Tropical deforestation and climate variability

A new tropical deforestation experiment has been performed, with the ARPEGE-Climat atmospheric global circulation model associated with the ISBA land surface scheme. Simulations are forced with observed monthly mean sea surface temperatures and thus inter-annual variability of the ocean system is taken into account. The local mean response to deforestation over Amazonia and Africa is relatively weak compared with most published studies and compensation effects are particularly important. However, a large increase in daily maximum temperatures is obtained during the dry season when soil water stress dominates. The analysis of daily variability shows that the distributions of daily minimum and maximum temperatures are noticeably modified with an increase in extreme temperatures. Daily precipitation amounts also indicate a weakening of the convective activity. Conditions for the onset of convection are less frequently gathered, particularly over southern Amazonia and western equatorial Africa. At the same time, the intensity of convective events is reduced, especially over equatorial deforested regions. The inter-annual variability is also enhanced. For instance, El Niño events generally induce a large drying over northern Amazonia, which is well reproduced in the control simulation. In the deforested experiment, a positive feedback effect leads to a strong intensification of this drying and a subsequent increase in surface temperature. The change in variability as a response to deforestation can be more crucial than the change of the mean climate since more intense extremes could be more detrimental for agriculture than an increase in mean temperatures.     

Tropical deforestation: Albedo and the surface-energy balance

Recent micrometeorological measurements for Amazonian rainforest are reviewed, emphasising those aspects of the radiation and heat balance which are likely to change with deforestation. The possible consequences of such deforestation are considered by examining the sensitivity of the surface energy balance to changes in those parameters which would be most drastically altered.     

Tropical storms: representation and diagnosis in climate models and the impacts of climate change

Tropical storms are located and tracked in an experiment in which a high-resolution atmosphere only model is forced with observed sea surface temperatures (SSTs) and sea ice. The structure, geographic distribution and seasonal variability of the model tropical storms show some similarities with observations. The simulation of tropical storms is better in this high-resolution experiment than in a parallel standard resolution experiment. In an anomaly experiment, sea ice, SSTs and greenhouse-gas forcing are changed to mimic the changes that occur in a coupled model as greenhouse-gases are increased. There are more tropical storms in this experiment than in the control experiment in the Northeast Pacific and Indian Ocean basins and fewer in the North Atlantic, Northwest Pacific and Southwest Pacific region. The changes in the North Atlantic and Northwest Pacific can be linked to El Niño-like behaviour. A comparison of the tracking results with two empirically derived tropical storm genesis parameters is carried out. The tracking technique and a convective genesis parameter give similar results, both in the global distribution and in the changes in the individual basins. The convective genesis parameter is also applied to parallel coupled model experiments that have a lower horizontal resolution. The changes in the global distribution of tropical storms in the coupled model experiments are consistent with the changes seen at higher resolution. This indicates that the convective genesis parameter may still provide useful information about tropical storm changes in experiments carried out with models that cannot resolve tropical storms.     

Tropical deforestation and atmospheric carbon dioxide

Recent estimates of the net release of carbon to the atmosphere from deforestation in the tropics have ranged between 0.4 and 2.5 × 10¹⁵ g yr^–1. Two things have happened to require a revision of these estimates. First, refinements of the methods used to estimate the stocks of carbon in the vegetation of tropical forests have produced new estimates that are intermediate between the previous high and low estimates of carbon stocks. When these revised estimates were used here to calculate the emissions of carbon from deforestation, the new range was 1.0–2.0 × 10¹⁵ g C.Second, the previous range of estimates of flux was based on rates of deforestation in 1980. Myers' recent estimate of the rates of tropical deforestation in 1989 is about 90% higher than the rates just 10 years ago. When these recent rates were used to calculate the current net flux of carbon to the atmosphere, the range was between 1.6 and 2.7 × 10¹⁵ g C.Other uncertainties expanded this range, however, to 1.1–3.6 × 10¹⁵ g C yr^–1. Three factors contributed about equally to the expanded range: rates of deforestation, the fate of deforested lands (permanent or temporary clearing), and carbon stocks of forests, including anthropogenic reductions of carbon stocks within forests (thinning or degradation).     

Probable impact of deforestation on hydrological processes

The various ways in which the forest cover may influence the atmospheric and soil processes controlling the hydrological cycle are examined. Case studies of extensive deforestation affecting the rainfall pattern are reviewed.     

Vegetation responses and feedbacks to climate: a review of models and processes

Vegetation changes both in stationary and changing climates. Such changes can significantly affect hydrological and climate dynamics. Probabilistic, inferential, empirical, statistical, threshold, ecophysiological, and mechanistic vegetation models provide tools and ideas to construct coupled climate and vegetation schemes to study climate/vegetation feedbacks. Their logic is discussed, typical applications are presented, and their usefulness is assessed. Developing coupled climate and vegetation schemes also implies tackling scaling issues explicitly. Just as the Courant-Friedrichs-Lewy (CFL) criterion guarantees that information is not transferred faster through space than time in climate models, information should be transmitted fast enough in vegetation models for the landscape to register vegetation responses. To guarantee that this is the case, a migration criterion, or m criterion, is proposed. The CFL criterion and the m criterion set formal constraints on the design of coupled atmosphere and vegetation schemes. In particular, the ratio of climate and vegetation space scales should be approximately five orders of magnitude less than the ratio of climate and vegetation time scales.     

Temporal patterns of deforestation and fragmentation in lowland Bolivia: implications for climate change

The lowlands of eastern and northeastern Bolivia are characterized by a transition between the humid evergreen forests of the Amazon Basin and the deciduous thorn-scrub vegetation of the Gran Chaco. Within this landscape lies one of the world’s best preserved areas: the ecoregion known as the Chiquitano dry forest, where deforestation patterns over a 30 year period were analyzed. Results indicate that the area of the natural cover was reduced from 97.21 % before 1976 to 82.10 % in 2008, causing significant change in the landscape, especially in the spatial configuration of forest cover. The density of forest fragments increased from 0.073 patches per 100 ha before 1976 to 0.509 in 2008, with a mean distance between patches of 151 and 210 m over the same period, leading to a considerable reduction in the fragment sizes, from 1,204 ha before 1976 to a mere 54 in 2008. This pattern, observed in forests, does not occur in the savannas because, on one hand the savanna area is much lower compared to that of forests, and on the other because the deforestation process tended to be concentrated within forested areas. Based on the observed patterns, it is possible that in the future the natural landscapes will be substituted principally by anthropic landscapes, if there is no change in the economic and land distribution policies. If this process continues, it will stimulate the expansion of mechanized agriculture and the colonization of new areas, which will lead to further deforestation and landscape fragmentation.     

The impact of deforestation on the hydrological cycle in the western Mediterranean: an ensemble study with two regional climate models

 A deforestation experiment is performed over the western Mediterranean, applying two different RCMs with differing domains and an ensemble technique to obtain a measure of their internal variability. The internal variability is used to assign statistical significance to the results, and also to discuss whether the models are sufficiently free to develop internal mesoscale processes. Considerable internal variability values found for hydrological variables even in autumn and winter seem to support the assumption that the models are free enough to be applied to such a sensitivity study. The combined use of two models, with strongly differing domains, and significance assigned through the use of internal variability should highlight responses to deforestation which are of physical origin and not a result dependent on one particular model. The overall significant response from both RCMs to deforestation is a reduction of evaporation (spring and summer, extending over the whole deforested zone) and a decrease in precipitation (late spring and summer, over some regions). A detailed analysis over subzones shows remarkable agreement between the two models over some of these subzones, showing non-local effects in precipitation response. Received: 8 February 2000 / Accepted: 12 January 2001     

Moisture-transfer coefficient for climate models

Recent parameterizations of the moisture-transfer coefficient from measurements in the field and from tuning of the ECMWF model are reviewed. A formula for the neutral transfer coefficient varying continuously with the wind velocity is proposed for climate models.     

A new framework for isolating individual feedback processes in coupled general circulation climate models. Part I: formulation

This paper proposes a coupled atmosphere–surface climate feedback–response analysis method (CFRAM) as a new framework for estimating climate feedbacks in coupled general circulation models with a full set of physical parameterization packages. The formulation of the CFRAM is based on the energy balance in an atmosphere–surface column. In the CFRAM, the isolation of partial temperature changes due to an external forcing or an individual feedback is achieved by solving the linearized infrared radiation transfer model subject to individual energy flux perturbations (external or due to feedbacks). The partial temperature changes are addable and their sum is equal to the (total) temperature change (in the linear sense). The decomposition of feedbacks is based on the thermodynamic and dynamical processes that directly affect individual energy flux terms. Therefore, not only those feedbacks that directly affect the TOA radiative fluxes, such as water vapor, clouds, and ice-albedo feedbacks, but also those feedbacks that do not directly affect the TOA radiation, such as evaporation, convections, and convergence of horizontal sensible and latent heat fluxes, are explicitly included in the CFRAM. In the CFRAM, the feedback gain matrices measure the strength of individual feedbacks. The feedback gain matrices can be estimated from the energy flux perturbations inferred from individual parameterization packages and dynamical modules. The inter-model spread of a feedback gain matrix would help us to detect the origins of the uncertainty of future climate projections in climate model simulations.     

An inter-comparison of regional climate models for Europe: model performance in present-day climate

The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation.     

Numerical Simulation for the Impact of Deforestation on Climate in China and Its Neighboring Regions

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North African climate variability. Part 1: Tropical thermocline coupling

Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000. The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF time scores. Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere. The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the Atlantic zonal circulation by a few months.     

North African climate variability. Part 2: Tropical circulation systems

Summary Tropical North African climate variability is investigated using a Sahel rainfall index and streamflow of the Nile River in the 20^th century. The mechanisms that govern tropical North Africa climate are diagnosed from NCEP reanalysis data in the period 1958–1998: spatially – using composite and correlation analysis, and temporally – using wavelet co-spectral analysis. The Sahelian climate is characterised by a decadal rhythm, whilst the mountainous eastern and equatorial regions exhibit interannual cycles. ENSO-modulated zonal circulations over the Atlantic/Pacific sector are important for decadal variations, and create a climatic polarity between South America and tropical North Africa as revealed through upper-level velocity potential and convection patterns. A more localised N–S shift in convection between the Sahel and Guinea coast is associated with the African Easterly Jet.     

A new framework for isolating individual feedback processes in coupled general circulation climate models. Part II: Method demonstrations and comparisons

We here use a coupled atmosphere-surface single column climate model to illustrate how the CFRAM, a new climate feedback analysis framework formulated in Part I of the two-part series papers, can be applied to isolate individual contributions to the total temperature change of a climate system from the external forcing alone, and from each of individual physical and dynamical processes associated with the energy transfer with the space and within the climate system. We demonstrate that the isolation of individual feedbacks in the CFRAM is achieved without referencing to a virtual climate system as in the online feedback suppression method. We show that partial temperature changes estimated by the online feedback suppression method include the “compensating effects” of other feedbacks when the feedback under consideration is suppressed. The partial temperature changes are addable in the CFRAM but they are not in the online feedback suppression method. We also apply the CFRAM to isolate the contributions to the lapse rate feedback from individual physical and dynamical feedback processes. We show that the lapse rate feedback includes not only the partial effect of each feedback that directly contributes to energy flux perturbations at the TOA (such as water vapor feedback), but also the total effects of those feedbacks that do not contribute to energy flux perturbations at the TOA (such as evaporation and moist convection feedbacks). Because the contributions to the lapse rate feedback from various physical and dynamical processes tend to cancel one another, the net lapse rate feedback is a residual of many large terms. This leads to a large uncertainty not only in estimating the lapse rate feedback itself, but also in other feedbacks whose effects are either partially or totally lumped into the lapse rate feedback.