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After running and winning on the most ambitious presidential platform on climate change in history, President-elect Joe Biden is now faced with delivering on his commitments.

Speculation has already begun regarding how this can be achieved, particularly if Republicans maintain a majority in the Senate. Yet most analyses to date have overlooked one of the most important tools at their disposal: building on the leadership of U.S. state and local governments in a new unprecedented era of climate federalism.

Now we face an even greater planetary threat—climate change—and state action has been one of the few bright spots in an overall grim U.S. policy picture. Thirty years ago, when I represented Louisiana Governor Buddy Roemer on a bipartisan National Governors’ Association (NGA) task force on climate change, we recognized the importance of national and global action. We also saw major roles for states in areas like electric power and transportation, where they hold significant authority over planning, investment, and regulation.

Where the federal government has largely dropped the ball on climate law and policy, states and cities from across the U.S. have stepped up to the plate. They sued EPA (successfully) to force regulation of carbon dioxide using Clean Air Act authority in Massachusetts v. EPA, and (unsuccessfully) to hold major polluters responsible for damage to their jurisdictions in Connecticut v. AEP. Meanwhile they moved forward in their own jurisdictions to promote clean energy, cut greenhouse gas emissions, and to respond to the impacts of climate change.

The ability to articulate where local climate authority is vulnerable has important consequences. First, it allows local governments to make a more reasoned risk calculus regarding the kind of climate action they are pursuing. Second, it provides policymakers and scholars with a further tool for precise discussions about local authority on climate and how far that local authority is likely to extend. And finally, it has significant substantive ramifications for climate progress. While many local governments are engaged in efforts to fill in climate regulatory gaps, there is substantial overlap between states unwilling to take action on climate change and those that will enact preemptive legislation in response to local attempts to take action. Thus, part of the federal government’s role can be to support local initiatives where they are occurring and to either regulate in a more top-down fashion or attempt to shore up local authority in places where such authority has been blocked by the state. For all of those reasons, localizing the conversation around climate federalism will remain important even during a federal administration inclined to take climate action.

The shifts in federal administration in 2016 and in 2020 have been accompanied by markedly different positions on how (and whether) to handle the climate crisis. Those swings show the importance of national elections as well as the potential fragility of depending on only a federal response to climate change.79 Thus, while comprehensive federal action on climate change is enormously welcome and needed, both Congress and the Biden Administration should also ensure that any action they take preserves the flexibility of and possibility for climate action at the state and local level as well. Federal climate action should work to include states as partners and responsible parties in any regulatory response. And where the federal government can empower climate action by local governments that would otherwise likely be subject to preemption by the state, it can have a lasting impact in shoring up the role of local governments as environmental actors within the United States.80 In short, federal tackling the climate crisis during the Biden Administration should include a clear-eyed view of the federal, state, and local possibilities for action, and work to empower a climate response from all points along the federalism spectrum.

President Joe Biden in his budget request to Congress is calling for increasing federal spending on Medicaid by providing coverage to the millions in the 10 states that have not expanded the health care program to more people under the Affordable Care Act.

But illustrating the wide divisions between Democrats and Republicans over the future of Medicaid, Republicans in Congress are proposing to reduce spending on Medicaid, arguing it has grown too much. According to the conservative Heritage Foundation, the program has nearly doubled—from $401.5 billion to $709 billion—since President Barack Obama signed the ACA into in 2010.

Most lawmakers — Republicans and Democrats alike — have declared the marquee safety-net programs of Medicare and Social Security off-limits for cuts as a divided Washington heads for a showdown over the national debt and government spending. Health programs for lower-income Americans, though, have gotten no such bipartisan assurances.

More than 20 million people gained Medicaid coverage in the past three years after Congress expanded access to the entitlement program during the covid-19 pandemic, swelling Medicaid’s population by about 30%. But enrollment will fall starting in April, when the pandemic-era changes end and states begin cutting coverage for Americans who are no longer eligible.

This is a critical moment for our nation. We must not allow struggling workers or our children, grandparents and disabled loved ones to fall back into hunger. SNAP helps a quarter of U.S. children avoid hunger. It also helps 6 million older adults on fixed incomes, 4 million non-elderly disabled people and more than 1 million low-income veterans. The majority of people on SNAP earn an income and don’t receive cash welfare benefits. SNAP puts food on the table for Americans of every color and in every zip code. The pandemic SNAP expansion successfully narrowed racial and ethnic inequities in food security, even though the majority of beneficiaries are white and participation is highest in rural areas and small towns.

The short-term, direct effects of the proposed reductions in SNAP spending include: Every $5 in federal expenditures on the Supplemental Nutrition Assistance Program generates $9 in community spending.70 As proposed, the SNAP benefits that would be lost through S. 3240 and H.R. 6083 could potentially generate between $3.3 million and $1 billion in community spending in 2013. Households that qualify for SNAP benefits spend an estimated 45 percent of their income on taxable goods, generating revenue via sales tax.72 In California, where data are available, if all eligible individuals participated in SNAP, $86 million of additional sales tax revenue would be generated for the state general fund.73 An additional $51 million would be generated for county budgets through state and local taxes. Decreased SNAP benefits may, therefore, have implications for state revenues. Every $1 billion reduction in SNAP will result in the loss of approximately 14,000 American jobs, including 7,000 jobs in food manufacturing and agriculture of which an estimated 3,000 are farm jobs. 74 S. 3240 would cut $4 billion to the Supplemental Nutrition Assistance Program, which would lead to an estimated 5,600 jobs lost annually. H.R. 6083 would cut $16 billion, which would lead to an estimated loss of 22,400 jobs annually. The potential effect of these cuts on jobs contradicts the current efforts to reduce unemployment.

Terror groups like al-Qaeda and ISIL (the Islamic State of Iraq and the Levant) have expressed interested, and have even attempted, to use biological agents in terror plots (Biodefense, 2016). The technical and administrative knowledge of biology and chemistry can be acquired globally in medical schools, research programs, and laboratories, making it difficult to prevent potential practitioners of bioterrorism from acquiring the required scientific knowledge (Chiodo, 2015). While at one time the ability to mutate strands was restricted to advanced research laboratories, rudimentary high school laboratories now have the ability to develop deadly biological agents (Garrett, 2012). With relative easy, terror groups could engineer the flu virus, making it deadlier (Selk, 2017). By combining traits of multiple strains and maximizing the virus’ natural properties, it could become highly transmittable (Farmer, 2107). Genetically engineered viruses have the potential to kill more people than nuclear weapons, governments remain underprepared for that threat (Selk, 2017).

Terrorists could be drawn to the use of biological agents because of the difficulty of detection and the ease at which some biological agents can naturally spread through a population (Bioterrorism, 2006). There are essentially three ways which agents could be acquired by terrorist: they could be stolen, created in laboratory environments, or collected naturally. A gloomy reality is that as the advancement of new technologies reduce the costs of genetic sequencing, it will become easier and less expensive to create novel organisms (Garrett, 2012). Bioterrorist attacks can be planned to induce maximum damage and panic with a minimum risk of early detection. Potential agents of attack are categorized by risk (rated as an, A, B, or a C) depending on the agent’s availability, ease of dissemination and transmission, and potential impact (Bioterrorism, 2006). Category A agents are considered the most dangerous and threatening to National Security. Ebola is categorized as a Category A bioagent because of its ability to cause mass panic and disruption and the special public health actions required for treating those infected.

To understand our vulnerability, and to determine what steps must be taken to end it, it is useful to ask about the very worst-case scenarios. Just how bad could a pandemic be? In science fiction, we sometimes encounter the idea of a pandemic so severe that it could cause the end of civilisation, or even of humanity itself. Such a risk to humanity’s entire future is known as an existential risk. We can say with certainty that the novel coronavirus, named Covid-19, does not pose such a risk. But could the next pandemic? To find out, and to put the current outbreak into greater context, let us turn to the past.

In 1347, death came to Europe. It entered through the Crimean town of Caffa, brought by the besieging Mongol army. Fleeing merchants unwittingly carried it back to Italy. From there, it spread to France, Spain and England. Then up as far as Norway and across the rest of Europe – all the way to Moscow. Within six years, the Black Death had taken the continent.

In the decades to come, advanced bioweapons could threaten human existence. Although the probability of human extinction from bioweapons may be low, the expected value of reducing the risk could still be large, since such risks jeopardize the existence of all future generations. We provide an overview of biotechnological extinction risk, make some rough initial estimates for how severe the risks might be, and compare the cost-effectiveness of reducing these extinction-level risks with existing biosecurity work. We find that reducing human extinction risk can be more cost-effective than reducing smaller-scale risks, even when using conservative estimates. This suggests that the risks are not low enough to ignore and that more ought to be done to prevent the worst-case scenarios. How worthwhile is it spending resources to study and mitigate the chance of human extinction from biological risks? The risks of such a catastrophe are presumably low, so a skeptic might argue that addressing such risks would be a waste of scarce resources. In this article, we investigate this position using a cost-effectiveness approach and ultimately conclude that the expected value of reducing these risks is large, especially since such risks jeopardize the existence of all future human lives. Historically, disease events have been responsible for the greatest death tolls on humanity. The 1918 flu was responsible for more than 50 million deaths,1 while smallpox killed perhaps 10 times that many in the 20th century alone.2 The Black Death was responsible for killing over 25% of the European population,3 while other pandemics, such as the plague of Justinian, are thought to have killed 25 million in the 6th century—constituting over 10% of the world's population at the time.4 It is an open question whether a future pandemic could result in outright human extinction or the irreversible collapse of civilization. A skeptic would have many good reasons to think that existential risk from disease is unlikely. Such a disease would need to spread worldwide to remote populations, overcome rare genetic resistances, and evade detection, cures, and countermeasures. Even evolution itself may work in humanity's favor: Virulence and transmission is often a trade-off, and so evolutionary pressures could push against maximally lethal wild-type pathogens.5,6 While these arguments point to a very small risk of human extinction, they do not rule the possibility out entirely. Although rare, there are recorded instances of species going extinct due to disease—primarily in amphibians, but also in 1 mammalian species of rat on Christmas Island.7,8 There are also historical examples of large human populations being almost entirely wiped out by disease, especially when multiple diseases were simultaneously introduced into a population without immunity. The most striking examples of total population collapse include native American tribes exposed to European diseases, such as the Massachusett (86% loss of population), Quiripi-Unquachog (95% loss of population), and the Western Abenaki (which suffered a staggering 98% loss of population).9 In the modern context, no single disease currently exists that combines the worst-case levels of transmissibility, lethality, resistance to countermeasures, and global reach. But many diseases are proof of principle that each worst-case attribute can be realized independently. For example, some diseases exhibit nearly a 100% case fatality ratio in the absence of treatment, such as rabies or septicemic plague. Other diseases have a track record of spreading to virtually every human community worldwide, such as the 1918 flu,10 and seroprevalence studies indicate that other pathogens, such as chickenpox and HSV-1, can successfully reach over 95% of a population.11,12 Under optimal virulence theory, natural evolution would be an unlikely source for pathogens with the highest possible levels of transmissibility, virulence, and global reach. But advances in biotechnology might allow the creation of diseases that combine such traits. Recent controversy has already emerged over a number of scientific experiments that resulted in viruses with enhanced transmissibility, lethality, and/or the ability to overcome therapeutics.13-17 Other experiments demonstrated that mousepox could be modified to have a 100% case fatality rate and render a vaccine ineffective.18 In addition to transmissibility and lethality, studies have shown that other disease traits, such as incubation time, environmental survival, and available vectors, could be modified as well.19-21 Although these experiments had scientific merit and were not conducted with malicious intent, their implications are still worrying. This is especially true given that there is also a long historical track record of state-run bioweapon research applying cutting-edge science and technology to design agents not previously seen in nature. The Soviet bioweapons program developed agents with traits such as enhanced virulence, resistance to therapies, greater environmental resilience, increased difficulty to diagnose or treat, and which caused unexpected disease presentations and outcomes.22 Delivery capabilities have also been subject to the cutting edge of technical development, with Canadian, US, and UK bioweapon efforts playing a critical role in developing the discipline of aerobiology.23,24 While there is no evidence of state-run bioweapons programs directly attempting to develop or deploy bioweapons that would pose an existential risk, the logic of deterrence and mutually assured destruction could create such incentives in more unstable political environments or following a breakdown of the Biological Weapons Convention.25 The possibility of a war between great powers could also increase the pressure to use such weapons—during the World Wars, bioweapons were used across multiple continents, with Germany targeting animals in WWI,26 and Japan using plague to cause an epidemic in China during WWII.27 Non-state actors may also pose a risk, especially those with explicitly omnicidal aims. While rare, there are examples. The Aum Shinrikyo cult in Japan sought biological weapons for the express purpose of causing extinction.28 Environmental groups, such as the Gaia Liberation Front, have argued that “we can ensure Gaia's survival only through the extinction of the Humans as a species … we now have the specific technology for doing the job … several different [genetically engineered] viruses could be released”(quoted in ref. 29). Groups such as R.I.S.E. also sought to protect nature by destroying most of humanity with bioweapons.30 Fortunately, to date, non-state actors have lacked the capabilities needed to pose a catastrophic bioweapons threat, but this could change in future decades as biotechnology becomes more accessible and the pool of experienced users grows.31,32 What is the appropriate response to these speculative extinction threats? A balanced biosecurity portfolio might include investments that reduce a mix of proven and speculative risks, but striking this balance is still difficult given the massive uncertainties around the low-probability, high-consequence risks. In this article, we examine the traditional spectrum of biosecurity risks (ie, biocrimes, bioterrorism, and biowarfare) to categorize biothreats by likelihood and impact, expanding the historical analysis to consider even lower-probability, higher-consequence events (catastrophic risks and existential risks). In order to produce reasoned estimates of the likelihood of different categories of biothreats, we bring together relevant data and theory and produce some first-guess estimates of the likelihood of different categories of biothreat, and we use these initial estimates to compare the cost-effectiveness of reducing existential risks with more traditional biosecurity measures. We emphasize that these models are highly uncertain, and their utility lies more in enabling order-of-magnitude comparisons rather than as a precise measure of the true risk. However, even with the most conservative models, we find that reduction of low-probability, high-consequence risks can be more cost-effective, as measured by quality-adjusted life year per dollar, especially when we account for the lives of future generations. This suggests that despite the low probability of such events, society still ought to invest more in preventing the most extreme possible biosecurity catastrophes.

But it is also true that hunger and food insecurity can lead to instability. There is a vicious feedback loop between conflict and hunger currently at play in dozens of countries around the world. War drives hunger and hunger drives war.

In 2017, the World Food Program USA (WFP USA) produced a report titled Winning the Peace: Hunger and Instability. It was a comprehensive review of the literature on “food-related instability”—or the ways that hunger drives conflict. At the time, U.S. lawmakers knew anecdotally that this relationship existed. Former Senate Agriculture Committee chair Pat Roberts (D-KS) said in a 2015 speech, “Show me a nation that cannot feed itself, and I’ll show you a nation in chaos.” Others, like Senator Lindsey Graham (R-SC), frequently warned that providing humanitarian assistance in some of the world’s most complex emergencies was necessary to avoid more costly—in both blood and treasure—engagements.

There is a rising danger of ‘food wars’ – conflicts over food, land and water – as the world food system begins to crack under the combined pressures of climate, water scarcity, toxicity and soil loss. In ‘Food or War’ – I show how food insecurity has driven conflict for thousands of years. A complete rethink of the current global food system is now urgent, to avoid widespread wars and migratory crises in the mid-century, as the human population surges past 10 billion and outruns its resources. “Ours is the Age of Food. Food is a central obsession in all cultures, nations, the media and society. Our future supply of food is filled with risk, and history tells us that lack of food leads to war. But this also presents spectacular opportunities for fresh human creativity and technological achievement.” Everyone depends on food, every single day. We take for granted it will always be there – but the scientific evidence suggests the global food system that fed us in the 20th Century is now in big trouble, from multiple causes. What the world most needs, before almost any other priority, is a new global food system that can feed ten billion people or more on a hot, overcrowded, resource-depleted Planet reliably, every single day – and do so for at least a century until we can bring human numbers safely back to a level the Earth can support. The alternative is catastrophe: a growing succession of climate-driven famines, mass migration out of affected regions, collapse of governments and outright wars. Anyone who doubts this need only look at our history, which records many such events.

There is a future for small farms. Or, to be more precise, there can be and should be a future for them. Given the dependence of ‘modern’ low-diversity agriculture on ‘traditional’ high-diversity agriculture, the long-term food security of humankind will depend on small farms and their continued provision of the environmental service of in situ conservation of crop genetic diversity. Policies to support small farms can be advocated, therefore, not merely as a matter of sympathy, or nostalgia, or equity. Such policies are also a matter of human survival. The diversity that underpins the sustainability of world agriculture did not fall from the sky. It was bequeathed to us by the 400 generations of farmers who have carried on the process of artificial selection since plants were first domesticated. Until recently, we took this diversity for granted. The ancient reservoirs of crop genetic diversity, plant geneticist Jack Harlan (1975, p. 619) wrote three decades ago, ‘seemed to most people as inexhaustible as oil in Arabia.’ Yet, Harlan warned, ‘the speed which enormous crop diversity can be essentially wiped out is astonishing.’ The central thesis of this essay is that efforts to conserve in situ diversity must go hand-in-hand with efforts to support the small farmers around the world who sustain this diversity. Economists and environmentalists alike by and large have neglected this issue. In thrall to a myopic notion of efficiency, many economists fail to appreciate that diversity is the sine qua non of resilience and sustainability. In thrall to a romantic notion of ‘wilderness,’ many environmentalists fail to appreciate that agricultural biodiversity is just as valuable – indeed, arguably more valuable from the standpoint of human well-being – as the diversity found in tropical rainforests or the spotted owls found in the ancient forests of the northwestern United States.

If the US maize supply were reduced, ethanol production would also decrease, leading to a greater demand for gas and oil. The United States has increased its domestic production of gas and oil following the shale revolution; however, the country remains a net energy importer. The United States would face a greater demand for international energy primarily from fossil fuels. Reliance on the maize monoculture is subject to vagaries as it can be impacted by a natural disasters, such as droughts, or an attack. If the monoculture is impacted, the United States will face a greater dependence on foreign fossil fuels and the potential for the country to become caught up in political entanglements with volatile energy producing countries. As one of the world's largest oil consumers, uncertainties concerning the maize monoculture and the lack of energy security means the United States is subject to the whims, powers, and price fluctuations of OPEC. The United States’ decision to use maize as a means to reduce foreign oil dependence is not efficient and creates a national security concern as it increases the domestic reliance on an uncertain commodity.

Farm size has become a key variable of interest in discussions surrounding food security, development, and the environment1. Most of the world’s farms are small—of the 570 million farms in the world, 84% are <2 ha in size2. Smallholders are facing growing pressure on their livelihoods from low prices in global markets and climate change-induced production losses3. Accordingly, smallholders have been the target of global development policies such as the Sustainable Development Goal (SDG) target 2.3, which seeks to support smallholders by increasing their productivity, incomes, and access to land. Many countries’ Intended Nationally Determined Contributions (INDCs) of the UN Conference of Parties on Climate Change (COP21) also aim to bolster smallholders’ adaptive capacity. Numerous scholars argue that smaller farms perform better than larger farms in terms of production, environmental, and socioeconomic outcomes4. On the basis of these arguments, scholars, policy makers, and social movements argue in favour of land reforms to redistribute farmland5,6. Although 84% of the world’s farms are <2 ha in size, they only constitute 12% of farmland—increasing the proportion of farmland in smaller farms will arguably increase its benefits. At the same time, consumers have increased their willingness to pay for products with labels associated with smaller farms7,8. Thus, there is a growing call for support for small farms. While this support is important, the performance of small farms in terms of productivity, resource efficiency, biodiversity, and greenhouse gas (GHG) emissions has itself remained highly contested9–14. Here, we synthesize the relationship between farm size and six socioeconomic and environmental outcomes, leveraging the past 50 yr of empirical evidence that directly assessed crop production, environmental performance, and economic outcomes as they relate to farm size. Our systematic assessment of the multidimensional outcomes related to farm size builds on past reviews that focused on single outcomes (for example, yield, economic performance, or biodiversity metrics for specific species)15–18, non-systematic reviews15,16, studies based on indirect measurements of farm size and the outcome variables of interest19–21, and studies with specific regional foci15,17. We present evidence from 118 studies (318 observations) from 51 countries on the relationship between relative farm size (along a continuum) and: (1) yields as value of crop output per area (value ha–1) or total crop production per area (kg ha–1), (2) crop diversity at species and varietal levels, (3) non-crop biodiversity at field and landscape levels, (4) resource-use efficiency as measured in terms of technical efficiency22, (5) GHG emissions per unit output, and (6) profit per unit area. Results Our analysis finds that smaller farms have higher yields and harbour greater crop diversity and higher levels of non-crop biodiversity at the field and landscape scales than larger farms (Table 1). We find no conclusive evidence for a relationship between farm size and resource-use efficiency, GHG emissions, or profit. In the remainder of this article, we will address each of these key findings in turn and discuss their implications for policy initiatives and consumer sup-port for small farms globally. Smaller farms have higher yields. Our synthesis shows that, in the literature, when primary studies assess yield across farm sizes, 79% (95% confidence interval, CI = 58–100%) of them report that smaller farms have higher yields (in either weight ha–1 and value ha–1terms) (Fig. 1). We also find that yields typically decrease by 5% for each hectare increase in farm size (−5% mean effect; 95% CI =−9 to −1%; Fig. 2a and Fig. 3), within the range of studied observations (mean = 7.5 ha; s.d. =22.7 ha). While the distribution of effects includes deviant cases (Fig. 3 and Supplementary Fig. 1), these new findings show that, on average, the available evidence supports the idea—which originated in the 1920s and has been studied extensively since the 1960s—that smaller farms are higher yielding than larger farms14,23,24. Moreover, we find that controlling for labour removed the effect—our model not controlling for labour has stronger effect size (although not statistically different from zero at the 95% confidence level) than the one that does. This result is in line with Sen’s 1964 prediction24 and subsequent literature25, that suggests that labour markets are an important reason for the inverse farm size–productivity relationship (Fig. 2b). Smaller farms have greater crop diversity. While many field studies have explored in situ crop diversity on small farms26–28, few directly measured the relationship between farm size and crop diversity. In our review, four studies show higher crop diversity on smaller farms, while three found the opposite; much too small a sample for statistical inference. But we previously conducted an in-depth quantitative analysis on the relationship between crop diversity and farm size across 55 countries and 154 crops using a newly harmonized dataset of nationality representative farmer surveys and agricultural censuses29. We found that, except for an unexplained dip in the 2–5 ha size range, there is a strong inverse relationship globally between farm size and the number of crop species found across the landscape—with higher species diversity within smaller farms than larger farms when controlling for area (Supplementary Fig. 2). Crop diversity on small farms is selected by farmers for a range of reasons such as improved nutrition30, market diversification31 and mitigation of drought risk32. Smaller farms harbour greater non-crop biodiversity. There are three key pathways by which smaller farms could be beneficial for non-crop biodiversity covered in the literature. The first is through ecological management practices, such as limited insecticide use and use of organic management practices. The second is through increased field edges (increased margin-to-field area ratio); increased field edges can lead to larger available breeding habitats for arthropods33,34, provide refuge for arthropods and smaller species to colonize after escaping recently disturbed fields35,36, increase the number of pollinators and beneficial predators within fields4,34 and act as conservation corridors for arthropods and small mammals37,38. The third is through landscape composition, with small-farm-dominated landscapes harbouring diverse land cover types such as forests and wetlands, fields of different crops or fields in different phenological stages of production39,40. In the studies we reviewed, there is evidence for all of these effects. When combined, 77% of studies (95% CI = 61–99%) reported that smaller farms and fields have greater biodiversity at the farm and landscape levels compared to larger farms and fields (Fig. 1).The three remaining variables we tested—GHG emissions, resource-use efficiency, and profits—did not show conclusive relationships for the effect size magnitude and sign (Figs. 2, 4, and 5), even though the majority of studies concluded that larger farms had greater resource efficiency than smaller farms (Fig. 1). For example, while the evidence we reviewed shows that GHG emissions per unit output tend to be higher on smaller farms, suggesting that smaller farms might be less efficient per unit output, the confidence intervals around this effect crossed zero (−4% mean effect; 95% CI =−10–2%). We found no clear difference between small or large farms in technical efficiency (our proxy for resource efficiency per unit output with 0% mean effect; 95% CI =−1–2%), even after controlling for a variety of moderating factors, such as access to credit, extension services or cooperative membership (Fig. 2c). See the Supplementary Information for further discussion of these results. Similarly, while profitability per unit area also declined with increasing farm size, statistical confidence in the effect was also low (−2% mean effect; 95% CI =−10–5%) (Fig. 2a). Discussion Our evidence review and meta-analysis of the current evidence base for these six outcomes associated with farm size finds strong support for the inverse farm size–productivity and farm size–diversity relationship (empirical findings that both have strong theoretical support24,41–43). While a couple of emerging studies44,45, with limited coverage, suggest that the inverse size–productivity relationship might simply be a result of measurement errors and while we were unable to rule out this possibility, our synthesis did find the inverse farm size–yield effect was removed when controlling for labour, suggesting that smaller farms may be more productive due to the availability of family labour24. Similarly, we recognize that socio-political context is important and small farms may not always be more biodiverse46 but most of the evidence base across a broad geo-graphic range of agricultural systems is in support of this positive relationship. An important caveat to interpreting the past literature for each of these outcomes are regional biases and variation. Regional biases are most evident in the biodiversity literature that has predominantly focused on higher-income countries, mainly in North America and Europe (Supplementary Fig. 4). Different regions may contain species that prefer environments that larger farms foster, either through their larger fields or different farm management techniques (Table 1). While we could not test for these factors explicitly due to our sample size, a dominant theme in the literature for other indica-tors (yield, profitability, and resource efficiency) was that a farm’s political, socioeconomic, and geographic context (a farmer’s access to training, credit, machinery, insurance, inputs, markets, and/or subsidies) may explain the farm size to outcome relationships. For instance, the relationships between farm size, resource efficiency, and profit were the most spatially heterogeneous across all variables examined. For certain smallholder-dominant countries (for example, India and Ethiopia) we found that smaller farms were more profitable, whereas larger farms were more profitable in countries dominated by large farms, higher incomes, and better rural infra-structure (for example, the United States). This may suggest that smallholders have better access to markets, inputs, and technologies in a smallholder-dominant system that may affect their profitability and resource efficiency (see Supplementary Information for expanded discussion). Systematic evidence syntheses, such as meta-analyses, are an iterative process47. For some of the outcomes (such as GHG emissions, profits, and resource-use efficiency) we were unable to identify consistent or confident outcomes from the existing literature. This may be due to the small number of included studies, limited cross-country analyses, or that these relationships are too context-specific to be generalized. Future work should continue to assess these outcomes and build on this study to include other important outcomes, such as mental and physical health of workers and farmers, employment opportunities, pesticide or fertilizer use efficiency, and other key ecosystem services, such as pollination, in addition to examining literature in languages other than English. New primary work is also needed to further explore the functional form of the effects we present here, and to explore how different socioecological and political conditions and measurement methods may mediate positive or negative outcomes across farm size classes. This could further inform policies on land reform (such as redistribution or consolidation) that address market failures so that such policies can maximize the multidimensional benefits of farming systems to society48.To support sustainable transitions in farming practices across a range of farm sizes, more evidence-based synthesis is needed at broad regional scales. Until recently, the role of farm size in the global food system has been largely assessed by independent case studies. As international commitments (for example, SDGs and COP21 INDCs) begin to evolve into actionable funding plans and as countries continue to decide on land use policies that directly affect the size of farms, it is critical to identify how farm size affects different social, economic and environmental outcomes. Our study lends evidence to boost support for policies targeting smallholders. Most of the world’s farms (84%) are operated by smallholders2 and smallholders in lower-income countries are also among the poorest people on the planet49. Our study shows that smallholders are both productive and stewards of biodiversity. Rewarding smaller farms for their conservation benefits may be one policy pathway towards supporting smallholders. Biodiversity could be promoted on larger farms by promoting more ecologically friendly management practices and increasing biodiversity refuges such as buffer strips and increased natural perimeters. These findings come at a time where donor countries need to invest an estimated US$14 billion annually to achieve the goal of SDG 2.3 to double the incomes and productivity of smallholders49. Our review adds to the motivation for these investments. We found that, despite smallholders’ increased yields and role in provision of ecosystem services, there is not enough evidence for equivalent gains in smallholders’ profits. Thus, development support for smallholders is imperative from multiple viewpoints: the data not only show that investing in smallholders could lead to humanitarian benefits but also to increases in food production and benefits to biodiversity.

Industrial agriculture is bringing about the mass extinction of life on Earth, according to a leading academic. Professor Raj Patel said mass deforestation to clear the ground for single crops like palm oil and soy, the creation of vast dead zones in the sea by fertiliser and other chemicals, and the pillaging of fishing grounds to make feed for livestock show giant corporations can not be trusted to produce food for the world. The author of bestselling book The Value of Nothing: How to Reshape Market Society and Redefine Democracy will be one of the keynote speakers at the Extinction and Livestock Conference in London in October. Organised by campaign groups Compassion in World Farming and WWF, it is being held amid rising concern that the rapid rate of species loss could ultimately result in the sixth mass extinction of life. This is just one reason why geologists are considering declaring a new epoch of the Earth, called the Anthropocene, as the fossils of soon-to-be extinct animals will form a line in the rocks of the future. The last mass extinction, which finished off the dinosaurs and more than three-quarters of all life about 65 million years ago, was caused by an asteroid strike that sent clouds of smoke all around the world, blocking out the sun for about 18 months. Prof Patel, of the University of Texas at Austin, said: “The footprint of global agriculture is vast. Industrial agriculture is absolutely responsible for driving deforestation, absolutely responsible for pushing industrial monoculture, and that means it is responsible for species loss. “We’re losing species we have never heard of, those we’ve yet to put a name to and industrial agriculture is very much at the spear-tip of that.” Speaking to The Independent, he pointed to a “dead zone” – an area of water where there is too little oxygen for most marine life – in the Gulf of Mexico that has grown to the same size as Wales because of vast amounts of fertiliser that has washed from farms in mainland US, into the Mississippi River and then into the ocean. “That dead zone isn’t an accident. It’s a requirement of industrial agriculture to get rid of the sh*t and the run-off elsewhere because you cannot make industrial agriculture workable unless you kick the costs somewhere else,” he said. “The story of industrial agriculture is all about externalising costs and exploiting nature.” The Amazon and surrounding lands in South America are also under increasing pressure from soy plantations. “Extinction is about the elimination of diversity. What happens in Brazil and other places is you get green deserts — monocultures of soy and nothing else. “Various kinds of chemistry is deployed to make sure it is only soy that’s grown on these mega-farms.

Bringing the wild back into our working lands may help prevent mass extinction BERKELEY, Calif. — With a body the size of a fist and wings that span more than a foot, the big brown bat must gorge on 6,000 to 8,000 bugs a night to maintain its stature. This mighty appetite can be a boon to farmers battling crop-eating pests. But few types of bats live on American farms. That’s because the current practice of monoculture – dedicating large swathes of land to a single crop – doesn’t give the bats many places to land or to nest. Diversifying working lands – including farmland, rangeland and forests – may be key to preserving biodiversity in the face of climate change, says a new review paper published this week in Science by conservation biologists at the University of California, Berkeley. Diversification could be as simple as adding trees or hedgerows along the edges of fields, giving animals like birds, bats and insects places to live, or as complex as incorporating a patchwork of fields, orchards, pasture and flowers into a single working farm. These changes could extend the habitat of critters like bats, but also much larger creatures like bears, elk and other wildlife, outside the boundaries of parks and other protected areas, while creating more sustainable, and potentially more productive, working lands. “Protected areas are extremely important, but we can’t rely on those on their own to prevent the pending sixth mass extinction,” said study co-author Adina Merenlender, a Cooperative Extension Specialist in the Department of Environmental Science, Policy and Management at UC Berkeley. “This is even more true in the face of climate change, because species will need to move around to adapt to shifts in temperature and climate.” Maintaining even small pieces of the original landscape – even a single tree- can help conserve the original diversity of species, Merenlender said. Clearing oak woodlands and shrublands to establish large vineyards hits many native species hard. Animals that are well adapted to urban and agricultural areas, such as mockingbirds, house finches and free-tail bats, continue to flourish, while animals that are more sensitive to disturbance, like acorn woodpeckers, orange-crowned warblers and big brown bats, begin to drop away. “If you can leave shrubs, trees and flowering plants, the habitat suitability — not just for sensitive birds but also for other vertebrates – goes way up,” Merenlender said. This is true not only in California’s vineyards, but on working lands around the world. Incorporating natural vegetation makes the farm more hospitable to more creatures, while reducing the use of environmentally degrading chemicals like herbicides, pesticides and man-made fertilizer. The ideal farming landscape includes woodland pastures and vegetable plots bumping up against orchards and small fields, said Claire Kremen, a professor in the Department of Environmental Science, Policy and Management. Integrating livestock produces manure which can fertilize the crops, while those same crops produce feed for livestock. Birds and bats provide pest control, and bees boost crop production by pollinating plants. “It is possible for these working landscapes to support biodiversity but also be productive and profitable,” Kremen said. “And ultimately, this is where we have to go. We just can’t keep mining our soils for their fertility and polluting our streams – in the end, this will diminish our capacity to continue producing the food that we need. Instead, we must pay attention to the species, from microbes to mammals, that supply us with critical services, like pollination, pest control and nutrient cycling”

SNAP Reduces Food Insecurity Households The significant, temporary increase in monthly SNAP benefits from ARRA helped reduce food insecurity by 2.2 percentage points and reduce very low food security by 2.0 percentage points among low-income households between December 2008 (pre-ARRA) and December 2009 (about eight months post-ARRA).133 Participation in SNAP for six months reduced the percentage of SNAP households that were food insecure by 6–17 percent, and reduced the percentage that were very low food secure by 12–19 percent, based on various estimates using a national sample of SNAP households.134 In another study, SNAP participation reduced the likelihood of being food insecure and very low food secure by 31 and 20 percent, respectively, based on a national sample of low-income households.135 Children Children in households that had participated in SNAP for six months were approximately one-third less likely to be food insecure than children in households recently approved for SNAP but not yet receiving it, based on a national sample of SNAP households with children.136 Among low-income households experiencing food insecurity among children, the odds of being food secure two years later were almost four times higher for SNAP participants compared to non-participants, according to a study that used national, longitudinal data.137 While food insecurity is dynamic and changes as families with children enter, participate in, and leave SNAP, a national study of more than 10,000 families found that participation in SNAP reduced the probability of child food insecurity.138 Among low-income families with children in the Three Cities Study (Boston, Chicago, and San Antonio), SNAP receipt reduced the probability of very low food security for households and for children.139 According to one estimate using national data, SNAP reduces childhood food insecurity by at least 8.1 percentage points “and perhaps much more.”140

SNAP Reduces Poverty and Deep Poverty Nationally, 3.6 million people — including 1.5 million children — were lifted above the poverty line in 2016 under the alternative poverty computation that counts SNAP benefits as income, based on Census Bureau data on poverty and income in the U.S.123 However, these estimates understate SNAP’s anti-poverty effects due to the underreporting of program participation in Census surveys. Making this adjustment for underreporting, SNAP lowers the poverty rate by 14 to 16 percent, according to analyses using national data in SNAP Matters: How Food Stamps Affect Health and Well-Being. 124 In addition, the anti-poverty effects are particularly strong when poverty rates rise during recessionary periods. Based on these and other findings contained in the book, the authors “conclude that SNAP is our nation’s most effective anti-poverty program for the nonelderly when adjusted for underreporting, one that is especially good at reducing extreme poverty — by over 50 percent — and also especially effective for poor families with children.” (Households are defined as living in extreme poverty when their cash income does not exceed $2 per person per day.) The average annual decline in the depth of child poverty when adding SNAP benefits to income was 15.5 percent, according to Current Population Survey data from 2000 to 2009.125 The effect was strongest in 2009, when the temporary increase in SNAP benefit levels from the American Recovery and Reinvestment Act (ARRA) began. In that year, SNAP benefits reduced the depth of child poverty by 20.9 percent.

Economic Impacts For struggling families and communities, SNAP is making a huge difference in their economic well-being and health. For example, SNAP is good for local economies – each dollar in federally funded SNAP benefits generates $1.79 in economic activity. Census Bureau data also show the crucial importance of federal nutrition programs and other supports for low-income families. When looking at the Census Bureau’s Supplemental Poverty Measure, which serves as an indicator of economic well-being, SNAP lifted 4.6 million people out of poverty in 2015. Health Impacts Numerous studies demonstrate that SNAP participation and the amounts of monthly SNAP benefits received by households affect the health and well-being of families and children. For instance, a Children’s HealthWatch brief (pdf) documents health improvements for young children who live in households that received increased SNAP benefits provided by the 2009 American Recovery and Reinvestment Act (ARRA). Following the increase, “children in families receiving SNAP were significantly more likely to be classified as ‘well’ than young children whose families were eligible but did not receive SNAP.” Read More: Massachusetts Inpatient Medicaid Cost Response to Increased Supplemental Nutrition Assistance Program Benefits The same study also found that the ARRA SNAP boost also had a positive impact on Medicaid costs. “Medicaid cost growth fell in Massachusetts after SNAP benefits increased, especially for people with chronic illnesses with high sensitivity to food insecurity.” Food Security Impacts There is strong evidence of SNAP’s effectiveness in alleviating food insecurity. Two studies included in the recent “Long-Term Benefits of the Supplemental Nutrition Assistance Program” White House Report demonstrate this point. Mabli and Ohls (2015) compared households’ state of food security before and after receiving SNAP benefits and found that “SNAP participation reduced the overall fraction of households that were food insecure and the fraction that were very low food secure by around 17 percent and 19 percent, respectively. Mabli and Worthington (2014) find even larger impacts on children’s food security in low-income households with children, showing that food insecurity among children fell by roughly 33 percent after their families had been receiving benefits for about six months.” Furthermore, the ARRA SNAP benefit boost was terminated effective November 2013 with negative impacts on families. In the wake of that cut, new research from Children’s HealthWatch finds that low monthly SNAP benefit levels and the November 2013 rollback in SNAP benefits have had “a measurable, negative effect on household and child food insecurity.”