Detection of Human Influence on a New, Validated 1500-Year Temperature Reconstruction

TLDR

Climate records over the last millennium place twentieth‑century warming in a broader historical context, yet millennial reconstructions reveal wide variability that challenges the reliability of current techniques and the uniqueness of recent warming. The study proposes a calibration method that preserves low‑frequency variance in temperature reconstructions. The authors apply this method to produce a new 1500‑year temperature reconstruction. The reconstruction shows substantial variability, matches independent borehole and other reconstructions, and detection analysis demonstrates that greenhouse‑gas warming is evident throughout the record, with roughly one‑third of first‑half‑twentieth‑century warming attributable to anthropogenic emissions and most second‑half warming also anthropogenic.

Abstract

Abstract Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate. High variability in reconstructions does not hamper the detection of greenhouse gas–induced climate change, since a substantial fraction of the variance in these reconstructions from the beginning of the analysis in the late thirteenth century to the end of the records can be attributed to external forcing. Results from a detection and attribution analysis show that greenhouse warming is detectable in all analyzed high-variance reconstructions (with the possible exception of one ending in 1925), and that about a third of the warming in the first half of the twentieth century can be attributed to anthropogenic greenhouse gas emissions. The estimated magnitude of the anthropogenic signal is consistent with most of the warming in the second half of the twentieth century being anthropogenic.