摘要:

In the historical record of solar activity the period from 1645 to 1715 is a singular epoch during which the number of sunspots decreased markedly for a generation. Known as the Maunder Minimum, this solar epoch coincided with the coldest part of the Little Ice Age (circa 1450 to 1850). We estimate the change at this time in the output of solar ultraviolet (UV) radiation at wavelengths from 120 to 300 nm, relative to contemporary observations. Since this portion of the solar UV spectrum determines ozone composition in the stratosphere, our results bear on the historical variability of ozone and its potential climatic effects. Between the Maunder Minimum and 1986 (the present day solar activity minimum between cycles 21 and 22) we estimate reductions of 64% in the irradiance of the Lyman α line of neutral hydrogen (at 121.6 nm), 8% at 200 nm, and 3.5% in the wavelength range from 210 to 250 nm. The reduction in the solar output from the entire spectral band between 120 and 300 nm is estimated to be 0.17 W/m2, which is approximately 6% of the change in the total solar irradiance of 2.7 W/m2previously estimated by us (Lean et al., 1992a) over the same time span. Because of this diminished UV output due to very low solar activity the Maunder Minimum total ozone concentration may have been 4% below its 1980 level. While the climatic consequences of such a change have yet to be determined, recent work by Haigh (1994) on modulation of radiative climate forcing by stratospheric ozone emphasizes the need to understand the role of UV irradiance variability as one forcing mechanism

ISSN:0886-6236    

DOI:10.1029/95GB00159

年代:1995

数据来源: WILEY

摘要:

The quantification of methane (CH4) emissions from global rice paddies is still highly uncertain. The extrapolation of CH4release rates from point measurements to regional level or empirically correlating methane emission rates with a few factors (e.g., temperature and rice primary production) is unlikely to yield reliable results. Reducing the uncertainties in estimates of current CH4emission and predicting its future change require a process model to simulate CH4emissions from various paddyland environments. CH4emission is an ecosystem process closely coupled to plant growth and soil organic matter decomposition. In this study a methane emission model was developed based on supplies of carbon substrate for methanogens by rice primary production and soil organic matter degradation, direct environmental controls on methanogenesis, and the balance between CH4production and consumption by methanotrophic oxidation. A validation of the model indicated its reasonable ability to calculate CH4emission rates and their seasonal variations. The model, when coupled with supporting data sets, would complement spatial and dynamic analysis of CH4emissions from rice paddies in the framework of climate‐plant‐soil interacti

ISSN:0886-6236    

DOI:10.1029/94GB03231

年代:1995

数据来源: WILEY

摘要:

A study (August 1990 to July 1991) of profiles of dissolved CH4concentrations, diffusive flux of CH4, and CH4production rates of 45 sites in the Lake Agassiz Peatlands in northern Minnesota shows that dissolved CH4deep in the peat (>1 m depth) mobilized easily to the vadose zone. During August 1990 the dissolved CH4concentrations at some depths at some sites were supersaturated with respect to one atmosphere partial pressure of CH4. At one site (2.5 m depth) the concentration of dissolved CH4in the peat pore‐water was 140 mg L−1. In July 1991, at no site did the concentration of dissolved CH4in the peat pore water exceed 40 mg L−1in the peat profile. The average calculated diffusive flux of CH4decreased from 95 to 45 mg m−2d−1between 1990 and 1991. Gaseous CH4was more in evidence in 1990 than in 1991. In 1990, CH4at many depths bubbled vigorously when peat pore water was sampled. At some sites there was sufficient pore pressure to eject slugs of water forcibly from piezometers. Similarly, dissolved inorganic carbon (DIC) consisting of H2CO3, CO2, HCO3−and CO32−decreased between the sampling times from an average for both bogs and fens in 1990 of 62 mg C L−1to 38 mg C L−1in 1991. A dynamic mechanism must exist which traps CH4deep in the peat column allowing gaseous CH4to build up, increasing dissolved CH4. Other times, CH4passes freely from deep peat to the vadose zone. We suggest as a hypothesis that a confining layer of trapped CH4bubbles forms at depth in the peat, trapping gaseous CH4. The duration of the “bubble confining layer” is uncertain. We propose two hypotheses. (1) The confining layer is usually present and deteriorates after a major climatic event such as a drought, or (2) the confining layer forms and collapses seasonally with seasonal variations in the

ISSN:0886-6236    

DOI:10.1029/95GB00634

年代:1995

数据来源: WILEY

摘要:

Phytomass (live plant mass) and net primary productivity are major components of the terrestrial carbon balance. A major location for phytomass storage is the subcontinent of Siberia, which is dominated by extensive reaches of taiga (boreal forest). The responsiveness of the phytomass component of the carbon pool is examined by comparing vegetation in the mid‐Holocene (4600–6000 years before present) to modern potential vegetation. The mid‐Holocene was warmer and moister in middle and northern Siberia than today, producing conditions ideal for boreal forest growth. As a result, both northern and middle taiga were dominated by shade‐tolerant dark‐needled species that thrive in moist climates. Today, shade‐tolerant dark‐needled taiga is restricted to western Siberia and the highlands of central Siberia, with its central and eastern components in the mid‐Holocene replaced today by light‐demanding light‐needled species with lower productivity and phytomass. Total phytomass in Siberia in the mid‐Holocene was 105.0 ± 3.1 Pg, compared to 85.9 ± 3.2 Pg in modern times, a loss of 19.1 ± 3.1 Pg of phytomass. The reduction in dark‐needled northern and middle taiga classes results in a loss of 28.8 Pg, while the expansion of the corresponding light‐needled taiga results in a gain of 13.5 Pg, a net loss of 15.3 Pg. The loss is actually greater, because the modern figures are for potential vegetation and not adjusted for agriculture and other anthropogenic disturbances. Given long periods for vegetation to approach equilibrium with climate, the phytomass component of the carbon pool is responsive to climate change. Changes in net primary productivity (NPP) for Siberia between the mid‐Holocene and the present were not as large as changes in phytomass. A minor decrease in NPP (0.6 Pg yr−1, 10%) has occurred under our cooler modern climate, primarily due to the shift from dark‐needled taiga in the mid‐H

ISSN:0886-6236    

DOI:10.1029/95GB00596

年代:1995

数据来源: WILEY

摘要:

Ring width and density measurements from the same trees can produce distinctly different climatic information. Ring width variations and recorded data in central and northern Alaska indicate annual temperatures increased over the past century, peaked in the 1940s, and are still near the highest level for the past 3 centuries. Density variations indicate summer temperatures are now warm but not above some previous levels occurring prior to this century. The early cooler period, corresponding to the Little Ice Age, was interrupted by brief warm intervals. The recent increase in temperatures combined with drier years may be changing the tree response to climate and raising the potential for some forest changes in Alaskan and other boreal forests.

ISSN:0886-6236    

DOI:10.1029/95GB00321

年代:1995

数据来源: WILEY

摘要:

In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90–130 Tg N yr−1(Tg = 1012g) on the continents. Human activities have resulted in the fixation of an additional ≈140 Tg N yr−1by energy production (≈20 Tg N yr−1), fertilizer production (≈80 Tg N yr−1), and cultivation of crops (e.g., legumes, rice) (≈40 Tg N yr−1). We can only account for part of this anthropogenic N. N2O is accumulating in the atmosphere at a rate of 3 Tg N yr−1. Coastal oceans receive another 41 Tg N yr−1via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr−1by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO3−, N2). The remaining 80 Tg N yr−1are either retained on continents in groundwater, soils, or vegetation or denitrified to N2. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N‐fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil‐fuel combustion. About two‐thirds of the increase will occur in Asia, which by 2020 will account for over half of

ISSN:0886-6236    

DOI:10.1029/95GB00158

年代:1995

数据来源: WILEY

摘要:

Understanding the role the oceans play in sequestering anthropogenic CO2is crucial to understanding global climate change. Correct parameterization of air‐sea flux of CO2is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO2solubility (Robertson and Watson, 1992). We re‐evaluate the effect of the cool thermal skin and present a high‐resolution seasonal estimate of its effect on the air‐sea flux of CO2. We compare air‐sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO2(Tans et al., 1990), we estimate that excluding the southern ocean, the surface skin correction increases the air‐sea flux of carbon by 0.48 Gt yr‐1. This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Gt yr−1. Conservatively, adding a 10% contribution from the southern ocean, we estimate a mean global increase in CO2flux due to the skin effect of 0.39 Gt C yr−1. This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO2flux anomaly of only 0.17 Gt C yr−1which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson and Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large‐scale flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (‐0.75°C) are found during the northern hemisphere winter in the regions of the Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient

ISSN:0886-6236    

DOI:10.1029/94GB03356

年代:1995

数据来源: WILEY

摘要:

An improved method has been developed for the separation of the natural and bomb components of the radiocarbon in the ocean. The improvement involves the use of a very strong correlation between natural radiocarbon and dissolved silica. This method is applied to radiocarbon measurements made on samples collected during the Geochemical Ocean Sections Study (GEOSECS), Transient Tracers in the Ocean (TTO) and South Atlantic Ventilation Experiment (SAVE) expeditions. On the basis of this new separation we provide not only an estimate of the global inventory of bomb14C at the time of the GEOSECS survey but also the distribution of bomb radiocarbon along four thermocline isopycnals in each ocean. We also document the evolution of the bomb14C inventory and penetration along thermocline isopycnals in the North Atlantic Ocean between the times of the GEOSECS (1972–1973) and TTO (1980–1982) surveys and in the South Atlantic Ocean between the times of the GEOSECS (1973) and SAVE (1987–1989) surveys. In addition, we show that the bomb tritium to bomb14C ratio (expressed in the tritium unit (TU) 81 units/100‰) for waters entering the thermocline of the northern hemisphere is about 9 times higher than for those entering the southern hemisphere thermocline. This contrast offers long‐term potential as an indicator of inter‐hemispheric transport of upper o

ISSN:0886-6236    

DOI:10.1029/95GB00208

年代:1995

数据来源: WILEY

摘要:

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio‐Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio‐Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio‐Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio‐Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio‐Ba fluxes in the deep sea. These data confirm Dymond et al.'s findings that settling particles have significantly higher Corg/bio‐Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio‐Ba to the seafloor. Instead, we propose that high Corg/bio‐Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio‐Ba ratios in settling particles could be used to estimate the input of continental or shelf‐derived refractory organic matte

ISSN:0886-6236    

DOI:10.1029/95GB00021

年代:1995

数据来源: WILEY

ISSN:0886-6236    

DOI:10.1029/95GB01108

年代:1995

数据来源: WILEY