Yesterday, my colleague Sarah Dry and I submitted the full manuscript of our forthcoming book ‘When Temperature Became Global’ to Princeton University Press. Following peer review, and any subsequent revisions, the book will enter production later this year and should hit the bookstores in the first quarter of 2027. Here is a brief synopsis of the book …

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Numbers shape the world around us in powerful ways and in this short book we tell the history of a hugely consequential number whose story has remained untold. Global temperature is today used to monitor global climate, to signal danger and to organize global climate policy. From the Paris Agreement to the concept of Net Zero, global temperature shapes global politics and our visions for the future. And yet it cannot be experienced by anyone anywhere on the planet and few people know how it is calculated. It is as important as Gross Domestic Product, yet its history remains unknown. 

We present the history of this concept in ten concise chapters, explaining what it is, how it has been made, and what it has been used for during the past 150 years. We seek to understand two things. How could an empirical global temperature index for the Earth be constructed that passed the test of scientific credibility, and thus hold political utility? And why have scientists sought to do so, at different times and places? 

Global temperature has not always had the iconic status it has today. We begin by tracing work by a range of independent researchers who for roughly one hundred years—from the 1870s onwards— developed a variety of large-scale temperature series as tools to test different theories of climate change. Our focus then sharpens in the years around 1980. In this period, scientists increasingly turned to large-scale temperature indices with a new objective: to distinguish possible anthropogenic global warming against a background of natural climatic variability. Researchers working independently at the Climatic Research Unit at the University of East Anglia, later in collaboration with the UK Met Office, and at the Goddard Institute for Space Studies at NASA developed new global temperature series which would definitively (if not immediately) transform global temperature from a tool used to study climatic change into what we know it as today: a trackable ‘emperor’ metric that structures global climate policy and discourse, and, by extension much planning for the future.

We conclude the book by reflecting on lessons of this history: that there might have been (and still are) other, perhaps better, ways of measuring the health of world climate, or indeed the well-being of its inhabitants. Global temperature has become central to the way we measure, understand and govern climate change, but as this history reveals, its emergence was not necessary or inevitable. Like any measure, it has advantages and disadvantages. Understanding its history helps us to see more clearly what those relative merits and demerits are. By extension, this history can, we hope, assist considerations of what other measures of the state of world climate might complement global temperature.

Mike Hulme, 15 January 2026