Doubled CO2 Conditions Research Articles

Can marine cloud brightening reduce coral bleaching?

AbstractIncreases in coral bleaching events over the last few decades have been largely caused by rising sea surface temperatures (SST), and continued warming is expected to cause even greater increases through this century. We use a Global Climate Model to examine the potential of marine cloud brightening (MCB) to cool oceanic surface waters in three coral reef provinces. Our simulations indicate that under doubled CO2 conditions, the substantial increases in coral bleaching conditions from current values in three reef regions (Caribbean, French Polynesia, and the Great Barrier Reef) were eliminated when MCB was applied, which reduced the SSTs at these sites roughly to their original values.

Effects of doubled CO<SUB>2</SUB> concentration on leaf photosynthesis, transpiration and water use efficiency of eight crop species

Aims Our objective was to elucidate the response of crop photosynthesis, transpiration and water use efficiency to atmospheric CO2 concentration. This has great significance to predicting crop productivity and water-demand changes under increasing atmospheric CO2 concentration. Methods The photosynthesis rate, transpiration rate and water use efficiency of eight crops (soybean (Glycine max), sweet potato (Ipomoea batatas), peanut (Arachis hypogaea), rice (Oryza sativa), cotton (Gossypium hirsu- tum), corn (Zea mays), sorghum (Sorghum vulgare) and millet (Setaria italica)) were studied under natural CO2 concentration, doubled CO2 concentration and natural CO2 concentration after doubled CO2 conditions. Important findings Doubled CO2 concentration increased the photosynthesis rate and decreased the transpira- tion rate, and therefore water use efficiency was more significantly increased. The increase of water use efficiency showed greater dependence on the increase of photosynthesis rate than the decrease of transpiration rate. The variations of photosynthesis rate and water use efficiency of C3 crops were larger than those of C4 crops. The ef- fect of photosynthesis rate of C3 crops on the water use efficiency was larger than that of C4 crops. The photosyn- thesis rate under natural CO2 concentration after doubled CO2 concentration was lower than that under natural CO2 concentration, but no significant difference was found for the transpiration rate. The photosynthetic capacity under natural CO2 concentration after doubled CO2 concentration was decreased mainly by the decreasing of some non-stomatal factors, including the protein content, activation levels and specific activity of the enzyme Rubisco.

Boreal lakes moderate seasonal and diurnal temperature variation and perturb atmospheric circulation: analyses in the Community Earth System Model 1 (CESM1)

ABSTRACTWe used a lake thermal physics model recently coupled into the Community Earth System Model 1 (CESM1) to study the effects of lake distribution in present and future climate. Under present climate, correcting the large underestimation of lake area in CESM1 (denoted CCSM4 in the configuration used here) caused 1 °C spring decreases and fall increases in surface air temperature throughout large areas of Canada and the US. Simulated summer surface diurnal air temperature range decreased by up to 4 °C, reducing CCSM4 biases. These changes were much larger than those resulting from prescribed lake disappearance in some present-day permafrost regions under doubled-CO2 conditions. Correcting the underestimation of lake area in present climate caused widespread high-latitude summer cooling at 850 hPa. Significant remote changes included decreases in the strength of fall Southern Ocean westerlies. We found significantly different winter responses when separately analysing 45-yr subperiods, indicating that relatively long simulations are required to discern the impacts of surface changes on remote conditions. We also investigated the surface forcing of lakes using idealised aqua-planet experiments which showed that surface changes of 2 °C in the Northern Hemisphere extra-tropics could cause substantial changes in precipitation and winds in the tropics and Southern Hemisphere. Shifts in the Inter-Tropical Convergence Zone were opposite in sign to those predicted by some previous studies. Zonal mean circulation changes were consistent in character but much larger than those occurring in the lake distribution experiments, due to the larger magnitude and more uniform surface forcing in the idealised aqua-planet experiments.

Doubled CO2‐induced cooling in the middle atmosphere: Photochemical analysis of the ozone radiative feedback

Simple radiative arguments predict that the impact of CO2 increases on the stratosphere and mesosphere should be a cooling and that the magnitude of the temperature change should be significantly larger than in the troposphere. Considering the temperature dependence of middle atmospheric gas‐phase ozone photochemistry, it is expected that the ozone response will generate a radiative feedback that mitigates the CO2‐induced cooling. The magnitude and vertical structure of this signal need to be characterised in order to distinguish the impact of future CO2 changes from other processes affecting the temperature evolution, such as changes in chlorine loading and water vapour trends. The Canadian Middle Atmosphere Model (CMAM) has been used in a process oriented study to examine the details of radiative and photochemical feedbacks under current and doubled CO2 conditions at low and middle latitudes. The model was run both with and without interactive chemistry in order to determine the importance of the radiative feedback through ozone changes on the CO2‐induced cooling signal. Changes in other greenhouse gases, ozone depleting substances, or SST's and sea ice coverage were not considered. The interactive model results show a substantial temperature decrease throughout most of the middle atmosphere with a maximum cooling of 10–12 K at the stratopause. In association with this temperature change, the ozone mixing ratio increases by 15–20% in the upper stratosphere and by 10–15% in the lower mesosphere. Results from the non‐interactive simulations show that the magnitude of the cooling is overestimated by up to ∼4.5 K when the ozone radiative feedback is not considered. In spite of the complexity of the ozone chemistry, the ozone increase at 30–70 km can be understood primarily as a result of the negative temperature dependence of the O + O2 + M → O3 + M reaction that controls odd oxygen partitioning. Additional partial contributions to the ozone increase below 60 km are provided by a decrease in the reaction rate coefficient of the Chapman reaction O + O3 → 2O2 and by a decrease in the NO2 abundance.

Height growth response of tree line black spruce to recent climate warming across the forest‐tundra of eastern Canada

Summary The northward expansion of the boreal forest vegetation zone is generally predicted under a warmer doubled CO2, but the delay associated with vegetation development processes often has been overlooked. In the subarctic forest‐tundra of northern Québec, reforestation of tundra uplands appears currently limited by the poor reproductive capacity of shrubby black spruce (Picea mariana), and the development of erect stems through accelerated height growth should be the first registered response to 20th century climate warming. The subarctic forest‐tundra is characterized by small‐ and large‐scale heterogeneity in topography, vegetation structure and climate. This spatial heterogeneity, added to the complexity of tree growth–climate relationships, can cause various growth responses of subarctic tree line black spruce to 20th century climate change. Twenty spruce populations at subarctic tree lines and seven isolated clones at the species limit were sampled along a > 300‐km latitudinal transect from the southern forest‐tundra to the shrub tundra. Height growth patterns of black spruce at tree line and above tree line were examined (i) over their life span, using dendrochronological dating of stem cross‐sections, and (ii) for the recent decades, using leader shoot elongation measurements. Indexed elongation chronologies were compared with regional climate data. Height growth of tree line trees generally decreased with increasing latitude. However, tree line trees in the northern forest‐tundra have experienced an acceleration of height growth since the 1970s, with their growth comparable to that of trees in the southern forest‐tundra. Height growth response of spruce trees appeared increasingly delayed from the northern forest‐tundra to the species limit. Above the subarctic tree line, wind‐exposed conditions obscured the decrease in height growth with latitude observed for tree line trees. Leader shoot elongation of spruce trees established on tundra hilltops appeared more controlled by summer heat sums than those at tree line all over the forest‐tundra, except at the arctic tree line. Winter precipitation also was linked to leader shoot elongation in some forest‐tundra sites. The increasing snow cover associated with recent warming appeared to have reduced the shoot elongation of spruce at forest margins showing the steepest slopes, hence subjected to snow overloading. In the northern forest‐tundra sites, the recent increase in height growth and positive trend in leader shoot elongation, consistent with a 1990s’ increase in heat sums, point to the development of spruce krummholz into erect growth forms. In the southern forest‐tundra, reforestation of tundra hilltops and northward expansion of the boreal forest predicted under doubled CO2 conditions could be delayed, as suggested by suppressed height growth of spruce above tree line.

Short-term volume changes of the Greenland ice sheet in response to doubled CO2 conditions

Short-term volume changes of the Greenland ice sheet in response to doubled CO2 conditions

Short-term volume changes of the Greenland ice sheet in response to doubled CO2 conditions

This paper focuses on the rôle of accumulation and cloudiness changes in the response of the Greenland ice sheet to global warming. Changes in accumulation or cloudiness were often neglected, or coupled to temperature changes. We used model output on temperature, precipitation and cloudiness from a GCM (ECHAM4 T106). The GCM output was used to drive the Greenland model that exists of a vertically averaged ice flow model, coupled to a 1D surface energy balance model that calculates the ablation. Variables are temperature, accumulation and cloudiness. Sensitivity experiments with this model show that changes in accumulation are very important for the ice sheet mass balance, whereas cloudiness is of secondary importance. If the Greenland model is forced by the GCM output, the Greenland model is found to contribute 70% less to sea level rise after 70 years than is indicated by the results presented in the IPCC report. This large discrepancy is mainly due to the fact that the enhanced ablation is strongly compensated by increased accumulation. Comparing the result obtained here with changes in mass balance derived directly from the same general circulation model, indicates a 20% larger contribution to sea level. This increase is due to changes in ice flow, and a different method for the ablation calculation.

Change in mass balance of polar ice sheets and sea level from high-resolution GCM simulations of greenhouse warming

AbstractFor projecting future sea level, the mass-balance changes on Greenland and Antarctica are considered to be crucial. Promising tools for such estimates are general circulation models (GCM). Until recently, a major impediment was their coarse grid resolution (3°-6°) causing substantial uncertainties in the mass-balance calculations of the poorly resolved ice sheets. The present study is based on a new climate-change experiment of the highest resolution currently feasible (1.1 °) performed with the ECHAM4 T106 GCM, thereby increasing confidence in the projected mass-balance and sea-level changes. This new experiment, with doubled CO2 concentration, suggests that the mass gain in Antarctica due to increased accumulation exceeds the melt-induced mass loss in Greenland by a factor of three. The resulting mass-balance change on both ice sheets is equivalent to a net sea-level decrease of 0.6 mm a"1 under doubled CO2 conditions. This may compensate for a significant portion of the melt-induced sea-level rise from the smaller glaciers and ice caps, thus leaving thermal expansion as the dominant factor for sea-level rise over the next decades. This compensating effect, however, no longer applies should atmospheric CO2 concentration reach levels well above "doubled the present value". On the contrary, under these conditions, the greenhouse warming would become large enough to induce substantial melting also on the Antarctic ice sheet, thereby significantly accelerating global sea-level rise.

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

Dimethylsulphide is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP) which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the microphysical properties of clouds relevant to climate change are sensitive to CCN density, it has been postulated that marine sulphur emissions may play a role in climate regulation. Here we examine the DMS cycle in the subantarctic Southern Ocean west of the atmospheric baseline station at Cape Grim, NW Tasmania, where a long time series of atmospheric data has been collected. The Southern Ocean is relatively free of anthropogenic sulphur emissions and thus sulphate aerosols will be mainly due to the biogenic source of DMS. An atmospheric general circulation model has been used to provide meteorological forcings under current and doubled atmospheric CO2 conditions. We have used an existing DMS production model to investigate the sensitivity of the sea-to-air flux to the simulated changes in temperature and wind speed at the ocean surface. Under doubled CO2 conditions sea-surface temperature is simulated to increase by 4 °C throughout the year. Annual mean surface wind speed is simulated to decrease by about 3%. The changes in temperature and wind speed cause a net increase of DMS transfer velocity in the range 3-11%. The increase in temperature will also increase the growth rate of phytoplankton. The annual integrated DMS flux is simulated to increase between 2% and 8% under doubled CO2 conditions. A median change of + 5% in DMS flux corresponds to an increase in the range 2-4% in cloud condensation nuclei (CCN) concentration and a perturbation to radiative forcing of — 0.29 W m-2, confirming a minor role for DMS-derived aerosols in climate regulation.

Great Lakes Hydrology Under Transposed Climates

Historical climates, based on 43 years of daily data from areas south and southwest of the Great Lakes, were used to examine the hydrological response of the Great Lakes to warmer climates. The Great Lakes Environmental Research Laboratory used their conceptual models for simulating moisture storages in, and runoff from, the 121 watersheds draining into the Great Lakes, over-lake precipitation into each lake, and the heat storages in, and evaporation from, each lake. This transposition of actual climates incorporates natural changes in variability and timing within the existing climate; this is not true for General Circulation Model-generated corrections applied to existing historical data in many other impact studies. The transposed climates lead to higher and more variable over-land evapotranspiration and lower soil moisture and runoff with earlier runoff peaks since the snow pack is reduced up to 100%. Water temperatures increase and peak earlier. Heat resident in the deep lakes increases throughout the year. Buoyancy-driven water column turnover frequency drops and lake evaporation increases and spreads more throughout the annual cycle. The response of runoff to temperature and precipitation changes is coherent among the lakes and varies quasi-linearly over a wide range of temperature changes, some well beyond the range of current GCM predictions for doubled CO2 conditions.

Elevation Dependency of the Surface Climate Change Signal: A Model Study

Abstract Results are presented from a present-day and a doubled CO2 experiment over the Alpine region with a nested regional climate model. The simulated temperature change signal shows a substantial elevation dependency, mostly during the winter and spring seasons, resulting in more pronounced warming at high elevations than low elevations. This is caused by a depletion of snowpack in doubled CO2 conditions and further enhanced by the snow–albedo feedback. This result is consistent with some observed temperature trends for anomalously warm years over the Alpine region and suggests that high elevation temperature changes could be used as an early detection tool for global warming. Changes in precipitation, as well as other components of the surface energy and water budgets, also show an elevation signal, which may have important implications for impact assessments in high elevation regions.

DROUGHTS OVER SOUTHERN AFRICA IN A DOUBLED-CO2 CLIMATE

The southern African region is susceptible to climatic extremes and particularly to extended dry periods. Possible changes in the probability of dry years under doubled-CO2 conditions are examined using output from the CSIRO nine-level general circulation model. Changes in annual mean rainfall are not expected to be significant. However, the model simulates an increase in the probability of dry years in the tropics, to the south-west of the subcontinent, as well as over the western and eastern parts of South Africa and southern Mozambique, where large percentage increases in the most intense dry spells are indicated. A decrease in the frequency of dry years is simulated over much of the interior of the subcontinent south of 10°S. In regions where the frequency of dry years decreases, the most severe events occur less often. The CSIRO nine-level model indicates a shift in the frequency distribution of daily rainfall events under doubled-CO2 conditions. A small change in the frequency distribution of daily rainfall events may have further implications for the frequency of mid-summer droughts during the peak summer rainfall period of December–February. Increases in the frequency of mid-summer droughts are simulated over the eastern part of the subcontinent south of 20°S.

Influences on surface energy fluxes in simulated present and doubled CO2 climates

The surface energy fluxes simulated by the CSIRO9 Mark 1 GCM for present and doubled CO2 conditions are analyzed. On the global scale the climatological flux fields are similar to those from four GCMs studied previously. A diagnostic calculation is used to provide estimates of the radiative forcing by the GCM atmosphere. For 1 × CO2, in the global and annual mean, cloud produces a net cooling at the surface of 31 W m−2. The clear-sky longwave surface greenhouse effect is 311 W m−2, while the corresponding shortwave term is −79 W m−2. As for the other GCM results, the CSIRO9 ΔCO2 surface warming (global mean 4.8°C) is closely related to the increased downward longwave radiation (LW →). Global mean net cloud forcing changes little. The contrast in warming between land and ocean, largely due to the increase in evaporative cooling (E) over ocean, is highlighted. In order to further the understanding of influences on the fluxes, simple physically based linear models are developed using multiple regression. Applied to both 1 × CO2 and ΔCO2 December–February mean tropical fields from CSIRO9, the linear models quite accurately (3–5 W m−2 for 1 × CO2 and 2–3 W m−2 for ΔCO2) relate LW → and net shortwave radiation to temperature, surface albedo, the water vapor column, and cloud. The linear models provide alternative estimates of radiative forcing terms to those from the diagnostic calculation. Tropical mean cloud forcings are compared. Over land, E is well correlated with soil moisture, and sensible heat with air-surface temperature difference. However an attempt to relate the spatial variation of LWt within the tropics to that of the nonflux fields had little success. Regional changes in surface temperature are not linearly related to, for instance, changes in cloud or soil moisture.

Comparison of Radiative and Physiological Effects of Doubled Atmospheric CO 2 on Climate

The physiological response of terrestrial vegetation when directly exposed to an increase in atmospheric carbon dioxide (CO 2 ) concentration could result in warming over the continents in addition to that due to the conventional CO 2 “greenhouse effect.” Results from a coupled biosphere-atmosphere model (SiB2-GCM) indicate that, for doubled CO 2 conditions, evapotranspiration will drop and air temperature will increase over the tropical continents, amplifying the changes resulting from atmospheric radiative effects. The range of responses in surface air temperature and terrestrial carbon uptake due to increased CO 2 are projected to be inversely related in the tropics year-round and inversely related during the growing season elsewhere.

Potential effects of global climatic change on the phenology and yield of maize in venezuela

Simulated impacts of global and regional climate change, induced by an enhanced greenhouse effect and by Amazonian deforestation, on the phenology and yield of two grain corn cultivars in Venezuela (CENIAP PB-8 and OBREGON) are reported. Three sites were selected:Turen, Barinas andYaritagua, representing two important agricultural regions in the country. The CERES-Maize model, a mechanistic process-based model, in theDecision Support System for Agrotechnology Transfer (DSSAT) was used for the crop simulations. These simulations assume non-limiting nutrients, no pest damage and no damage from excess water; therefore, the results indicate only the difference between baseline and perturbed climatic conditions, when other conditions remain the same. Four greenhouse-induced global climate change scenarios, covering different sensitivity levels, and one deforestation-induced regional climate change scenario were used. The greenhouse scenarios assume increased air temperature, increased rainfall and decreased incoming solar radiation, as derived from atmospheric GCMs for doubled CO2 conditions. The deforestation scenarios assume increased air temperature, increased incoming solar radiation and decreased rainfall, as predicted by coupled atmosphere-biosphere models for extensive deforestation of a portion of the Amazon basin. Two baseline climate years for each site were selected, one year with average precipitation and another with lower than average rainfall. Scenarios associated with the greenhouse effect cause a decrease in yield of both cultivars at all three sites, while the deforestation scenarios produce small changes. Sensitivity tests revealed the reasons for these responses. Increasing temperatures, especially daily maximum temperatures, reduce yield by reducing the duration of the phenological phases of both cultivars, as expected from CERES-Maize. The reduction of the duration of the kernel filling phase has the largest effect on yield. Increases of precipitation associated with greenhouse warming have no effects on yield, because these sites already have adequate precipitation; however, the crop model used here does not simulate potential negative effects of excess water, which could have important consequences in terms of soil erosion and nutrient leaching. Increases in solar radiation increased yields, according to the non-saturating light response of the photosynthesis rate of a C4 plant like corn, compensating for reduced yields from increased temperatures in deforestation scenarios. In the greenhouse scenarios, reduced insolation (due to increased cloud cover) and increased temperatures combine to reduce yields; a combination of temperature increase with a reduction in solar radiation produces fewer and lighter kernels.

Synoptic scale disturbances of the Indian summer monsoon as simulated in a high resolution climate model

Hamburg atmospheric general circulation model ECHAM3 at T106 resolution (1.125' lat.Aon.) has considerable skill in reproducing the observed seasonal reversal of mean sea level pressure, the location of the summer heat low as well as the position of the monsoon trough over the Indian subcontinent. The present-day climate and its seasonal cycle are realistically simulated by the model over this region. The model simulates the structure, intensity, frequency, movement and lifetime of monsoon depressions remarkably well. The number of monsoon depressions/storms simulated by the model in a year ranged from 5 to 12 with an average frequency of 8.4 yr-', not significantly different from the observed climatology. The model also simulates the interannual variability in the formation of depressions over the north Bay of Bengal during the summer monsoon season. In the warmer atmosphere under doubled CO2 conditions, the number of monsoon depressions/cyclonic storms forming in Indian seas in a year ranged from 5 to 11 with an average frequency of 7.6 yr-', not significantly different from those inferred in the control run of the model. However, under doubled CO2 conditions, fewer depressions formed in the month of June. Neither the lowest central pressure nor the maximum wind speed changes appreciably in monsoon depressions identified under simulated enhanced greenhouse conditions. The analysis suggests there will be no significant changes in the number and intensity of monsoon depressions in a warmer atmosphere.

The influence of climate change and the timing of stratospheric warmings on Arctic ozone depletion

Satellite data are presented showing the timing of sudden warmings in the lower stratosphere during the winters 1979–1992. A three‐dimensional dynamical‐radiative‐photochemical model is used to establish how Arctic ozone depletion will respond to a doubling of CO2 according to the timing of the warmings. In a series of idealized experiments the timing of the warmings is varied by specifying different geopotential wave amplitudes at the 316‐mbar model lower boundary. Results from a “transient climate change experiment” show that the chosen wave amplitudes are appropriate for both the current and the doubled CO2 atmosphere. For doubled CO2 the experiments show that any significant risk of an Arctic ozone hole will be confined to those years with only a late stratospheric warming. In all other years the results suggest that springtime total ozone over the Arctic is more likely to increase by a small amount due to a combination of slower homogeneous chemistry and changes in transport. The predictions obtained from the idealized studies are then tested by prescribing at the model lower boundary the observed geopotential wave amplitudes from two specific years with late winter warmings. Doubling CO2 amounts produced no significant increase in ozone depletion with the 1989 wave amplitudes, but with 1990 wave amplitudes, an Arctic ozone hole occurred with minimum column of 187 Dobson Units. This contrasting response is attributed to the large midwinter pulse in the 1989 wave amplitudes compared to the less dramatic and shorter timescale fluctuations in the 1990 wave amplitudes. It is concluded that under doubled CO2 conditions an Arctic ozone hole is likely to occur in years with late stratospheric warmings following winters in which there were no significant pulses in the upper tropospheric planetary wave amplitudes.

Implications of climate change scenarios for soil erosion potential in the USA

AbstractAtmospheric general circulation models (GCMs) project that increasing atmospheric concentrations of CO2 and other greenhouse gases May, result in global changes in temperature and precipitation over the next 40‐100 years. Equilibrium climate scenarios from four GCMs run under doubled CO2 conditions were examined for their effect on the climatic potential for sheet and rill erosion in the conterminous USA. Changes in the mean annual rainfall factor (R) in the Universal Soil Loss Equation (USLE) were calculated for each cropland, pastureland and rangeland sample point in the 1987 National Resources Inventory. Projected annual precipitation changes were assumed to be from differences in either storm frequency or storm intensity. With all other USLE factors held constant these changes in R translated to changes in the sheet and rill erosion national average of +2 to +16 per cent in croplands, ‐2 to +10 per cent in pasturelands and ‐5 to +22 per cent in rangelands under the eight scenarios. Land with erosion rates above the soil loss tolerance (T) level and land classified as highly erodible (eredibility index >8) also increased slightly. the results varied from model to model, region to region and depended on the assumption of frequency versus intensity changes. These results show the range of sensitivity of soil erosion potential by water under projected climate change scenarios. However, actual changes in soil erosion could be mitigated by alterations in cropping patterns and other management practices, or possibly by increased crop growth and residue production under higher atmospheric CO2 concentrations.

Potential CO2-induced climate effects on North American wheat-producing regions

The environmental requirements for growth of winter, spring, and fallsown spring wheats in North America are specified and compared to temperature results from the control run of the Goddard Institute for Space Studies general circulation model (GISS GCM) and observed precipitation in order to generate a simulated map of current wheat production regions. The simulation agrees substantially with the actual map of wheat-growing regions in North America. Results from a doubled CO2 run of the climate model are then used to generate wheat regions under the new climatic conditions. In the simulation, areas of production increase in North America, particularly in Canada, due to increased growing degree units (GDU). Although wheat classifications may change, major wheat regions in the United States remain the same under simulated doubled CO2 conditions. The wheat-growing region of Mexico is identified as vulnerable due to high temperature stress. Higher mean temperatures during wheat growth, particularly during the reproductive stages, may increase the need for earlier-maturing, more heat-tolerant cultivars throughout North, America. The soil moisture diagnostic of the climate model is used to analyze potential water availability in the major wheat region of the Southern Great Plains.