The impact of deforestation on cloud cover over the Amazon arc of deforestation

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Abstract

Atmospheric general circulation model (AGCM) simulations predict that a complete deforestation of the Amazon basin would lead to a significant climate change; however, it is more difficult to determine the amount of deforestation that would lead to a detectable climate change. This paper examines whether cloudiness has already changed locally in the Brazilian arc of deforestation, one of the most deforested regions of the Amazon basin, where over 15% of the primary forest has been converted to pasture and agriculture. Three pairs of deforested/forested areas have been selected at a scale compatible with that of climate model grids to compare changes in land cover with changes in cloudiness observed in satellite data over a 10-year period from 1984 to 1993. Analysis of cloud cover trends suggests that a regional climate change may already be underway in the most deforested part of the arc of deforestation. Although changes in cloud cover over deforested areas are not significant for interannual variations, they are for the seasonal and diurnal distributions. During the dry season, observations show more low-level clouds in early afternoon and less convection at night and in early morning over deforested areas. During the wet season, convective cloudiness is enhanced in the early night over deforested areas. Generally speaking, the results suggest that deforestation may lead to increased seasonality; however, some of the differences observed between deforested and forested areas may be related to their different geographical locations.

Introduction

The Amazon forest is the largest continuous tropical forest ecosystem in the world. Deforestation is rapidly progressing in Amazonia, particularly in the Brazilian arc of deforestation (southeast part of the Amazonian basin). The mean rate of gross deforestation in the Legal Amazon (Brazilian part of the Amazon basin) from 1978 to 1998 was 0.66%/year. In 20 years, 13.2% of the primary forests of Brazil were converted to pasture and agriculture (INPE, 2000). Other Amazonian countries such as Peru (Gentry & Lopez-Parodi, 1980), Colombia, Venezuela, Ecuador, and Bolivia (Myers, 1982) also have high rates of deforestation. If deforestation continues at this rate, most of the Amazonian tropical forest will disappear in less than 100 years (Nobre, Sellers, & Shukla, 1991).

In recent years, the possible impact of complete deforestation of the Amazon on regional and global climate has been studied using atmospheric general circulation model (AGCM) simulations. Costa and Foley (2000) summarized the results of eight recent AGCM experiments and compared them with their own results. All AGCMs predict higher surface temperatures and lower evapotranspiration rates as the two major effects of complete Amazonian deforestation. On the other hand, predictions concerning precipitation vary greatly: while the majority of simulations indicate a decrease in precipitation, some simulations show an increase Dirmeyer & Shukla, 1991, Polcher & Laval, 1994. In addition, all of these AGCM simulations totally disagree on the magnitude (and even the sign) of runoff changes after deforestation (Costa & Foley, 2000).

The Amazon basin is a major heat source in the general atmospheric circulation system and large-scale deforestation should have a strong impact on the local, regional, and global climate. However, such climatic change has not yet been observed in Amazonia even though large areas have already been deforested. The key element of climate in these regions is the rainfall. Unfortunately, it is difficult to accurately estimate this parameter due to the limited density of the rain gauge network. Using a different approach, Richey, Nobrey, and Deser (1989) analysed long-term (1903–1985) river flow data for the Manacapuru river near Manaus and concluded that the river flow fluctuation was marked by a 2- to 3-year periodicity and that there was no statistically significant change on the decadal time scale. Another approach consists in analysing the cloud cover, given that long-time series of satellite observations are now available. Using the monthly mean outgoing long-wave radiation (OLR) data from the NOAA polar-orbiting satellites and monthly rainfall totals at Belem and Manaus in the past 15 years, Chu, Yu, and Hastenrath (1994) found indications of a slight rainfall increase (related to an increase in convection) associated with deforestation over almost all of the Amazon basin (see Fig. 1). The most significant increase in convection was found in the western equatorial part of Amazonia, along the eastern slope of the Andes, where rainfall is most abundant. The rainfall series at Belem and Manaus also show positive trends (although the 5% significance level is not reached using Mann–Kendall rank statistics).

To understand the possible impact of deforestation on cloudiness, some useful information is provided by local experiments such as the Anglo-BRazilian Amazonian Climate Observation Study (ABRACOS; Gash, Nobre, Roberts, & Victoria, 1998) or the Large-Scale Biosphere Atmosphere (LBA; LBA, 1996) experiment. Observations of the dynamics of the convective boundary layer from ABRACOS provide evidence that the lower moisture availability over pasture land develops a deeper boundary layer because of higher sensible heating, especially during the dry season (Fisch, Culf, & Nobre, 1996). Also, modelling simulations by Avissar and Liu (1996) and observations during the ABRACOS experiment by Dias and Regnier (1996) show that the surface inhomogeneity caused by the interruption of the tropical rain forest by patches of pasture induces differences in the surface sensible heat fluxes. These differences can affect surface fluxes and generate mesoscale circulation in (and even above) the boundary layer, which in turn may trigger cumulus convection and increase the recycling of water Dalu et al., 1996, Pielke et al., 1998. Observations in South Amazonia have shown that deforestation can result in an increase in dry season afternoon fair weather cumulus clouds (Cutrim, Martin, & Rabin, 1995). Thus, the increase in surface albedo that accompanies deforestation could be augmented by a cloud-induced increase in atmospheric albedo.

It is generally difficult to compare observations and AGCM simulations because AGCM studies assume deforestation over the entire basin, which is not yet the case even though large areas of Amazonia have been converted from forest to pasture and agricultural land. The climatic response to deforestation is probably nonlinear and AGCM studies do not provide an indication of the amount of deforestation that leads to significant changes in the behavior of the atmospheric processes. It is therefore important to determine whether the present extent of deforestation has already started a climate change process that can be detected in available observation datasets.

This paper examines the climatic effects of land cover changes on local cloudiness, focusing on the Amazonian arc of deforestation. The aim is to determine whether cloud cover changes due to land cover changes are already detectable at a scale compatible with climate model grid cells (GCs). Today, long series of satellite observations are available and, at the same time, land use changes in the Legal Amazon have been accurately estimated by different programs INPE, 2000, Tropical Rain Forest Information Center (TRFIC), 2000. Our methodology is based on the comparison of cloud feature trends at several time scales for pairs of reference and deforested areas with similar elevations and locations. The period of study was determined by available datasets and extends from January 1984 to December 1993.

The data used are presented in Section 2. Section 3 deals with the selection and the characteristics of the grid cells used as reference and deforested areas. The results concerning cloud cover variations at different time scales are given in Section 4 and discussed in Section 5.

Section snippets

Cloud data

The cloud dataset used in this study comes from the International Satellite Cloud Climatology Project (ISCCP; Rossow & Schiffer, 1991) and covers a 10-year period from 1984 to 1993. We used the stage D2 dataset that gives monthly averaged data for the eight 3-h time intervals of the diurnal cycle. From the 130 ISCCP D2 variables describing the cloud cover characteristics, we selected the following parameters: total cloud cover (TCC), cloud top pressure (CTP), cloud vertical optical thickness

Sampling criteria

The sampling methodology was based on the following criteria:

  • Area samples should correspond to ISCCP 2.5°×2.5° grid cells.

  • Amazon GCs were classified as deforested and nondeforested, based on the TRFIC maps of deforestation. Nondeforested GCs were those with less than 5% of the GC area deforested and a deforestation rate of less than 0.4%/year. Deforested GCs were those in the vicinity of nondeforested GCs with the largest deforestation area and the highest deforestation rate.

  • Another criterion

Interannual variability

The interannual variability of the mean cloud cover and precipitation for the three GC pairs is illustrated for target areas R1 and D1 in Fig. 4. TCC and HCC show maxima in 1985 and 1989 and minima in 1987, 1988, 1992, and 1993. The TCC minima are associated with El Niño Southern Oscillation (ENSO) events whereas TCC maxima are associated with La Niña Southern Oscillation (LNSO) events. Six of the 10 years of study are concerned by LNSO or ENSO events while 1984, 1986, 1990, and 1991 are

Summary and discussion

Cloud cover comparisons of deforested and forested areas suggest that a regional climate change is already underway in the most deforested part of the arc of deforestation. For a climate model grid scale and a 10-year study period, the results do not allow us to make a statistically significant conclusion on the hypothesis of less convection over deforested regions on the interannual time scale. This is due essentially to the strong interannual impact of the LNSO/ENSO events compared to the

Acknowledgements

This work is part of a joint project of the Institut de Recherche pour le Développement (IRD) and the Conselho Nacional de Desenvolvimento Cientifico e Tecnólogico (CNPq), under support no. 910153/98-1 and 690089/01-5. This work was also partially funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), under support no. 99/06045-7. It has been coordinated by the Aerospace Technical Center/Space and Aeronautic Institute/Atmospheric Science Division (CTA/IAE/ACA) of São José

References (23)

  • J. Polcher et al.

    The impact of African and Amazonian deforestation on tropical climate

    Journal of Hydrology

    (1994)
  • R. Avissar et al.

    Three-dimensional numerical study of shallow convective clouds and precipitation induced by land surface forcing

    Journal of Geophysical Research

    (1996)
  • P.-S. Chu et al.

    Detecting climate change concurrent with deforestation in the Amazon basin: Which way has it gone?

    Bulletin of the American Meteorological Society

    (1994)
  • M.H. Costa et al.

    Combined effects of deforestation and doubled atmospheric CO2 concentrations on the climate of Amazonia

    Journal of Climate

    (2000)
  • A.D. Culf et al.

    Radiation, temperature and humidity over forest and pasture in Amazonia

  • E. Cutrim et al.

    Enhancement of cumulus clouds over deforested lands in Amazonia

    Bulletin of the American Meteorological Society

    (1995)
  • G.A. Dalu et al.

    Heat and momentum fluxes induced by thermal inhomogeneities

    Journal of Atmospheric Sciences

    (1996)
  • M.A.F.S. Dias et al.

    Simulation of mesoscale circulations in a deforested area of Rondônia in the dry season

  • P.A. Dirmeyer et al.

    The impact on climate of Amazon deforestation in a GCM with interactive clouds

  • G. Fisch et al.

    Modelling convective boundary layer growth in Rondônia

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