Simultaneous Regional Detection of Land‐Use Changes and Elevated GHG Levels: The Case of Spring Precipitation in Tropical South America

Abstract

A decline in dry season precipitation over tropical South America has a large impact on ecosystem health of the region. Results here indicate that the magnitude of negative trends in dry season precipitation in the past decades exceeds the estimated range of trends due to natural variability of the climate system defined in both the preindustrial climate and during the 850–1850 millennium. The observed drying is associated with an increase in vapor pressure deficit. The univariate detection analysis shows that greenhouse gas (GHG) forcing has a systematic influence in negative 30‐year trends of precipitation ending in 1998 and later on. The bivariate attribution analysis demonstrates that forcing by elevated GHG levels and land‐use change are attributed as key causes for the observed drying during 1983–2012 over the southern Amazonia and central Brazil. We further show that the effect of GS signal (GHG and sulfate aerosols) based on RCP4.5 scenario already has a detectable influence in the observed drying. Thus, we suggest that the recently observed “drier dry season” is a feature which will continue and intensify in the course of unfolding anthropogenic climate change. Such change could have profound societal and ecosystem impacts over the region.

A Recent Systematic Increase in Vapor Pressure Deficit over Tropical South America

“…5c). These results are in line with the recently detected externally forced “drier dry season” feature over the region 24 .

Figure 5Long-term trend in surface fluxes and parameters over 1987–2016. ( a ) Externally forced trend in total could cover in comparison with 400 pseudo-realizations of unforced trends derived from 12,000-year Pre-industrial simulations, ( b ) in downwelling shortwave radiation, ( c ) in precipitation and ( d ) in dew point.

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“…S4), the following attribution analysis is focused on dry ASO months where the observed trends in VPD are found to be larger than natural variability-generated trends and need to be explained by external drivers. Our attribution analysis is based on assessing the amplitude of the response of VPD to each external forcing from the observations via the estimation of scaling factors 21,24 ( See Methods ). The null hypothesis is that the observed change in VPD is drawn from a hypothetical population of a climate disturbed by a specific external influence.…”

“…This results in greater potential evapotranspiration over land compared to the ocean 19 , and in land-atmosphere feedbacks that amplify the increase of aridity over land 23 . On the supply side of the water balance, the reduction of dry season rainfall over the south and southeastern Amazon in the recent decades has been determined to be well beyond the ranges expected from natural internal variability of the climate system 24 . The detected “drier dry season” was attributed to simultaneous effects of increasing GHG concentrations and changes in LU over 1983–2012 time period 24 .…”

Fire Affects Asymbiotic Nitrogen Fixation in Southern Amazon Forests

“…That transition zone is fire-prone due to a seasonal climate (~5-to 6-month dry season) and mosaic-type distribution of open savannas characterized by a discontinuous cover of fire-adapted woody species with thick bark in a continuous grass understory and fire-sensitive forests dominated by trees with thin bark and lacking grass understory (Hart et al, 2005;Hoffmann et al, 2009;Ratnam et al, 2011). Recent increases in dry season length (Agudelo et al, 2018;Fu et al, 2013) and intensity (Barkhordarian, von Storch, Behrangi, et al, 2018, Barkhordarian, von Storch, Zorita, et al, 2018 can escalate the likelihood of fires throughout the season in that region (Barlow et al, 2019). As a result, without proper fire management, positive feedbacks between fire frequency, dry season length, land-use change, and management (i.e., the use of fire on pastoral and agricultural land) are expected to facilitate shifts fromfire-sensitive to fire-adapted vegetation (Brando et al, 2014;Cano-Crespo et al, 2015;Dantas et al, 2015;Flores et al, 2017;Hansen et al, 2008;Hoffmann et al, 2012;Lovejoy & Nobre, 2018;Silverio et al, 2013).…”

Anthropogenic Soils Promote Biodiversity in Amazonian Rainforests

“…Although deforestation rates in Amazonia have been showing 64 a generally decreasing trend over the last decade, human activities in the region were still 65 responsible for losses of 7,900 km 2 of its natural vegetation in 2018 alone 1 . Many forested 66 areas have become highly fragmented, and may be reaching tipping points where 67 biodiversity and ecosystem functions may be dramatically affected 2,3 , potentially leading to 68 cascading effects that impact ecosystem services over a much larger area 4,5 . 69…”