Wherever scientists meet, they are increasingly questioning whether humanity is getting a grip on the major contemporary challenges raised by scientific research. The lead article by Behrens et al. on the obesity and climate “co-crises” illustrates why (1). It raises fundamentally important issues about the way we live today and the drivers of existence.

First, the role of science is sometimes described as simply offering data. The question of what to do about problems is left to the realm of policy and politics, not science. Behrens et al. cut through this “facts-versus-actions” distinction. The topics of obesity and climate are vast in themselves, but when both are seen to connect, it is surely correct to ask whether science’s contribution of data, analysis, and advice regarding both has altered the trajectory of events for the better. If not, why not? Is science consigned to plotting the trajectory of falling over metaphorical cliffs? Surely not. The World Health Organization reported that the incidence of overweight had exceeded that of underweight by 1995 (2). The United Nations Framework Convention on Climate Change was signed by 154 states (plus the European Commission) in Rio de Janeiro in 1992 (3). The lack of sufficient action on these matters since those dates is not due to insufficient data or advice given to policymakers; indeed, there has been an avalanche of research.

Second, the article by Behrens et al. illustrates one of the greatest successes of science in recent decades—the capacity of researchers and methods to pool data at multiple scales, from micro to macro and from within and across disciplines. The comprehension of complexity now possible is remarkable. This has partly been enabled by advances in technology—computer power, datasets, data pooling, and the capacity of microchips—but also the scientific imagination: asking “what might be found if...?” The effect is clear. The range of knowledge and the capacity to stand “above” the minutiae of particular studies are now almost taken for granted. We now know how to amalgamate bodies of knowledge and distinguish among methods for doing so. But the desire to do this is not modern. Many advances in science were achieved when boundaries were pushed, and when different bodies of knowledge were juxtaposed. Recognition of this process informed, for instance, Thomas Kuhn’s great thesis that science develops when paradigms—frameworks of understanding—are found wanting and replaced by newer frameworks that explain, or make sense of, the previously denied or ignored (4).

Behrens et al.’s work suggests we are in such a moment of paradigmatic change today; the fusion of data on the rise of obesity and climate change makes denial of their connection harder to maintain. In this respect, Behrens et al.’s article sits alongside other powerful challenges to public policy in the vein of the EAT-Lancet Commissions of 2019 and 2025 (5) and the Syndemic of Obesity, Undernutrition, and Climate Change Commission (6). These challenge the attention policymakers are giving to the co-existence and reinforcement of climate, obesity, and societal formation of food systems.

Third, asking “why?” inevitably leads to “what can be done?” The schism is not pure versus applied science. Science historians sometimes suggest that these questions are reversed and that a concern for change leads to questioning of the status quo. Twentieth-century historians, for instance, argued that the explosion of innovation and commercial exploitation characterizing the development of industrialization was directly associated with the loosening of religious strictures on knowledge (7). As previously powerful European religions fragmented, it became possible to question norms and think “outside the box”. The spread of broad ecological and interdisciplinary thinking that began, in the West, in the “gentleman science” era of the mid-18th and 19th centuries extended across societies and reframed economic activity. Questioning existing frameworks of knowledge and daring to ask “why” characterize the growth of science. In the past, only those with private funds, facilities, and sufficient mental “space” in their lives could both ask why and pursue answering the question. Technical change, more than pure science, shaped the emergence of disciplines. This was gradually replaced by professionalized and stricter disciplinary focuses. Now, as Behrens et al. show, science can again capture the scientific imagination by offering data on a scale that was unthinkable a few decades ago. Surely, progress must follow from this power. But perhaps what is missing is a “narrative” of how to unlock the lock-ins.

Fourth, there is a worrying possibility that apathy might be setting in. If glucagon-like peptide (GLP)-1 receptor agonist drugs can seemingly cure obesity, why worry about it? Why not look to such technical fixes? We meet a friend who was previously obese but now finds a slimmer echo. Those who can afford the bill can cure obesity. This is the tantalizing prospect offered by some. In effect, it sidesteps rather than rebuts the systemic analysis of obesity—that social conditions determine population outcomes. Instead, the chimera of personalized medicine seems to offer a resolution. But does it? These drugs come at immense cost, and the parent pharmaceutical companies are subject to the vagaries of stockholder values. The equivalent technical-fix approach to climate change is carbon capture—but few climatologists expect this to achieve the scale necessary to tame, let alone reverse, global warming (8).

Fifth, there is the matter of the public, not science. Behrens et al. expose the gap between science and policymakers, but will pressure from the public succeed where science is failing? We discern the obesity pandemic when walking down the street or when comparing photos across generations, for example, within families or on beaches or in crowds. At the same time, we experience climate change in the weather wherever we are. We should not need to read vast reports on obesogenic trends or witness glaciers melting. Immense scientific data and bodies of knowledge on obesity and climate change dovetail with human observation. What matters, however, is not the trends but our acceptance of their normalization. We barely remark on it. We take more notice when the previously overweight shrink. But both trends can coexist. The world gets fatter overall while some individuals get thinner, thanks to the efficacy of a medical revolution. Meanwhile, politicians shrug their shoulders and huge externalized costs are dumped on future generations (9). This gap between people’s lived experience of these new normals and the need to tackle the implications of scientific data remains firmly in the “too-difficult-to-handle” policy box.

The policy–evidence–action gap can no longer be ignored or categorized as a problem only for specialists or policymakers. It is everyone’s problem. The scale of the obesity, food, and climate challenge requires a major recalibration of food supply and availability. Humanity is currently living with the outcomes of the post-World War II, mid-20th century experiment with food. Seventy years of apparent success have, in fact, breached planetary boundaries (10). Life itself is threatened. The Malthusian problem of population growth exceeding the capacity of food supply may have been answered in the last century by a combination of economic wealth and technical advancement, notably the Haber–Bosch system for fixing nitrogen from the air. However, that model of pumping nutrients into the soil to accelerate plant growth is running out of room. It has also created vast environmental problems. The challenges now facing humanity and its diverse food systems are how to balance modes of consumption, production, and distribution with ecosystems, livable economies, and population health.

If policymakers fail to act rationally in response to such data, at what level might shocks and a rising scale of damage drag them into action? Or will a combination of public alarm and societal disruption provide the missing incentive?