JP

Nitrogen effect on carbon-water coupling in forests, grasslands, and shrublands in the arid western U.S.

As greenhouse gases, including CO2, accumulate in the atmosphere, the western United States is predicted to undergo large-scale climate warming and reduced summer precipitation in the coming decades. In this study we explore the role of these climate changes with elevated CO2 to determine the plant physiological response on primary productivity and associated feedbacks on evapotranspiration (ET) and runoff using a biogeochemistry model, TEM-Hydro, with downscaled climate data for the western United States from the NCAR CCSM3 A2 scenario. Net primary productivity increases by 32% in forests due to feedbacks between warmer temperatures and enhanced nitrogen mineralization but decreases in shrublands by 24% due to excessive drying and reduced nitrogen mineralization. Warming directly increases nitrogen mineralization rates but indirectly decreases them by reducing soil moisture, so the net effect is highly dependent on climatic conditions within each biome. Increased soil moisture resulting from larger water use efficiency from the elevated CO2 leads to more net nitrogen mineralization in forests, which reduces N-limiting conditions. The effect of CO2 on stomatal conductance is therefore enhanced because of its effect on reducing nitrogen limiting conditions. Runoff decreases over the 21st century by 22% in forests, 58% in grasslands, and 67% in shrublands due to the reduced precipitation in each region but is modulated by the plant-induced changes in ET. The role of moisture limitation is therefore a crucial regulator of nitrogen limitation, which determines the future productivity and water availability in the West.

© 2011 American Geophysical Union

Nitrous oxide (N2O) isotopic composition in the troposphere: instrumentation, observations at Mace Head, Ireland, and regional modeling

Nitrous oxide (N2O) is a significant greenhouse gas and main contributor to stratospheric ozone destruction. Surface measurements of N2O mole fractions have been used to attribute source and sink strengths, but large uncertainties remain. Stable isotopic ratios of N2O (here considered 14N15N16O, 15N14N16O, 14N14N18O, relative to the abundant 14N14N16O) linked to source and sink isotopic signatures can provide additional constraints on emissions and counter-balancing stratospheric sink. However, the isotopic composition in the troposphere has been regarded and measured as a fixed value, limited by insufficient measurement precision and few data.

This thesis provides the foundation for high-frequency, high-precision measurements and utilization of N2O tropospheric isotopic composition. This is achieved through the development of a new measuring capability with sufficient precision to detect the subtle signals of N2O isotopic composition in tropospheric air and uniquely fully-automated and high-frequency capable. This instrument was applied to produce the first set of tropospheric air observations gathered at a remote research station covering a full annual cycle, paired with air origin information, and providing a valuable assessment of tropospheric composition and its potential utility. The first regional model of tropospheric N2O isotopic composition was developed for further assessment of expected variability and utility of isotopic composition data.

The optimized fully-automated, liquid-cryogen-free pre-concentration device coupled to continuous flow IRMS resulted in 15N site-specific precisions markedly improved over other systems of 0.11 and 0.14‰ (1σ) for δ15Nα and δ15Nβ, respectively, and among the best bulk composition precisions of 0.05 and 0.10‰ for δ15N bulk and δ18O, respectively. The high-precision, non-continuous flask observations of N2O 15N site-specific composition (January 2010 to January 2011; Mace Head Atmospheric Research Station, Ireland) detected statistically significant signals on short-term and annual timescales, and when analyzed with air history information showed consistencies with source-receptor relationships. No seasonal cycle could be detected in the low-frequency observations, but regional model scenarios of the stratospheric seasonal signal resulted in amplitudes at the cusp of current measurement capabilities.

This thesis illustrated detectable variations in tropospheric N2O isotopic composition which can potentially reduce uncertainty in the N2O budget with high-frequency, high-precision observations, now feasible by the instrumentation developed here.

Nonlinearities, feedbacks and critical thresholds within the earth's climate system

The Earth's climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm. While this is widely accepted, there is a relatively poor understanding of the different types of nonlinearities, how they manifest under various conditions, and whether they reflect a climate system driven by astronomical forcings, by internal feedbacks, or by a combination of both. In this paper, after a brief tutorial on the basics of climate nonlinearity, we provide a number of illustrative examples and highlight key mechanisms that give rise to nonlinear behavior, address scale and methodological issues, suggest a robust alternative to prediction that is based on using integrated assessments within the framework of vulnerability studies and, lastly, recommend a number of research priorities and the establishment of education programs in Earth Systems Science. It is imperative that the Earth's climate system research community embraces this nonlinear paradigm if we are to move forward in the assessment of the human influence on climate.

© 2004 Kluwer Academic Publishers

Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface waters of the western North Atlantic Ocean have been constrained because ambient concentrations are typically at or below the detection limits of standard colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods — determinations of nitrate plus nitrite (N + N) by chemiluminescence and soluble reactive phosphorus (SRP) by the magnesium induced co-precipitation (MAGIC) protocol — to surface waters along a transect from the Sargasso Sea at 26oN through the Gulf Stream at 37oN, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compared with data from the BATS program, and the HOT station in the Pacific Ocean, permitting cross-ecosystem comparisons. Microbial populations were analyzed along the transect, and an attempt was made to interpret their distributions in the context of the measured nutrient concentrations.
     Surface concentrations of N + N and SRP during the March 1998 transect separated into 3 distinct regions, with the boundaries corresponding roughly to the locations of the BATS station (~31oN) and the Gulf Stream (~31oN). Although N + N and SRP co-varied, the [N + N]:[SRP] molar ratios increased systematically from ~1 to 10 in the southern segment, remained relatively constant at ~40-50 between 31oN and 37oN, then decreased again systematically to ratios ≤ 10% north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (≥ 90%) the total dissolved N (TDN) and P (TDP) pools except in the northern portion of the transect. The [DON]:[DOP] molar ratios were relatively invariant (~30-60) across the entire transect.

© 2001 Elsevier Science

Observations of High Rates of NO2-HONO Conversion in the Nocturnal Atmospheric Boundary Layer in Kathmandu, Nepal

Nitrous acid (HONO) plays a significant role in the atmosphere, especially in the polluted troposphere. Its photolysis after sunrise is an important source of hydroxyl free radicals (OH). Measurements of nitrous acid and other pollutants were carried out in the Kathmandu urban atmosphere during January–February 2003, contributing to the sparse knowledge of nitrous acid in South Asia. The results showed average nocturnal levels of HONO (1.7±0.8 ppbv), NO2 (17.9±10.2 ppbv), and PM10 (0.18±0.11 mg m−3) in urban air in Kathmandu. Surprisingly high ratios of chemically formed secondary [HONO] to [NO2] (up to 30%) were found, which indicates unexpectedly efficient chemical conversion of NO2 to HONO in Kathmandu. The ratios of [HONO]/[NO2] at night were found to be much higher than previously reported values from measurements in urban air in Europe, North America and Asia. The influences of aerosol surface, ground reactive surface, and relative humidity on NO2-HONO chemical conversion were discussed. The high humidity, strong and low inversion layer at night, and high aerosol pollution burden in Kathmandu may explain the particularly efficient conversion of NO2 to HONO.

Obstacles to global CO2 trading: A familiar problem

There are many obstacles to the development of an international CO2 emissions trading system, but the biggest is a feature that is often assumed: the existence of a single national system. Once a national system is in place, an international system will develop naturally more as a matter of self-interested trade than as international agreement. Meeting the Kyoto targets will create a scarcity; and the scarcity requires that use and the associated rent be allocated somehow. This allocation--deciding who gets what--is a familiar problem and the largest impediment to the creation of a national system, and thus of an international regime of CO2 emissions trading. The paper reviews the various instruments by which such the Kyoto target might be met from the standpoint of the allocation of the scarce use and the associated rent. In particular, the paper emphasizes that existing users will largely continue to use the scarce resource and that they now actively exercise the incipient right to the proposed scarcity. Creation of the scarcity and the allocation of rights raise fundamental issues of equity that lie pre-eminently in the political realm. The author observes that the creation of the scarcity and the allocation of rights are fused and that agreement on one will occur only as there is agreement on the other. Nevertheless, such problems have been solved before --for land and for SO2 permits--although in both cases the conditions were easier than what is now proposed for CO2.An international CO2 trading system will develop from a national allowance system for the same reasons that trading can be expected to occur domestically. However, the unavoidable requirement of certification and verification will impede access to non-Annex B sources of emission reduction, and at the same time encourage countries with such sources to accept Annex B limits. The negotiation of such limits raises the same problems of allocation as faced at the national level, only on a global scale; and there is even less agreement here. Nevertheless, the discussion on global allocation will not begin in earnest until a national system creates the trade opportunities that will make an Annex B limit worth pursuing. The development of an international system for CO2 emissions trading should not be expected to be either quick or easy, but to occur only by accretion and mostly as a matter of self-interested trade.

Copyright American Council for Capital Formation Center for Policy

Obstacles To Global CO2 Trading: A Familiar Problem

There are many obstacles to the development of an international CO2 emissions trading system, but the biggest is a feature that is often assumed: the existence of a single national system. Once a national system is in place, an international system will develop naturally more as a matter of self-interested trade than as international agreement. Meeting the Kyoto targets will create a scarcity; and the scarcity requires that use and the associated rent be allocated somehow. This allocation--deciding who gets what--is a familiar problem and the largest impediment to the creation of a national system, and thus of an international regime of CO2 emissions trading. The paper reviews the various instruments by which such the Kyoto target might be met from the standpoint of the allocation of the scarce use and the associated rent. In particular, the paper emphasizes that existing users will largely continue to use the scarce resource and that they now actively exercise the incipient right to the proposed scarcity. Creation of the scarcity and the allocation of rights raise fundamental issues of equity that lie pre-eminently in the political realm. The author observes that the creation of the scarcity and the allocation of rights are fused and that agreement on one will occur only as there is agreement on the other. Nevertheless, such problems have been solved before --for land and for SO2 permits--although in both cases the conditions were easier than what is now proposed for CO2.An international CO2 trading system will develop from a national allowance system for the same reasons that trading can be expected to occur domestically. However, the unavoidable requirement of certification and verification will impede access to non-Annex B sources of emission reduction, and at the same time encourage countries with such sources to accept Annex B limits. The negotiation of such limits raises the same problems of allocation as faced at the national level, only on a global scale; and there is even less agreement here. Nevertheless, the discussion on global allocation will not begin in earnest until a national system creates the trade opportunities that will make an Annex B limit worth pursuing. The development of an international system for CO2 emissions trading should not be expected to be either quick or easy, but to occur only by accretion and mostly as a matter of self-interested trade.

Ocean heat uptake in transient climate change: Mechanisms and uncertainty due to subgrid-scale eddy mixing

The ocean heat uptake (OHU) is studied using the Massachusetts Institute of Technology (MIT) ocean general circulation model (OGCM) with idealized ocean geometry. The OGCM is coupled with a statistical-dynamic atmospheric model. The simulation of OHU in the coupled model is consistent with other coupled ocean-atmosphere GCMs in a transient climate change when CO2 concentration increases by 1% per year. The global average surface air temperature increases by 1.78C at the time of CO2 concentration doubling (year 70). The ocean temperature increases by about 1.08C near the surface, 0.18C at 1000 m in the Pacific, and 0.38C in the Atlantic. The maximum overturning circulation (MOTC) in the Atlantic at 1350 m decreases by about 4.5 Sv (1 Sv [10^6 m^3 per second). The center of MOTC drifts upward about 300 m, and therefore a large OTC anomaly (14 Sv) is found at 2700 m. The MOTC recovers gradually, but the OTC anomaly at 2700 m does not seem to recover after CO2 concentration is kept constant during 400-yr simulation period.
        The diagnosis of heat flux convergence anomaly indicates that the warming in the lower latitudes of the Atlantic is associated with large-scale advection. But, the warming in the higher latitudes is associated with the heat brought down from the surface by convection and eddy mixing. In global average, the treatments of convection and eddy mixing are the two main factors affecting the OHU.
        The uncertainty of OHU due to subgrid-scale eddy mixing is studied. In the MIT OGCM this mixing is a combination of Gent-McWilliams bolus advection and Redi isopycnal diffusion (GMR), with a single diffusivity being used to calculate the isopycnal and thickness diffusion. Experiments are carried out with values of the diffusivity of 500, 1000, and 2000 m^2/sec. The total OHU is insensitive to these changes. The insensitivity is mainly due to the changes in the vertical heat flux by GMR mixing being compensated by changes in the other vertical heat flux components.
        In the Atlantic when the diffusivity is reduced from 1000 to 500 m^2/sec, the surface warming can penetrate deeper. Therefore, the warming decreases by about 0.158C above 2000 m but increases by about 0.158C below 2500 m. Similarly, when the diffusivity is increased from 1000 to 2000 m2 s21, the surface warming becomes shallower; the warming increases by about 0.28C above 1000 m but decreases by about 0.28C below 1000 m. These changes in the vertical distribution of the OHU also contribute to the insensitivity of the total OHU to changes in the GMR mixing. The analysis of heat flux convergence indicates that the difference of OHU seems to be associated with the MOTC circulation.

© 2003 American Meterological Society

Oceanic sources, sinks and transport of atmospheric CO2

We synthesize estimates of the contemporary net air-sea CO2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO2 (pCO2) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a−1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in the high latitudes. Both estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO2 and a strong uptake of anthropogenic CO2. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO2-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1 (inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1 (pCO2-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a−1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.

© 2009 American Geophysical Union

Pages

Subscribe to JP