Paleoclimatologists reconstructing temperatures from centuries before direct measurement rely heavily on proxies—tree rings, ice cores, coral bands—whose physical properties correlate with past temperature or precipitation. Tree-ring width, in particular, has served as a workhorse proxy because wide rings generally form in favorable growing years and narrow rings in unfavorable ones, and because trees in many regions can be cross-dated to the calendar year, giving proxy records unusual chronological precision.

The proxy's convenience, however, conceals an asymmetry that complicates its use for reconstructing the twentieth century's warmest decades. In several high-latitude and high-altitude tree populations, ring width tracks temperature closely up to a point, but above some threshold, additional warmth ceases to widen rings further—the trees may instead become moisture-limited, or heat-stressed, even as temperatures continue to climb. Where this divergence occurs, reconstructions built from tree-ring width alone understate just how anomalous recent warming truly is, since the proxy quietly stops responding at the exact moment its accuracy matters most.

Researchers have proposed supplementing ring width with other tree-ring properties, such as wood density late in the growing season, which in some species continues to respond to temperature past the point where width saturates. Yet incorporating a second proxy is not a simple corrective: density measurements require different, more destructive sampling techniques, are available for fewer sites and shorter historical windows, and their own relationship to temperature must independently be validated against the instrumental record before they can be trusted to fill the gap that width leaves open.