Uncertain sinks in the shrubs.

alancing the global carbon budget just got more difficult. For decades, there has been debate over what happens to the carbon dioxide released from the burning of fossil fuels and clearing of tropical rainforests. About half of it accumulates in the atmosphere 1,2 , prompting concerns about global warming. The ocean and land take up the rest, acting as carbon sinks, and understanding where and why these sinks occur is essential for managing them in the future. Now, new field measurements cast doubt on the estimated size of one potentially large but poorly quantified carbon sink — that of woody shrubs when they encroach into and replace grassland, a process that has been thought to lock up large amounts of extra carbon. As they describe on page 623 of this issue, Jackson et al. 3 show that there are smaller increases in carbon storage than anticipated, and in some cases actual reductions. There is general agreement that land in the Northern Hemisphere acts as an important carbon sink. Yet estimates of its exact size, and the specific locations and contributing factors, vary greatly, due in part to different measurement approaches. Calculations based on measurements of carbon dioxide and oxygen in the atmosphere 1,2 nearly always produce larger estimated sinks for North America and Eurasia (0.6–2.7 ǂ10 15 grams — petagrams — of carbon per year) than do those based on ground measurements of forest growth (0.6–0.7 Pg C yr ǁ1). Last year, however, a carbon budget produced for the continental United States partly reconciled top-down (atmospheric) and bottom-up (land-based) methods by including estimates of both forest and non-forest sinks 4. Together, the bottom-up estimates (0.4–0.7 Pg C yr ǁ1) roughly matched those from the top-down approach (0.7DŽ0.5 Pg C yr ǁ1). This reconciliation depended on the existence of a rather large sink (about 0.13 Pg C yr ǁ1) in the rangelands of the western United States 4,5. The suppression of fires and overgrazing have favoured the expansion of trees and woody shrubs into grasslands 6 , over an estimated area of as much as 220 million hectares in the United States 4,5. This 'woody encroachment' formed 18–34% of the total estimated sink for carbon in the continental United States — it was by far the largest non-forest sink, and the least secure term in the land-based carbon budget 4. Jackson and colleagues' measurements 3 add further uncertainty to these estimates. …