Large-scale hydrological responses to tropical deforestation

Macroscale Modelling of the Hydrosphere (Proceedings of the Yokohama Symposium, July 1993). V IAHSPubl.no. 214,19^3. *

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Large-scale hydrological responses to tropical deforestation

T. B. DURBIDGE & A. HENDERSON-SELLERS Climatic Impacts Centre, School of Earth Sciences, Macquarie University, Sydney, New South Wales 2109, Australia

Abstract Deforestation, especially tropical deforestation, is a geopolitical issue: tropical forests contain the vast majority of the world's species, deforestation contributes about 20% of the current flux of carbon dioxide to the atmosphere, and, it is often argued, that sustainable agriculture is impossible in these regions. Two regions have been particularly identified as areas where the impacts of the extensive removal of moist, tropical forest are potentially critical to the sustained activities of humans; these are the Amazon basin in South America and Southeast Asia. These regions contain the largest, contiguous tropical forest resource remaining on Earth and play essential roles in the regional climate and hydrology. The deforestation of the Amazon region has been the subject of a number of previous investigations which differ significantly in experimental design and results. Therefore part of this research provides further insight into the understanding of the consequences of deforestation. As a first step to extending these deforestation investigations, the Southeast Asian region presents a useful arena to examine model sensitivities to the different surface and atmospheric features and permits insight into the teleconnections that exist between the regions at a global scale. Results show that two elements of the macroscale hydrosphere are impacted by tropical deforestation in Southeast Asia: evaporation is significantly decreased while surface runoff is increased. In addition, we find a weakening in the Hadley circulation over the deforested area of Southeast Asia. The Walker circulation is most strongly affected in July when the Atlantic and east Pacific cells are replaced by large-scale descending motion from ~ 140°W to ~ 100°E.

WHY STUDY TROPICAL DEFORESTATION? The claim is often advanced that the removal of tropical rainforests will substantially alter the climate, either by adding C0 2 to the atmosphere, thereby enhancing the greenhouse effect, or by increasing the global surface albedo (Bolin, 1977; Woodwell etal, 1978; Sagan etal., 1979; Hampicke, 1980; Potter et ah, 1981; Shukla & Mintz, 1982). Indeed the "devastating" impact on the climate is often cited as an important reason for protecting tropical forest. Whatever the truth of these claims, the tropical forests of the world offer excellent scientific and pertinent political motivation for study of the sensitivity of global climate models to land-atmosphere interactions. The last decade has witnessed an increasing level of sophistication in the models applied to this problem. In all there have been six global climate model (GCM) studies of the climatic impact of tropical deforestation published in the literature: Henderson-Sellers & Gornitz (1984), Dickinson & Henderson-Sellers (1988), Lean & Warrilow (1989), Shukla et al. (1990), Nobre et al. (1991), Mylne & Rowntree (1992) and Henderson-Sellers et al. (1993).

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The model used here is an updated version of the currently available NCAR Community Climate Model (CCM1). A full description of CCM1 is given in Williamson et al. (1987) and circulation statistics from seasonal and perpetual January and July simulations of the standard version of CCM1 are given in Williamson & Williamson (1987). CCMl-Oz is a modified version of CCM1 which includes the current version of the Biosphere-Atmosphere Transfer Scheme (BATS IE) and a mixed-layer, slab ocean of 50 m depth. The mixed layer ocean model includes a three-layer ice model sub-component and a standard q-flux scheme to correct for ocean advection of energy and the prescription of a fixed mixed layer depth. CCMl-Oz includes a number of modifications to the physics subroutines including the Slingo clouds and radiation updates (Slingo, 1989). The model simulates full seasonal and diurnal cycles. Review of a number of standard global fields shows that the general circulation of the atmosphere is well simulated. The experiment conducted is to modify the specified land use in a total of twenty seven R15 grid elements. In all cases the ecotype was changed from tropical moist forest to scrub grassland, the soil texture made finer by two classes and the soil colour made lighter by two classes. For Amazonia, the 18 points were modified and in South East Asia, 9 points were deforested (Fig. 1).

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This evaluation focusses primarily on the large-scale hydrological responses to deforestation in Southeast Asia. The Southeast Asian region is important as a secondary focus of the simulations described in Henderson-Sellers et al. (1993) because it contains the next largest area of contiguous tropical forest to the Amazon basin and in considerably different land- surface and climatic regimes. Also, as tropical deforestation is being investigated in the context of the global climate system, the teleconnections that may exist between the two regions can be investigated.


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REGIONAL-SCALE IMPACTS OF INSTANTANEOUS TROPICAL DEFORESTATION IN SOUTHEAST ASIA The Southeast Asian Region's seasonal cycle is dominated by two main features of the regional climate. The first is the monsoon circulation, which is resolved into distinct and opposite phases of the wet and the dry seasons. The second is the seasonal passage of the Intertropical Convergence Zone (ITCZ), to which the monsoon circulation is related. The ITCZ follows the solar migration passing over the region of Southeast Asia approximately three months out of phase with the monsoon circulation. Climatic variables within a region bounded by latitudes 20°N and 11°S and longitudes 95°E and 150°E containing the Southeast Asia land points have been spatially averaged. Assessment of the impact of the instantaneous deforestation has been based here on both physical reasoning and on determination of statistically significant responses. The statistical test employed is a Student's t test. The averaging, which excludes the ocean points, has been used to represent the natural variation of the climate for this region in the form of plots showing ±2 standard errors: about a 95% confidence interval. Differences in the climate in the deforested case outside these error bars (i.e. changes that exceed the natural variability of the particular climate feature) are likely to be statistically significant. The plots of evaporation for the 72 months of the control and deforested simulations (Fig. 2(a)) reveal an overall decrease in evaporation. The decrease is between about 5 and 15 W m"2 for the spatially averaged region in the dry and wet seasons, respectively. The seasonal cycle is maintained for all the years of the simulation but the slight upward trend of the peak evaporation values for the control situation is not seen in the deforested situation. Figure 2(b) shows that the decreases are mostly well outside the range of natural variation. The absorbed solar radiation at the surface shows an overall decrease of about 10 W m"2 while maintaining the seasonal pattern; the differences are well outside the natural variability of the region and are significant. The net radiation shows a similar seasonal cycle to the absorbed solar radiation with an overall decrease of about 10 W m"2 which is at a maximum in the mid-year period. The temperatures of the soil surface and root zone show very similar changes with a decrease of about 0.5 to 1.0 K peaking in May, which is well outside the natural variability of the region. The Stevenson Screen temperature shows a less marked decrease of between 0 and 0.6 K with the same maximum in May and a similar seasonal cycle. The months showing no change are, of course, inside the 95 % confidence range, but all other differences are outside the natural variation. The diurnal skin temperature range increases by between 1.2 and 2.5 K for all months and well exceeds the confidence interval of ±2 standard errors. Total runoff and especially surface runoff (Figs 2(c) and (d)) show increases of 0.5 mm and 1.0 mm, respectively, that lie outside the 95% confidence interval and show a marked reduction in annual variation for the last 2 years of the simulation. Total precipitation (Fig. 2(e)) shows a slight increase of about 1 mm day"1, which exceeds the 95% confidence interval, for January but shows a decrease of about 2 mm day"1 in the wet season for the deforested case, with little change during other months. The plots showing the depth of water on the foliage show only a general pattern of increase during the months of the dry season and a decrease during the period of the wet

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season. The variation in total cloud shows almost no change due to deforestation, except for slight increases in April and October corresponding to the passage of the ITCZ. The water in the root zone (Fig. 2(f)) shows a very marked decrease of about 100 mm due to deforestation for all months but this is due to the prescribed nature of the soil parameters for this experiment. Overall, the most significant impact on Southeast Asia's regional macroscale hydrology is the reduction in the evaporative flux. This occurs preferentially in the rainy season when there is also a decrease in precipitation. Surface runoff is increased year-round.

LARGE-SCALE IMPACTS OF TROPICAL DEFORESTATION: WALKER AND HADLEY CIRCULATION CHANGES It has been hypothesized that large-scale land-surface disturbance, especially if it occurs coincidentally in time in more than one geographical location, might prompt circulation changes beyond the area of prescribed change. The only previous GCM experiment which was for a long enough time-period to permit this type of investigation (Henderson-Sellers & Gornitz, 1984) did not detect changes "external" to the deforested area. As this investigation differs in a number of important ways from this earlier study (viz: higher spatial resolution GCM, more complex land-surface scheme, better regional climate prediction for the present-day) it was thought useful to investigate the possibility of larger-scale changes once again. An anticipated outcome of tropical deforestation is a reduction in the vertical ascent over the deforested region(s) caused by the reduction in evaporation and the smaller turbulent exchanges, themselves resulting from the imposed decreases in canopy extent and vegetation roughness length. This, in turn, might affect either the Walker or Hadley circulations in the area and, more importantly, the effects of deforestation in two regions might interact, perhaps re-inforcing the "external" disturbance they prompt. Here we assess these possibilities by considering the changes in the vertical velocity (Pa s"1 ) over the two deforested regions. Figure 3 depicts a latitudinal cross-section designed to capture the cells of the Walker circulation and a longitudinal cross-section selected to depict the latitudinal movement of the ITCZ in the deforested region of interest: Southeast Asia. Figures 3(a)-(d) shows the Walker circulation for the control (a) and (b), the deforestation experiment (c) and (d) for January and July, respectively. The control experiment shows the cells of the Walker circulation very clearly in both months. In both seasons there is large-scale ascent over the Amazon basin (~50°W), over Southeast Asia (~ 100°E) and over the western Pacific (~ 140°E-180°E). In January (Fig. 3(a)) there is additionally an ascending limb over tropical Africa (~25°E) but this does not appear in the July cross-section (Fig. 3(b)). Also ascent over the Amazon is significantly weaker in July than in January because the ITCZ lies well to the north of the basin in July. The regions of largest change seem to be the Amazon basin ( ~ 50°W) in January and for the eastern edge of Southeast Asia in July (~ 140°E). The effects of deforestation are easier to assess by direct comparison of the control and experiment vertical velocities but note that the ranges (but not the contour interval) differ slightly. In both seasons the ascent over the Amazon is considerably diminished

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hydrological responses to tropical deforestation

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in the deforestation experiment. The effect is greater in January because the ascent is very much stronger in the January control climate. This result is in agreement with the finding of Henderson-Sellers & Gornitz (1984). The impact of deforestation on the Walker circulation is essentially to remove the Atlantic and east Pacific cells in July so that descent predominates from ~ 140°W to ~ 100°E. Figures 3(e)-(h) depict the Hadley circulation as captured by a cross-section through the atmosphere extending from 90GE to 112°E which was selected to encompass the mainland areas of Southeast Asia where deforestation is imposed. The control case (Figs 3(e) and (f)) shows the position of the ITCZ on the equator in January and centred at about 12°N in July. The effects of deforestation are very much smaller in this region than the changes described by Henderson-Sellers et al. (1993) for South America. Nonetheless, the vertical ascent in the ITCZ is seen to be slightly diminished in both seasons (but note the difference in contour intervals between Figs 3(g) and (h)). Overall, the impact on the general circulation of the atmosphere can be detected in the cells of both the Hadley and the Walker circulations. In the case of the Hadley circulation, results show diminished ascent in the ITCZ over both deforested areas in all seasons. The impacts are smaller over Southeast Asia than in the Amazon in the case of the Walker circulation but teleconnections between the two deforested regions are suggested. This possibility is the topic of current research.

Acknowledgements This research comprises part of a Model Evaluation Consortium for Climate Assessment (MECCA, Phase 1) study. It has also been supported in part by the Australian Research Council, the Climatic Impacts Centre of Macquarie University, the Australian Federal Department of Arts, Sports, Environment, and Territories (DASET) and the National Center for Atmospheric Research (USA). This is Contribution no 92/13 of the Climatic Impacts Centre.

REFERENCES Bolin, B. (1977) Changes of land biota and their importance for the carbon cycle. Science 196, 613-621. Dickinson, R. E. & Henderson-Sellers, A. (1988) Modelling tropical deforestation: a study of GCM land-surface parameterizations. Quart. J. Roy. Meteorol. Soc. 114(B), 439-462. Hampicke, V. (1980) The role of the biosphere. In: Interactions of Energy and Climate (ed. by W. Bach, J. Pankrath & J. Williams), 149-167. Reidel, Dordrecht. Henderson-Sellers, A. & Gornitz, V. (1984) Possible climatic impacts of land cover transformations, with particular emphasis on tropical deforestation. Climatic Change 6, 231-258. Henderson-Sellers, A., Durbidge, T. B., Pitman, A. J., Dickinson, R. E., Kennedy, P. J. &McGuffie, K. (1993) Tropical deforestation: modelling local to regional-scale climate change. /. Geophys. Res. (in press). Lean, J. 8c Warrilow, D. A. (1989) Simulation of the regional climatic impact of Amazon deforestation. Nature 342, 411-413. Mylne, M . F. & Rowntree, P. R. (1992, Modelling the effects of albedo change associated with tropical deforestation. Climatic Change 21, 317-343. Nobre, C. A., Sellers, P. J. & Shukla, J. (1991) Amazonian deforestation and regional climate change. J. Clim. 4, 957-988. Potter, G. L., Ellsaesser, H. W., MacCracken, M. C. & Ellis, J. S. (1981) Albedo changeby man: test of climatic effects. Nature 291, 47-50. Sagan, C., Toon, 0. B. & Pollack, J. B. (1979) Anthropogenic albedo changes and the earth's climate. Science 206, 1363-1368. Shukla, J. & Mintz, Y. (1982) Influence of land-surface évapotranspiration on the Earth's climate. Science 215, 1498-1501.

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Shukla, } . , Nobre, C. & Sellers, P. J. (1990) Amazon deforestation and climate change. Science 247, 1322-1325. Slingo, A. (1989) A GCM parameterization for the shortwave radiative properties of water clouds. J. Atmos. Sci. 46, 1419-1427. Williamson, G. S. & Williamson, D. L. (1987) Circulation statistics from seasonal and perpetual January and July simulations with the NCAR Community Climate Model (CCM1): R15. National Center for Atmospheric Research, Technical Note, NCAR/TN-302+STR. Williamson, D. L., Kiehl, J. T., Ramanathan, V., Dickinson, R. E. & Hack, J. J. (1987) Description of the NCAR Community Climate Model (CCM1). National Center for Atmospheric Research, Technical Note, NCAR/TN-285+STR. Woodwell, G. M., Whittaker, R. H., Reiner, W. A., Likens, G. E., Detwicke, C. C. & Botkin, D. B. (1978) The biota and the world carbon budget. Science 199, 141-146.