# The global extent of the grassland biome and implications for the terrestrial carbon sink

## TL;DR

This paper shows that grassland coverage has been misestimated by over 50 million km², affecting global carbon sink calculations.

## Contribution

The study corrects grassland mapping errors using expert validation, leading to a revised carbon stock estimate of 155.02 Pg.

## Key findings

- Grassland coverage was corrected to 22.8% of terrestrial land (30.1 million km²).
- Misestimation of grasslands led to significant errors in soil carbon stock projections.
- Inconsistent definitions and classification flaws caused prior mapping inaccuracies.

## Abstract

Land cover data are commonly used to model the terrestrial carbon (C) sink, yet these data have wide margins of error that significantly alter estimates of global C storage. Here we demonstrate this data vulnerability in grasslands, which are critical to C cycling but whose estimated distribution has varied by >50 million km2 (3.5–42% of the Earth’s terrestrial surface). Comparing multiple high-resolution land cover products with expertly annotated grassland data from six continents, we show sources of mapping error and discuss C implications based on 2023 United Nations (UN) FAO estimates. Past misidentification arose from inconsistent definitions on grassland identity and classification flaws especially relating to woody plant cover. Correcting these errors adjusted grassland coverage to 22.8% of the terrestrial land base (30.1 million km2), elevating UN projections of soil C stocks to 155.02 Pg (0–30 cm depth). These findings underscore the challenges of biome mapping for ecosystem accounting and policy, when lacking field-validated remotely sensed data.

By combining satellite observations with ground-based data and expert validation, this analysis demonstrates considerable misestimation of grassland extent and thereby carbon stock estimates in previous global assessments based on remote sensing.