Significance: Could the intensification of pasture-based cattle ranching allow Brazil to protect its forests and reduce its greenhouse gas (GHG) emissions while increasing its agricultural production? Would these benefits be substantially undermined by increased deforestation and GHGs triggered abroad? We model two policies for increasing cattle ranching productivity in Brazil: a tax on conventional pasture and a subsidy for semi-intensive pasture. Either policy could considerably mitigate global GHGs by limiting future deforestation in Brazil. The GHG benefits would be roughly ten times greater than the emissions triggered by policies stemming from ( i ) increased cattle production abroad (under the tax) and ( ii ) increased beef consumption (under the subsidy). Agricultural intensification policies may help emerging economies to balance agricultural development and forest protection.

Hundreds of millions of the world's poorest people directly depend on smallholder farming systems. These people now face a changing climate and associated societal responses. We use mapping and a literature review to juxtapose the climate fate of smallholder systems with that of other agricultural systems and population groups. Limited direct evidence contrasts climate impact risk in smallholder agricultural systems versus other farming systems, but proxy evidence suggests high smallholder vulnerability. Smallholders distinctively adapt to climate shocks and stressors. Their future adaptive capacity is uncertain and conditional upon the severity of climate change and socioeconomic changes from regional development. Smallholders present a greenhouse gas (GHG) mitigation paradox. They emit a small amount of CO₂ per capita and are poor, making GHG regulation unwarranted. But they produce GHG-intensive food and emit disproportionate quantities of black carbon through traditional biomass energy. Effectively accounting for smallholders in mitigation and adaption policies is critical and will require innovative solutions to the transaction costs that enrolling smallholders often imposes. Together, our findings show smallholder farming systems to be a critical fulcrum between climate change and sustainable development.

Nearly half of recent decades' global forest loss occurred in the Amazon and Cerrado (tropical savanna) biomes of Brazil, known as the arc of deforestation. Despite prior analysis in individual river basins, a generalizable empirical understanding of the effect of deforestation on streamflow across this region is lacking. We frame land use change in Brazil as a natural experiment and draw on in situ and remote sensing evidence in 324 river basins covering more than 3 × 10⁶ km² to estimate streamflow changes caused by deforestation and agricultural development between 1950 and 2013. Deforestation increased dry season low flow by between 4 and 10 percentage points (relative to the forested condition), corresponding to a regional‐ and time‐averaged rate of increase in specific streamflow of 1.29 mm/year², equivalent to a 4.08 km³/year² increase, assuming a stationary climate. In conjunction with rainfall and temperature variations, the net (observed) average increase in streamflow over the same period was 0.76 mm/year², or 2.41 km³/year². Thus, net increases in regional streamflow in the past half century are 58% of those that would have been experienced with deforestation given a stationary climate. This study uses a causal empirical analysis approach novel to the water sciences to verify the regional applicability of prior basin‐scale studies, provides a proof of concept for the use of observational causal identification methods in the water sciences, and demonstrates that deforestation masks the streamflow‐reducing effects of climate change in this region.