Hotter Nights, Hidden Consequences: An Overlooked Dimension of Climate Change.

Global temperatures are shifting in complex ways due to climate change. While early research focused on rising mean temperatures and its effect on biological outcomes, recent work has shifted toward understanding the influence of temperature variability. In particular, many studies investigate temperature variation by symmetrically expanding daily temperature ranges around a fixed mean or by increasing daytime maximums. Although these approaches isolate specific aspects of temperature change, they often fail to capture how climate change is actually reshaping daily temperature cycles. In this perspective paper, we use climate data across three geographic scales to illustrate a striking and consistent pattern: daily minimum temperatures are rising faster than daily maximums, effectively reducing daily temperature range. A global analysis reveals that nighttime minimum temperatures are increasing more rapidly than daytime maximums across most land areas worldwide, especially at higher latitudes and elevations. At the continental scale, North American climate data show that asymmetric warming occurs year-round, with the strongest effects in winter. Regional patterns reveal especially strong nighttime warming in mountainous regions like the Rocky and Pacific Mountain systems. Locally, hourly data from Paradise, Nevada show nighttime temperatures have risen by over 4°C since the 1950s, while daytime highs remained stable, reducing daily temperature range by more than 4°C. We then synthesize findings from 84 studies that directly investigated biological responses to nighttime warming. Nearly half (47%) of the orders studied were plants, highlighting major taxonomic gaps in animal and microbial systems. Most studies (57%) were in organismal biology, yet few were hypothesis driven. Across taxa, asymmetric warming alters energetics, increases metabolic costs, and affects both thermal performance traits (e.g., metabolism, activity) and threshold-dependent traits (e.g., phenology, sex determination). We highlight evidence that nighttime warming may enhance or inhibit cellular recovery from heat stress (Heat Stress Recovery Hypotheses), shift species interactions, disrupt pollination networks, and reshape community structure. We conclude with a call for broader research across taxa, life stages, and ecological contexts, and recommend experimental, field-based, and modeling approaches tailored to disentangle the unique effects of asymmetric warming. Understanding asymmetric warming is not just a research gap-it's a pressing ecological imperative essential for predicting and mitigating climate change impacts on biodiversity.