Tropical forests are critically important for human livelihoods, climate stability, and biodiversity conservation but remain threatened (1). Recent years have seen major strides in documenting historical and annual tropical forest loss with satellites (2). Now, a convergence of satellite technologies and analytical capabilities makes it increasingly possible to monitor deforestation in near real time, on the scale of days, weeks, or months, rather than years (3, 4). This advance creates greater potential for near–real-time action as well and could play a key role in achieving local, national, and international forest, biodiversity, and climate policy goals, as there is a global imperative to address deforestation. Challenges remain, however, to attaining effective policy action based on the new technology. On the basis of lessons learned from pioneering work in Brazil and Peru, we suggest at least two key factors for successfully linking the technical and policy realms. On the technical side, it is critical to capitalize on continually improving satellite technology to better detect, understand, and prioritize deforestation events. On the policy side, institution building, along with related civil-society engagement, is needed to facilitate effective action within complex government frameworks. We outline a five-step protocol for near–real-time tropical deforestation monitoring, with the goal of bridging the gap between technology and policy.
More and Better Eyes in the Sky
The number of Earth observation satellites, and the quality and accessibility of the imagery they provide, has greatly improved in recent years (see the first figure) (5), making satellite imagery the most consistent and effective tool for large-scale forest monitoring. Satellite-based monitoring has four key considerations: spatial resolution, temporal resolution, sensor type, and data access. Spatial resolution (that is, pixel size) has been steadily increasing since the 1970s (see the first figure), trending from coarse (>250 m) to medium (10 to 30 m) to high (<5 m). Temporal resolution (frequency of imagery for any given location) has also improved. Until recently, there had been a trade-off between spatial and temporal resolution, with higher-resolution sensors covering less area per day. For example, NASA’s coarse-resolution MODIS (Moderate Resolution Imaging Spectroradiometer) sensor collects optical imagery of every point daily, whereas medium-resolution Landsat has a revisit time of 16 days. However, constellations of miniature satellites (such as the 175 satellites of the company Planet) address the trade-off, by providing high-resolution (3 m) optical imagery with near-daily frequency (6).