A comprehensive quantification of global nitrous oxide sources and sinks.

Nitrous oxide (N2O), like carbon dioxide, is a long-lived greenhouse gas that accumulates in the atmosphere. Over the past 150 years, increasing atmospheric N2O concentrations have contributed to stratospheric ozone depletion1 and climate change2, with the current rate of increase estimated at 2 per cent per decade. Existing national inventories do not provide a full picture of N2O emissions, owing to their omission of natural sources and limitations in methodology for attributing anthropogenic sources. Here we present a global N2O inventory that incorporates both natural and anthropogenic sources and accounts for the interaction between nitrogen additions and the biochemical processes that control N2O emissions. We use bottom-up (inventory, statistical extrapolation of flux measurements, process-based land and ocean modelling) and top-down (atmospheric inversion) approaches to provide a comprehensive quantification of global N2O sources and sinks resulting from 21 natural and human sectors between 1980 and 2016. Global N2O emissions were 17.0 (minimum-maximum estimates: 12.2-23.5) teragrams of nitrogen per year (bottom-up) and 16.9 (15.9-17.7) teragrams of nitrogen per year (top-down) between 2007 and 2016. Global human-induced emissions, which are dominated by nitrogen additions to croplands, increased by 30% over the past four decades to 7.3 (4.2-11.4) teragrams of nitrogen per year. This increase was mainly responsible for the growth in the atmospheric burden. Our findings point to growing N2O emissions in emerging economies-particularly Brazil, China and India. Analysis of process-based model estimates reveals an emerging N2O-climate feedback resulting from interactions between nitrogen additions and climate change. The recent growth in N2O emissions exceeds some of the highest projected emission scenarios3,4, underscoring the urgency to mitigate N2O emissions.

Historical increases in land-derived nutrient inputs may alleviate effects of a changing physical climate on the oceanic carbon cycle.

The implications of climate change and other human perturbations on the oceanic carbon cycle are still associated with large uncertainties. Global-scale modelling studies are essential to investigate anthropogenic perturbations of oceanic carbon fluxes but, until now, they have not considered the impacts of temporal changes in riverine and atmospheric inputs of P and N on the marine net biological productivity (NPP) and air-sea CO2 exchange (FCO2 ). To address this, we perform a series of simulations using an enhanced version of the global ocean biogeochemistry model HAMOCC to isolate effects arising from (1) increasing atmospheric CO2  levels, (2) a changing physical climate and (3) alterations in inputs of terrigenous P and N on marine carbon cycling over the 1905-2010 period. Our simulations reveal that our first-order approximation of increased terrigenous nutrient inputs causes an enhancement of 2.15 Pg C year-1 of the global marine NPP, a relative increase of +5% over the simulation period. This increase completely compensates the simulated NPP decrease as a result of increased upper ocean stratification of -3% in relative terms. The coastal ocean undergoes a global relative increase of 14% in NPP arising largely from increased riverine inputs, with regional increases exceeding 100%, for instance on the shelves of the Bay of Bengal. The imprint of enhanced terrigenous nutrient inputs is also simulated further offshore, inducing a 1.75 Pg C year-1 (+4%) enhancement of the NPP in the open ocean. This finding implies that the perturbation of carbon fluxes through coastal eutrophication may extend further offshore than that was previously assumed. While increased nutrient inputs are the largest driver of change for the CO2 uptake at the regional scale and enhance the global coastal ocean CO2 uptake by 0.02 Pg C year-1 , they only marginally affect the FCO2 of the open ocean over our study's timeline.

Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

Nitrous oxide (N2 O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N2 O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N2 O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N2 O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half-degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N2 O emissions of 10.6-19.8 Gmol N year-1 (148-277 Gg N year-1 ), with reservoirs producing most N2 O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N2 O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.

Identifying the provenance and quantifying the contribution of dust sources in EPICA Dronning Maud Land ice core (Antarctica) over the last deglaciation (7-27 kyr BP): A high-resolution, quantitative record from a new Rare Earth Element mixing model.

Antarctic ice cores have revealed the interplay between dust and climate in the Southern Hemisphere. Yet, so far, no continuous record of dust provenance has been established through the last deglaciation. Here, using a new database of 207 Rare Earth Element (REE) patterns measured in dust and sediments/soils from well-known potential source areas (PSA) of the Southern Hemisphere, we developed a statistical model combining those inputs to provide the best fit to the REE patterns measured in EPICA Dronning Maud Land (EDML) ice core (E. Antarctica). Out of 398 samples measured in the EDML core, 386 samples have been un-mixed with statistical significance. Combined with the total atmospheric deposition, we quantified the dust flux from each PSA to EDML between 7 and 27 kyr BP. Our results reveal that the dust composition was relatively uniform up until 14.5 kyr BP despite a large drop in atmospheric deposition at ∼18 kyr with a large contribution from Patagonia yielding ∼68 % of total dust deposition. The remaining dust was supplied from Australia (14-15 %), Southern Africa (∼9 %), New Zealand (∼3-4 %) and Puna-Altiplano (∼2-3 %). The most striking change occurred ∼14.5 kyr BP when Patagonia dropped below 50 % on average while low-latitude PSA increased their contributions to 21-23 % for Southern Africa, 13-21 % for Australia and âˆ¼ 4-10 % for Puna-Altiplano. We argue that this shift is linked to long-lasting changes in the hydrology of Patagonian rivers and to sudden acceleration of the submersion of Patagonian shelf at 14.5 kyr BP, highlighting a relationship between dust composition and eustatic sea level. Early Holocene dust composition is highly variable, with Patagonian contribution being still prevalent, at ∼50 % on average. Provided a good coverage of local and distal PSA, our statistical model based on REE pattern offers a straightforward and cost-effective method to trace dust source in ice cores.

Continental shelves as a variable but increasing global sink for atmospheric carbon dioxide.

It has been speculated that the partial pressure of carbon dioxide (pCO2) in shelf waters may lag the rise in atmospheric CO2. Here, we show that this is the case across many shelf regions, implying a tendency for enhanced shelf uptake of atmospheric CO2. This result is based on analysis of long-term trends in the air-sea pCO2 gradient (ΔpCO2) using a global surface ocean pCO2 database spanning a period of up to 35 years. Using wintertime data only, we find that ΔpCO2 increased in 653 of the 825 0.5° cells for which a trend could be calculated, with 325 of these cells showing a significant increase in excess of +0.5 µatm yr-1 (p < 0.05). Although noisier, the deseasonalized annual data suggest similar results. If this were a global trend, it would support the idea that shelves might have switched from a source to a sink of CO2 during the last century.