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Past estimates of Greenland Ice Sheet accumulation rates have been multiyear climatologies based on ice/firn cores and coastal precipitation records. Existing annually resolved estimates have incompletely quantified uncertainty, owing primarily to incomplete spatial coverage. This study improves upon these shortcomings by calibrating annual (1958–2007) solid precipitation output from the Fifth Generation Mesoscale Model modified for polar climates (Polar MM5) using firn core and meteorological station data. The calibration employs spatial interpolation of regionally derived linear correction functions. Residual uncertainties exhibit coherent spatial patterns, which are modeled via spatial interpolation of root mean squared errors. Mean 1958–2007 Greenland Ice Sheet annual accumulation rate is 337 ± 48 mm/yr water equivalent (w.e.) or 591 ± 83 Gt/yr. Annual estimates contain one standard deviation uncertainties of 74 mm/yr w.e., 22%, or 129 Gt/yr. Accumulation rates in southeast Greenland are found to exceed 2000 mm/yr w.e. and to dominate interannual variability in Greenland Ice Sheet total accumulated mass, representing 31% of the whole. Accumulation rates in the southeast are of sufficient magnitude to affect the sign of Greenland mass balance during some years. The only statistically significant temporal change in total ice sheet accumulation in the 1958–2007 period occurred between 1960 and 1972, when a simultaneous accumulation increase and decrease occurred in west and east Greenland, respectively. No statistically significant uniform change in ice sheet‐wide accumulation is evident after 1972. However, regional changes do occur, including an accumulation increase on the west coast post‐1992. The high accumulation rates of 2002–2003 appear to be confined to the southeast.

Using an assemblage of four ice cores collected around the Pacific basin, one of the first basinwide histories of Pacific climate variability has been created. This ice core–derived index of the interdecadal Pacific oscillation (IPO) incorporates ice core records from South America, the Himalayas, the Antarctic Peninsula, and northwestern North America. The reconstructed IPO is annually resolved and dates to 1450 CE. The IPO index compares well with observations during the instrumental period and with paleo-proxy assimilated datasets throughout the entire record, which indicates a robust and temporally stationary IPO signal for the last ∼550 years. Paleoclimate reconstructions from the tropical Pacific region vary greatly during the Little Ice Age (LIA), although the reconstructed IPO index in this study suggests that the LIA was primarily defined by a weak, negative IPO phase and hence more La Niña–like conditions. Although the mean state of the tropical Pacific Ocean during the LIA remains uncertain, the reconstructed IPO reveals some interesting dynamical relationships with the intertropical convergence zone (ITCZ). In the current warm period, a positive (negative) IPO coincides with an expansion (contraction) of the seasonal latitudinal range of the ITCZ. This relationship is not stationary, however, and is virtually absent throughout the LIA, suggesting that external forcing, such as that from volcanoes and/or reduced solar irradiance, could be driving either the ITCZ shifts or the climate dominating the ice core sites used in the IPO reconstruction.

Background Glacier ice archives information, including microbiology, that helps reveal paleoclimate histories and predict future climate change. Though glacier-ice microbes are studied using culture or amplicon approaches, more challenging metagenomic approaches, which provide access to functional, genome-resolved information and viruses, are under-utilized, partly due to low biomass and potential contamination. Results We expand existing clean sampling procedures using controlled artificial ice-core experiments and adapted previously established low-biomass metagenomic approaches to study glacier-ice viruses. Controlled sampling experiments drastically reduced mock contaminants including bacteria, viruses, and free DNA to background levels. Amplicon sequencing from eight depths of two Tibetan Plateau ice cores revealed common glacier-ice lineages including Janthinobacterium , Polaromonas , Herminiimonas , Flavobacterium , Sphingomonas , and Methylobacterium as the dominant genera, while microbial communities were significantly different between two ice cores, associating with different climate conditions during deposition. Separately, ~355- and ~14,400-year-old ice were subject to viral enrichment and low-input quantitative sequencing, yielding genomic sequences for 33 vOTUs. These were virtually all unique to this study, representing 28 novel genera and not a single species shared with 225 environmentally diverse viromes. Further, 42.4% of the vOTUs were identifiable temperate, which is significantly higher than that in gut, soil, and marine viromes, and indicates that temperate phages are possibly favored in glacier-ice environments before being frozen. In silico host predictions linked 18 vOTUs to co-occurring abundant bacteria ( Methylobacterium , Sphingomonas , and Janthinobacterium ), indicating that these phages infected ice-abundant bacterial groups before being archived. Functional genome annotation revealed four virus-encoded auxiliary metabolic genes, particularly two motility genes suggest viruses potentially facilitate nutrient acquisition for their hosts. Finally, given their possible importance to methane cycling in ice, we focused on Methylobacterium viruses by contextualizing our ice-observed viruses against 123 viromes and prophages extracted from 131 Methylobacterium genomes, revealing that the archived viruses might originate from soil or plants. Conclusions Together, these efforts further microbial and viral sampling procedures for glacier ice and provide a first window into viral communities and functions in ancient glacier environments. Such methods and datasets can potentially enable researchers to contextualize new discoveries and begin to incorporate glacier-ice microbes and their viruses relative to past and present climate change in geographically diverse regions globally. 8eoz6b1Gq7e8M2dM_SBXdy Video Abstract

Resolving discrepancies in long-term Holocene temperature trends between climate models and proxy records is essential to improve future climate projections. However, uncertainties in paleoclimate reconstructions limit their ability to constrain models. This study compares ice core-derived oxygen isotope records with isotope-enabled climate simulations to evaluate model performance and investigate Holocene temperature variability. Our results show that simulated and observed oxygen isotope trends in Greenland and West Antarctica are generally consistent, driven by orbital forcing. However, the model underestimates the early Holocene peak and subsequent decline observed in Greenland records. The most striking mismatch appears in tropical mountains, where the model shows a slight increase in isotopic trends, while proxy records indicate a clear decline. The mechanisms controlling this decreasing isotopic trend remain unclear and cannot be fully explained by temperature or hydroclimate changes alone. Addressing this tropical mountain oxygen isotope conundrum requires efforts to improve the model and paleoclimate interpretations. Analysis of Holocene climate forcing mechanisms suggests a general agreement between ice core oxygen isotope records and climate models in Greenland and West Antarctica. However, contrasting model-data isotopic trends in tropical mountains challenge interpretations and call for further investigation by the paleoclimate community.

Three lines of evidence for abrupt tropical climate change, both past and present, are presented. First, annually and decadally averaged$\\delta^{18}C$and net mass-balance histories for the last 400 and 2,000 yr, respectively, demonstrate that the current warming at high elevations in the mid- to low latitudes is unprecedented for at least the last 2 millennia. Second, the continuing retreat of most mid- to low-latitude glaciers, many having persisted for thousands of years, signals a recent and abrupt change in the Earth's climate system. Finally, rooted, soft-bodied wetland plants, now exposed along the margins as the Quelccaya ice cap (Peru) retreats, have been radiocarbon dated and, when coupled with other widespread proxy evidence, provide strong evidence for an abrupt mid-Holocene climate event that marked the transition from early Holocene (pre-5,000-yr-B.P.) conditions to cooler, late Holocene (post-5,000-yr-B.P.) conditions. This abrupt event, ≈5,200 yr ago, was widespread and spatially coherent through much of the tropics and was coincident with structural changes in several civilizations. These three lines of evidence argue that the present warming and associated glacier retreat are unprecedented in some areas for at least 5,200 yr. The ongoing global-scale, rapid retreat of mountain glaciers is not only contributing to global sea-level rise but also threatening freshwater supplies in many of the world's most populous regions.

A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (Ta‾) and net snow accumulation (c˙‾). We estimate a total ice-sheet-wide FAC of 26 800±1840 km3, of which 6500±450 km3 resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area (Ta‾≤-19 ∘C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (Ta‾>-19 ∘C and c˙‾≤600 mm w.e. yr−1), FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between 150±100 Gt and 540±440 Gt, respectively, from the top 10 m and entire firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and firn retention capacity.

Six ice cores from Kilimanjaro provide an ~11.7-thousand-year record of Holocene climate and environmental variability for eastern equatorial Africa, including three periods of abrupt climate change: ~8.3, ~5.2, and ~4 thousand years ago (ka). The latter is coincident with the \"First Dark Age,\" the period of the greatest historically recorded drought in tropical Africa. Variable deposition of F- and Na+ during the African Humid Period suggests rapidly fluctuating lake levels between ~11.7 and 4 ka. Over the 20th century, the areal extent of Kilimanjaro's ice fields has decreased ~80%, and if current climatological conditions persist, the remaining ice fields are likely to disappear between 2015 and 2020.

Ice core data are combined with Regional Atmospheric Climate Model version 2 (RACMO2) output (1958–2010) to develop a reconstruction of Greenland ice sheet net snow accumulation rate,Ât (G), spanning the years 1600–2009. Regression parameters from regional climate model (RCM) output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values. Each core site’s residual variance is used to inversely weight the cores’ respective contributions. The interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the RCM data. Uncertainty and significance of changes is measured using statistical models. A 12% or 86 Gt yr−1increase in ice sheet accumulation rate is found from the end of the Little Ice Age in ∼1840 to the last decade of the reconstruction. This 1840–1996 trend is 30% higher than that of 1600–2009, suggesting an accelerating accumulation rate. The correlation ofÂt (G) with the average surface air temperature in the Northern Hemisphere (SATNH t ) remains positive through time, while the correlation ofÂt (G) with local near-surface air temperatures or North Atlantic sea surface temperatures is inconsistent, suggesting a hemispheric-scale climate connection. An annual sensitivity ofÂt (G) to SATNH t of 6.8% K−1or 51 Gt K−1is found. The reconstuction,Ât (G), correlates consistently highly with the North Atlantic Oscillation index. However, at the 11-yr time scale, the sign of this correlation flips four times in the 1870–2005 period.