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We thank the Bren School of Environment Science and Management of the University of California Santa Barbara for support leading to the initial publication. This work was also supported by a grant overseen by the French ‘Programme Investissement d’Avenir’ as part of the ‘Make Our Planet Great Again’ programme (reference: 17-MPGA-0004) and by a National Science Foundation grant (LTER-1831944). More

Rapeseed crops depend on pollinators such as bees.Credit: fotokostic/iStock/Getty

Farming trends deplete pollinators
Most cultivated crops depend on insect pollinators, such as bees, but global crop trends are leaving pollinators worse off.
Using data from the United Nations’ Food and Agriculture Organization, an international team, led by Marcelo Aizen at the National University of Comahue in Rio Negro, Argentina, assessed changes in the amount of land used for agriculture and the types of crops cultivated between 1961 and 2016. During that time, the area of land used to grow crops increased by around 40%, and pollinator-dependent cropland more than doubled. Soya bean, rapeseed and oil palm — crops associated with deforestation and diversity loss — account for much of the expansion and for the increase in pollinator dependence.

But although the land used has increased, crop diversity has remain largely the same since 2000. Producers have opted for large-scale cultivation of one crop. That’s a problem because monocultures don’t provide pollinators with a stable, year-round supply of food. This ultimately leads to a fall in insect numbers, lower yields and increased deforestation as demand for land surges.
Greater reliance on crops that are dependent on single-species pollinators, coupled with declining pollinator populations, could cause problems for food security. Poorer regions will be the hardest hit by crop failures, but higher-income countries that rely on imported food will also be affected.
Rotating a diverse range of crops on a single piece of land could help to stem the decline in pollinator populations. Planting native flowers and hedgerows on agricultural land and restoring neighbouring natural environments could also preserve pollinator habitats.
Glob. Change Biol. 25, 3516–3527 (2019)
US household food waste calculated
Working out how much food goes uneaten in an individual household is notoriously difficult. Comprehensive data on how much food ends up in the bin does not exist. But Yang Yu and Edward Jaenicke at Pennsylvania State University in University Park used a new method to overcome the lack of data.
Instead of trying to measure food waste directly, Yu and Jaenicke calculated a household’s ability to efficiently convert food brought into the household into the energy required to maintain the body weight of its residents. First, they obtained data on food purchases from around 4,000 households that took part in the 2012 US Department of Agriculture’s National Household Food Acquisition and Purchase Survey. The authors then calculated the metabolic energy requirements of the people living in each household from attributes such as height, weight, age and gender. The amount of food waste was estimated according to the difference between the household’s food inputs and its members’ energy requirements, not accounting for overeating.
The study showed that the average household wasted close to one-third of the food that it bought, which means that the United States wastes an estimated US$240 billion worth of food per year. The most efficient household in the study wasted about 9% of its food. Healthier diets created more waste than unhealthier diets, owing to the greater proportion of fruit and vegetables. Higher-income households wasted about 50% more food than lower-income households, and small households wasted more per person than large households.
Am. J. Agric. Econ. 102, 525–547 (2020)
Hidden hunger a global problem
There is more than enough food to feed the global population. But local patterns of production still leave 10% of the world’s people with insufficient calories, and more than half with inadequate quantities and variety of micronutrients — known as hidden hunger.
These are findings of a detailed analysis of food production by Ozge Geyik and colleagues at Deakin University in Burwood, Australia. The team gathered data on the nutrient content of 174 individual foods produced across 177 countries between 1995 and 2015. The researchers analysed whether individual countries and regions could meet the energy needs of their populations, as well as supply them with protein, iron, zinc, vitamin A, vitamin B12 and folate.
The study is one of the first to take such a detailed look at global patterns of nutrient production using disaggregated food data over time. Previous work has typically grouped foods into broad categories, such as cereals, dairy and vegetable oils, which can lead to under- or overestimates of specific nutrients.
Global food production increased steadily over the two decades, and outpaced increases in food requirements. However, on a regional level, the analysis found that more than half of the countries in Africa and Asia were not producing enough calories for their populations.
In 2015, more than 20% of the global population lived in countries with inadequate iron, vitamin A, vitamin B12 and folate production. Food production often fell short in multiple nutrients. More than 70% of countries with nutrient shortfalls produced inadequate amounts of iron, vitamin A and folate. And more than one-fifth of those not producing enough nutrients, fell short by more than half of what was necessary for their population.
The authors suggest that countries with nutrient deficiencies could prioritize the production of foods that contain the nutrients that their population needs. For example, in places where protein production is adequate, shifting production to protein sources that are higher in vitamin A and iron could alleviate these nutrient shortfalls. Adding micronutrients directly to soils and the leaves of crop plants is another possible solution.
Glob. Food Sec. 24, 100355 (2020)
Nutrient recycling possibilities mapped
The age-old practice of fertilizing crops with livestock manure has been reimagined in a study led by Sheri Spiegal from the US Department of Agriculture in Las Cruces, New Mexico. In the study, the team introduces the concept of a manureshed — land around livestock farms that could benefit from the nutrient-rich manure that those farms produce.
Spiegal and her colleagues mapped a patchwork of more than 3,000 counties across the United States. They classified counties as manure sources if they could supply nutrients in manure from livestock, or sinks if the crops grown could use the nutrients from manure.
The work reveals a surfeit of opportunity to recycle nutrients. The researchers identified counties that could recycle nitrogen and phosphorous nutrients at the local county level, as well as four regional manuresheds — in the northwest, southwest, central and southeast United States — where clusters of source counties could join together to develop sustainable redistribution programmes over longer distances. The work suggests a pathway towards removing manure from areas where it can pollute the local environment and delivering it to nutrient-poor agricultural lands, easing the reliance on commercial fertilizers that pollute the environment and deplete finite natural resources. But the authors note that further research — on how best to recover and transport manure, for instance — will be needed to turn the vision into a reality.
Agric. Syst. 182, 102813 (2020)
Intervention trade-offs assessed
Transforming the way land is managed and food is produced could shore up food supplies and address the challenges of climate change and biodiversity loss. But an assessment of proposed interventions reveals that few are up to the task of protecting both livelihoods and the environment.
Pamela McElwee from Rutgers University in New Brunswick, New Jersey, and her colleagues assessed the benefits and trade-offs of 40 proposed changes to land management, food-production chains and the management of environmental risks. The potential interventions are outlined in the 2019 report from the Intergovernmental Panel on Climate Change, and include improving management of livestock, reforestation, reducing consumer and retail food waste and management of urban sprawl.

The authors assessed each of the actions against the United Nations’ 17 Sustainable Development Goals (SDGs), as well as 18 measures from the Nature’s Contributions to People (NCP) framework, which was drawn up by scientists associated with the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services in 2017. This framework is intended to recognize nature’s social, cultural, spiritual and religious significance, as well as its role in providing food, clean water and healthy air.
The analysis revealed that several interventions carried unintended negative consequences. The production of bioenergy, either with or without carbon capture, planting forests and commercial crop insurance all had potentially negative consequences for both SDGs and NCPs. For example, bioenergy had large negative impacts on maintaining land biodiversity, freshwater quality and food production, despite providing affordable clean energy. About one-third of the interventions proposed had no substantial trade-offs. These included improving water management, increasing soil organic carbon content, reducing pollution, reducing post-harvest losses and fire management.
The analysis could help decision-makers to assess environmental or developmental policies to avoid unintended trade-offs, the authors say.
Glob. Change Biol. 26, 4691–4721 (2020) More

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A farmer inspects her maize crop, grown using a ‘push–pull’ approach.Credit: The ‘Push–Pull’ Farming System: Climate-smart, sustainable agriculture for Africa/ICIPE/Green Ink Ltd UK

On paper, the global agriculture sector has done an admirable job of keeping pace with a growing population. According to the United Nations’ Food and Agriculture Organization, agricultural output per person has increased by 50% since 1960 — impressive, considering the number of mouths to feed has more than doubled.
But the reality is messier. Many people, including those in high-income nations, lack reliable access to nutritious food. And food security is an ongoing struggle for people in poorer regions. Even transient disruptions can have far-reaching consequences. One article1 described the global food supply as being “on a razor’s edge” — weather events or natural disasters in one part of the world can cause the price of grain everywhere to spike by more than 50%. “Globally, we have to increase food production by 60%, and in some areas we have to increase by 100%,” says P. V. Vara Prasad, a crop ecophysiologist at Kansas State University, Manhattan.

Over the past 50 years, producers increased agricultural output in much of the world through the ‘green revolution’. But this revolution has been environmentally harmful, relying heavily on chemical pesticides and fertilizers that have inflicted lasting damage on the soil and water supply. Natural biodiversity has been sacrificed to create vast monoculture fields. And in many low-income nations, survival depends on coaxing greater productivity from existing plots as more and more people scramble for limited resources, says Bernard Vanlauwe, a soil scientist based in Nairobi at the International Institute of Tropical Agriculture.
Many agricultural researchers are now looking to a set of practices known as sustainable intensification. The specifics vary depending on the setting, but a growing number of examples from around the world highlight the possibility of a second green revolution — one that might better live up to its name.
Many roads to sustainability
The concept of sustainable intensification was popularized2 in 1997 by Jules Pretty, an environmental scientist at the University of Essex in Colchester, UK. His goal was to challenge the idea that increasing yield is inherently incompatible with environmental health, with an agricultural philosophy that encompasses parameters such as biodiversity and water quality as well as the social and economic welfare of farmers. Researchers have defined the scope of sustainable intensification in different ways, but the big picture, says Pretty, entails recognizing that agriculture is inexorably connected with the environment and designing cultivation strategies accordingly. “Components of sustainable systems tend to be multifunctional,” he says. “You want a diverse system that provides support to pollinators, fixes nitrogen and provides a break against insects.” Advocates of sustainable intensification recognize that global agriculture can’t be reinvented in one fell swoop and that progress will come from incremental steps that improve efficiency, as well as more-dramatic measures that redesign the farming landscape.
Lucas Garibaldi, an agroecologist at the National University of Río Negro in Bariloche, Argentina, has focused on pollinators as a crucial component of what he calls ecological intensification. “Crop yield depends not only on the count of pollinators, but also on the biodiversity of pollinators,” says Garibaldi. “Millions of honeybees alone will not replace the function of diverse species of wild bees and butterflies and birds.” He notes that different bees pollinate different crops, but also allow more efficient pollination for some plant species. To create a haven for these airborne assistants, Garibaldi advocates minimizing pesticide use and including non-agricultural zones in farmland. These could be wild-plant borders that surround fields or just hedgerow-like strips of flowers that are appealing to the bees that traverse them.
Growing a mix of crops can have many benefits, including attracting pollinators. Conventional monoculture leaves soil exposed for much of the year, Garibaldi says. This creates opportunities for weeds to grow — necessitating herbicides — or leaves soil susceptible to erosion. With multiple crops or rotation throughout the year, more durable root systems that densely and extensively permeate the ground can be established, reinforcing the soil and preventing the nutrient depletion associated with long-term monoculture.

Crops rely on pollinators such as bees. Credit: Chris Gomersall/2020VISION/naturepl.com

Diversity can also eliminate the need for pesticides. Pretty says around 180,000 farmers in Kenya, Uganda and Tanzania now use push–pull cropping practices when growing maize. They plant grasses around the edges of maize plots that produce chemicals that ‘pull’ a common pest, the maize stalk borer (Busseola fusca), away from crops, while the maize itself attracts parasitic wasps that prey on the stalk borer. The farmers also intersperse legumes of the genus Desmodium with the maize that enrich the soil with nitrogen, and produce compounds that ‘push’ away pests and kill off a genus of invasive weed known as Striga.
Sustainable soil management is a thorny issue, particularly in resource-limited settings. Vanlauwe notes that nutrient depletion is one of the greatest threats to yield for African farmers, making a hard-line approach to sustainability unrealistic. “People who say you can trigger agricultural development in Africa without fertilizer do not have on the ground experience,” he says. But there are environmentally friendly ways to feed the soil. Jo Smith, a soil scientist at the University of Aberdeen, UK, has been equipping farmers in Africa and Asia with anaerobic digesters — simple systems that use microbes to convert animal manure into biogas for fuel and leave a nutrient-rich bioslurry. “It’s like giving them a little fertilizer factory — it gives you available ammonium that the crop can take up quickly,” she says. The biogas is also less harmful than conventional fuels, reducing household air pollution and improving quality of life, Smith adds.
Much of the world’s farming takes place on smallholder plots. One study3 estimated that one-third of the global food supply is produced on farms of less than two hectares. This fragmentation can make it challenging to introduce sustainable intensification practices. “Smallholder production systems are absolutely risk-averse,” says Vanlauwe. “Falling from earning US$100 to $50 a month can be the difference between being not-hungry and being hungry.”
Close collaboration with individual farmers is needed, but this is difficult to achieve at scale. Fortunately, smallholders are increasingly participating in collectives that can accelerate information sharing and reduce the risk associated with adopting new cultivation strategies. In August4, Pretty and his colleagues reported that, worldwide, around 8 million such groups have formed over the past two decades. “That’s about 240 million people working in collective-action efforts around areas like irrigation, forest management, pest management and water,” says Pretty. By partnering with these groups, researchers can design programmes that are more likely to be compatible with social, cultural and environmental conditions, and establish local networks of collaborators to facilitate the dissemination of information.
Some governments are also taking a more active role. Ethiopia, for example, has focused on aspects of ecological repair by establishing ‘exclosure’ areas for depleted soils. “Areas are fenced off, and after about ten years the land starts to recover,” Smith says.
In China, Fusuo Zhang, a plant-nutrition specialist at the China Agricultural University in Beijing, and his colleagues are working with government officials to mobilize an effort to help smallholder farmers across the nation transition to more evidence-based, sustainable cultivation. This includes selecting seed varieties that are suited to a given plot, using modelling techniques to guide planting based on levels of sunlight, water and nutrients, and optimizing the timing and density of seed planting. “We sent faculty members and groups of students to live among the farmers in the villages, and work with them to try to change their management,” says Zhengxia Dou, an agricultural scientist at the University of Pennsylvania in Philadelphia, who collaborated with Zhang’s team. By 2015, the effort had grown to include nearly 21 million farmers across China, who, on average, achieved a more than 10% boost in yield while using around 15% less fertilizer and reducing their greenhouse-gas output5.
Many farmers in India are embracing a national programme known as zero-budget natural farming (ZBNF). This cultivation strategy involves using soil microbes and mulch rather than synthetic fertilizers to enrich lands. Farmers in several Indian states are pursuing the approach, including around half a million farmers in Andhra Pradesh. But some scientists are concerned that the approach is untested and unproven. Last year, Panjab Singh, president of the National Academy of Agricultural Sciences in Delhi, told the newspaper The Hindu, “We are worried about the impact on farmers’ income, as well as food security.”

Smith concurs. “It was a political move, not a scientific move,” she says, adding that the natural farming approach has “not been properly trialled”. To assess the technique, she and her colleagues modelled the long-term impact of ZBNF on soil health. They found that the approach could meaningfully and sustainably improve nitrogen levels for low-yield lands, but that it would offer little benefit to farms already achieving high yields6. They concluded that a more targeted implementation of ZBNF is needed to protect overall national food security. Smith remains largely positive about ZBNF, which has been gaining momentum among farmers. “There’s a lot of good things about it, but it needs more science,” she says.
Outside national initiatives, smallholder sustainable intensive farming requires targeted investment and efforts to support social and economic stability. Vanlauwe contends that, in many parts of sub-Saharan Africa, environmental and political conditions mean that many farmers will continue to struggle at the margins for the foreseeable future. Still, he sees a path towards economic mobility. “Give them access to credit they pay back over time, and invest in integration and value-chains so they can get rid of or sell excess produce,” he says. “It’s about creating incentives and access systems.”
But durable change also requires building local expertise in crop and soil research, and in ecosystems. Many specialists in these areas are also involved with international education and training. For example, as director of the Feed the Future Innovation Lab for Collaborative Research on Sustainable Intensification, Prasad has helped to coordinate undergraduate- and graduate-level agriculture programmes in places such as Senegal, Cambodia and Bangladesh. Normally, these programmes take on a few dozen students at a time, but the shift to online training as a result of the coronavirus pandemic could prove to be a long-term gain for capacity building. “We are now talking to about 500 or even 1,000 students,” he says. More

Calculation of solar elevation and azimuth angles versus time
For our numerical calculations, the solar elevation angle θs(t) from the horizon and the solar azimuth angle αs(t) from south (axis y, Fig. 7A) were calculated as a function of time t with an algorithm based on a semi-analytical approximation (analytical Kepler’s orbits modified with astronomical perturbations) and the planetary theory VSOP 87 (Variations Séculaires des Orbites Planètaires) of Bretagnon and Francou30. This method is valid for the 1950–2050 period with an accuracy of 0.01°. Using this algorithm, we calculated the geocentric ecliptical, then the geocentric equatorial, and finally the geocentric horizontal coordinates of the Sun, resulting in the values of θs(t) and αs(t).
Diurnal cloudiness
Total cloud cover (TCC) time series of high temporal resolution (1 h) were evaluated for the period 01.01.2009–31.12.2018 from the ERA5 reanalysis of the European Centre for Medium-Range Weather Forecasts31. The geographic coverage is global with a native spatial resolution of 0.25° × 0.25° ≈ 27 km × 27 km. Climatological mean values of TCC were determined by averaging for each hour of each calendar day of every year in the vegetative period of sunflowers. Since TCC is a dimensionless relative parameter in the range 0–1 (0 is clear sky, 1 is overcast), the hourly climatological means are equivalent to the time-dependent probability 0 ≤ σ(t) ≤ 1 of cloudy situation. We determined the diurnal cloud probability function σ(t) in July, August and September in Boone County (Kentucky, USA, 39° N, − 84.75° E, Fig. 2A), central Italy (41.0° N, 15.0° E, Fig. 2B), central Hungary (47.0° N, 19.0° E, Fig. 2C), and south Sweden (58.0° North, 13.0° East, Fig. 2D). The cloudiness data used in our calculations correspond to the decade between 2009 and 2018. Because similar data are not readily available for the period when sunflowers were domesticated, we assume in this work that the data obtained in the last decade is historically representative. The validity of this assumption can be evaluated when paleo-climatological cloudiness data become available.
Measurement of the elevation angle of mature sunflower heads versus time
In a sunflower plantation at Budaörs (near Budapest), we measured the elevation angle θn of the normal vector of the mature head of the same 100 sunflowers as a function of time t, approximately weakly from 6 July to 11 September 2020. The studied sunflowers were individuals in a given row of the plantation.
Measurement of the absorption spectra of mature sunflower heads
The absorption spectra A(λ) of young (2 weeks after anthesis) and old (4 weeks after anthesis) inflorescence and back of mature sunflower heads were measured in the field with an Ocean Optics STS-VIS spectrometer (Ocean Insight, Largo, USA) in July 2020. Measurements were performed under total overcast conditions to ensure isotropic diffuse skylight illumination. At first, the reflection spectrum of the inflorescence/back was determined as follows: a spectrum was measured by directing the spectrometer’s head on the target at a distance of 5 cm, then another spectrum was registered by pointing the spectrometer to the overcast sky. In the laboratory these two spectra were divided by each other. Finally, assuming that all non-reflected light was absorbed, the absorption spectrum A(λ) = 1 − R(λ) was obtained by subtracting the reflection spectrum R(λ) from 1. Absorption spectra were measured for 3 sunflowers and then averaged.
Calculation of sky irradiance absorbed by a sunflower inflorescence
In the x–y-z reference frame of Fig. 7A, let the normal vector of a mature sunflower inflorescence be

$$underline {text{n}} = , left( {{text{cos}}theta_{{text{n}}} cdot {text{sin}}alpha_{{text{n}}} ,{text{ cos}}theta_{{text{n}}} cdot {text{cos}}alpha_{{text{n}}} ,{text{ sin}}theta_{{text{n}}} } right),$$
(2)

where axes x and y point to west and south, axis z points vertically upward, the elevation angle − 90° ≤ θn ≤  + 90° is measured from the horizontal (θn  > 0°: above the horizon, θn  More

Time-series sampling
Air samples were collected in Yurga (55.711 N, 84.937 E), where the average temperatures range from 6 to 24 °C in summer (June–August), and from − 21 to − 6 °C in winter (November–March) (the open source service https://weatherspark.com/). Meteorological characteristics (temperature, relative humidity, and wind direction) during the time series are represented in Fig. S1 and S2. Specifically, air samplers were positioned at an open-air balcony (~ 4 m above the ground level under a concrete canopy) of a five-storey residential setting. Samples were collected in duplicates (i.e., two technical replicates) with two high flowrate and filter-based air samplers (SASS3100, Research International, USA). The first set of samples were collected during three time periods (1:00–3:00, 9:00–11:00, and 15:00–17:00) on 26 and 28 July 2017; the second set was also collected during three time periods (9:00–11:00, 15:00–17:00, and 21:00–23:00) within consecutive days from 2 to 5 December 2017, the third set was collected during four time periods (1:00–3:00, 9:00–11:00, 15:00–17:00, and 21:00–23:00) within consecutive days from 27 August to 2 September, 2018. In total, 78 samples in 39 time intervals were collected and used for preparation of 62 sequencing libraries (Table S1).
High volumetric, filter-based air samplers (SASS3100, Research International, USA) were used in this study, with SASS bioaerosol electret filters (6 cm diameter, expected 50% efficiency for 0.5 µm particle size, Research International, USA) as the filter medium. Sampling was performed at 300 L/min air flowrate for 2 h. After sampling, the SASS filters were stored at − 20 °C. During transport from Siberia to Singapore, the samples were hand-carried with cooling.
Sample blanks
In each sampling set, blanks were also collected as controls. The blanks consisted of 12 filter blank samples (FB) and three reagent blank samples (RB). The filter blank samples were collected by installing a new filter on the air sampler at the sampling location for about 5 s. The filter was then collected and analysed with the same protocol as the time-series samples. Reagent blank samples involved extractions performed with extraction reagents without any filter.
Details on metagenomic analysis for blanks are provided in the Supplementary section (Fig. S11).
DNA extraction
Technical replicates were isolated separately. For processing, the SASS filter was first transferred into a sterile 5 mL tube. Phosphate buffered saline (pH 7.2) with 0.1% (v/v) Triton X-100 (2 mL, PBS-T) was added to the 5 mL tube as the wash buffer. Using tweezers, the SASS filter in the tube was moved up and down a few times to let the PBS-T penetrate the filter. The tube was then sonicated for 1 min in a sonication bath without heating to dislodge the biomass from the filter. After sonication, the filter was squeezed with tweezers and the PBS-T with suspended particles was transferred into a sterile 50 mL conical tube to complete the first washing step. This washing step was repeated three times for each filter sample, using fresh 2 mL PBS-T for each repeat. At the end of the second and third repeats, the filter was transferred into the barrel of a 10 mL syringe, placed in the same 50 mL conical tube containing the wash liquid. The 50 mL tube with the syringe and SASS filter was then centrifuged at 5000×g for 2 min to remove any leftover PBS-T. The expected total recovered supernatant volume from the three washes for each sample was 6 mL, which contained the captured airborne particles.
Upon completion of the wash steps, the supernatant was subsequently filtered through a 0.02 µm Anodisc filter (Whatman, UK) using a vacuum manifold (DHI, Denmark). The Anodisc was finally transferred into a 5 mL bead tube provided in the DNeasy PowerWater Kit (Qiagen, Germany) for DNA extraction.
DNA extraction from the Anodisc was mostly performed following the standard protocol of the DNeasy Power Water Kit with the following modifications to increase DNA yield. Briefly, 0.1 mg/mL (final) Proteinase K was added to the lysis buffer (solution PW1) prior to the initial 55 °C incubation. The initial incubation time at 55 °C was also prolonged from the recommended 10 min to overnight incubation. After initial incubation, the sample tubes were vortexed for 3 min and subsequently placed into an ultrasonic bath (Elmasonic, USA) for sonication at 65 °C for 30 min29, followed by another 5 min vortex. The remaining extraction steps were completed as instructed in the manufacturer’s protocol.
In the first and second time series (SUMMER 2017 and WINTER 2017), the DNA isolated from the technical replicates was pooled to provide sufficient material for sequencing.
Metagenomic sequencing
For the metagenomic sequencing and NGS data processing, we used standardised procedures and pipelines described in detail elsewhere1. Extracted air DNA samples were quantitated on a Qubit 2.0 fluorometer, using the Qubit dsDNA HS (High Sensitivity) Assay Kit (Invitrogen). Immediately prior to library preparation, sample quantitation was repeated on a Promega QuantiFluor fluorometer, using Invitrogen’s Picogreen assay.
Next-generation sequencing libraries were prepared with Swift Biosciences’ Accel-NGS 2S Plus DNA Library Kit, following the instructions provided in the kit. With the exception of samples that had a concentration of  More