Continuing population and consumption growth will mean that the global demand for food will increase for at least another 40 years. Growing competition for land, water, and energy, in addition to the overexploitation of fisheries, will affect our ability to produce food, as will the urgent requirement to reduce the impact of the food system on the environment. The effects of climate change are a further threat. But the world can produce more food and can ensure that it is used more efficiently and equitably. A multifaceted and linked global strategy is needed to ensure sustainable and equitable food security, different components of which are explored here.

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processes. Biological or land-based forms of CO 2 utilization can generate economic value in the form of, for example, wood products for buildings, increased plant yields from enhanced soil carbon uptake, and even the production of biofuel and bio-derived chemicals. We use this broader definition deliberately; by thinking functionally, rather than narrowly about specific processes, we hope to promote dialogue across scientific fields, compare costs and benefits across pathways, and consider common techno-economic characteristics across pathways that could potentially assist in the identification of routes towards the mitigation of climate change. In this Perspective, we consider a non-exhaustive selection of ten CO 2 utilization pathways and provide a transparent assessment of the potential scale and cost for each one. The ten pathways are as follows: (1) CO 2-based chemical products, including polymers; (2) CO 2-based fuels; (3) microalgae fuels and other microalgae products; (4) concrete building materials; (5) CO 2 enhanced oil recovery (CO 2-EOR); (6) bioenergy with carbon capture and storage (BECCS); (7) enhanced weathering; (8) forestry techniques, including afforestation/reforestation, forest management and wood products; (9) land management via soil carbon sequestration techniques; and (10) biochar. These ten CO 2 utilization pathways can also be characterized as 'cycling', 'closed' and 'open' utilization pathways (Fig. 1, Table 1, Supplementary Materials). For instance, many (but not all) conventional industrial utilization pathways-such as CO 2-based fuels and chemicals-tend to be 'cycling': they move carbon through industrial systems over timescales of days, weeks or months. Such pathways do not provide net CO 2 removal from the atmosphere, but they can reduce emissions via industrial CO 2 capture that displaces fossil fuel use. By contrast, 'closed' pathways involve utilization and nearpermanent CO 2 storage, such as in the lithosphere (via CO 2-EOR or BECCS), in the deep ocean (via terrestrial enhanced weathering) or in mineralized carbon in the built and natural environments. Finally, 'open' pathways tend to be based in biological systems,

It is now clear that fished populations can fluctuate more than unharvested stocks. However, it is not clear why. Here we distinguish among three major competing mechanisms for this phenomenon, by using the 50-year California Cooperative Oceanic Fisheries Investigations (CalCOFI) larval fish record. First, variable fishing pressure directly increases variability in exploited populations. Second, commercial fishing can decrease the average body size and age of a stock, causing the truncated population to track environmental fluctuations directly. Third, age-truncated or juvenescent populations have increasingly unstable population dynamics because of changing demographic parameters such as intrinsic growth rates. We find no evidence for the first hypothesis, limited evidence for the second and strong evidence for the third. Therefore, in California Current fisheries, increased temporal variability in the population does not arise from variable exploitation, nor does it reflect direct environmental tracking. More fundamentally, it arises from increased instability in dynamics. This finding has implications for resource management as an empirical example of how selective harvesting can alter the basic dynamics of exploited populations, and lead to unstable booms and busts that can precede systematic declines in stock levels.

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