Structurally intact tropical forests sequestered ~50% of global terrestrial carbon uptake over the 1990s and early 2000s, removing ~15% of anthropogenic CO 2 emissions 1 – 3 . Climate-driven vegetation models typically predict that this tropical forest ‘carbon sink’ will continue for decades 4 , 5 . Here, we assess trends in the carbon sink using 244 structurally intact African tropical forests spanning 11 countries, we compare them with 321 published plots from Amazonia and investigate the underlying drivers of the trends. The carbon sink in live aboveground biomass in intact African tropical forests has been stable for the three decades to 2015, at 0.66 Mg C ha -1 yr -1 (95% CI:0.53-0.79), in contrast to the long-term decline in Amazonian forests 6 . Thus, the carbon sink responses of Earth’s two largest expanses of tropical forest have diverged. The difference is largely driven by carbon losses from tree mortality, with no detectable multi-decadal trend in Africa and a long-term increase in Amazonia. Both continents show increasing tree growth, consistent with the expected net effect of rising atmospheric CO 2 and air temperature 7 – 9 . Despite the past stability of the African carbon sink, our data suggest a post-2010 increase in carbon losses, delayed compared to Amazonia, indicating asynchronous carbon sink saturation on the two continents. A statistical model including CO 2 , temperature, drought and forest dynamics accounts for the observed trends and indicates a long-term future decline in the African sink, while the Amazonian sink continues to rapidly weaken. Overall, the uptake of carbon into Earth’s intact tropical forests peaked in the 1990s. Given that the global terrestrial carbon sink is increasing in size, observations indicating greater recent carbon uptake into the Northern hemisphere landmass 10 reinforce our conclusion that the intact tropical forest carbon sink has already saturated. This tropical forest sink saturation and ongoing decline has consequences for policies to stabilise Earth’s climate.

OBITUARY Wallace Broecker, geochemist who sounded global-warming alarm p.34 RENEWABLES Call for more hydropower undervalues the benefits of wetlands p.33 MATHEMATICS A romp through the history of calculus, from the Greeks to DNA p.32 TECHNOLOGY Coders, up close and personal p.30 K eeping global warming below 1.5 °C to avoid dangerous climate change 1 requires the removal of vast amounts of carbon dioxide from the atmosphere, as well as drastic cuts in emissions. The Intergovernmental Panel on Climate Change (IPCC) suggests that around 730 billion tonnes of CO 2 (730 petagrams of CO 2 , or 199 petagrams of carbon, Pg C) must be taken out of the atmosphere by the end of this century 2. That is equivalent to all the CO 2 emitted by the United States, the United Kingdom, Germany and China since the Industrial Revolution. No one knows how to capture so much CO 2. Forests must play a part. Locking up carbon in ecosystems is proven, safe and often affordable 3. Increasing tree cover has other benefits, from protecting biodiversity to managing water and creating jobs. The IPCC suggests that boosting Regenerate natural forests to store carbon Plans to triple the area of plantations will not meet 1.5 °C climate goals. New natural forests can, argue Simon L. Lewis, Charlotte E. Wheeler and colleagues. Reforesting of burnt areas in Kalimantan province, Indonesia.

Arbuscular mycorrhizal fungi (AMF) are ecologically important root symbionts of most terrestrial plants. Ecological studies of AMF have concentrated on differences between species; largely assuming little variability within AMF species. Although AMF are clonal, they have evolved to contain a surprisingly high within-species genetic variability, and genetically different nuclei can coexist within individual spores. These traits could potentially lead to within-population genetic variation, causing differences in physiology and symbiotic function in AMF populations, a consequence that has been largely neglected. We found highly significant genetic and phenotypic variation among isolates of a population of Glomus intraradices but relatively low total observed genetic diversity. Because we maintained the isolated population in a constant environment, phenotypic variation can be considered as variation in quantitative genetic traits. In view of the large genetic differences among isolates by randomly sampling two individual spores, <50% of the total observed population genetic diversity is represented. Adding an isolate from a distant population did not increase total observed genetic diversity. Genetic variation exceeded variation in quantitative genetic traits, indicating that selection acted on the population to retain similar traits, which might be because of the multigenomic nature of AMF, where considerable genetic redundancy could buffer the effects of changes in the genetic content of phenotypic traits. These results have direct implications for ecological research and for studying AMF genes, improving commercial AMF inoculum, and understanding evolutionary mechanisms in multigenomic organisms.