Showing papers in "Tellus B in 1999"

The annual net flux of carbon to the atmosphere from changes in land use 1850–1990*

Abstract: Rates of land-use change, including clearing for agriculture and harvest of wood, were reconstructed from statistical and historic documents for 9 world regions and used, along with the per ha changes in vegetation and soil that result from land management, to calculate the annual flux of carbon between land and atmosphere. Between 1850 and 1990, changes in land use are calculated to have added 124 PgC to the atmosphere, about half as much as released from combustion of fossil fuels over this period. About 108 PgC are estimated to have been transferred from forests to the atmosphere as a result of human activity, 2/3 from tropical forests and 1/3 from temperate zone and boreal forests. Another 16 PgC were lost from non-forests, largely as a result of cultivation of mid-latitude grassland soils. About 800 × 10 6 ha of forest were cleared for agricultural purposes, and approximately 2000 × 10 6 ha were harvested. Conversion of forests to agricultural lands released 105 PgC; harvest of wood released about 20 PgC. These estimates of release include the accumulations of carbon in wood products (17 PgC) and woody debris (4 PgC), the losses of carbon from oxidation of wood products, woody debris, and soil organic matter (373 PgC in total), and the accumulations of carbon in forests recovering from harvest and in the fallows of shifting cultivation (249 PgC). Over the decade of the 1980s the annual net flux of carbon from changes in land use averaged about 2.0 PgC yr −1 , higher than the 1.6 PgC yr −1 estimated previously. Almost all of this flux was from tropical regions, where rates of deforestation averaged approximately 15 × 10 6 ha yr −1 . Outside the tropics, regrowth of forests logged in earlier years largely balanced the losses of carbon from oxidation of wood products. DOI: 10.1034/j.1600-0889.1999.00013.x

A 1000-year high precision record of δ 13 C in atmospheric CO 2

Abstract: We present measurements of the stable carbon isotope ratio in air extracted from Antarctic ice core and firn samples. The same samples were previously used by Etheridge and co-workers to construct a high precision 1000-year record of atmospheric CO 2 concentration, featuring a close link between the ice and modern records and high-time resolution. Here, we start by confirming the trend in the Cape Grim in situ δ 13 C record from 1982 to 1996, and extend it back to 1978 using the Cape Grim Air Archive. The firn air δ 13 C agrees with the Cape Grim record, but only after correction for gravitational separation at depth, for diffusion effects associated with disequilibrium between the atmosphere and firm, and allowance for a latidudinal gradient in δ 13 C between Cape Grim and the Antarctic coast. Complex calibration strategies are required to cope with several additional systematic influences on the ice core δ 13 C record. Errors are assigned to each ice core value to reflect statistical and systematic biases (between ± 0.025‰ and ± 0.07‰); uncertainties (of up to ± 0.05‰) between core-versus-core, ice-versus-firn and firn-versus-troposphere are described separately. An almost continuous atmospheric history of δ 13 C over 1000 years results, exhibiting significant decadal-to-century scale variability unlike that from earlier proxy records. The decrease in δ 13 C from 1860 to 1960 involves a series of steps confirming enhanced sensitivity of δ 13 C to decadal timescale-forcing, compared to the CO 2 record. Synchronous with a ‘‘Little Ice Age’′ CO 2 decrease, an enhancement of δ 13 C implies a terrestrial response to cooler temperatures. Between 1200 AD and 1600 AD, the atmospheric δ 13 C appear stable. DOI: 10.1034/j.1600-0889.1999.t01-1-00005.x

Reconstructing the recent carbon cycle from atmospheric CO2, δ13C and O2/N2 observations*

Abstract: This paper presents an attempt to recover the space–time structure of fluxes of CO 2 to the atmosphere over the period 1980–1995 from atmospheric concentration and isotopic composition measurements. The technique used is Bayesian synthesis inversion in which sources are aggregated into large regions and their strengths adjusted to match observed concentrations. The sources are constrained by prior estimates based on a priori knowledge. The input data are atmospheric CO 2 concentration measurements from the NOAA/CMDL network, 13 CO 2 composition and O2/N2 ratios measured at Cape Grim, Tasmania by CSIRO Atmospheric Research. The primary findings are a relatively large long-term mean ocean uptake of CO 2 , and seasonal fluxes over land with similar integrated magnitude, but smaller peak amplitude, compared with those derived by Fung and co-workers. Predicted interannual variability is smaller than reported in previous studies. The largest contributor is the oceanic tropics where fluxes vary on the time scale of the southern oscillation. There is evidence of longer time-scale variation in land uptake. Increases in ocean uptake and northern land uptake in the early 1990s are consistent with a response to the Mt. Pinatubo eruption. DOI: 10.1034/j.1600-0889.1999.t01-1-00008.x

Is there a “continental shelf pump” for the absorption of atmospheric CO2?

Abstract: Based on the results obtained in the East China Sea, we propose a new term, Continental Shelf Pump , as a mechanism for the absorption of atmospheric CO 2 . We investigated the carbonate system of the East China Sea along a single observation line traversing its central part on 5 cruises in various seasons. The directly observed fugacity of CO 2 dissolved in the surface water decreased with decreasing salinity and temperature as well as nutrient content. The relation has been expressed as a simple equation of these 3 parameters. Putting the observed data on the parameters in the various parts of the East China Sea in various months into this equation, we have obtained 55 ± 5 ppm as an annual mean fugacity deficit of CO 2 in the surface water of the East China Sea, which nearly equals the directly measured mean fugacity along the observation line. The net absorption flux estimated from the fugacity deficit has agreed with the amount of carbonate transported out of the East China Sea calculated for the distributions of total dissolved carbonate and alkalinity. The distributions of density and total dissolved carbonate reveal the cause of this large deficiency, described as follows. The shallower shelf zone is more cooled than the open sea when heat is lost from the surface. This cooling produces denser water, which together with photosynthetic activity, accelerates the absorption of CO 2 in the shelf zone. The absorbed CO 2 is transformed to organic carbon and regenerated especially at the shallow bottom. Isopycnal mixing (advection and diffusion) transports the denser coastal water, especially the bottom water enriched in dissolved and particulate carbon, into the subsurface layer of the open oceans. The transport continues in the layer below the pycnocline even in the warm season and maintains the low fugacity of CO 2 in the surface water of the shelf zone. This is the continental shelf pump. The pump would account for a net oceanic uptake of CO 2 of 1 GtC/ yr, if the world continental shelf zone would absorb the atmospheric CO 2 at the rate observed in the East China Sea. DOI: 10.1034/j.1600-0889.1999.t01-2-00010.x

Direct effects of CO2 concentration on growth and isotopic composition of marine plankton

Abstract: The assessment of direct effects of anthropogenic CO 2 increase on the marine biota has received relatively little attention compared to the intense research on CO 2 -related responses of the terrestrial biosphere. Yet, due to the rapid air–sea gas exchange, the observed past and predicted future rise in atmospheric CO 2 causes a corresponding increase in seawater CO 2 concentrations, [CO 2 ], in upper ocean waters. Increasing [CO 2 ] leads to considerable changes in the surface ocean carbonate system, resulting in decreases in pH and the carbonate concentration, [CO 2 −3 ]. These changes can be shown to have strong impacts on the marine biota. Here we will distinguish between CO 2 -related responses of the marine biota which (a) potentially affect the ocean's biological carbon pumps and (b) are relevant to the interpretation of diagnostic tools (proxies) used to assess climate change on geological times scales. With regard to the former, three direct effects of increasing [CO 2 ] on marine plankton have been recognized: enhanced phytoplankton growth rate, changing elemental composition of primary produced organic matter, and reduced biogenic calcification. Although quantitative estimates of their impacts on the oceanic carbon cycle are not yet feasible, all three effects increase the ocean's capacity to take up and store atmospheric CO 2 and hence, can serve as negative feedbacks to anthropogenic CO 2 increase. With respect to proxies used in palaeo-reconstructions, CO 2 -sensitivity is found in carbon isotope fractionation by phytoplankton and foraminifera. While CO 2 - dependent isotope fractionation by phytoplankton may be of potential use in reconstructing surface ocean p CO 2 at ancient times, CO 2 -related effects on the isotopic composition of foraminiferal shells confounds the use of the difference in isotopic signals between planktonic and benthic shells as a measure for the strength of marine primary production. The latter effect also offers an alternative explanation for the large negative swings in δ 13 C of foraminiferal calcite between glacial and interglacial periods. Changes in [CO 2 −3 ] affect the δ 18 O in foraminiferal shells. Taking this into account brings sea surface temperature estimates for the glacial tropics closer to those obtained from other geochemical proxies. DOI: 10.1034/j.1600-0889.1999.00023.x

Climate change feedback on the future oceanic CO2 uptake

Abstract: Output from a coupled atmosphere–ocean model forced by the IS92a greenhouse gas scenario was used to investigate the feedback between climate change and the oceanic uptake of CO 2 To improve the climate simulation, we used Gent and co-workers eddy parameterization in the ocean and a prognostic equation for export production from the upper ocean For the period of 1850 to 2100, the change in the oceanic uptake of CO 2 with climate was separated into 3 feedbacks (i) Climate change warmed the sea-surface temperature which increased the partial pressure of CO 2 in the surface ocean and reduced the accumulated ocean uptake by 48 Gt C (ii) Climate change reduced meridional overturning and convective mixing and increased density stratification in high latitudes which slowed the transport of anthropogenic CO 2 into the ocean interior and reduced the cumulative ocean CO 2 uptake by 41 Gt C (iii) Climate change altered “natural” cycling of carbon in the ocean which increased the cumulative ocean CO 2 uptake by 33 Gt C The change in natural carbon cycling with climate change was dominated by 2 opposing factors First, the supply of nutrients to the upper ocean decreased which reduced the export of organic matter (by 15% by year 2100) and produced a net CO 2 flux out of the ocean However, associated with the reduced nutrient supply was the reduction in the supply of dissolved inorganic carbon to the upper ocean, which produced net CO 2 flux into the ocean For our model, the latter effect dominated By the year 2100, the combinations of these 3 climate change feedbacks resulted in a decrease in the cumulative oceanic CO 2 uptake of 56 Gt C or 14% of the 402 Gt C of oceanic CO 2 uptake predicted by a run with no climate change Our total reduction in oceanic CO 2 uptake with climate change for the 1850 to 2100 period was similar to the 58 Gt C reduction in oceanic CO 2 uptake predicted by Sarmiento and Le Quere However, our consistency with this previous estimate is misleading By including the Gent and co-workers eddy parameterization in the ocean, we reduced the positive feedback between climate change and the oceanic uptake of CO 2 from 169 to 89 Gt C (80 Gt C change) This reduction reflects a decrease in both sea surface warming and anthropogenic forcing feedbacks By using a prognostic parameterization of export production, we reduced the negative feedback response of the natural carbon cycle to climate change from 111 to 33 Gt C (78 Gt C) These 2 large offsetting changes in the ocean response to climate change produced only a net change of 2 Gt C This resulted in a net reduction in oceanic uptake of 2 Gt C from the previous study DOI: 101034/j1600-08891999t01-1-00012x

The sensitivity of terrestrial carbon storage to historical climate variability and atmospheric CO2 in the United States

Abstract: We use the Terrestrial Ecosystem Model (TEM, Version 41) and the land cover data set of the international geosphere–biosphere program to investigate how increasing atmospheric CO 2 concentration and climate variability during 1900–1994 affect the carbon storage of terrestrial ecosystems in the conterminous USA, and how carbon storage has been affected by land-use change The estimates of TEM indicate that over the past 95 years a combination of increasing atmospheric CO 2 with historical temperature and precipitation variability causes a 42% (43 Pg C) decrease in total carbon storage of potential vegetation in the conterminous US, with vegetation carbon decreasing by 72% (32 Pg C) and soil organic carbon decreasing by 19% (11 Pg C) Several dry periods including the 1930s and 1950s are responsible for the loss of carbon storage Our factorial experiments indicate that precipitation variability alone decreases total carbon storage by 95% Temperature variability alone does not significantly affect carbon storage The effect of CO 2 fertilization alone increases total carbon storage by 44% The effects of increasing atmospheric CO 2 and climate variability are not additive Interactions among CO 2 , temperature and precipitation increase total carbon storage by 11% Our study also shows substantial year-to-year variations in net carbon exchange between the atmosphere and terrestrial ecosystems due to climate variability Since the 1960s, we estimate these terrestrial ecosystems have acted primarily as a sink of atmospheric CO 2 as a result of wetter weather and higher atmospheric CO 2 concentrations For the 1980s, we estimate the natural terrestrial ecosystems, excluding cropland and urban areas, of the conterminous US have accumulated 782 Tg C yr −1 because of the combined effect of increasing atmospheric CO 2 and climate variability For the conterminous US, we estimate that the conversion of natural ecosystems to cropland and urban areas has caused a 182% (177 Pg C) reduction in total carbon storage from that estimated for potential vegetation The carbon sink capacity of natural terrestrial ecosystems in the conterminous US is about 69% of that estimated for potential vegetation DOI: 101034/j1600-0889199900021x

The annual fCO2 cycle and the air–sea CO2 flux in the sub‐Antarctic Ocean

Abstract: The sub-Antarctic zone (SAZ) lies between the subtropical convergence (STC) and the sub-Antarctic front (SAF), and is considered one of the strongest oceanic sinks of atmospheric CO 2 . The strong sink results from high winds and seasonally low sea surface fugacities of CO 2 ( f CO 2 ), relative to atmospheric f CO 2 . The region of the SAZ, and immediately south, is also subject to mode and intermediate water formation, yielding a penetration of anthropogenic CO 2 below the mixed layer. A detailed analysis of continuous measurements made during the same season and year, February – March 1993, shows a coherent pattern of f CO 2 distributions at the eastern (WOCE/SR3 at about 145°E) and western edges (WOCE/I6 at 30°E) of the Indian sector of the Southern Ocean. A strong CO 2 sink develops in the Austral summer (Δ f CO 2 < − 50 μatm) in both the eastern (110°−150°E) and western regions (20°−90°E). The strong CO 2 sink in summer is due to the formation of a shallow seasonal mixed-layer (about 100 m). The CO 2 drawdown in the surface water is consistent with biologically mediated drawdown of carbon over summer. In austral winter, surface f CO 2 is close to equilibrium with the atmosphere (Δ f CO 2 ± 5 μatm), and the net CO 2 exchange is small compared to summer. The near-equilibrium values in winter are associated with the formation of deep winter mixed-layers (up to 700 m). For years 1992–95, the annual CO 2 uptake for the Indian Ocean sector of the sub Antarctic Zone (40°−50°S, 20°−150°E) is estimated to be about 0.4 GtC yr −1 . Extrapolating this estimate to the entire sub-Antarctic zone suggests the uptake in the circumpolar SAZ is approaching 1 GtC yr −1 . DOI: 10.1034/j.1600-0889.1999.t01-3-00008.x

Three-dimensional transport and concentration of SF6 A model intercomparison study (TransCom 2)

Abstract: Sulfur hexafluoride (SF 6 ) is an excellent tracer of large-scale atmospheric transport, because it has slowly increasing sources mostly confined to northern midlatitudes, and has a lifetime of thousands of years. We have simulated the emissions, transport, and concentration of SF 6 for a 5-year period, and compared the results with atmospheric observations. In addition, we have performed an intercomparison of interhemispheric transport among 11 models to investigate the reasons for the diVerences among the simulations. Most of the models are reasonably successful at simulating the observed meridional gradient of SF 6 in the remote marine boundary layer, though there is less agreement at continental sites. Models that compare well to observations in the remote marine boundary layer tend to systematically overestimate SF 6 at continental locations in source regions, suggesting that vertical trapping rather than meridional transport may be a dominant control on the simulated meridional gradient. The vertical structure of simulated SF 6 in the models supports this interpretation. Some of the models perform quite well in terms of the simulated seasonal cycle at remote locations, while others do not. Interhemispheric exchange time varies by a factor of 2 when estimated from 1-dimensional meridional profiles at the surface, as has been done for observations. The agreement among models is better when the global surface mean mole fraction is used, and better still when the full 3-dimensional mean mixing ratio is used. The ranking of the interhemispheric exchange time among the models is not sensitive to the change from station values to surface means, but is very sensitive to the change from surface means to the full 3-dimensional tracer fields. This strengthens the argument that vertical redistribution dominates over interhemispheric transport in determining the meridional gradient at the surface. Vertically integrated meridional transport in the models is divided roughly equally into transport by the mean motion, the standing eddies, and the transient eddies. The vertically integrated mass flux is a good index of the degree to

Carbon dioxide emissions from fossil‐fuel use, 1751–1950

Abstract: Newly compiled energy statistics allow for an estimation of the complete time series of carbon dioxide (CO 2 ) emissions from fossil-fuel use for the years 1751 to the present. The time series begins with 3 × 10 6 metric tonnes carbon (C). This initial flux represents the early stages of the fossil-fuel era. The CO 2 flux increased exponentially until World War I. The time series derived here seamlessly joins the modern 1950 to present time series. Total cumulative CO 2 emissions through 1949 were 61.0 × 10 9 tonnes C from fossil-fuel use, virtually all since the beginning of the Industrial Revolution around 1860. The rate of growth continues to grow during present times, generating debate on the probability of enhanced greenhouse warming. In addition to global totals, national totals and 1° global distributions of the data have been calculated. DOI: 10.1034/j.1600-0889.1999.t01-3-00002.x

The rôle of thermohaline circulation in climate

Abstract: This article discusses the role of the THC in climate, based upon the results of several numerical experiments which use a coupled ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of NOAA, USA. The first part of the article explores the mechanism which is responsible for the abrupt climate change such as the Younger Dryas event using the coupled model. In response to the freshwater discharge into high north Atlantic latitudes over a period of 500 years, the THC in the Atlantic Ocean weakens, reducing surface air temperature over the northern north Atlantic Ocean, the Scandinavian Peninsula, and the circumpolar ocean and Antarctic Continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces the multidecadal fluctuation in the intensity of the THC, which generates the almost abrupt change of climate. It is noted that similar but much weaker oscillation of the THC is also evident in the control integration of the coupled model without freshwater forcing. The irregular oscillation of the THC mentioned above appears to be related to the fluctuation of the Subarctic Gyre and associated east Greenland current, yielding the evolution of the surface salinity anomaly which resembles that of “great salinity anomaly”. The second part of this article describes the response of a coupled ocean-atmosphere model to the doubling and quadrupling of atmospheric carbon dioxide over centuries time-scale. In one integration, the CO 2 concentration increases by 1%/year (compounded) until it reaches 4 × the initial value at the 140th year and remains unchanged thereafter. In another integration, the CO 2 concentration also incleases at the rate of 1%/year until it reaches 2 × the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the CO 2 -quadrupling integration is the gradual disappearance of thermohaline circulation in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the north Atlantic by the 250 years of the integration and remains very weak until the 900th year. However, it begins to restore the original intensity by the 1600th year. In the CO 2 -doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years, but almost recovers its original intensity by the 500th year. The weakening of the THC moderates temporarily the greenhouse warming over the north Atlantic Ocean and its vicinity. In both numerical experiments described above, the initial weakening of the THC results from the capping of oceanic surface by relatively fresh, low-density water, which surpresses the convective cooling of water in the sinking region of the THC. DOI: 10.1034/j.1600-0889.1999.00008.x

Simulation of the transport of Rn222 using on‐line and off‐line global models at different horizontal resolutions: a detailed comparison with measurements,

Abstract: The short-lived radionuclide Rn 222 is emitted at a fairly constant rate from the continents and is a good surrogate for studying the transport of “air pollution” from polluted continental areas to clean, remote regions. The large concentration gradients of 2–3 orders of magnitude which exist between the continents and the remote atmosphere present a major challenge to the modelling of horizontal and vertical atmospheric transport. We use the global off-line tracer transport model TM3 at 3 different resolutions. Input to the model consists of meteorological data for the year 1993 obtained from the European Centre for Medium Range Weather Forecasts (ECMWF). The same meteorological data is used to constrain the climate model ECHAM4-T42-L19. Using these meteorological data, Rn 222 simulations are used to evaluate and document model performance and associated uncertainties. High time-resolution measurements made at 2 continental stations, 2 stations under continental influence and 4 remote sites, and aircraft measurements obtained during the NARE aircraft campaign are used for a detailed comparison. Although in specific regions there are inter-model differences of up to a factor of 2 in the calculated boundary layer concentrations, these differences are not translated into a better performance of either model for the stations used for comparison. We generally obtain high correlations of model results and measurements; these range from r = 0.6–0.8 for the continental and coastal stations and 0.5–0.6 for the remote sites. Calculated mean concentrations and corresponding standard deviations generally agree favourably with observations, lending credibility to the usefulness of our models for evaluating transport of air pollutants from continental sources to remote regions. The main cause of model deviations is probably related to uncertainties in the meteorological input data set provided by the ECMWF model and to a lesser extent by our knowledge of the spatial distribution of Rn 222 emissions and uncertainties involving sub-grid scale parameterization of vertical transport, e.g., diffusion and convection. DOI: 10.1034/j.1600-0889.1999.t01-2-00001.x

A first‐order analysis of the potential rôle of CO2 fertilization to affect the global carbon budget: a comparison of four terrestrial biosphere models

Abstract: We compared the simulated responses of net primary production, heterotrophic respiration, net ecosystem production and carbon storage in natural terrestrial ecosystems to historical (1765 to 1990) and projected (1990–2300) changes of atmospheric CO 2 concentration of four terrestrial biosphere models: the Bern model, the Frankfurt Biosphere Model (FBM), the High-Resolution Biosphere Model (HRBM) and the Terrestrial EcosystemModel (TEM). The results of the model intercomparison suggest that CO 2 fertilization of natural terrestrial vegetation has the potential to account for a large fraction of the so-called ‘‘missing carbon sink’′ of 2.0 Pg C in 1990. Estimates of this potential are reduced when the models incorporate the concept that CO 2 fertilization can be limited by nutrient availability. Although the model estimates differ on the potential size (126 to 461 Pg C) of the future terrestrial sink caused by CO 2 fertilization, the results of the four models suggest that natural terrestrial ecosystems will have a limited capacity to act as a sink of atmospheric CO 2 in the future as a result of physiological constraints and nutrient constraints on NPP. All the spatially explicit models estimate a carbon sink in both tropical and northern temperate regions, but the strength of these sinks varies over time. Differences in the simulated response of terrestrial ecosystems to CO 2 fertilization among the models in this intercomparison study reflect the fact that the models have highlighted different aspects of the effect of CO 2 fertilization on carbon dynamics of natural terrestrial ecosystems including feedback mechanisms. As interactions with nitrogen fertilization, climate change and forest regrowth may play an important role in simulating the response of terrestrial ecosystems to CO 2 fertilization, these factors should be included in future analyses. Improvements in spatially explicit data sets, whole-ecosystem experiments and the availability of net carbon exchange measurements across the globe will also help to improve future evaluations of the role of CO 2 fertilization on terrestrial carbon storage. DOI: 10.1034/j.1600-0889.1999.00017.x

Seasonal and inter-annual variation of CO2 flux between a temperate forest and the atmosphere in Japan

Abstract: The objective of this research is to elucidate the seasonal and inter-annual variations of CO 2 exchanges between the atmosphere and a temperate deciduous forest in Japan and to elucidate their relation to meteorological conditions. The uptake rates of CO 2 from October 1993 to December 1996 were estimated from field measurements of CO 2 concentrations and meteorological conditions using a tower. Net of uptake rate of CO 2 was positive (uptake by forest ecosystems) from June to September and negative (release to the air) from October to April. Averages of integrated uptake rates of CO 2 were 840, − 450 and 390 gCO 2 /m 2 /year (2.3, − 1.2 and 1.1 tC/ha/year) for daytime, night and whole day (net), but they had notable inter-annual variation due to the differences of averaged insolation and temperature each summer of 1994 to 1996. The errors of CO 2 flux due to topographical conditions were investigated through comparison with heat budgets. CO 2 uptake rate estimated by tower measurement might be underestimation of 40%, therefore, above net-uptake value, 1.1 tC/ha/year became 1.8. This value of uptake rate was smaller than the results obtained in other temperate deciduous forests. The causes of this are partially in the difference of the height of the site and the short active period of the present forest. According to the CO 2 flux measurements in several forests including the present one, the forest ecosystems could be a large sink of CO 2 , however, more data of the CO 2 flux is needed at the various forests and latitudes to reduce the uncertainty of estimation of CO 2 uptake on a global scale. DOI: 10.1034/j.1600-0889.1999.00020.x

Soil carbon storage in plantation forests and pastures: land-use change implications

Abstract: Afforestation may lead to an accumulation of carbon (C) in vegetation, but little is known about changes in soil C storage with establishment of plantation forests. Plantation forest carbon budget models often omit mineral soil C changes from stand-level C budget calculations, while including forest floor C accumulation, or predict continuous soil C increases over several rotations. We used national soil C databases to quantify differences in soil C content between pasture and exotic pine forest plantations dominated by P. radiata (D. Don), and paired site studies to quantify changes in soil C with conversion of pasture to plantation forest in New Zealand. Overall, mineral soil C to 0.10 m was 20–40% lower under pine for all soil types (p < 0.01) except soils with high clay activity (HCA), where there was no difference. Similar trends were observed in the 0.1–0.3 m layer. Moreover, mineral soil C to 0.1 m was 17–40% lower under pine than pasture in side-by-side comparisons. The only non-significant difference occurred at a site located on a HCA soil ( p = 0.08). When averaged across the site studies and the national databases, the difference in soil C between pasture and pine was about 16 t C ha −1 on non-HCA soils. This is similar to forest floor C averaged across our individual sites (about 20 t C ha −1 ). The decrease in mineral soil C could result in about a 15% reduction in the average C sequestration potential (112 t C ha −1 ) when pasture is converted to exotic plantation forest on non-HCA soils. The relative importance of this change in mineral soil C will likely vary depending on the productivity potential of a site and harvest impacts on the forest floor C pool. Our results emphasize that changes in soil C should be included in any calculations of C sequestration attributed to plantation forestry. DOI: 10.1034/j.1600-0889.1999.00015.x

The interannual variability of the global carbon cycle

Abstract: (1999). The interannual variability of the global carbon cycle. Tellus B: Chemical and Physical Meteorology: Vol. 51, No. 2, pp. 210-212.

CO emissions from degrading plant matter.

Abstract: CO emissions from degrading deciduous leaf and grass matter have been investigated in laboratory and field measurements. CO emissions are induced both photochemically and thermally. Photochemical CO production can be described by a 2nd-order polynomial in light intensity and exhibits a hysteresis effect, not previously reported. Humid material showed higher CO emissions than dry material. A preliminary, relative action spectrum for the photochemically induced CO emissions is presented. Although UV irradiation caused most of the CO production, visible light also caused up to 40% of the emissions. We propose a cleavage of the cellulose chain as the important step prior to CO production. Thermal CO emissions from degrading plant material obey an Arrhenius type equation (presented for several species in this paper), but emissions are lower than those induced photochemically. During our field measurements on dry grasses in a South African savanna we found a strong influence of incident radiation intensity and temperature on measured CO fluxes. Solely photochemical CO production from the grasses is calculated by subtraction of soil fluxes and thermally induced grass CO emissions from the total CO emissions. CO emissions and hysteresis differ between the grasses investigated and may be interpreted by the grass' colour and their architecture. Deposition of CO on the soils was much lower than CO emission from the dry grasses during daytime. Nighttime data show that possible thermal CO production from the grasses may partially compensate for CO deposition on the soils for several hours after sunset depending on temperature. DOI: 10.1034/j.1600-0889.1999.t01-4-00003.x

The macroturbulence of the troposphere

Abstract: Eddy length scales, eddy velocity scales, and the amplitude of eddy fluxes in the mid-latitude troposphere are discussed, primarily from the qualitative perspective provided by studies of quasi-geostrophic turbulence. The utility of a diffusive picture for the near surface poleward flux of heat is emphasized, as is the extent to which a full closure theory for the troposphere, including the interior potential vorticity fluxes, must revolve around this theory for the heat flux. A central problem in general circulation theory is then to determine which factors control the horizontal diffusivity near the surface. The baroclinic eddy production problem has distinctive features that make it stand out from other inhomogeneous turbulence problems such as Benard convection and laboratory shear flows, the crucial point being that there can be scale separation between the eddies and the scale of the mean flow inhomogeneity in the direction of the relevant transport. This scale separation makes diffusive closures more compelling. In addition, it allows one to compute diffusivities from models of homogeneous turbulence. DOI: 10.1034/j.1600-0889.1999.00006.x

Three‐dimensional transport and concentration of SF6

Abstract: Sulfur hexafluoride (SF 6 ) is an excellent tracer of large-scale atmospheric transport, because it has slowly increasing sources mostly confined to northern midlatitudes, and has a lifetime of thousands of years. We have simulated the emissions, transport, and concentration of SF 6 for a 5-year period, and compared the results with atmospheric observations. In addition, we have performed an intercomparison of interhemispheric transport among 11 models to investigate the reasons for the differences among the simulations. Most of the models are reasonably successful at simulating the observed meridional gradient of SF 6 in the remote marine boundary layer, though there is less agreement at continental sites. Models that compare well to observations in the remote marine boundary layer tend to systematically overestimate SF 6 at continental locations in source regions, suggesting that vertical trapping rather than meridional transport may be a dominant control on the simulated meridional gradient. The vertical structure of simulated SF 6 in the models supports this interpretation. Some of the models perform quite well in terms of the simulated seasonal cycle at remote locations, while others do not. Interhemispheric exchange time varies by a factor of 2 when estimated from 1-dimensional meridional profiles at the surface, as has been done for observations. The agreement among models is better when the global surface mean mole fraction is used, and better still when the full 3-dimensional mean mixing ratio is used. The ranking of the interhemispheric exchange time among the models is not sensitive to the change from station values to surface means, but is very sensitive to the change from surface means to the full 3-dimensional tracer fields. This strengthens the argument that vertical redistribution dominates over interhemispheric transport in determining the meridional gradient at the surface. Vertically integrated meridional transport in the models is divided roughly equally into transport by the mean motion, the standing eddies, and the transient eddies. The vertically integrated mass flux is a good index of the degree to which resolved advection vs. parameterized diffusion accomplishes the meridional transport of SF 6 . Observational programs could provide a much better constraint on simulated chemical tracer transport if they included regular sampling of vertical profiles of nonreactive trace gases over source regions and meridional profiles in the middle to upper troposphere. Further analysis of the SF 6 simulations will focus on the subgrid-scale parameterized transports. DOI: 10.1034/j.1600-0889.1999.00012.x

Gravity waves in the mesosphere generated by tropospheric convention

Abstract: The observed cold temperatures in the summer mesosphere are dynamically maintained primarily through upwelling induced in response to the action of a zonal drag force caused by the breaking of upward propagating gravity waves. Tropospheric convective storms are believed to be important sources of gravity waves in the summer mesosphere, but little is known about the characteristics of mesospheric gravity waves generated by convection. As a first attempt to model such waves a nonhydrostatic cloud-resolving numerical model is used to simulate a 2-dimensional squall line in a domain of width 2048 km and depth 90 km. The simulation produces a broad spectrum of convectively generated gravity waves. These propagate into the middle atmosphere, forming a fan-like pattern of waves with amplitudes increasing with height, and eventually reach breaking amplitudes in the mesosphere. The resultant mesospheric wave-breaking produces strong zonal forcing, which is eastward to the east of the storm center and westward to the west of the storm center. Breaking of upward propagating waves also generates high frequency downward propagating secondary waves of short horizontal wavelength, and long vertical wavelength. The secondary waves have only a small influence on the net vertical transfer of momentum, but produce a strong signature in perturbation vertical velocity, featuring alternating positive and negative interference with the primary upward propagating modes. DOI: 10.1034/j.1600-0889.1999.00005.x

Long‐term variability in the global carbon cycle inferred from a high‐precision CO2 and δ13C ice‐core record

Abstract: The new high precision Law Dome ice core record of CO 2 and δ 13 CO 2 is used with a 1-D global carbon cycle model to investigate natural variability in the carbon cycle and the anthropogenic CO 2 perturbation, focusing on variations on time-scales of centuries. A major feature of the ice core record is the decrease in CO 2 , and increase in δ 13 C, through the ‘‘Little Ice Age’′ period (roughly 1550–1800). We show that this observed decrease in CO 2 is consistent with the effect of decreased temperature on either terrestrial or oceanic exchange, however the increase in δ 13 C favors a terrestrial response to cooling. We perform single deconvolution model calculations which generally give good agreement with observed variations in CO 2 , δ 13 C and Δ 14 C data for different reservoirs and due to both natural and anthropogenic causes. The fit to prebomb Δ 14 C is improved by using an ice core 10 Be record to represent the natural production of 14 C due to cosmic rays, however, the uncertainties in interpreting the 10 Be are as yet too large to use prebomb Δ 14 C to better constrain the model parameters. DOI: 10.1034/j.1600-0889.1999.t01-1-00009.x

Human impact on the atmospheric sulfur balance

Abstract: The paper reviews the development in our understanding of the atmospheric part of the global sulfur cycle, including the role played by C.-G. Rossby and his colleagues in the 1950s, and presents a brief assessment of the current knowledge. Measurements of the concentrations of sulfur compounds in air, precipitation, ice cores and sea water during the past 25 years, together with recent development in three-dimensional tracer transport modeling, have resulted in a reasonably consistent picture of the burdens and fluxes of the main sulfur compounds in the atmosphere. It is clear that man's activities, in particular the burning of fossil fuels, are having a large impact on the atmospheric sulfur balance. Even on a global scale, the man-made emissions of gaseous sulfur compounds are likely to be two to three times as large as the natural sources. In and around the most heavily industrialized regions this ratio exceeds ten over extended areas. Nevertheless, there are several important issues that need to be resolved. Some of these are directly linked to the urgent problem of reducing the uncertainty in the estimate of direct and, in particular, indirect climate forcing due to man-made sulfate aerosols. One such issue is the magnitude of the wet scavenging of SO 2 and aerosol sulfate during upward transport into and within the free troposphere in connection with convective and frontal cloud systems which has a decisive influence on the sulfate concentrations in the upper troposphere. Another uncertain process is the rate of oxidation of SO 2 in cloud droplets and on aerosol particles. A fundamental question that remains to be answered is to what degree man-made sulfur emissions have increased the number of aerosol particles that can act as cloud condensation nuclei. DOI: 10.1034/j.1600-0889.1999.00009.x

Anthropogenic versus natural sources of atmospheric sulphate from an Alpine ice core

Abstract: Opposite to greenhouse gases, sulphate aerosol particles are expected to cause climate cooling, but uncertainties exist about source variability and strength. We analysed an ice core from a European glacier to quantify source strengths of aerosol-borne sulphate over a 200-year period. Sulphate from emissions of SO 2 increased by more than an order of magnitude during this century. This anthropogenic source is responsible for about 80% of total sulphate in the industrial period, and reflects emissions of west European countries. In the pre-industrial period mineral dust was the dominant contributor, followed by sulphate from SO 2 emissions with volcanoes or biomass burning as possible sources. DOI: 10.1034/j.1600-0889.1999.t01-4-00006.x

Can a strong atmospheric CO 2 rectifier effect be reconciled with a “reasonable” carbon budget?

Abstract: Atmospheric CO 2 accumulates near the Earth's surface because of relatively deeper vertical mixing when photosynthesis is active than when it is not. Some models simulate an excess of more than 2.5 ppmv CO 2 in the remote Northern Hemisphere due to this ‘‘rectification’′ of an annually balanced terrestrial carbon cycle. The covariance between CO 2 flux and vertical mixing, and the resulting vertical structure of CO 2 are generally consistent with field data at local scales, but it is difficult to reconcile such a strong rectifier signal with current ideas about the global carbon budget. A rectifier effect of 2.5 ppmv at northern flask sampling stations implies an unreasonably strong northern sink of atmospheric CO 2 , and a corresponding source in the tropics or Southern Hemisphere. DOI: 10.1034/j.1600-0889.1999.t01-1-00010.x

Spatial and temporal variation in the carbon and oxygen stable isotope ratio of respired CO2 in a boreal forest ecosystem

Abstract: We measured the stable isotope ratio of respired carbon dioxide at two spatial scales in a black spruce forest in northern Canada: CO 2 released from the forest floor and CO 2 released from the entire ecosystem at night. Despite wide variation in the δ 13 C values of organic matter among above-ground plant species, and along a continuum from moss through to the mineral soil, the carbon isotope ratio of respired CO 2 was quite similar to the δ 13 C value for the dominant black spruce foliage. The CO 2 released from the forest floor during the fall was slightly enriched in 13 O compared to CO 2 respired by the entire ecosystem, perhaps because soil respiration contributes a larger percentage to total ecosystem respiration later in the year as the soil warms. Short-term changes in the oxygen isotope ratio of precipitation and variation in enrichment of 18 O during evaporation and transpiration had significant effects on the δ 18 O value of respired CO 2 . Changes in the oxygen isotope ratio of water in moss tissue can have a large effect on total ecosystem respired CO 2 both because a large surface area is covered by moss tissue in this ecosystem and because the equilibration between CO 2 diffusing through the moss and water in moss tissue is very rapid. During the summer we observed that the δ 18 O value of CO 2 respired from the forest floor was relatively depleted in 18 O compared to CO 2 respired from the entire ecosystem. This was because water in black spruce foliage had higher δ 18 O values than moss and soil water, even at night when transpiration had stopped. DOI: 10.1034/j.1600-0889.1999.00018.x

Comparison of methods to determine the anthropogenic CO2 invasion into the Atlantic Ocean

Abstract: A comparison of different methods for estimating the anthropogenic CO 2 burden in the Atlantic Ocean is performed using referenced, high quality total dissolved inorganic carbon (DIC) data. The dataset is from two cruises through the center of the basin between 62°N and 43°S in 1991 and 1993. The specific anthropogenic input is determined utilizing empirical procedures as described in Gruber et al. (1996) and Chen and Millero (1979) to correct for remineralization and to estimate preanthropogenic endmembers. These estimates are compared with output of the Princeton ocean biogeochemical model and the NCAR ocean model. The results show that the specific inventories of anthropogenic carbon agree to within 20% but with different storage and uptake patterns. The empirical estimates differ because of assumptions about mixing and winter outcrop endmembers. The same remineralization quotients (Redfield ratios) were used for each method. Varying these constants within the range of literature values causes changes in specific inventories of similar magnitude as the differences observed with different methodologies. Comparison of anthropogenic CO 2 uptake and chlorofluorocarbon ages suggests that the anthropogenic CO 2 penetration in the North Atlantic is too shallow following the procedure according to Gruber et al. (1996), and too deep using those of Chen and Millero (1979). The results support these previous observations in that the uptake of CO 2 in the North Atlantic is disproportionate to its surface area. This is caused by a combination of deep water formation and deep winter mixed layers. DOI: 10.1034/j.1600-0889.1999.00027.x

A 3‐dimensional study of δ18O in atmospheric CO2: contribution of different land ecosystems

Abstract: Land biospheric carbon exchange associated with respiration and photosynthesis exerts a major control on the oxygen isotope composition (δ 18 O) of atmospheric CO 2 especially with respect to the seasonal cycle. In particular, an important feature that requires our attention is the phase of the seasonal cycle of δ 18 O which lags CO 2 by one month in the Arctic. We have developed a global parameterization of the land biotic exchange of 180 in CO 2 , which has been prescribed in an atmospheric 3-D transport model in order to simulate the global atmospheric distribution of δ 18 O. Furthermore, we have separated in the model the specific contribution of different regions of the globe to the seasonal and latitudinal variation of δ 18 O. The model simulated values are compared in detail with atmospheric observations made at 22 different remote stations. The respective role of respiration vs. photosynthesis in determining the phase and amplitude of the δ 18 O seasonal cycle is also analysed. Based on a good agreement between our model simulation and the atmospheric observations, we observe that the large seasonal cycle of δ 18 O at high latitudes is mainly due to the respiratory fluxes of all extra-tropical ecosystems while for CO 2 the relative contributions of photosynthesis and respiration to the overall seasonal cycle are similar. Geographically, the CO 2 exchanges with the northern Siberian ecosystem dominate the δ 18 O seasonality at all remote stations of the northern hemisphere, reflecting the strongly continental climate of that region. OI: 10.1034/j.1600-0889.1999.t01-2-00006.x

Satellite sea surface temperature: a powerful tool for interpreting in situ pCO2 measurements in the equatorial Pacific Ocean

Abstract: In order to determine the seasonal and interannual variability of the CO 2 released to the atmosphere from the equatorial Pacific, we have developed p CO 2 -temperature relationships based upon shipboard oceanic CO 2 partial pressure measurements, p CO 2 , and satellite sea surface temperature, SST, measurements. We interpret the spatial variability in p CO 2 with the help of the SST imagery. In the eastern equatorial Pacific, at 5°S, p CO 2 variations of up to 100 μatm are caused by undulations in the southern boundary of the equatorial upwelled waters. These undulations appear to be periodic with a phase and a wavelength comparable to tropical instability waves, TIW, observed at the northern boundary of the equatorial upwelling. Once the p CO 2 signature of the TIW is removed from the Alize II cruise measurements in January 1991, the equatorial p CO 2 data exhibit a diel cycle of about 10 matm with maximum values occurring at night. In the western equatorial Pacific, the variability in p CO 2 is primarily governed by the displacement of the boundary between warm pool waters, where air–sea CO 2 fluxes are weak, and equatorial upwelled waters which release high CO 2 fluxes to the atmosphere. We detect this boundary using satellite SST maps. East of the warm pool, Δ P is related to SST and SST anomalies. The 1985–97 CO 2 flux is computed in a 5° wide latitudinal band as a combination of Δ P and CO 2 exchange coefficient, K , deduced from satellite wind speeds, U . It exhibits up to a factor 2 seasonal variation caused by K -seasonal variation and a large interannual variability, a factor 5 variation between 1987 and 1988. The interannual variability is primarily driven by displacements of the warm pool that makes the surface area of the outgassing region variable. The contribution of Δ P to the flux variability is about half the contribution of K . The mean CO 2 flux computed using either the Liss and Merlivat (1986) or the Wanninkhof (1992) K – U parametrization amounts to 0.11 GtC yr −1 or to 0.18 GtC yr −1 , respectively. The error in the integrated flux, without taking into account the uncertainty on the K – U parametrization, is less than 31%. DOI: 10.1034/j.1600-0889.1999.00025.x

Continuous measurements of CO and H2 deposition velocities onto an andisol: uptake control by soil moisture

Abstract: Carbon monoxide (CO) and hydrogen (H 2 ) net deposition velocities from the atmosphere onto soil and carbon dioxide (CO 2 ) effluxes to the atmosphere have been measured in an andisol field in Tsukuba, Japan by the open-flow chamber method. The deposition velocities of CO and H 2 were closely correlated ( R = 0.87), with a ratio of 1.55, which was attributed to the difference in molecular diffusivities. However, the deposition velocities did not exhibit a direct relationship with the CO 2 efflux. Deposition velocities of CO and H 2 ranged from 0.00 to 0.06 cm s −1 and from 0.00 to 0.10 cm s −1 , respectively, and were closely related to the level of the surface soil moisture (0–5 cm) and were higher in plowed plots than in compacted plots. CO deposition velocity was slightly lower in the daytime due to higher production rates affected by the soil temperature. These findings indicate that microbial CO and H 2 consumption was limited by transport resistance in the soil and that the in situ CO and H 2 uptake rates may be limited by a higher soil moisture level. CO and H 2 deposition was estimated to be restricted to the surface soil (possibly only the top 2–3 cm). CH 4 and CO 2 gas profiles were also related to the variation of the soil moisture level. DOI: 10.1034/j.1600-0889.1999.t01-2-00009.x

Carbon budget in the East China Sea in spring

Abstract: Results of total dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements made in the East China Sea (ECS) during a geochemical expedition of KEEP (Kuroshio Edge Exchange Processes) program in May of 1996 show that ECS is a CO 2 sink during the spring season. The mean diVerence of f CO 2 (fugacity of CO 2 ) between the atmosphere and surface water is calculated to be 28 matm, and the resulting net CO 2 invasion flux is 2.1 mol m −2 yr −1 , which gives about 0.03 GtC/ yr of CO 2 uptake in this continental shelf in spring. This study supports the notion that the shelf regions can be a significant CO 2 sink. The riverine alkalinity, which discharges into ECS, is estimated to be 1,743 μmol/kg on the basis of a linear relationship between TA and salinity. The observed salinity-normalized alkalinity in ECS is higher than that in the open sea, and this excess alkalinity is estimated to be 42 μmol/kg −1 . With the known rate of the Changjiang discharge, this excess TA gives a mean residence time of 1.2 years for the continental shelf water in the ECS. The DIC in the ECS is also found to be higher than that in the open sea. This excess DIC is estimated to be about 76 ± 70 μmol/kg −1 , which is equal to a net carbon input to ECS of 3.9 ± 3.6 mol m −2 yr −1 . Based on the riverine alkalinity input, the equivalent riverine carbon flux from Changjiang discharge is estimated to be about 1.8 mol m −2 yr −1 . With net CO 2 invasion flux of 2.1 ± 2.8 mol m −2 yr −1 , the remaining 0 ± 4.6 mol m −2 yr −1 could come from remineralization of organic matter derived from biological pump in the shelf or terrestrial sources. Although this preliminary carbon budget implies that gas exchange and riverine input are the main sources of excess carbon in ECS, the contribution of biological carbon flux can not be ruled out because of the large uncertainty associated with these estimates. DOI: 10.1034/j.1600-0889.1999.00028.x