The science of GEDI is centered on answering three key questions, given below. GEDI answers these questions through science objectives that require collecting data on forest structure needed to quantify the current and future state of forest resources related to carbon and biodiversity.

 GEDI science questions and objectives.

1. What is the carbon balance of the Earth’s forests?
2. How will the land surface mitigate atmospheric CO₂ concentrations in the future?
3. How does forest structure affect habitat quality and biodiversity?

1. Quantify the distribution of aboveground carbon stored in vegetation
2. Quantify the effects of vegetation disturbance and recovery on carbon storage
3. Quantify the potential for existing and new/regrowing forests to sequester carbon in the future
4. Quantify the spatial and temporal distribution of habitat structure and its influence on habitat quality and biodiversity

Forests absorb (or “sequester”) CO₂ from the atmosphere and act as carbon “sinks”. This absorbed carbon is used to drive plant growth and is stored as biomass. About 50% of tree’s weight or biomass is in the form of carbon. When forests are disturbed through natural or human causes, such as fire or deforestation, they release this stored carbon back into the atmosphere. The release of this forest carbon (a carbon “source”) is the second largest source of atmospheric CO₂ (behind fossil fuel emissions). But forests are estimated to absorb about ⅓ of the fossil carbon emitted per year.

Annual deforestation from 2000 to 2016 mapped using Landsat imagery. The yellow to red color gradient indicates deforestation occurring closer to 2000 or 2016, respectively.

There is great uncertainty about the magnitudes of carbon emitted and absorbed by forests. This uncertainty is captured in the idea of a “missing sink” of carbon. When we do a global accounting of the sources and sinks of carbon, which should be equal because the carbon must go somewhere, there is an imbalance: there is some emitted carbon that cannot be accounted for. One hypothesis is that forests act as this missing sink.

Young, rapidly regrowing forests absorb carbon at higher rates than mature forests. What impact will cutting down forests or planting new forests have on concentrations of atmospheric CO₂ through time?

Illustration showing major elements of the global carbon cycle.

Quantifying the carbon sequestration trajectory of existing forests as well as reforestation and afforestation on other lands is key to creating better management and policy aimed at reducing CO₂ emission. The best way to ask “what if” questions is through prognostic ecosystem models that allow us to model the impacts of land use change that may result from increased urbanization, changing agricultural activities, or implementation of policy goals such as maintaining or increasing forest areas. The key to utilizing such models is accurate information on the current biomass status of forests. This provides an accurate starting point from which potential carbon sequestration trajectories can be estimated. GEDI provides these data, and when married to ecosystem models, provides the capacity for evaluating alternative conservation and development strategies.

Species declines can be primarily attributed to loss and fragmentation of habitat, particularly in tropical biomes where species abundance has decreased over the past three decades by about 50%. Rapid rates of species extinctions (~50,000 per year) are projected to rise in this century unless measures are taken to reduce deforestation, expand and connect protected area networks, and mitigate the negative effects of climate change.

Forest canopy structure, such as measured by GEDI, is key towards helping us understand the status of forest biological diversity at regional and global scales. For example, the diversity and abundance of many bird species are known to depend critically on forest canopy profiles, that is, how leaves and branches are arranged vertically.