From Heat Summation to Climate Adaptation: Lessons from 75 Years of the Winkler Index for Agroclimatic Metric Design

Kaitlin Libbey, Ava Andrews, Amanda Vigil, Diego Vasquez, James Ronald Campbell, Elisabeth Jane Forrestel 

Agroclimatic indices are foundational tools for matching crop cultivars to suitable climates, yet the accelerating pace of climate change is exposing the limits of indices built on mean conditions. The Winkler Index (WI) is a regional categorization system based on heat summation informed by wines made from grapes throughout California in the 1940s to 1960s, and it remains one of the most widely used agroclimatic indices in viticulture. This systematic review characterizes the main uses, themes, and contexts of the WI across 679 publications from 61 countries over 75 years, to identify broadly transferable principles for agroclimatic index design. The successes stem from its ability to predict a highly nuanced outcome—cultivar suitability and wine quality—from a simple, communicable metric. Its documented limitations, including a single thermal metric, fixed calendar thresholds, inability to capture extremes, variability, or evaporative demand, and coarse spatial resolution, mirror the failures of analogous indices across cropping systems. As climate change intensifies the mismatch between mean-condition indices and increasingly volatile growing environments, the WI's trajectory offers a template for the next generation of bioclimatic indices across agricultural systems.