Interactions between rainfall, deforestation and fires during recent years in the Brazilian Amazonia

Abstract

Understanding the interplay between climate and land-use dynamics is a fundamental concern for assessing the vulnerability of Amazonia to climate change. In this study, we analyse satellite-derived monthly and annual time series of rainfall, fires and deforestation to explicitly quantify the seasonal patterns and relationships between these three variables, with a particular focus on the Amazonian drought of 2005. Our results demonstrate a marked seasonality with one peak per year for all variables analysed, except deforestation. For the annual cycle, we found correlations above 90% with a time lag between variables. Deforestation and fires reach the highest values three and six months, respectively, after the peak of the rainy season. The cumulative number of hot pixels was linearly related to the size of the area deforested annually from 1998 to 2004 (r2=0.84, p=0.004). During the 2005 drought, the number of hot pixels increased 43% in relation to the expected value for a similar deforested area (approx. 19000km2). We demonstrated that anthropogenic forcing, such as land-use change, is decisive in determining the seasonality and annual patterns of fire occurrence. Moreover, droughts can significantly increase the number of fires in the region even with decreased deforestation rates. We may expect that the ongoing deforestation, currently based on slash and burn procedures, and the use of fires for land management in Amazonia will intensify the impact of droughts associated with natural climate variability or human-induced climate change and, therefore, a large area of forest edge will be under increased risk of fires.

Figure 1

Monthly time series of (a) mean rainfall (mm) derived from the TRMM dataset (January 1998–December 2006), (b) cumulative deforested area (km²) from the INPE-DETER dataset (April 2004–September 2006) and (c) cumulative number of hot pixel detections from NOAA-12 dataset (May 1998–December 2006) within the limits of the Brazilian Legal Amazonia. Dashed lines in (a) correspond to the s.d. of the mean monthly rainfall (n=6705 pixels). Grey bars indicate the dry season length for each year (months with rainfall below 100 mm month⁻¹).

Figure 2

Maps of the Brazilian Amazonia showing (a) the total cumulative deforested area based on the INPE-DETER dataset until 2004 (yellow) and in 2005 (red), and the annual cumulative number of hot pixel detections in 2005 from NOAA-12 dataset over (b) areas deforested until 2004, (c) areas deforested in 2005 and (d) forested areas in 2005.

Figure 3

Spectral analysis on the monthly time series showing (a–c) the power spectra (Fourier periodograms) of (a) hot pixels and deforestation (n=30), (b) rainfall and deforestation (n=30) and (c) rainfall and hot pixels (n=103). The value at each temporal frequency gives the relative strength of the corresponding periodic component in each series. (d–f) The coherency spectra for the relationship between (d) hot pixels and deforestation, (e) rainfall and deforestation and (f) rainfall and hot pixels are shown. The value for each frequency gives the correlation (similar to the Pearson's correlation coefficient) between the corresponding periodic components in both signals. (g–i) The phase spectra between (g) hot pixels and deforestation, (h) rainfall and deforestation and (i) rainfall and hot pixels. Here, the values indicate the lag between the periodic components in both signals at each frequency (at one cycle yr⁻¹, 2π, π and π/2 are equivalent to 12, 6 and 3 months time lag, respectively). The dashed lines in (d–i) indicate the 95% bilateral pointwise CI computed using a four-month smoothing window. The arrows indicate the phase shift for the coherent annual frequency. (a) Light grey circles, fires; dark grey squares, deforestation. (b) Light grey circles, rain; dark grey squares, deforestation. (c) Light grey circles, rain; dark grey squares, fires.

Figure 4

Linear regression between the annual cumulative number of hot pixels and the annual cumulative deforested area between 1998 and 2004 derived from the INPE-PRODES dataset (n=7, p=0.004). It shows the linear fit, indicated by the grey arrow and the coefficient of determination (r²). Note that 2005 is not included in the regression due to its anomalous characteristic as a function of the drought.