The End of the Amazon Carbon Sink? (AMSINK)

How forests respond to climate change and the increasing concentration of carbon dioxide in the atmosphere affects the course of climate change itself. The response of these ecosystems thus determine how much humanity needs to reduce greenhouse gas emissions to keep global heating within agreed limits. Understanding these is especially critical for the Amazon due to its huge importance for Earth’s climate. While the need to know is urgent, the challenge is enormous. Satellites, experiments and computer models all help, but long-term, tree-by-tree measurements in many sites are essential to understand how forests are accelerating or slowing the rate of climate change.

So far intact Amazon forests have provided a huge ‘sink’ for carbon, slowing climate change. But a decade ago there were already signs this had weakened with tree death rates increasing, partly as a result of severe droughts. Projections suggest the sink may now be finished. If so we have lost a key ally in stabilising Earth’s climate. But with no ground analysis since 2011 a big gap between knowledge and projection has opened up. The forecast is based on old measurements, has large uncertainty, and crucially fails to account for the huge natural variability of forests. AMSINK will resolve this. It uses ambitious new measurements, analysis and integration to discover how and why South American forests are changing, where and why they are able to resist climate change, and what the future holds. In short to understand the dynamics and destiny of this critical system.

Our central idea is that the biomass carbon sink depends on the fundamental nature of forests - their climate, nutrients, soil water, species and diversity, and turnover rate - so that in many places net uptake of carbon may persist well beyond simple predictions based on climate-change. For example, while we know that scarce nutrients limit forest growth and droughts kill trees, (1) one third of the Amazon has relatively-rich soils where nutrients may allow long-term growth increases and (2) up to half is prone to waterlogging so some drying could benefit trees by aerating the soil, while (3) tree resistance to drought is likely to vary greatly with species.

To know what is happening to S American forests, where and why therefore requires a huge, integrated and ambitious measurement effort. We must look widely at many forests for a long time. We need holistic science.

To make this possible AMSINK takes three key steps. First we will unite 15 forest plot networks, each examining different locations and aspects of change across tropical S America, to create a connected ‘team-of-teams’ spanning 8 million km2 of forest. Second we will enrich these, with comparable nutrient, moisture and species drought-sensitivity measurements across the networks. Third, for the most valuable long-term Amazon plots we will take careful consistent tree-by-tree monitoring into the mid-2020s. In total AMSINK will integrate 1000 plots into a globally standardized framework and track more than half-a-million trees for up to 62 years. It will analyse these to discover the distribution, dynamics and trend of the biomass carbon sink to 2026, determine its drivers, and test our hypotheses of forest resistance to climate change. Our new findings will be applied to forecast future growth, mortality and carbon trends across the region, providing critical insights for national policy-makers, global agreements, & fellow scientists.

In all AMSINK makes the first continent-scale integrated measurement and analysis of how nature controls the long-term climate-change sensitivity of tropical forests and their carbon balance. It replaces projection with new measures to 2026, novel insight into which sink processes are changing where, critical understanding of why, and new constraints on how their future will unfold. It delivers exceptional scientific value for UK taxpayer support and vital, timely discoveries for policy-makers.