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Many governments agreed to limit global mean temperature change to below 2 °C, yet this level has not been assessed scientifically. A synthesis of the literature suggests that temperature is the best available target quantity, but a safe level is uncertain.
The moon Phobos will eventually either disintegrate to form a ring or crash into Mars. Observational constraints and geotechnical considerations suggest that Phobos will partially break apart into a ring, with stronger fragments impacting Mars.
Low soil fertility can limit crop productivity, which in turn constrains the ability of poor households to invest in improving soils. This self-reinforcing feedback can trap households in chronic poverty for years or even generations.
Earth's composition differs from its meteoritic precursors. An evaluation of the evidence suggests that some material could have been lost to space during collisions, which may explain Earth's unusual plate tectonic regime and habitable climate.
The continents have a puzzling structure — a transition occurs at mid-lithospheric depths. A synthesis of geological data indicates that stress-induced sliding along crystal grain boundaries may be responsible forforthe transition.
An iris effect in tropical cloud-cover was controversially proposed as a negative climate change feedback that is not represented in climate models. If such an effect exists, it could go some way to reconciling climate models and observations.
El Niño diversity and its genesis are debated. An overview of existing work along with a fuzzy clustering analysis and simulations suggest that the asymmetry, irregularity and extremes of El Niño result from westerly wind bursts.
Our understanding of the interactions between clouds, circulation and climate is limited. Four central research questions — now tractable through advances in models, concepts and observations — are proposed to accelerate future progress.
