Glacial termination: sensitivity to orbital and CO2 forcing in a coupled climate system model

 To study glacial termination and related feedback mechanisms, a continental ice dynamics model is globally and asynchronously coupled to a physical climate (atmosphere-ocean-sea ice) model. The model performs well under present-day, 11 kaBP (thousand years before present) and 21 kaBP perpetual forcing. To address the ice-sheet response under the effects of both perpetual orbital and CO₂ forcing, sensitivity experiments are conducted with two different orbital configurations (11 kaBP and 21 kaBP) and two different atmospheric CO₂ concentrations (200 ppmv and 280 ppmv). This study reveals that, although both orbital and CO₂ forcing have an impact on ice-sheet maintenance and deglacial processes, and although neither acting alone is sufficient to lead to complete deglaciation, orbital forcing seems to be more important. The CO₂ forcing has a large impact on climate, not uniformly or zonally over the globe, but concentrated over the continents adjacent to the North Atlantic. The effect of increased CO₂ (from 200 ppmv to 280 ppmv) on surface air temperature has its peak there in winter associated with a reduction in sea-ice extent in the northern North Atlantic. These changes are accompanied by an enhancement in the intensity of the meridional overturning and poleward ocean heat transport in the North Atlantic. On the other hand, the effect of orbital forcing (from 21 kaBP to 11 kaBP) has its peak in summer. Since the summer temperature, rather than winter temperature, is found to be dominant for the ice-sheet mass balance, orbital forcing has a larger effect than CO₂ forcing in deglaciation. Warm winter sea surface temperature arising from increased CO₂ during the deglaciation contributes to ice-sheet nourishment (negative feedback for ice-sheet retreat) through slightly enhanced precipitation. However, the precipitation effect is totally overwhelmed by the temperature effect. Our results suggest that the last deglaciation was initiated through increasing summer insolation with CO₂ providing a powerful feedback. Received: 22 February 2000 / Accepted: 17 September 2000     

World-wide increase in tropospheric methane, 1978–1983

Tropospheric concentrations of methane in remote locations have averaged a yearly world-wide increase of 0.018±0.002 parts per million by volume (ppmv) during the period from January 1978 to December 1983. The concentrations in the north temperate zone are always greater than those in the south temperate zone by 7±1% because the major methane sources are all predominantly located in the northern hemisphere. The average world-wide tropospheric concentration of methane in dry air was 1.625 ppmv at the end of 1983, measured against an NBS standard certified as 0.97 ppmv (but with an accuracy of only ±1%). The world-wide concentration increases are described by a linear equation with a standard deviation of 0.003 ppmv for ten different collection periods during 1978–1983. The precision of measurement of the methane concentration in the atmospheric samples and in the standard was measured to be ±0.4% for each. Repetitive measurements of an air sample collected in November 1977 have shown the same concentration for six years with a standard deviation for these data of ±0.003 ppmv.The causes for the steady increase in methane concentration in the troposphere cannot be fixed with certainty from present data. Contributing causes can include increases in the source strengths from cattle and rice fields. The atmospheric concentrations of CO, CH₄ and HO are all closely coupled with one another, and increased concentrations of CO and/or CH₄ should cause reduced concentrations of HO, which in turn should lengthen the atmospheric lifetimes of CO and CH₄.Among other physical and chemical effects, a increase of 0.18 ppmv per decade should contribute a greenhouse warming of about 0.04°C per decade. Other secondary contributions to the greenhouse effect from increases in CH₄ may arise from methane-induced increases in stratospheric H₂O, in tropospheric O₃, and in numerous other trace species whose concentration is controlled by reaction with HO radicals.An increased CH₄ source strength may result from the effect of increasing atmospheric temperatures on the known aqueous biological CH₄ sources, such as swamps, and may be an added consequence of the greenhouse effect.     

Aircraft measurements and model simulations of stratospheric ozone and N2O: implications for chemistry and transport processes in the models

Airborne measurements of stratospheric ozone and N₂O from the SCIAMACHY (Scanning Imaging Absorption Spectrometer) Validation and Utilization Experiment (SCIA-VALUE) are presented. The campaign was conducted in September 2002 and February–March 2003. The Airborne Submillimeter Radiometer (ASUR) observed stratospheric constituents like O₃ and N₂O, among others, spanning a latitude from 5°S to 80°N during the survey. The tropical ozone source regions show high ozone volume mixing ratios (VMRs) of around 11 ppmv at 33 km altitude, and the altitude of the maximum VMR increases from the tropics to the Arctic. The N₂O VMRs show the largest value of 325 ppbv in the lower stratosphere, indicating their tropospheric origin, and they decrease with increasing altitude and latitude due to photolysis. The sub-tropical and polar mixing barriers are well represented in the N₂O measurements. The most striking seasonal difference found in the measurements is the large polar descent in February–March. The observed features are interpreted with the help of SLIMCAT and Bremen Chemical Transport Model (CTMB) simulations. The SLIMCAT simulations are in good agreement with the measured O₃ and N₂O values, where the differences are within 1 ppmv for O₃ and 15 ppbv for N₂O. However, the CTMB simulations underestimate the tropical middle stratospheric O₃ (1–1.5 ppmv) and the tropical lower stratospheric N₂O (15–30 ppbv) measurements. A detailed analysis with various measurements and model simulations suggests that the biases in the CTMB simulations are related to its parameterised chemistry schemes.     

Airborne measurements of savanna fire emissions and the regional distributio of pyrogenic pollutants over Western Africa

We measured CO₂, CO, CH₄, H₂, and NO₂ in air masses polluted by savanna fires over Côte d'Ivoire, western Africa. Elevated concentrations of these trace gases were found in fire plumes and also in extensive haze layers. Trace gas mixing ratios ranged as high as 605 ppmv for CO₂, 14.8 ppmv for CO, 2.7 ppmv for CH₄, 4.2 ppmv for H₂, and 25 ppbv for NO₂. We compare our emission ratios to those obtained in previous field and laboratory studies. The emission ratios, expressed as an average and as a range or as an average only, were: dCO/dCO₂ 5.3×10^–2 (3–18×10^–2); dCH₄/dCO 5.3×10^–2; dH₂/dCO 2.4×10^–1 and dNO₂/dCO₂ 1.8×10^–4 (1.5–2.2×10^–4). The values found match those found during similar measurements, though our results point to rather vigorous burning in the savanna of western Africa.     

Dangerous anthropogenic interference, dangerous climatic change, and harmful climatic change: non-trivial distinctions with significant policy implications

Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC) calls for stabilization of greenhouse gas (GHG) concentrations at levels that prevent dangerous anthropogenic interference (DAI) in the climate system. However, some of the recent policy literature has focused on dangerous climatic change (DCC) rather than on DAI. DAI is a set of increases in GHGs concentrations that has a non-negligible possibility of provoking changes in climate that in turn have a non-negligible possibility of causing unacceptable harm, including harm to one or more of ecosystems, food production systems, and sustainable socio-economic systems, whereas DCC is a change of climate that has actually occurred or is assumed to occur and that has a non-negligible possibility of causing unacceptable harm. If the goal of climate policy is to prevent DAI, then the determination of allowable GHG concentrations requires three inputs: the probability distribution function (pdf) for climate sensitivity, the pdf for the temperature change at which significant harm occurs, and the allowed probability (“risk”) of incurring harm previously deemed to be unacceptable. If the goal of climate policy is to prevent DCC, then one must know what the correct climate sensitivity is (along with the harm pdf and risk tolerance) in order to determine allowable GHG concentrations. DAI from elevated atmospheric CO₂ also arises through its impact on ocean chemistry as the ocean absorbs CO₂. The primary chemical impact is a reduction in the degree of supersaturation of ocean water with respect to calcium carbonate, the structural building material for coral and for calcareous phytoplankton at the base of the marine food chain. Here, the probability of significant harm (in particular, impacts violating the subsidiary conditions in Article 2 of the UNFCCC) is computed as a function of the ratio of total GHG radiative forcing to the radiative forcing for a CO₂ doubling, using two alternative pdfs for climate sensitivity and three alternative pdfs for the harm temperature threshold. The allowable radiative forcing ratio depends on the probability of significant harm that is tolerated, and can be translated into allowable CO₂ concentrations given some assumption concerning the future change in total non-CO₂ GHG radiative forcing. If future non-CO₂ GHG forcing is reduced to half of the present non-CO₂ GHG forcing, then the allowable CO₂ concentration is 290–430 ppmv for a 10% risk tolerance (depending on the chosen pdfs) and 300–500 ppmv for a 25% risk tolerance (assuming a pre-industrial CO₂ concentration of 280 ppmv). For future non-CO₂ GHG forcing frozen at the present value, and for a 10% risk threshold, the allowable CO₂ concentration is 257–384 ppmv. The implications of these results are that (1) emissions of GHGs need to be reduced as quickly as possible, not in order to comply with the UNFCCC, but in order to minimize the extent and duration of non-compliance; (2) we do not have the luxury of trading off reductions in emissions of non-CO₂ GHGs against smaller reductions in CO₂ emissions, and (3) preparations should begin soon for the creation of negative CO₂ emissions through the sequestration of biomass carbon.     

Simulated changes in vegetation distribution, land carbon storage, and atmospheric CO2 in response to a collapse of the North Atlantic thermohaline circulation

It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed. Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ¹³C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude on the CO₂ fertilisation feedback mechanism. The CO₂ changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows multi-millennial CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North.     

Effects of Climate Change on Carbon Storage in Boreal Forests of China: A Local Perspective

The BIOME3 model was used to simulate the distribution patterns and carbon storage of the horizontal, zonal boreal forests in northeast and northwest China using a mapping system for vegetation patterns combined with carbon density estimates from vegetation and soils. The BIOME3 prediction is in reasonable good agreement with the potential distribution of Chinese boreal forests. The effects of changing atmospheric CO₂ concentration had a nonlinear effect on boreal forest distribution, with 3.5–10.8% reduced areas for both increasing and decreasing CO₂. In contrast, the increased climate together with and without changing CO₂ concentration showed dramatic changes in geographic patterns, with 70% reduction in area and disappearance of almost boreal forests in northeast China. The baseline carbon storage in boreal forests of China is 4.60 PgC (median estimate) based on the vegetation area of actual boreal forest distribution. If taking the large area of agricultural crops into account, the median value of potential carbon storage is 6.92 PgC. The increasing (340–500 ppmv) and decreasing CO₂ concentration (340–200 ppmv) led to decrease of carbon storage, 0.33 PgC and 1.01 PgC respectively compared to BIOME3 potential prediction under present climate and CO₂ conditions. Both climate change alone and climate change with CO₂ enrichment (340–500 ppmv) reduced largely the carbon stored in vegetation and soils by ca. 6.5 PgC. The effect of climate change is more significant than the direct physiological effect of CO₂ concentration on the boreal forests of China, showing a large reduction in both distribution area and carbon storage.     

Probabilistic simulations of the impact of increasing leaf-level atmospheric carbon dioxide on the global land surface

Using a climate model with a sophisticated land surface scheme, simulations were conducted to explore the impact of increases in leaf-level carbon dioxide (CO₂) on evaporation, temperature and other land surface quantities. Fifty-one realizations were run, for each of four Januarys and four Julys for CO₂ concentrations at leaf-level of 280, 375, 500, 650, 840 and 1,000 ppmv. Atmospheric CO₂ concentration was held constant at 375 ppmv in all experiments. Statistically significant decreases in evaporation and increases in temperature occur in specific regions as leaf-level CO₂ is increased from 280 to 375 ppmv. These same areas expand geographically, and the magnitude of the changes increase as leaf-level CO₂ is increased further suggesting that changes are caused by the increase in leaf-level CO₂ and are not internal model variability. As leaf-level CO₂ is increased further, larger areas of the continental surface are affected by increasing amounts and a statistically significant change in precipitation is seen. The increase in leaf-level CO₂ from 280 ppmv to 375 ppmv causes statistically significant changes in the evaporation over 12% of continental surfaces in July. This increases to 25% at 500 ppmv, 35% at 650 ppmv, 41% at 840 ppmv and 47% at 1,000 ppmv. This affects temperature and rainfall by similar amounts, generally in coincident regions. An analysis of these results over key regions shows that the probability density functions of the latent heat flux and temperature are affected non-uniformly. There is a shift in the latent heat flux probability density function to lower values, mainly through the reduction in the upper tail of the distribution. The temperature probability density function shifts to higher values, mainly through an increase in the upper tail of the distribution indicating that the impact is focussed on extremes. Given that there are a suite of well evaluated land surface models that include the biogeochemical effects of increasing CO₂ we suggest that the inclusion of such a model should be a recommended component of climate models used in future assessment reports by the Intergovernmental Panel on Climate Change.     

Cost–benefit analysis of climate change dynamics: uncertainties and the value of information

We analyze climate change in a cost–benefit framework, using the emission and concentration profiles of Wigley et al. (Nature 379(6562):240–243, 1996). They present five scenarios that cover the period 1990–2300 and are designed to reach stabilized concentration levels of 350, 450, 550, 650 and 750 ppmv, respectively. We assume that the damage cost in each year t is proportional to the corresponding gross world product and the square of the atmospheric temperature increase (ΔT(t)). The latter is estimated with a simple two-box model (representing the atmosphere and deep ocean). Coupling the damage cost with the abatement cost, we interpolate between the five scenarios to find the one that is optimal in the sense of minimizing the sum of discounted annual (abatement plus damage) costs over a time horizon of N years. Our method is simpler than ‘traditional’ models with the same purpose, and thus allows for a more transparent sensitivity study with respect to the uncertainties of all parameters involved. We report our central result in terms of the stabilized emission level E _o and concentration level p _o (i.e. their values at t = 300 years) of the optimal scenario. For the central parameter values (that is, N = 150 years, a discount rate r _dis = 2%/year and a growth rate r _gro = 1%/year of gross world product) we find E _o  = 8.0 GtCO₂/year and p _o = 496 ppmv. Varying the parameters over a wide range, we find that the optimal emission level remains within a remarkably narrow range, from about 6.0 to 12 GtCO₂/year for all plausible parameter values. To assess the significance of the uncertainties we focus on the social cost penalty, defined as the extra cost incurred by society relative to the optimum if one makes the wrong choice of the emission level as a result of erroneous damage and abatement cost estimates. In relative terms the cost penalty turns out to be remarkably insensitive to errors. For example, if the true damage costs are three times larger or smaller than the estimate, the total social cost of global climate change increases by less than 20% above its minimum at the true optimal emission level. Because of the enormous magnitude of the total costs involved with climate change (mitigation), however, even a small relative error implies large additional expenses in absolute terms. To evaluate the benefit of reducing cost uncertainties, we plot the cost penalty as function of the uncertainty in relative damage and abatement costs, expressed as geometric standard deviation and standard deviation respectively. If continued externality analysis reduces the geometric standard deviation of relative damage cost estimates from 5 to 4, the benefit is 0.05% of the present value G _tot of total gross word product over 150 years (about $3.9 × 10¹⁵), and if further research reduces the standard deviation of relative abatement costs from 1 to 0.5, the benefit is 0.03% of G _tot .     

Sensitivity of the LLN climate model to the astronomical and CO2 forcings over the last 200 ky

 The Louvain-la-Neuve climate model (here referred to as the LLN 2-D model has been used extensively to simulate the Northern Hemisphere ice volume under both the insolation and CO₂ forcings. The period analysed here covers the last 200 ky. First, sensitivity analyses to constant CO₂ concentration were performed. The model was accordingly forced by insolation changes only, the CO₂ concentration being kept constant to respectively 210, 250 and 290 ppmv. Results show that the simulated ice volume variations are comparable to the geological reconstructions only when the CO₂ concentration is low (210 ppmv) and that the sensitivity of the simulated Northern Hemisphere ice volume to CO₂ is not constant through time. Second, three CO₂ reconstructions were used to force the LLN 2-D model in addition to insolation. Results show (1) a better agreement with the SPECMAP oxygen isotope time series, in particular as far as the amplitude of the signal is concerned, and (2) that the simulated Northern Hemisphere ice volume is not very sensitive to the slight differences between these three reconstructions.     

In situ Measurement of H2O and CH4 with Telecommunication Laser Diodes in the Lower Stratosphere: Dehydration and Indication of a Tropical Air Intrusion at Mid-Latitudes

Telecommunication laser diodes emitting near 1.39 m and 1.65 m in combination with direct-differential absorption spectroscopy are efficient tools to monitor in situ stratospheric H₂O andCH₄ with a good precision error (a few percents), a high temporal resolution (ranging from 10 ms to 1 s), a large dynamic range in the concentration measurements (four orders of magnitude) and a high selectivity in the analyte species. To illustrate the capability of laser probing technique, we report balloonborne H₂Oand CH₄ simultaneous measurements obtained on October 2001 atmidlatitudes (43° N). The H₂O vertical profile achieved with the lasersensor in the lower stratosphere is compared with the H₂O data yielded by a balloonborne frost-point hygrometer. The total hydrogen mixing ratio in the lower stratosphere, 2[CH₄] + [H₂O], appears to beconstant at 7.5 ± 0.1 ppmv. Nevertheless, an unexpected largedehydration of 0.5 ppmv was detected by both the laser sensor and thehygrometer between 16 km and 23 km. We suspect the occurrence of a tropicalair intrusion into mid-latitudes. We support this interpretation using a high-resolution advection model for potential vorticity.     

Climate Change to the End of the Millennium

Anthropogenic climate change will continue long after anthropogenic CO₂ emissions cease. Atmospheric CO₂, global warming and ocean circulation will approach equilibrium on the millennial timescale, whereas thermal expansion of the ocean, ice sheet melt and their contributions to sea level rise are unlikely to be complete. Atmospheric CO₂ in year 3000 depends non-linearly on the total amount of CO₂ emitted and is very likely to exceed the present level of ∼380 ppmv. CO₂ is doubled for ∼2500 GtC emitted, quadrupled if all ∼5000 GtC of conventional fossil fuel resources are emitted, and increases by a factor of ∼32 if a further 20,000 GtC of exotic fossil fuel resources are emitted. Global warming in year 3000 will also depend on climate sensitivity to doubling CO₂, which is most probably ∼3 ^∘C but highly uncertain. Thermal expansion will contribute 0.5–2 m to millennial sea level rise for each doubling of CO₂. The Greenland ice sheet could melt completely within the millennium under > 8×CO₂, adding a further ∼7 m to sea level. The rate of melt depends on the magnitude of forcing above a regional warming threshold of 1–3 ^∘C. The West Antarctic ice sheet could be threatened by 4–10 ^∘C local warming, and its potential contribution to millennial sea level rise exceeds current maximum estimates of ∼1 m. The fate of the ocean thermohaline circulation may depend on the rate as well as the magnitude of forcing.     

Recent climatic change in Australia: Implications for a CO2-warmed earth

A significant change in mean precipitation occurred over much of Australia between 1913–45 and 1946–78. This is described on a seasonal basis and related to possible changes in the atmospheric circulation. It now appears that during this time mean surface temperatures in the mid southern latitude zone increased by up to 1 °C. This temperature change could be at least partly due to an increase in atmospheric CO₂ concentrations from about 260 ppmv in the early nineteenth century. In any case the observed temperature increase is similar to the predicted future effects of a 50% increase in atmospheric CO₂ concentrations. Thus the climatic change which occurred earlier this century is at least a good analogy for the effects of a CO₂-induced global warming which is expected to occur over a similar time interval in the future. This allows the construction of more detailed and quantitative climate scenarios. The most noteworthy conclusion is that marked changes in the seasonally of precipitation should be anticipated, with seasonal changes in some areas being of the order of 50% or more for a doubling of CO₂ content. The results are in general consistent with earlier more qualitative scenarios for Australia.     

Millennial timescale carbon cycle and climate change in an efficient Earth system model

A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8–3.3°C and predict atmospheric CO₂ close to present observations. Six idealized total fossil fuel CO₂ emissions scenarios are used to explore a range of 1,100–15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO₂ approaches equilibrium in year 3000 at 420–5,660 ppmv, giving 1.5–12.5°C global warming. The ocean is a robust carbon sink of up to 6.5 GtC year⁻¹. Under ‘business as usual’, the land becomes a carbon source around year 2100 which peaks at up to 2.5 GtC year⁻¹. Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6°C local warming is only briefly exceeded if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7 m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4–10 m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in two out of three model versions, but only under extreme emissions including exotic fossil fuel resources.     

Monin-Obukhov Functions for Standard Deviations of Velocity

The origins of Monin-Obukhov similarity theory (MOST) are briefly reviewed, as a context for the analysis of signals from sonic anemometers operating in the surface layer over a Utah salt flat. At this site (over the interval of these measurements) the neutral limit for the normalized vertical velocity standard deviation (σ _w /u _*) deviates markedly from what has generally been regarded as the standard value (i.e. about 1.3), suggesting (since others have also reported such deviations) that this Monin-Obukhov constant is not, in fact, universal. New (but tentative) formulae are suggested for σ _w and for the longitudinal standard deviation σ _u .     

An ozone depletion event in the sub-arctic surface layer over Hudson Bay,Canada

During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) program, aircraft flights during April 7–11, 2000 revealed a large area air mass capped below ∼500 m altitude over Hudson Bay, Canada in which ozone was reduced from normal levels of 30–40 ppbv to as low as 0.5 ppbv. From some of the in-situ aircraft measurements, back-trajectory calculations, the tropospheric column of BrO derived from GOME satellite measurements, and results from a regional model, we conclude that the event did not originate from triggering of reactive halogen release in the sub-Arctic region of Hudson Bay but resulted from such an event occurring at higher latitudes over the islands of the northern Canada Archipelago and nearby Arctic Ocean with subsequent transport over a distance of 1,000–1,500 km to Hudson Bay. BrO _x remained active during this transport despite considerable changes in the conditions of the underlying surface suggesting that chemical recycling during transport dominated any local halogen input from the surface. If all of the tropospheric column density of BrO is distributed uniformly within the surface layer, then the mixing ratio of BrO derived from the satellite measurements is at least a factor of 2–3 larger than derived indirectly from in situ aircraft measurements of the NO/NO₂ ratio.     

Oceanic transport and storage of carbon emissions

Using a global carbon cycle model (GLOCO) that considers seven terrestrial biomes, surface and deep ocean layers based on the HILDA model and a single mixed atmosphere, we analyzed the response of atmospheric CO₂ concentration and oceanic DIC and DOC depth profiles to additions of carbon to the atmosphere and ocean. The rate of transport of carbon to the deepest oceanic layers is rather insensitive to the atmosphereic-ocean surface gas exchange coefficient over a wide range, hence discrepancies between researchers on the precise global average value of this coefficient do not significantly affect predictions of atmospheric response to anthropogenic inputs. Upwelling velocity, on the other hand, amplifies oceanic response by increasing primary production in the upper ocean layers, resulting in a larger flux into DOC and sediments and increased carbon storage; experiments to reduce the uncertainty in this parameter would be valuable.The location of the carbon addition, whether it is released in the atmosphere or in the middle of the oceanic thermocline, has a significant impact on the maximum atmospheric CO₂ concentration (pCO₂) subsequently reached, suggesting that oceanic burial of a significant fraction of carbon emissions (e.g. via clathrate hydrides) may be an important management option for limiting pCO₂ buildup. Our analysis indicates that the effectiveness of ocean burial decreases asymptotically below about 1000 m depth. With a constant emissions scenario (at 1990 levels), pCO₂ at year 2100 is reduced from 501 ppmv considering all emissions go to the atmosphere, to 422 ppmv with ocean burial at a depth of 1000 m of 50% of the fossil fuel emissions. An alternative scenario looks at stabilizing pCO₂ at 450 ppmv; with no ocean burial of fossil fuel emissions, the rate of emissions has to be cut drastically after the year 2010, whereas oceanic burial of 2 GtC/yr allows for a smoother transition to alternative energy sources.     

Temperature, CO2 and the growth and development of wheat: Changes in the mean and variability of growing conditions

The experiment described here resulted from simulation analyses of climate-change studies that highlighted the relative importance of changes in the mean and variance of climatic conditions in the prediction of crop development and yield. Growth and physiological responses of four old cultivars of winter wheat, to three temperature and two carbon dioxide (CO₂) regimes (350 or 700 ppmv) were studied in controlled environment chambers. Experimental results supported the previous simulation analyses. For plants experiencing a 3 °C increase in day and night temperatures, relative to local long-term mean temperatures (control treatment), anthesis and the end of grain filling were advanced, and grain and dry matter yields were reduced by 27% and 18%, respectively. Increasing the diurnal temperature range, but maintaining the same mean temperature as the control, reduced the maximum leaf area (27%) and grain yield (13%) but did not affect plant development. Differences among the temperature treatments in both phyllochron interval and anthesis date may have resulted from differences between measured air, and unmeasured plant, temperatures, caused by evaporative cooling of the plants. Thermal time (base = 0 °C), calculated from air temperature, from anthesis to the end of grain filling was about 650 °C d for all cultivars and treatments. Doubling ambient CO₂ concentration to 700 ppmv reduced maximum leaf area (21%) but did not influence plant development or tiller numbers.     

NUMERICAL SIMULATION OF THE INFLUENCE OFO3,CO2 AND SPECTRUM VARIATION ON THE PHOTOSYNTHESIS OF CROP CANOPY*

Coupled the photosynthesis with transpiration and adjustment of stoma,a dynamic ecological model for simulating the canopy photosynthesis of winter wheat was established by scaling up from the biochemical scale to canopy scale,in which the effects of O₃,CO₂ and solar spectrum on crop photosynthesis were fully considered.Validation of the model against the data measured with CI-301PS portable photosynthesis analyzer showed that the leaf photosynthesis model passed the correlation significance test and had a fairly high accuracy.Numerical analysis showed that the canopy photosynthesis rate would be reduced by 29% if the O₃ concentration increases from 0 ppbv to 200 ppbv,whereas the canopy photosynthesis rate would increase by about 37% while the CO₂ concentration increases from 330 ppmv to 660 ppmv,and the canopy photosynthesis rate would be reduced by 27%0 or so under the condition that the spectrum coefficient changed from 0.5 to 0.4.If the O₃ concentration reached 200 ppbv at noon on the typical sunny day with higher radiation,the canopy photosynthesis will be reduced slightly in the suburb area where the pollution is serious and the photochemical fog is easy to be formed,contrast with that in the clear region and regardless of the climate change,due to the fact that the positive effect of CO₂ on crop photosynthesis can not compensate the negative effect of O₃ on crop photosynthesis.The canopy photosynthesis will be reduced by 35% or so than the BASE value at present,when the spectrum of photosynthetic active radiation(PAR) reduces to 0.4 or so.     

Vertical Structure of Selected Turbulence Characteristics above an Urban Canopy

Summary In this paper the results of an urban measurement campaign are presented. The experiment took place from July 1995 to February 1996 in Basel, Switzerland. A total of more than 2000 undisturbed 30-minute runs of simultaneous measurements of the fluctuations of the wind vector u′, v′, w′ and the sonic temperature θ _s ′ at three different heights (z=36, 50 and 76 m a.g.l.) are analysed with respect to the integral statistics and their spectral behaviour. Estimates of the zero plane displacement height d calculated by the temperature variance method yield a value of 22 m for the two lower levels, which corresponds to 0.92 h (the mean height of the roughness elements). At all three measurement heights the dimensionless standard deviation σ _w /u _* is systematically smaller than the Monin-Obukhov similarity function for the inertial sublayer, however, deviations are smaller compared to other urban turbulence studies. The σ_θ/θ_* values follow the inertial sublayer prediction very close for the two lowest levels, while at the uppermost level significant deviations are observed. Profiles of normalized velocity and temperature variances show a clear dependence on stability. The profile of friction velocity u _* is similar to the profiles reported in other urban studies with a maximum around z/h=2.1. Spectral characteristics of the wind components in general show a clear dependence on stability and dimensionless measurement height z/h with a shift of the spectral peak to lower frequencies as thermal stability changes from stable to unstable conditions and as z/h decreases. Velocity spectra follow the −2/3 slope in the inertial subrange region and the ratios of spectral energy densities S _w (f)/S _u (f) approach the value of 4/3 required for local isotropy in the inertial subrange. Velocity spectra and spectral peaks fit best to the well established surface layer spectra from Kaimal et al. (1972) at the uppermost level at z/h=3.2. Received September 26, 1997 Revised February 15, 1998