Potential climate-related impacts on future crop yield are a major societal concern. Previous projections of the Agricultural Model Intercomparison and Improvement Project's Global Gridded Crop Model Intercomparison based on CMIP5 identified substantial climate impacts on all major crops, but associated uncertainties were substantial. Here we report new 21st-century projections using ensembles of latest-generation crop and climate models. Results suggest markedly more pessimistic yield responses for maize, soybean and rice compared to the original ensemble. Mean end-of-century maize productivity is shifted from +5% to −6% (SSP126) and from +1% to −24% (SSP585)-explained by warmer climate projections and improved crop model sensitivities. In contrast, wheat shows stronger gains (+9% shifted to +18%, SSP585), linked to higher CO 2 concentrations and expanded high-latitude gains. The 'emergence' of climate impacts consistently occurs earlier in the new projectionsbefore 2040 for several main producing regions. While future yield estimates remain uncertain, these results suggest that major breadbasket regions will face distinct anthropogenic climatic risks sooner than previously anticipated.

The idea that a quantum magnet could act like a liquid crystal, breaking spin-rotation symmetry without breaking time-reversal symmetry, holds an abiding fascination. However, the very fact that spin nematic states do not break time-reversal symmetry renders them "invisible" to the most common probes of magnetism -they do not exhibit magnetic Bragg peaks, a static splitting of lines in NMR spectra, or oscillations in µSR. Nonetheless, as a consequence of breaking spin-rotation symmetry, spin-nematic states do possess a characteristic spectrum of dispersing excitations which could be observed in experiment. With this in mind, we develop a symmetry-based description of long-wavelength excitations in a spin-nematic state, based on an SU(3) generalisation of the quantum non-linear sigma model. We use this field theory to make explicit predictions for inelastic neutron scattering, and argue that the wave-like excitations it predicts could be used to identify the symmetries broken by the otherwise unseen spin-nematic order.

The spin-nematic state has proved elusive, due to the difficulty of observing the order parameter in experiment. In this article we develop a theory of spin excitations in a field-induced spinnematic state, and use it to show how a spin-nematic order can be indentified using inelastic neutron scattering. We concentrate on 2-dimensional frustrated ferromagnets, for which a two-sublattice, bond-centered spin-nematic state is predicted to exist over a wide range of parameters. First, to clarify the nature of spin-excitations, we introduce a soluble spin-1 model, and use this to derive a continuum field theory, applicable to any two-sublattice spin-nematic state. We then parameterise this field theory, using diagrammatic calculations for a realistic microscopic model of a spin-1/2 frustrated ferromagnet, and show how it can be used to make predictions for inelastic neutron scattering. As an example, we show quantitative predictions for inelastic scattering of neutrons from BaCdVO(PO4)2, a promising candidate to realise a spin-nematic state at an achievable h ∼ 4T. We show that in this material it is realistic to expect a ghostly Goldstone mode, signalling spin-nematic order, to be visible in experiment.PACS numbers: 75.10.Jm, 75.40.Gb S=1/2 J 1 −J 2 [Eq. 1], for ferromagnetic J 1 and antiferromagnetic J 2 [5,9,24]. This model is believed to describe a number of quasi-two dimensional magnets, including BaCdVO(PO 4 ) 2 [25]. S=1/2 J 1 −J 2 [Eq. 1], as described in Section VI of this article, for parameters J 1 = −3.6 K and J 2 = 3.2 K relevant to BaCdVO(PO 4 ) 2 25 . All predictions have been convoluted with a gaussian of standard deviation 0.006 meV to mimic experimental resolution. Equivalent results for a powder sample are shown in Fig. 2.