Increased dry-season length over southern Amazonia in recent decades and its implication for future climate projection.

We have observed that the dry-season length (DSL) has increased over southern Amazonia since 1979, primarily owing to a delay of its ending dates (dry-season end, DSE), and is accompanied by a prolonged fire season. A poleward shift of the subtropical jet over South America and an increase of local convective inhibition energy in austral winter (June-August) seem to cause the delay of the DSE in austral spring (September-November). These changes cannot be simply linked to the variability of the tropical Pacific and Atlantic Oceans. Although they show some resemblance to the effects of anthropogenic forcings reported in the literature, we cannot attribute them to this cause because of inadequate representation of these processes in the global climate models that were presented in the Intergovernmental Panel on Climate Change's Fifth Assessment Report. These models significantly underestimate the variability of the DSE and DSL and their controlling processes. Such biases imply that the future change of the DSE and DSL may be underestimated by the climate projections provided by the Intergovernmental Panel on Climate Change's Fifth Assessment Report models. Although it is not clear whether the observed increase of the DSL will continue in the future, were it to continue at half the rate of that observed, the long DSL and fire season that contributed to the 2005 drought would become the new norm by the late 21st century. The large uncertainty shown in this study highlights the need for a focused effort to better understand and simulate these changes over southern Amazonia.

Call for caution regarding the efficacy of large-scale afforestation and its hydrological effects.

Large-scale afforestation programmes are generally presented as effective ways of increasing the terrestrial carbon sink while preserving water availability and biodiversity. Yet, a meta-analysis of both numerical and observational studies suggests that further research is needed to support this view. The use of inappropriate concepts (e.g., the biotic pump theory), the poor simulation of key processes (e.g., tree mortality, water use efficiency), and the limited model ability to capture recent observed trends (e.g., increasing water vapour deficit, terrestrial carbon uptake) should all draw our attention to the limitations of available theories and Earth System Models. Observations, either based on remote sensing or on early afforestation initiatives, also suggest potential trade-offs between terrestrial carbon uptake and water availability. There is thus a need to better monitor and physically understand the observed fluctuations of the terrestrial water and carbon cycles to promote suitable nature-based mitigation pathways depending on pre-existing vegetation, scale, as well as baseline and future climates.

How can event attribution science underpin financial decisions on Loss and Damage?

With climate extremes hitting nations across the globe, disproportionately burdening vulnerable developing countries, the prompt operation of the Loss and Damage fund is of paramount importance. As decisions on resource disbursement at the international level, and investment strategies at the national level, loom, the climate science community's role in providing fair and effective evidence is crucial. Attribution science can provide useful information for decision makers, but both ethical implications and deep uncertainty cannot be ignored. Considering these aspects, we articulate a vision that integrates established attribution methods and multiple lines of evidence within a coherent logical framework.