Amazonian forest dieback under climate-carbon cycle projections for the 21st century

Summary The first GCM climate change projections to include dynamic vegetation and an interactive carbon cycle produced a very significant amplification of global warming over the 21st century. Under the IS92a business as usual emissions scenario CO₂ concentrations reached about 980ppmv by 2100, which is about 280ppmv higher than when these feedbacks were ignored. The major contribution to the increased CO₂ arose from reductions in soil carbon because global warming is assumed to accelerate respiration. However, there was also a lesser contribution from an alarming loss of the Amazonian rainforest. This paper describes the phenomenon of Amazonian forest dieback under elevated CO₂ in the Hadley Centre climate-carbon cycle model.     

A Green Planet Versus a Desert World: Estimating the Maximum Effect of Vegetation on the Land Surface Climate

We quantify the maximum possible influence of vegetation on the global climate by conducting two extreme climate model simulations: in a first simulation (desert world), values representative of a desert are used for the land surface parameters for all non glaciated land regions. At the other extreme, a second simulation is performed (green planet) in which values are used which are most beneficial for the biosphere's productivity. Land surface evapotranspiration more than triples in the presence of the green planet, land precipitation doubles (as a second order effect) and near surface temperatures are lower by as much as 8 K in the seasonal mean resulting from the increase in latent heat flux. The differences can be understood in terms of more absorbed radiation at the surface and increased recycling of water. Most of the increase in net surface radiation originates from less thermal radiative loss and not from increases in solar radiation which would be expected from the albedo change. To illustrate the differences in climatic character and what it would imply for the vegetation type, we use the Köppen climate classification. Both cases lead to similar classifications in the extra tropics and South America indicating that the character of the climate is not substantially altered in these regions. Fundamental changes occur over Africa, South Asia and Australia, where large regions are classified as arid (grassland/desert) climate in the desert world simulation while classified as a forest climate in the green planet simulation as a result of the strong influence of maximum vegetation on the climate. This implies that these regions are especially sensitive to biosphere-atmosphere interaction.     

The greening of the McGill Paleoclimate Model. Part I: Improved land surface scheme with vegetation dynamics

The formulation of a new land surface scheme (LSS) with vegetation dynamics for coupling to the McGill Paleoclimate Model (MPM) is presented. This LSS has the following notable improvements over the old version: (1) parameterization of deciduous and evergreen trees by using the models climatology and the output of the dynamic global vegetation model, VECODE (Brovkin et al. in Ecological Modelling 101:251–261 (1997), Global Biogeochemical Cycles 16(4):1139, (2002)); (2) parameterization of tree leaf budburst and leaf drop by using the models climatology; (3) parameterization of the seasonal cycle of the grass leaf area index; (4) parameterization of the seasonal cycle of tree leaf area index by using the time-dependent growth of the leaves; (5) calculation of land surface albedo by using vegetation-related parameters, snow depth and the models climatology. The results show considerable improvement of the models simulation of the present-day climate as compared with that simulated in the original physically-based MPM. In particular, the strong seasonality of terrestrial vegetation and the associated land surface albedo variations are in good agreement with several satellite observations of these quantities. The application of this new version of the MPM (the green MPM) to Holocene millennial-scale climate changes is described in a companion paper, Part II.

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Multiforced statistical assessments of greenhouse-gas-induced surface air temperature change 1890–1985

Based on univariate correlation and coherence analyses and considering the physical basis of the relationships, a simple multiforced (multiple) statistical concept is used which correlates observational climatic time series simultaneously with volcanic, solar, ENSO, and the anthropogenic greenhouse gases forcing. This is appropriate to remove some natural climate noise in the observed data and to evaluate the components (signals) possibly due to the anthropogenic greenhouse gas forcing (CO₂, or equivalent CO₂ implying additional gases) during industrial time. In this paper, we apply this technique to 100 global box data time series 1890–1985, of the surface air temperature, using observed data from Hansen and Lebedeff. The results are presented in terms of latitudinal-seasonal and regional trends, where the observed trend patterns are compared with the hypothetical signals (statistical assessments) possibly due to anthropogenic greenhouse forcing. These latter signals can be amplified to enable a comparison with corresponding results from general circulation model (GCM) CO₂ doubling experiments. These observed-statistical assessments lead to results which are, at least qualitatively and in respect to the zonal mean temperatures, very similar to some GCM experiments indicating the maximum CO₂ doubling signals (statistical assessment > 12 K) in the arctic winter. However, these signals are moderate in the tropics and in the Southern Hemisphere (global average 2.8–4.4 K). As far as the industrial signals are concerned (observed period) these signals are somewhat larger (maximum 7 K, global average 0.5–0.9 K) than the observed trends (maximum 5 K, global average 0.5 K). Phase shifts of cause and effect may amplify these signals but are very uncertain.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

A meteorological anomaly in Palestine 33 centuries ago: How did the sun stop?

Summary The meteorological conditions pertaining to the famous biblical episode during which Joshua stopped the sun when the Hebrew pursued the Amorites (Joshua 10, 10–13) are analyzed. The story, which cannot be interpreted as an anomaly in the astronomic motions, could be interpreted from the point of view of atmospheric optics. The event occurred at sundown, after an exceptional hailstorm, which left the ground of the valley where the Amorites stayed covered with ice. This resulted in a big drop in the air temperature, forming the conditions for a superior mirage. The Hebrew were in another nearby place, untouched by hail, on hot soil, in the usual daytime conditions of inferior mirage. When they reached the place of the Amorites, their optical path inverted curvature, displacing upwards the sun which was setting. Hail in Palestine is a quite rare meteorological event and the superior mirage would seem decidedly rare in the subtropical climate. Given that the event has been described in detail it could not have been invented, as the superior mirage was then unknown, in particular its connection with hailstorms. This would thus lead to the conclusion that the miracle described should be interpreted as a meteorological event which actually did happen.     

The convective boundary layer over pasture and forest in Amazonia

Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.     

Quantifying the Impact of Global Climate Change on Potential Natural Vegetation

Impacts of climate change on vegetation are often summarized in biome maps, representing the potential natural vegetation class for each cell of a grid under current and changed climate. The amount of change between two biome maps is usually measured by the fraction of cells that change class, or by the kappa statistic. Neither measure takes account of varying structural and floristic dissimilarity among biomes. An attribute-based measure of dissimilarity (V) between vegetation classes is therefore introduced. V is based on (a) the relative importance of different plant life forms (e.g. tree, grass) in each class, and (b) a series of attributes (e.g. evergreen-deciduous, tropical-nontropical) of each life form with a weight for each attribute. V is implemented here for the most used biome model, BIOME 1 (Prentice, I. C. et al., 1992). Multidimensional scaling of pairwise V values verifies that the suggested importance values and attribute weights lead to a reasonable pattern of dissimilarities among biomes. Dissimilarity between two maps (V) is obtained by area-weighted averaging of V over the model grid. Using V, present global biome distribution from climatology is compared with anomaly-based scenarios for a doubling of atmospheric CO2 concentration (2 × CO2), and for extreme glacial and interglacial conditions. All scenarios are obtained from equilibrium simulations with an atmospheric general circulation model coupled to a mixed-layer ocean model. The 2 × CO2 simulations are the widely used OSU and GFDL runs from the 1980's, representing models with low and high climate sensitivity, respectively. The palaeoclimate simulations were made with CCM1, with sensitivity similar to GFDL. V values for the comparisons of 2 × CO2 with present climate are similar to values for the comparisons of the last interglacial and mid-Holocene with present climate. However, the two simulated 2 × CO2 cases are much more like each other than they are to the simulated interglacial cases. The largest V values were between the last glacial maximum and all other cases, including the present. These examples illustrate the potential of V in comparing the impacts of different climate change scenarios, and the possibility of calibrating climate change impacts against a palaeoclimatic benchmark.     

Toward an improved global network for determination of tropospheric ozone climatology and trends

An examination of typical tropospheric ozone variability on daily, monthly, annual and interannual timescales and instrumental precision indicates that the current ozonesonde network is insufficient to detect a trend in tropospheric ozone of 1% per year at the 2 level even at stations with records a decade in length. From a trend prediction analysis we conclude that in order to detect a 1% per year trend in a decade or less it will be necessary to decrease the time between observations from its present value of 3–7 days to 1 day or less. The spatial distribution of the current ozonesonde stations is also inadequate for determining the global climatology of ozone. We present a quantitative theory taking into account photochemistry, surface deposition, and wind climatology to define the effectively sampled region for an observing station which, used in conjunction with the instrumental precision and the above prediction analysis, forms the basis for defining a suitable global network for determining regional and global ozone climatology and trends. At least a doubling of the present number of stations is necessary, and the oceans, most of Asia, Africa, and South America are areas where more stations are most needed. Differential absorption lidar ozone instruments have the potential for far more frequent measurements of ozone vertical profiles and hence potentially more accurate climatology and trend determinations than feasible with ozonesondes but may produce a (fair weather) biased data set above the cloud base. A strategy for cloudy regions in which either each station utilizes both lidars and sondes or each station is in fact a doublet comprised of a near-sea-level lidar and a proximal-mountain-top lidar could serve to minimize this bias.     

Trace gas exchange at the air/water interface: Measurements of mass accommodation coefficients

A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO₃ (0.01), N₂O₅ (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.     

Observed variations of σθ and σΦ in the nocturnal drainage flow in a deep valley

The variations of and in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average and in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.In the drainage flow, the hourly average varies inversely with wind speed according to the relation u 0.7ms^-1. The vertical standard deviation is much less enhanced by complex terrain than the horizontal standard deviation. The observed values are predicted fairly well by the local similarity theory.Oak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 andOak Ridge Associated Universities (ORAU) Summer Research Participant at ATDD in 1987 and     

Application of a “Big-Tree” model to regional climate modeling: a sensitivity study

Summary ¶In order to better understand land-atmosphere interactions and increase the predictability of climate models, it is important to investigate the role of forest representation in climate modeling. Corresponding to the big-leaf model commonly employed in land surface schemes to represent the effects of a forest, a so called big-tree model, which uses multi-layer vegetation to represent the vertical canopy heterogeneity, was introduced and incorporated into the National Center for Atmospheric Research (NCAR) regional climate model RegCM2, to make the vegetation model more physically based. Using this augmented RegCM2 and station data for China during 1991 Meiyu season, we performed 10 experiments to investigate the effects of the application of the big-tree model on the summer monsoon climate.With the big-tree model incorporated into the regional climate model, some climate characteristics, e.g. the 3-month-mean surface temperature, circulation, and precipitation, are significantly and systematically changed over the model domain, and the change of the characteristics differs depending on the area. Due to the better representation of the shading effect in the big-tree model, the temperature of the lower layer atmosphere above the plant canopy is increased, which further influences the 850hPa temperature. In addition, there are significant decreases in the mean latent heat fluxes (within 20–30W/m²) in the three areas of the model domain.The application of the big-tree model influences not only the simulated climate of the forested area, but also that of the whole model domain, and its impact is greater on the lower atmosphere than on the upper atmosphere. The simulated rainfall and surface temperature deviate from the originally simulated result and are (or seem to be) closer to the observations, which implies that an appropriate representation of the big-tree model may improve the simulation of the summer monsoon climate.We also find that the simulated climate is sensitive to some big-tree parameter values and schemes, such as the shape, height, zero-plane displacement height and mixing-length scheme. The simulated local/grid differences may be very large although the simulated areal-average differences may be much lower. The area-average differences in the monthly-mean surface temperature and heat fluxes can amount to 0.5°C and 4W/m², respectively, which correspond to maximum local/grid differences of 3.0°C and 40W/m² respectively. It seems that the simulated climate is most sensitive to the parameter of the zero-plane displacement among the parameters studied.     

Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model

The effect of changes in zonal and meridional atmospheric moisture transports on Atlantic overturning is investigated. Zonal transports are considered in terms of net moisture export from the Atlantic sector. Meridional transports are related to the vigour of the global hydrological cycle. The equilibrium thermohaline circulation (THC) simulated with an efficient climate model is strongly dependent on two key parameters that control these transports: an anomaly in the specified Atlantic–Pacific moisture flux (F_a) and atmospheric moisture diffusivity (K_q). In a large ensemble of spinup experiments, the values of F_a and K_q are varied by small increments across wide ranges, to identify sharp transitions of equilibrium THC strength in a 2-parameter space (between Conveyor On and Off states). Final states from this ensemble of simulations are then used as the initial states for further such ensembles. Large differences in THC strength between ensembles, for identical combinations of F_a and K_q, reveal the co-existence of two stable THC states (Conveyor On and Off)—i.e. a bistable regime. In further sensitivity experiments, the model is forced with small, temporary freshwater perturbations to the mid-latitude North Atlantic, to establish the minimum perturbation necessary for irreversible THC collapse in this bistable regime. A threshold is identified in terms of the forcing duration required. The model THC, in a Conveyor On state, irreversibly collapses to a Conveyor Off state under additional freshwater forcing of just 0.1 Sv applied for around 100 years. The irreversible collapse is primarily due to a positive feedback associated with suppressed convection and reduced surface heat loss in the sinking region. Increased atmosphere-to-ocean freshwater flux, under a collapsed Conveyor, plays a secondary role.     

Monoterpene concentrations in and above a forest of scots pine

Diurnal and vertical ambient air measurements of the monoterpenes have been made in and above a Scots pine (Pinus sylvestris) forest of central Sweden, within the boreal northern coniferous biome. Sampling was done with Tenax TA, and analysis by GC and ion trap detection. Daytime mixing ratios were on the order of tenths of a ppbv from the forest floor to the top of the forest, and a factor of 2 or 3 lower above the forest. Mixing ratios at night were at the ppbv level, highest near the forest floor and the crown, and decreased with height above the forest. The highest total concentration observed was 8 ppbv inside the forest at 3 am (GMT). The average terpene composition was 3-carene 32%, -pinene 29%, limonene 18%, -pinene 10%, -phellandrene 7%, camphene 5%, and sabinene at less than 2%. The 3-carene/-pinene ratio varied with wind direction and speed, relative humidity, and wet/dry vegetation, but not with ozone or NO₂ concentration, solar radiation, or temperature. Variations in the observed terpene composition at the sampling site are mainly caused by the influence of other vegetation in the vicinity of the site. It would seem that wet Scots pine emits more 3-carene relative to -pinene than does dry pine.     

Flux–Profile Relationships Over a Fetch Limited Beech Forest

The influence of an internal boundary layer and a roughness sublayer on flux–profile relationships for momentum and sensible heat have been investigated for a closed beech forest canopy with limited fetch conditions. The influence was quantified by derivation of local scaling functions for sensible heat flux and momentum (_h and _m) and analysed as a function of atmospheric stability and fetch. For heat, the influences of the roughness sublayer and the internal boundary layer were in agreement with previous studies. For momentum, the strong vertical gradient of the flow just above the canopy top for some wind sectors led to an increase in _m, a feature that has not previously been observed. For a fetch of 500 m over the beech forest during neutral atmospheric conditions, there is no height range at the site where profiles can be expected to be logarithmic with respect to the local surface. The different influence of the roughness sublayer on _h and _m is reflected in the aerodynamic resistance for the site. The aerodynamic resistance for sensible heat is considerably smaller than the corresponding value for momentum.     

A study of the sensitivity of bare soil evaporation schemes to soil surface wetness,using the coupled soil moisture and surface temperature prediction model,BARESOIL

Summary The performance of evaporation schemes with and approach and their combination within resistance representation of evaporation from bare soil surface is discussed. For this purpose nine schemes, based on different functions of or , on the ratio of the volumetric soil moisture content and its saturated value are used.The quality of the chosen schemes has been evaluated using the results of time integration by the coupled soil moisture and surface temperature prediction model, BARESOIL, using in situ data. A sensitivity analysis was made using two sets of data derived from the volumetric soil moisture content of the top soil layer. One with values below the wilting point (0.17 m³m^–3) and the second with values above 0.20m³m^–3. Data sets were obtained at the experimental site Rimski anevi, Yugoslavia, from the bare surface of a chernozem soil.With 4 Figures     

Paleoclimate data constraints on climate sensitivity: The paleocalibration method

The relationship between paleoclimates and the future climate, while not as simple as implied in the paleoanalog studies of Budyko and others, nevertheless provides sufficient constraints to broadly confirm the climate sensitivity range of theoretical models and perhaps eventually narrow the model-derived uncertainties. We use a new technique called paleocalibration to calculate the ratio of temperature response to forcing on a global mean scale for three key intervals of Earth history. By examining surface conditions reconstructed from geologic data for the Last Glacial Maximum, the middle Cretaceous and the early Eocene, we can estimate the equilibrium climate sensitivity to radiative forcing changes for different extreme climates. We find that the ratios for these three periods, within error bounds, all lie in the range obtained from general circulation models: 2–5 K global warming for doubled atmospheric carbon dioxide. Paleocalibration thus provides a data-based confirmation of theoretically calculated climate sensitivity. However, when compared with paleodata on regional scales, the models show less agreeement with data. For example, our GCM simulation of the early Eocene fails to obtain the temperature contrasts between the Equator and the Poles (and between land and ocean areas) indicated by the data, even though it agrees with the temperature data in the global average. Similar results have been reported by others for the Cretaceous and for the Last Glacial Maximum.The U.S. Government right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.     

Statistics of Surface-Layer Turbulence Over Terrain with Metre-Scale Heterogeneity

Refuge has patchy vegetation in sandy soil. During midday and at night, the surface sources and sinks for heat and moisture may thus be different. Although the Sevilleta is broad and level, its metre-scale heterogeneity could therefore violate an assumption on which Monin-Obukhov similarity theory (MOST) relies. To test the applicability of MOST in such a setting, we measured the standard deviations of vertical (_w) and longitudinal velocity (_u), temperature (_t), and humidity (_q), the temperature-humidity covariance (¯tq), and the temperature skewness (S_t). Dividing the former five quantities by the appropriate flux scales (u_*, _*, and q_*) yielded the nondimensional statistics _w/u_*, _u/u_*, _t/|t_*|, _q/|q_*|, and ¯tq/t_*q_*. _w/u_*, _t/|t_*|, and S_t have magnitudes and variations with stability similar to those reported in the literature and, thus, seem to obey MOST. Though _u/u_* is often presumed not to obey MOST, our _u/u_* data also agree with MOST scaling arguments. While _q/|q_*| has the same dependence on stability as _t/|t_*|, its magnitude is 28% larger. When we ignore ¯tq/t_*q_* values measured during sunrise and sunset transitions – when MOST is not expected to apply – this statistic has essentially the same magnitude and stability dependence as (_t/t_*)². In a flow that truly obeys MOST, (_t/t_*)², (_q/q_*)², and ¯tq/t_*q_* should all have the same functional form. That (_q/q_*)² differs from the other two suggests that the Sevilleta has an interesting surface not compatible with MOST. The sources of humidity reflect the patchiness while, despite the patchiness, the sources of heat seem uniformly distributed.     

Equilibrium evaporation beneath a growing convective boundary layer

Expressions for the equilibrium surface Bowen ratio ( _s ) and equilibrium evaporation are derived for a growing convective boundary layer (CBL) in terms of the Bowen ratio at the top of the mixed layer _i and the entrainment parameter A _R . If A_R is put equal to zero, the solution for _s becomes-that previously obtained for the zero entrainment or closed box model. The Priestley-Taylor parameter is also calculated and plotted in terms ofA _R and _i . Realistic combinations of the atmospheric parameters give values of in the range 1.1 to 1.4.     

Experiments On Buoyant Plume Dispersion In A Laboratory Convection Tank

Plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focusis on highly-buoyant plumes that loft near or become trapped in the CBL capping inversion and resistdownward mixing. Such plumes are defined by dimensionless buoyancy fluxes F_* 0.1, where F_* = F_b/(U w_* ² z_i), F_b is the stack buoyancy flux,U is the mean wind speed, w_* is the convective velocity scale, and z_i is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, _c) concentration fields as a function of F_* and the dimensionless distance X = w_*x/(U z_i), where x is the distance downstream of the source.The experiments reveal the following mainresults: (1) For 3 X 4and F_* 0.1, the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below z_i with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff. (2) The lateral dispersion (_y) variation with X is consistent with Taylor's theory for _* 0.1 and a buoyancy-enhanced dispersion, _y/z_i F_* ^1/3X^2/3, forF_* = 0.2 and 0.4. (3) The entrapment, the plume fraction above z_i, has a mean (E) that follows a systematic variationwith X and F_*, and a variability (_e/E) that is broad ( 0.3 to 2) near the source but subsides to 0.25 far downstream. (4) Vertical profiles of the concentration fluctuation intensity (c/C) are uniform for z < z_i and X > 1.5, but exhibit significant increases: (a) at the surface and close to the source (X 1.5), and(b) in the entrainment zone. (5) The cumulative distribution functions (CDFs) of the scaled concentration fluctuations (c/_c) separate into mixed-layer and entrainment-layer CDFs for X 2, with the mixed-layer group collapsing to a single distribution independent of z.These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous buoyant releases in a convection tank. They also are the first tank experiments to demonstrate agreement with field observations of: (1) the scaled ground-level concentration along the plume centreline, and (2) the dimensionless lateral dispersion _y/z_i of buoyant plumes.     

Public expectation as an element of human perception of climate change

Human expectations regarding weather and climate sometimes lead to perceptions of climate change which are not supported by observational evidence. This paper analyses two very characteristic complaints about current climate in Switzerland, i.e., the lack of snow in winter and the lack of sunshine in summer, through a statistical investigation of climatological data. As one major problem of public perception of climate in mid-latitude regions is linked to the strong variability of the climatic parameters, the paper suggests means of presenting climatic data which include a measure of this variability. Such presentations would help overcome the common confusion between the terms weather and climate, and stress the fact that short-term extreme events are not necessarily indicative of a long-term shift in climate.