Forest carbon budget estimates show discrepancies between satellite data and national registry information. International research explains how to align them
by Matteo Cavallito
Calculating the net carbon flux in forests remains a complex task based on variable methods that return different results. This is no small problem for analysts as well as policymakers, who are called upon to assess the effectiveness of climate mitigation initiatives undertaken. The discrepancies that still exist between the different estimates, however, can be overcome. This is supported by an international study recently published in the journal Carbon Balance and Management that involved authors from several universities and research centers.
Satellites and inventories
At the heart of the discussion is the calculation of net forest carbon flux, or the balance between the amount of element released and the amount of element sequestered by these ecosystems. The figure is measured as the average value per hectare over a given period. In the survey, the team of scientists, led by Viola Heinrich, a researcher in the School of Geographical Sciences at the University of Bristol, described the causes behind the gap between estimates based on satellite data and country estimates reported in national greenhouse gas inventories.
Satellites, observes a statement released by the European Space Agency, “provide independent information to show how forests change over time, which allows rates of carbon flux to be estimated.” These measurements can then be compared with results reported in inventories. These are typically compiled by combining statistics on surveyed activities with specific emission factors or models. And they often return divergent results from those measured from space.
At the root of the forest data discrepancies
According to Heinrich, at the root of the gaps between satellite data and those reported in inventories is the ability to assess whether or not the observed area is managed by humans. The case of Brazil, an epicenter of global deforestation observations, is illustrative. “Between 2001 and 2020, the EO-derived estimates of all forest-related emissions and removals indicate that Brazil was a net sink of carbon (minus 200 million tons per year),” the study explains.
“While Brazil’s National Greenhouse Gas Inventories reported a net carbon source (800 million tons per year).”
According to top-down observations, in short, the forest was absorbing more carbon than it was releasing while the records stated otherwise. The fact, Heinrich further explains, is that “National reports can define an area of forest as managed, but satellites cannot distinguish whether they are managed or natural.” When the datasets are adapted by adding this information, however, everything changes.
Aligning emission estimates
By including the variable related to the category of forest examined (managed or natural) and intervening in the methodology, in fact, the authors achieved a substantial alignment of the satellite data with those of the Brazilian National Greenhouse Gas Inventory. In fact, the analysis from space adjusted in this way returned a negative balance of 600 million tons per year of CO2 (amount absorbed minus amount released), quite comparable to that indicated in the inventory.
The authors also analyzed available space data for two other countries, Indonesia and Malaysia. In the latter case, in particular, results from satellites and logs show “a considerable difference.” The survey turned up “specific aspects where differing approaches may explain divergence, including uncertainties and inaccuracies,” Heinrich said. The study, she added, “highlights the importance of enhanced transparency, as set out by the Paris Agreement, to enable alignment between different approaches for independent measuring and verification.”