It has been suggested that rainfall in the Amazon decreases if forest loss exceeds some threshold, but the specific value of this threshold remains uncertain. Here, we investigate the relationship between historical deforestation and rainfall at different geographical scales across the Southern Brazilian Amazon (SBA). We also assess impacts of deforestation policy scenarios on the region’s agriculture. Forest loss of up to 55–60% within 28 km grid cells enhances rainfall, but further deforestation reduces rainfall precipitously. This threshold is lower at larger scales (45–50% at 56 km and 25–30% at 112 km grid cells), while rainfall decreases linearly within 224 km grid cells. Widespread deforestation results in a hydrological and economic negative-sum game, because lower rainfall and agricultural productivity at larger scales outdo local gains. Under a weak governance scenario, SBA may lose 56% of its forests by 2050. Reducing deforestation prevents agricultural losses in SBA up to US$ 1 billion annually.
Amazonian deforestation is causing notable changes in the hydrological cycle by altering important precipitation characteristics. This study uses daily rainfall time series data from 112 rain gauges and a recent yearly 1‐km land use data set covering the period from 1974 to 2012 to evaluate the effects of the extent of deforestation at different spatial scales on the onset of the rainy season and on the duration of dry spells in southern Amazonia. Correlation analyses indicate a delay in the onset of 0.12–0.17 days per percent increase in deforestation. Analysis of cumulative probability density functions emphasizes that the likelihood of rainy season onset occurring earlier than normal decreases as the local deforestation fraction increases. In addition, the probability of occurrence of dry spells in the early and late rainy season is higher in areas with greater deforestation. The delayed onset and longer dry spell events in highly deforested areas increase the climate risk to agriculture in the region.
Past studies presented evidence that deforestation may affect the precipitation seasonality in southern Amazon. This study uses daily rainfall data from Tropical Rainfall Measurement Mission 3B42 product and a recent yearly 1‐km land use dataset to evaluate the quantitative effects of deforestation on the onset, demise and length of the rainy season in southern Amazon for a period of 15 years (1998–2012). Additionally, we use the Niño4 index, zonal wind data and deforestation data to explain and predict the interannual variability of the onset of the rainy season. During this period, onset has delayed ~0.38 ± 0.05 days per year (5.7 ± 0.75 days in 15 years), demise has advanced 1.34 ± 0.76 days per year (20 ± 11.4 days in 15 years) and the rainy season has shortened by 1.81 ± 0.97 days per year (27 ± 14.5 days in 15 years). Onset, demise and length also present meridional and zonal gradients linked to large‐scale climate mechanisms. After removing the effects related to geographical position and year, we verified a relationship between onset, demise and length and deforestation: Onset delays ~0.4 ± 0.12 day, demise advances ~1.0 ± 0.22 day and length decreases ~0.9 ± 0.34 day per each 10% deforestation increase relative to existing forested area. We also present empirical evidence of the interaction between large‐scale and local‐scale processes, with interannual variation of the onset in the region explained by Niño4 sea surface temperature anomalies, Southern Hemisphere subtropical jet position, deforestation and their interactions (r2 = 69%, p < .001, mean absolute error = 2.7 days).
A intensificação do processo de aquecimento do planeta causado pelo aumento na emissão de gases do efeito estufa (GEE) representa inúmeros riscos para os ecossistemas naturais. Mudanças na composição atmosférica podem alterar variáveis climáticas em diversas regiões do planeta, dentre elas a Bacia Amazônica Brasileira (BAB). Considerada a maior bacia hidrográfica do planeta, esta região influencia o clima de outras partes do Brasil e da América Latina. Portanto, alterações na BAB podem acarretar em desequilíbrios ambientais em outras regiões. Dentre os efeitos causadas pelas mudanças climáticas, estão as modificações nos padrões de evapotranspiração, importante reguladora do ciclo hidrológico. Neste artigo simulamos as alterações futuras na evapotranspiração de referência (ETo) na BAB decorrentes de um contínuo aumento nas emissões de GEE (cenário RCP 8.5). Utilizamos a plataforma Dinamica EGO para implementar um modelo espacialmente explícito baseado no método de Penman-Monteith padronizado pela FAO. A validação foi realizada comparando estatisticamente os resultados entre as simulações e as observações. Dessa forma, comprovamos que o modelo representa de forma próxima ao real o processo de evapotranspiração na BAB. A sazonalidade projetada da ETo se manteve estatisticamente similar à observada até o ano de 2050, com o aumento da ETo durante a estação seca e diminuição durante a estação chuvosa. A ETo apresentou um padrão de distribuição espacial com maiores valores na porção leste, se estendendo no sentido norte-sul. Este padrão acompanha a distribuição dos valores de temperatura e saldo de radiação, além de coincidir com o arranjo espacial do desmatamento. Um forçamento radiativo de 8,5 w/m² em todo o planeta poderá aumentar a ETo na BAB, devido à elevação nos valores de temperatura e saldo de radiação solar. Este aumento é mais evidente na região nordeste se estendendo progressivamente para o sudoeste da bacia. Ao longo do século 21, as futuras alterações nos padrões de ETo podem trazer grandes problemas para as práticas agrícolas e para o abastecimento hídrico na BAB e em outras partes do País. Em suma, as mudanças climáticas globais, refletindo em alterações na ETo em conjunto com o aumento do desmatamento, podem acarretar em uma desregulação do balanço hídrico. Os nossos resultados reforçam a necessidade da adoção de ações governamentais efetivas com o objetivo de mitigar os efeitos das intensas emissões de GEE
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