Lonnie G. Thompson

The oxygen isotope (δ ¹⁸ O _ice ) record from a ∼310-meter core drilled on the Guliya ice cap (GIC) in 2015 (GP2015) demonstrates reproducibility with a ∼309-meter core drilled nearby in 1992. Construction of the timescale for a composite of the GIC cores used ¹⁰ Be and ³⁶ Cl to locate the 41–thousand year Laschamp geomagnetic excursion and a match to a δ ¹⁸ O _atm -dated core from the Guliya Summit. Comparison of the δ ¹⁸ O record from an Asian Monsoon speleothem with the GIC δ ¹⁸ O _ice supports a timescale back to 128 thousand years, the oldest known so far from Tibet. The δ ¹⁸ O _ice record shows orbital variations in phase with June insolation and polar ice core stable isotope records through the last glacial cycle, although the relationship is reversed since the deglaciation. Future investigation is required to resolve this observation. Radiocarbon-dated ice from the glacier margin supports the presence of glacial-stage ice in the GIC. The ³⁶ Cl/ ¹⁰ Be ratios in GP2015 confirm that GIC ice is older than 100 thousand years.

Speleothem oxygen isotope records (δ ¹⁸ O _s ) from the Asian monsoon region provide valuable insights into past hydroclimate variability, yet their interpretation relies on understanding the controls on modern precipitation isotopes (δ ¹⁸ O _p ). Although numerous explanations for the spatiotemporal variability of δ ¹⁸ O _p across this region have been proposed, the primary controls remain unresolved. Here we propose a hypothesis to explain δ ¹⁸ O _p variability from δ ¹⁸ O _p records at 46 sites along a transect spanning the Asian monsoon region. Our results show that seasonally low δ ¹⁸ O _p values migrate gradually westward along the east–west transect. We further demonstrate that the corresponding westward migration of atmospheric updraft, particularly convection, controls δ ¹⁸ O _p variations along the transect. In contrast to previous interpretations emphasizing monsoon moisture advection, our findings highlight atmospheric updraft as an important control on δ ¹⁸ O _p variations across the Asian monsoon region. These findings provide an alternative perspective on interpreting the climate signals preserved in Asian δ ¹⁸ O _s records.

Tropical wetlands are widely considered the largest natural source of atmospheric methane (CH ₄ ) ^1–4 . However, uncertainties about wetland extent and CH ₄ production have led to large variations in modelled CH ₄ emission trends ^2,3 . Most historical reconstructions rely on data from polar ice cores, which cannot fully resolve the tropical contribution ^5,6 to the CH ₄ budget. Here we present a 2,000-year record of atmospheric CH ₄ concentrations from ice cores drilled from the South Peak summit of Nevado Huascarán (Summit Core A, SCA; −9.122° S, −77.605° W; 6,768 m asl). We find that the trends and magnitudes of our CH ₄ record are broadly consistent with polar records ⁷ . Our δ ¹³ C-CH ₄ measurements (from approximately 1530 CE to 1999 CE) align with isotope values ⁸ consistent with a dominant tropical CH ₄ source. Integration of our record into an atmospheric four-box model suggests a sustained equatorial dominance of CH ₄ source strength over the past two millennia. Our findings indicate that equatorial CH ₄ emissions are higher than previous estimates based only on polar ice core data, supporting the long-standing hypothesis that low-latitude CH ₄ emissions dominated pre-industrial (PI) CH ₄ variability ^5,6 . These results demonstrate the importance of tropical ice cores on the reconstruction of CH ₄ variability and latitudinal distribution.

The westerlies moisture transport underpins water security for over two billion people dependent on the Asian water towers (AWTs). However, the mechanisms by which large-scale westerlies-advected moisture is integrated into the AWTs’ atmospheric water budget remain poorly understood due to observational gaps. Here, we combine three-dimensional observations of atmospheric water vapor stable isotopes with isotope-enabled modeling. We identify the conveyor mechanism that regulates the vertical moisture transport under calm conditions during the winter-spring period when the westerlies are dominant. Sharp vertical isotopic gradients show that large-scale westerlies-advected moisture is predominantly confined aloft, while local residual moisture persists near the surface. Our results show the interplay of the westerlies’ subsidence at night with thermodynamically distinct local residual air, yielding thermal inversions and condensation that suppresses vertical mixing and decouples moisture between the free troposphere and the atmospheric boundary layer. This process constitutes a primary pathway for integrating westerlies-advected moisture into the local moisture budget without precipitation, sustaining near-surface moisture accumulation. Our results provide critical benchmarks for improving atmospheric models, refining climate projections of the intensifying water cycle over the AWTs, and advancing interpretations of isotopic records in regional climatic archives.

In addition to warming and hydrological changes, glacier retreat is influenced by mineral dust and other light absorbing impurities. However, very little is known about the presence of mineral dust in the tropical Andes, making it difficult to model the radiative impact of dust on glacial surfaces. We present two 60‐year (1960–2019) ice core dust records from the col and summit of Earth's highest tropical mountain (Nevado Huascarán, Cordillera Blanca, Peru) and use the Snow, Ice, and Aerosol Radiative model (SNICAR) to investigate dust‐driven albedo changes over time. Our results show that, although dust concentrations have trended upward for several decades, mineral dust has only a marginal effect on the albedo of the high‐altitude glaciers at Huascarán. We estimate that by the end of the century, the reduction in surface albedo due to mineral dust will translate to an additional 15–70 mm w.e. of ablation on the col and 16–57 mm w.e. on the summit, or less than ∼5% of the average net accumulation rates of today.

Chapter 4 explores the question: how do we read the clues about past climate preserved in Earth’s ice? This exploration begins by describing the natural archive of glacial ice and the types of paleoclimate data recorded in that archive. These data provide information about past regional to global-scale climate variability, and well as external forcing mechanisms at play during those times. Ice cores from polar ice sheets generally record a longer history, but ice cores from low latitude mountains provide paleoclimate information that is more directly linked to human civilizations and indigenous peoples’ histories. Melting of the cryosphere has accelerated in recent decades due to warming temperatures, endangering water resources for societies dependent on glacial meltwater and endangering coastal settings due to sea level rise.

The persistent discrepancy between climate models and proxy records regarding temperature variations throughout the Holocene remains evident in our latest isotope‐enabled transient simulations (iTRACE). This study quantifies model sensitivity to insolation forcing and investigates the origins of the uncertainties in mid‐Holocene temperatures by combining iTRACE with multi‐model simulations from the Palaeoclimate Model Intercomparison Project (PMIP4) component of the latest phase of the Coupled Model Intercomparison Project (CMIP6). We show that inter‐model uncertainties in temperature response exceed the ensemble mean signals, particularly over the Northern Hemisphere and during boreal winter. The weak temperature response observed in most models, compared to paleoclimate reconstructions, is attributed to low sensitivity to insolation forcing, arising from insufficient local‐to‐regional feedbacks over northern mid‐to‐high latitudes. Additionally, model uncertainties in boreal summer temperatures originate from uncertainties in sea ice‐albedo feedback and shortwave‐cloud feedback. In boreal winter, model temperature uncertainties are attributed to uncertainties in air moisture content, cloud cover, and ocean heat release to the subpolar atmosphere. The mid‐Holocene temperature response is modulated by the interplay of multiple internal feedbacks within the climate system, in addition to a direct response to insolation forcing. Finally, we argue that improving the mid‐Holocene temperature simulations in mid‐high latitudes will also enhance tropical temperature simulations, as these regions are linked through meridional atmospheric heat transport.