Many climate change mitigation strategies rely on strong projected growth in biomass energy, supported by literature estimating high future bioenergy potential. However, expectations to 2050 are highly divergent. Examining the most widely cited studies finds that some assumptions in these models are inconsistent with the best available evidence. By identifying literature-supported, up-to-date assumptions for parameters including crop yields, land availability, and costs, we revise upper-end estimates of potential biomass availability from dedicated energy crops. Even allowing for the conversion of virtually all 'unused' grassland and savannah, we find that the maximum plausible limit to sustainable energy crop production in 2050 would be 40-110 EJ yr À1 . Combined with forestry, crop residues, and wastes, the maximum limit to long-term total biomass availability is 60-120 EJ yr À1 in primary energy. After accounting for current trends in bioenergy allocation and conversion losses, we estimate maximum potentials of 10-20 EJ yr À1 of biofuel, 20-40 EJ yr À1 of electricity, and 10-30 EJ yr À1 of heating in 2050. These findings suggest that many technical projections and aspirational goals for future bioenergy use could be difficult or impossible to achieve sustainably.

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Lakes are highly sensitive recorders of climate processes, but are extremely difficult to correlate precisely to ice-core and marine records, especially in the absence of reliable radiocarbon dates. Relative paleointensity (RPI) of Earth's magnetic field is an independent method of correlating high-resolution climate records, and can be applied to both marine and terrestrial sediments, as well as (inversely) correlated to the cosmogenic nuclide records preserved in ice cores. Here we present the correlation of an RPI record from Mono Lake, California to GLOPIS, the Global PaleoIntensity Stack, which increases the age estimation of the basal Mono Lake sediments by N 20 000 yr (20 kyr), from ∼40 ka (kyr before present) to 67 ka. The Mono Lake sediments thus preserve paleoclimatic records of most of the last glacial period, from 67 to 14 ka. In addition, the paleointensity-based age of 40 ka for the geomagnetic excursion preserved at Mono Lake indicates that this is a record of the global Laschamp excursion.

Summary• We report the first investigation of changes in electron partitioning via the alternative respiratory pathway (AP) and alternative oxidase (AOX) protein abundance in field-grown plants and their role in seasonal acclimation of respiration.• We sampled two alpine grasses native to New Zealand, Chionochloa rubra and Chionochloa pallens, from field sites of different altitudes, over 1 yr and also intensively over a 2-wk period.• In both species, respiration acclimated to seasonal changes in temperature through changes in basal capacity (R 10 ) but not temperature sensitivity (E 0 ). In C. pallens, acclimation of respiration may be associated with a higher AOX : cytochrome c oxidase (COX) protein abundance ratio. Oxygen isotope discrimination (D), which reflects relative changes in AP electron partitioning, correlated positively with daily integrated photosynthetically active radiation (PAR) in both species over seasonal timescales. Respiratory parameters, the AOX : COX protein ratio and D were stable over a 2-wk period, during which significant temperature changes were experienced in the field.• We conclude that respiration in Chionochloa spp. acclimates strongly to seasonal, but not to short-term, temperature variation. Alternative oxidase appears to be involved in the plant response to both seasonal changes in temperature and daily changes in light, highlighting the complexity of the function of AOX in the field.

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