Mudanças climáticas no Semiárido Brasileiro: impactos da intensificação da aridez no vigor da vegetação

Abstract

The Brazilian semiarid (BSR) is one of the most populous semi-arid zones, highly dependent on water resources for socioeconomic activities, and one of the most biodiverse regions in the world, with a predominance of the Caatinga biome and enclaves of Cerrado and Atlantic Forest. Water conditions, through aridity from the perspective of climate change, can affect the vigor of vegetation in the SAB. Understanding the effects of future climate changes in this region is crucial for environmental planning. This thesis aims to understand the influence of arid conditions in future perspectives (2061 – 2080) on the vigor of SAB vegetation. To fulfill this proposal, I) future scenarios were established for the behavior of ETo, and II) the aridity levels and their implications on climatic classes and zones with susceptibility to desertification in the SAB considering IPCC scenarios, that is, the Shared Socioeconomic Pathways (SSP). Two SSPs were used, 126, representing an optimistic scenario about climate change, and 585, comprising pessimistic conditions for the end of the century. In mapping ETo, five machine learning algorithms were applied (BRNN, Cubist, Earth, Linear Regression, and Random Forest (RF)) considering the current scenario and SSPs 126 and 585. Cubist showed the best metrics among the algorithms tested (R² = 0.98 and RMSE = 0.07 mm day-¹). By the end of the century (2061 – 2080), ETo rates are projected to increase (+3 to +15% in SSPs 126 and 585). Some regions of the SAB may experience an increase of > 600 mm year-¹. This increase in ETo will lead to the intensification of aridity, which will make the SAB significantly (p-value <0.05) drier since the most arid lands will expand by 46,000 km² in SSP 126 and 125,600 km² in SSP 585 concerning the current scenario. These arid conditions could change the region’s climate types. The semi-arid climate is expected to expand 56,500 to 140,400 km² over humid climates. While arid climates will expand from 4100 to 21,500 km². These dry lands will be more susceptible to desertification processes. Zones of “high” susceptibility could expand by 622,400 to 706,300 km², while areas with “very high” susceptibility should expand to between 4400 and 21,700 km² in SSPs 126 and 585, respectively. The most affected areas will be the Central (Bahia and Pernambuco) and Southern (northern Minas Gerais) parts of the SAB. To understand the effects of aridity on vegetation vigor (represented by the Normalized Difference Vegetation Index – NDVI), we retrained the same algorithms previously used in the second chapter, in this case, RF achieved better performance (R² = 0.72). The results showed that aridity will induce significant changes in the vigor of SAB vegetation. The caatingas, humid and dry tropical forests, and ecotonal zones are expected to suffer significant losses in vegetative vigor in the coming decades. Meanwhile, savannas are predicted to undergo marked greening. Our results reveal that climate change is expected to affect SAB vegetation domains severely.