This project addressed the globally important issue of how to sustain tropical Amazon forests and intensifying tropical agriculture in a rapidly changing climate. This project found that climate change now threatens crop yield gains in Brazil and Brazil's plans to further increase agricultural production. It also showed that Brazil is fast approaching a climatic limit to rainfed intensive agriculture across much of the zone of cropland expansion at the edge of the Cerrado and Amazon forest biomes. Between 1980 and 2019, a warming and drying climate pushed 28% of current agricultural lands out of the optimum climate space for agriculture and that number projected to reach 51% by 2030 and 74% by 2060 (Fig. 1). While adaptations like irrigation and drought-resistant crop cultivars can provide some relief, this project showed that tropical forests and savannas stabilize local temperature and humidity, sustain rainfall, and control dry season length. By slowing ongoing climate changes, forest conservation and restoration could help avert the near-term collapse of rainfed agriculture in the drier parts of Brazil.

Throughout the global tropics, this project estimated that deforestation has caused a 1 degree C increase in land surface temperature and a 7% decrease in water evaporated by plants (Fig. 2). This temperature change is already as large as global temperature changes caused by increasing greenhouse gases in the atmosphere. Southeast Asia and the Amazon contributed the most to this change, given the large-scale deforestation occurring in those regions. Project results suggest that the Congo is even more sensitive to deforestation and that for every 10% of its area deforested, temperature will increase by ~0.5 degree C and water evaporation by plants decrease by 64 mm per year. Complete deforestation of the tropics could lead to a ~4 degree C increase in temperature and 30% decrease in water evaporated back to the atmosphere. This leads to the conclusion that conservation of tropical forests is a priority to limit global warming to 2 degree C.

This project also found that climate warming is driving a large part of the tropics out of its historical biomass equilibrium, primarily by increasing dry-season severity. These climatic changes could eventually lead to losses of up to between 14.4 and 23.9 Pg C across the tropics. Carbon losses are concentrated in the Americas (10.5 Pg C), where the humid regions shrink by 4.2%. Changes in Africa and Asia are smaller because they contain less carbon to begin with, and the projected regional drying is less severe. The predicted changes in tropical biomass could further increase atmospheric CO₂ concentrations, prompting positive vegetation-atmosphere feedbacks.

This project also identified mechanisms that constrain the environmental impacts of intensifying tropical crop agriculture in the Brazil cropland frontier region. First, cropland soils are highly weathered and deep (to > 8 meters) and have the ability to adsorb negatively-charged ions (cations) from percolating drainage water. Deep soils retain large amounts of nitrogen in the form of nitrate that would otherwise run off to streams and rivers (Fig. 3). This project measured relatively minor differences in the amount of nitrate and other nutrients in streams draining cropland compared with intensive cropland. This suggests that under current levels of nitrogen fertilizer applications, that fertilized cropland will not generate nitrogen runoff in the same way that occurs in most temperate croplands such as in the Mississippi River basin. Second, this project found that under current cropping practices and fertilizer application rates, that the emissions of the greenhouse gas nitrous oxide and the toxic and ozone-producing gas nitric oxide were relatively low. These finding suggest that if deforestation ends and climate is stabilized that current cropping practices could be sustained with a lower level of releases of reactive nitrogen than occur in intensive temperate crop agriculture.

This project has produced more than 24 peer reviewed papers and numerous talks at professional society meetings, universities, and panels for the scientific community. Many of the outcomes produced, particularly the negative impacts of climate on agricultural productivity, are highly relevant to shaping policies on agricultural land management, climate, land use, and regional development in Brazil. This project routinely includes presentation of results and discussions of solutions to Brazilian agricultural industry leaders and federal and state government leaders. Through the project's partner in Brazil (IPAM Amazonia), the project presented before the Brazilian congress and produced numerous policy briefs for the ministries of agriculture and environment, the supreme court, and the office of the president. As a result, the project outcomes have emphasized the economic and climatic importance of keeping tropical forests standing and have directly influenced Brazilian and international policy.  

Last Modified: 12/05/2023
Modified by: Michael T Coe