Perspectives

The careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters. A synthesis and analysis of data from two Mesozoic ocean anoxic events and the Palaeocene-Eocene thermal maximum applies these techniques to constrain past ocean deoxygenation.

The Sustainable Development Goals map out a broad spectrum of objectives. Analytical tools in form of the Index and Dashboards provide a starting point to set national baselines, and allow comparison of the SDGs with other indices of well-being.

The only temperature profile of the lowermost Venusian atmosphere appears unstable. Compositional heterogeneity due to density-driven separation of N₂ from CO₂ gas in the lower atmosphere of Venus may be a viable explanation.

Dehydration of subducting slabs could create a reservoir of water in the overlying mantle. A synthesis of thermal model results, however, shows that slab dehydration is slow over geological time scales, so such reservoirs are probably rare.

The atmosphere can hold more water in a warming climate, which may lead to more extreme rainfall events. An analysis suggests that links ofrainfall extremes with daily temperature variations do not provide a reliable basis for projections.

External metal inputs to oceans affect ocean productivity and metal cycling. A synthesis of researchreveals that internal processes such as metal retention, recycling and remineralizationare also important.

The net carbon flux on land comprises emissions from land-use change and uptake by vegetation. An evaluation of vegetation models suggests that emissions from land-use change, and thus the terrestrial sink, may be substantially underestimated.

Volcanic eruptions can release large amounts of climatically active gases. An emerging view stresses the role of the size and chemical composition of the plume, including its water content, in controlling the climatic effects of an eruption.

Atmospheric oxygen levels increased in two stages. This two-step rise of oxygen may be a natural consequence of lowered oxidative capacity caused by the emergence of felsic continents and the growth of a continental carbon reservoir.

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.

Scientific confidence in climate change effects is much higher for aspects related to global patterns of surface temperature, than for circulation. Circulation will remain hard to predict, necessitating a risk-based approach to decision making.

Some giant ore deposits are formed from magma, but the precise controls on their formation are unclear. A Perspective article analyses the distribution of some diamond, platinum-group element and gold deposits worldwide, and suggests that the structure and composition of sub-continental lithospheric mantle could play a role in ore genesis.