Reduction in carbon uptake during turn of the century drought in western North America

Fossil fuel emissions aside, temperate North America is a net sink of carbon dioxide at present1–3. Year-to-year variations in this carbon sink are linked to variations in hydroclimate that affect net ecosystem productivity3,4. The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming5–8. Here, we examine the effect of the turn of the century drought in western North America on carbon uptake in the region, using reanalysis data, remote sensing observations and data from global monitoring networks. We show that the area-integrated strength of the western North American carbon sink declined by 30–298 Tg C yr−1 during the 2000–2004 drought. We further document a pronounced drying of the terrestrial biosphere during this period, together with a reduction in river discharge and a loss of cropland productivity. We compare our findings with previous palaeoclimate reconstructions7 and show that the last drought of this magnitude occurred more than 800 years ago. Based on projected changes in precipitation and drought severity, we estimate that the present mid-latitude carbon sink of 177–623 Tg C yr−1 in western North America could disappear by the end of the century. From 2000 to 2004, western North America (25–50N, 100–125W) experienced a protracted drought (Fig. 1), the most extreme five-year average Palmer Drought Severity Index (PDSI, see Methods) event since 1200. The drought was evident in several hydroclimatic indicators: precipitation, soil moisture, evaporative fraction and latent heat (Fig. 1). Although precipitation recovered in the latter part of 2004 this did not ameliorate the drought condition until 2005 owing to the inherent lag between precipitation inputs and terrestrial biosphere response after multiyear precipitation deficits. Similarly, the cumulative effect over five consecutive drought years led to a clear reduction in water availability as indicated by decreased runoff in all major water basins of the western United States (Fig. 2). The California and Upper Colorado basins had 50% less runoff during the drought with the Lower Colorado River Basin showing a 10% reduction in runoff persisting through 2007. The drought’s