, and the University of East Anglia, and the US National Science Foundation. Following the initial terms of reference, five scientific papers were published in the peer-reviewed scientific literature, and these are listed at the end of this report.

The Nitrogen Study and its Terms of Reference, 2013-2017

Although the early work of Working Group 38 did consider some aspects of the deposition and impacts of atmospheric nitrogen species on the ocean, it was recognized that this was a significant and complex scientific issue that required a more in-depth study. Thus, at GESAMP’s 39th session in 2012, Members approved additional terms of reference for continued work of GESAMP WG 38 to address issues related to the impact of the atmospheric deposition of anthropogenic nitrogen to the ocean. An abbreviated form of the new Terms of Reference were as follows:

  • Update the geographical estimates of atmospheric anthropogenic nitrogen deposition to the global ocean;
  • Re-evaluate the magnitude and impact of atmospheric nitrogen deposition on marine biogeochemistry;
  • Provide a more reliable estimate of the impact of atmospheric anthropogenic nitrogen deposition on the production of additional nitrous oxide in the ocean and its subsequent emission to the atmosphere;
  • Evaluate the extent to which anthropogenic nitrogen, delivered to the coastal zone via rivers, is transported to the open ocean; and
  • Make a detailed estimate of the input and impact of anthropogenic nitrogen in the area of the Northern Indian Ocean and the South China Sea.

To address these new terms of reference, a highly successful and productive workshop on “The Atmospheric Deposition of Nitrogen and Its Impact on Marine Biogeochemistry” was held at the University of East Anglia in Norwich, United Kingdom, from 11 to 14 February 2013. Twenty-three scientists participated in the workshop. Sub-groups began the development of a number of scientific papers covering the task areas above. Seven papers have been published in the peer-reviewed scientific literature and they are listed at the end of this report. This nitrogen workshop was supported by WMO, IMO, the University of East Anglia and the US National Science Foundation through SCOR.

Results of the Nitrogen Study

The results of the nitrogen workshop were synthesized in GESAMP Reports and Studies 97, “The Magnitude and Impacts of Anthropogenic Atmospheric Nitrogen Inputs to the Ocean”. This synthesis by GESAMP WG 38 provides new current best estimates of nitrogen inputs to the ocean from the atmosphere (39 TgN y-1), and for context it made comparable estimates of inputs from rivers (34 TgN y-1) and natural biological nitrogen fixation (164 TgN y-1). Most of the atmospheric nitrogen input reaches the open ocean beyond the shelf break, while a substantial part of the riverine input is trapped on the shelf. Both the riverine and atmospheric nitrogen inputs have been substantially increased by human activity, with the atmosphere now the main vehicle by which anthropogenic nitrogen reaches the open ocean. The atmospheric input of nitrogen is estimated to now be almost 4 times that in 1850, and even in 1850 conditions were not pristine.

Atmospheric deposition of nitrogen to the oceans involves several distinct chemical components, each of approximately the same magnitude; oxidised nitrogen, primarily nitrate aerosol and nitric acid; reduced nitrogen, primarily ammonium aerosol and ammonia; and a poorly characterised organic nitrogen component. Identification of sources is important for the effective management of nitrogen inputs to the ocean. The main anthropogenic source of oxidised nitrogen is fossil fuel combustion on land plus an increasingly important source from fuel combustion on ships, while for reduced nitrogen the primary anthropogenic emission source is from intensive agriculture. There is also an important but poorly understood natural recycling of ammonia and organic nitrogen between the atmosphere and the oceans. The quantification of the net magnitude of atmospheric nitrogen inputs to the ocean and their impact is sensitive to the uncertainties in the magnitude of this recycling.

Atmospheric nitrogen emissions come predominantly from areas of high fossil fuel combustion and high rates of intensive agriculture. The largest emission sources are in North America, Europe, India and South-East Asia. Models based on future emission scenarios suggest that total nitrogen inputs to the oceans will change little between now and 2050, but that emissions are likely to increase over southern Asia and decline over North America and Europe. The largest inputs of nitrogen to the oceans occur downwind of these large emission sources over the North Atlantic, Northern Indian and north-west Pacific Oceans. Impacts of this atmospheric deposition on the marine environment have been previously suggested for the north-west Pacific, and impacts in this region and the Northern Indian Ocean are likely to increase in the future, based on the emission scenarios considered. Such impacts may include increases in phytoplankton production, and in the north-western Indian Ocean this may lead to increases in the emissions of the greenhouse gas N2O from the low oxygen waters that occur naturally at depth in this region.

More generally the impact of nitrogen deposition to the ocean will be an increase in primary production in regions that are currently nitrogen limited, which include the surface waters of tropical ocean gyres. The increase in ocean production at the present day compared to 1850 levels is estimated to be about 0.4%, with an associated increase in the ocean uptake of CO2 of 0.15Pg C y-1. This estimate is very sensitive to assumptions about feedbacks that involve atmospheric nitrogen deposition suppressing nitrogen fixation. There is also, on a longer time scale, a sensitivity to feedbacks in which increasing nitrogen inputs to the ocean increase primary production and organic matter inputs to the deep ocean, increasing denitrification and anammox and leading to increased emissions of N2 and N2O gas.

Through these papers significant advances have been made in our understanding of atmospheric nitrogen deposition and its impacts on ocean biogeochemistry. A number of scientific gaps remain, providing new opportunities and challenges. Therefore we recommend additional work on:

  • The sources of atmospheric organic nitrogen,
  • the magnitude and significance of recycling of ammonia and organic nitrogen from the oceans,
  • the parameterisation of deposition, particularly dry deposition
  • an improved data base, particularly on wet deposition over the oceans
  • the extent and thresholds for suppression of nitrogen fixation by ambient surface water dissolved nitrogen
  • the retention of nitrogen within shelf systems, and particularly how the rates of bacterial processes that convert fixed nitrogen back to dinitrogen gas (denitrification and the related anammox both of which convert Nr into inert nitrogen gas) vary with temperature
  • the impacts of possible changes in aerosol acidity over coming decades on the atmospheric delivery of nutrients to the oceans
  • increasing atmospheric carbon dioxide concentrations are already driving ocean acidification and this is likely to increase in coming decades. The impacts of the changing ocean pH on ocean emissions of atmospherically important gases including nitrogen species, but also other gases, needs to be evaluated.

We also recommend further work in the following regions, which are particularly sensitive to likely changes in atmospheric deposition:

  • the Northwest Pacific where deposition fluxes are expected to grow and where there may already be impacts from the current inputs,
  • the northern Indian Ocean, an important source region for N2O, which receives a large atmospheric input that is argued to already be increasing plankton productivity,
  • areas of the Mediterranean and North Atlantic where primary production is phosphorus- or iron-limited and hence where additional nitrogen deposition may lead to different nutrient biogeochemical responses to those in other ocean areas where nitrogen is the primary limiting nutrient.

Activities of WG 38, 2017-2018

From 27 February to March 2 2017 two linked workshops took place at the University of East Anglia (UEA), Norwich, United Kingdom (see photo of attendees below) under the auspices of GESAMP Working Group 38 and sponsored by WMO, NSF, SCOR, SOLAS and UEA. Recognising that the acidity of atmospheric aerosols will change in the future as the balance of emissions of acid and alkali precursors to the atmosphere will change over coming decades and that ocean pH will also change due to anthropogenic CO2 emissions, these two workshops focussed on the impacts of these changes on air-sea exchange and the wider Earth system.

Workshop on the Impact of Ocean Acidification on Fluxes of Atmospheric non CO2 Climate-Active Species

The following is taken from the abstract of the paper by Hopkins et al. (2020), derived from this workshop and listed below. Surface ocean biogeochemistry and photochemistry regulate ocean-atmosphere fluxes of trace gases critical for Earth’s atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO2) accompanying ocean acidification (OA), with potential for future climate feedbacks. Current understanding is reviewed (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N2O), ammonia, and halocarbons. The focus is on DMS, for which available information is considerably greater than for other trace gases. OA-sensitive regions are highlighted, such as polar oceans and upwelling systems, and the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes is discussed. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, a proposal is presented combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable future predictions of future global marine trace gas fluxes.

Workshop on Changing Atmospheric Nutrient Solubility

The following is taken from the abstract of the paper by Baker et al. (2020), derived from this workshop and listed below. Anthropogenic emissions of nitrogen and sulfur oxides and ammonia have altered the pH of aerosol, cloudwater and precipitation, with significant decreases over much of the marine atmosphere. Some of these emissions have led to an increased atmospheric burden of reactive nitrogen and its deposition to ocean ecosystems. Changes in acidity in the atmosphere also have indirect effects on the supply of labile nutrients to the ocean. For nitrogen, these changes are caused by shifts in the chemical speciation of both oxidized (NO3- and HNO3) and reduced (NH3 and NH4+) forms that result in altered partitioning between the gas and particulate phases that affect transport. Other important nutrients, notably iron and phosphorus, are impacted because their soluble fractions increase due to exposure to low pH environments during atmospheric transport. These changes affect the magnitude, distribution and deposition mode of individual nutrient supply to the ocean. Changes in deposition mode also affect the extent to which nutrient deposition interacts with the sea-surface microlayer during transport into bulk seawater. The ratios of nitrogen, phosphorus, iron and other trace metals in atmospheric deposition are also affected. Since marine microbial populations are sensitive to nutrient supply ratio, the consequences of atmospheric acidity change include shifts in ecosystem composition, in addition to overall changes in marine productivity. Nitrogen and sulfur oxide emissions are decreasing in many regions, but ammonia emissions are much harder to control. The acidity of the atmosphere is therefore expected to decrease in the future, with further implications for nutrient supply to the ocean.

Two additional papers based on the work on atmospheric acidity have been published which have focused particularly on the sources of iron to the oceans: Myriokefalitakis, et al 2018 and Ito et al., 2019, and they are listed below. With encouragement from WMO, several members of WG38 also contributed to a paper looking at future strategies for making cost effective measurements of atmospheric parameters over the oceans using ships of opportunity. This paper (Smith et al. (2019) is also listed below.

Activities of WG 38, 2019-2020

New Workshop in 2020

In the past WG 38 has focused on the scientific aspects of the deposition of chemicals to the ocean. In 2019 we began the development of a new type of workshop. This new workshop would bring together scientists as well as ocean managers and policymakers to address issues related to the importance of atmospheric deposition to a specific area of the ocean. This new workshop has the title: What is the potential role of atmospheric deposition in driving ocean productivity in the Madagascar Channel and Southwest Indian Ocean? - A case for an Adaptive-Dynamic Management approach within Large Marine Ecosystems.

The Agulhas Current Large Marine Ecosystem in the southwest Indian Ocean is an important region of periodic large-scale algal blooms which are readily identifiable by satellite observations. Paradoxically, these blooms are found during the austral summer months when much of the region is devoid of nutrients in the surface layer of the ocean and hence productivity would be expected to be low. Marine natural resources are of considerable societal importance in this region in the context of food security. Given the extent of these blooms, it is probable that they are having an impact on the regional trophic structures, which sustain the all-important local fisheries. Mozambique and Madagascar are the countries most likely to be affected as fisheries make up 70% and 46% respectively of their national food security baskets. Yet the drivers of these blooms are not understood, despite several scientific investigations, meaning that predicting their future persistence and the resulting societal impacts is challenging. This further constrains any long-term sustainable planning and management and thus threatens human welfare. One possibility that has not been evaluated yet is the role of atmospheric deposition of nutrients in creating and/or sustaining these blooms. The southwest Indian Ocean is a region of relatively high atmospheric deposition driven by biomass burning and industrial emissions in central and southern Africa. Several dated studies suggest that there is a distinct atmospheric flow path associated, at least in part, with biomass burning, which brings nitrogen, iron and other atmospheric nutrients and pollutants into the Mozambique Channel, as well as the greater south-western Indian and Southern Ocean regions. More recently, models are now producing scientific evidence which can be used to evaluate this atmospheric deposition, and therefore test the hypothesis that the deposition helps to support the blooms. With these advancements, now is an appropriate time to attempt to understand some of the main controls and drivers of the algal blooms in this region and formulate relevant management and policy advice related to the associated natural resources, particularly the commercial and small-scale fisheries. The complexities and uncertainties of the environmental drivers of the blooms and their societal importance make this a challenging and important region in which to consider the most appropriate approach to the formulation of policy advice, providing a test case within which to evaluate new approaches to adaptive environmental management. Resource and human-wellbeing managers are frequently confronted by trends arising from scientific investigation and research which, although not proven at the higher ‘confidence-limits’ level, are sufficiently strong and repetitive as to raise concerns at the decision-making level. A more dynamic, adaptive management approach has been proposed frequently over the last decade that would analyse such trends to identify the strength of the ‘weight-of-evidence’ and decide whether it is sufficient to warrant consequent management and policy decisions. At its meeting in New York in 2019, GESAMP therefore endorsed the setting up of a new workshop led by WG38 with the objectives as below.


The Workshop has a series of related objectives.

  • To evaluate the current knowledge of the atmospheric inputs into the southwest Indian Ocean and scientific evidence for the factors that control algal blooms in this region, including the potential role of atmospheric deposition, and the confidence in our understanding of these factors.
  • To debate the associated potential impacts and management implications with a broader group of stakeholders/experts (including social scientists and economists)
  • To present this information to decision-makers at the senior management and policy level for their response and advice on adaptive management steps
  • To identify the feasibility of institutionalising such an adaptive/dynamic management process at the regional level and linking it into national management processes.
  • In parallel with this process, to introduce young and emerging scientists to the debate and the science involved and to build capacity for this dialogue within the region.

The plan is for this new workshop to take place at Nelson Mandela University in Port Elizabeth, South Africa. Currently the dates are set for 12-15 October 2020.

European Geosciences Union Meeting

For the sixth year in a row WG 38 organized a session on atmospheric input of chemicals to the ocean for the European Geosciences Union meeting held in Vienna, Austria in April 2019 – “Air-Sea Exchanges: Impacts on Biogeochemistry and Climate”. WG 38 is also sponsoring for the seventh time a similar session at the European Geosciences Union meeting in April 2020.

To view a list of the most recent publictions of WG38 please click here