Ice cores from low latitudes can provide a wealth of unique information about past climate in the tropics, but they are difficult to recover and few exist. Here, we report annually resolved ice core ...records from the Quelccaya ice cap (5670 meters above sea level) in Peru that extend back ~1800 years and provide a high-resolution record of climate variability there. Oxygen isotopic ratios (δ¹⁸O) are linked to sea surface temperatures in the tropical eastern Pacific, whereas concentrations of ammonium and nitrate document the dominant role played by the migration of the Intertropical Convergence Zone in the region of the tropical Andes. Quelccaya continues to retreat and thin. Radiocarbon dates on wetland plants exposed along its retreating margins indicate that it has not been smaller for at least six millennia.
This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal ...high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.
Glacier loss on Kilimanjaro continues unabated Thompson, L.G; Brecher, H.H; Mosley-Thompson, E ...
Proceedings of the National Academy of Sciences - PNAS,
11/2009, Letnik:
106, Številka:
47
Journal Article
Recenzirano
Odprti dostop
The dramatic loss of Kilimanjaro's ice cover has attracted global attention. The three remaining ice fields on the plateau and the slopes are both shrinking laterally and rapidly thinning. Summit ice ...cover (areal extent) decreased almost equal to1% per year from 1912 to 1953 and almost equal to2.5% per year from 1989 to 2007. Of the ice cover present in 1912, 85% has disappeared and 26% of that present in 2000 is now gone. From 2000 to 2007 thinning (surface lowering) at the summits of the Northern and Southern Ice Fields was almost equal to1.9 and almost equal to5.1 m, respectively, which based on ice thicknesses at the summit drill sites in 2000 represents a thinning of almost equal to3.6% and almost equal to24%, respectively. Furtwängler Glacier thinned almost equal to50% at the drill site between 2000 and 2009. Ice volume changes (2000-2007) calculated for two ice fields reveal that nearly equivalent ice volumes are now being lost to thinning and lateral shrinking. The relative importance of different climatological drivers remains an area of active inquiry, yet several points bear consideration. Kilimanjaro's ice loss is contemporaneous with widespread glacier retreat in mid to low latitudes. The Northern Ice Field has persisted at least 11,700 years and survived a widespread drought almost equal to4,200 years ago that lasted almost equal to300 years. We present additional evidence that the combination of processes driving the current shrinking and thinning of Kilimanjaro's ice fields is unique within an 11,700-year perspective. If current climatological conditions are sustained, the ice fields atop Kilimanjaro and on its flanks will likely disappear within several decades.
Observed changes in the surface elevation of the Greenland Ice Sheet are caused by ice dynamics, basal elevation change, basal melt, surface mass balance (SMB) variability, and by compaction of the ...overlying firn. The last two contributions are quantified here using a firn model that includes compaction, meltwater percolation, and refreezing. The model is forced with surface mass fluxes and temperature from a regional climate model for the period 1960–2014. The model results agree with observations of surface density, density profiles from 62 firn cores, and altimetric observations from regions where ice-dynamical surface height changes are likely small. In areas with strong surface melt, the firn model overestimates density. We find that the firn layer in the high interior is generally thickening slowly (1–5 cm yr−1). In the percolation and ablation areas, firn and SMB processes account for a surface elevation lowering of up to 20–50 cm yr−1. Most of this firn-induced marginal thinning is caused by an increase in melt since the mid-1990s and partly compensated by an increase in the accumulation of fresh snow around most of the ice sheet. The total firn and ice volume change between 1980 and 2014 is estimated at −3295 ± 1030 km3 due to firn and SMB changes, corresponding to an ice-sheet average thinning of 1.96 ± 0.61 m. Most of this volume decrease occurred after 1995. The computed changes in surface elevation can be used to partition altimetrically observed volume change into surface mass balance and ice-dynamically related mass changes.
A high-resolution ice core record from Dasuopu, Tibet, reveals that this site is sensitive to fluctuations in the intensity of the South Asian Monsoon. Reductions in monsoonal intensity are recorded ...by dust and chloride concentrations. The deeper, older sections of the Dasuopu cores suggest many other periods of drought in this region, but none have been of greater intensity than the greatest recorded drought, during 1790 to 1796 A.D. of the last millennium. The 20th century increase in anthropogenic activity in India and Nepal, upwind from this site, is recorded by a doubling of chloride concentrations and a fourfold increase in dust. Like other ice cores from the Tibetan Plateau, Dasuopu suggests a large-scale, plateau-wide 20th-century warming trend that appears to be amplified at higher elevations.
Atmospheric warming over the Greenland Ice Sheet during the last 2 decades has increased the amount of surface meltwater production, resulting in the migration of melt and percolation regimes to ...higher altitudes and an increase in the amount of ice content from refrozen meltwater found in the firn above the superimposed ice zone. Here we present field and airborne radar observations of buried ice layers within the near-surface (0–20 m) firn in western Greenland, obtained from campaigns between 1998 and 2014. We find a sharp increase in firn-ice content in the form of thick widespread layers in the percolation zone, which decreases the capacity of the firn to store meltwater. The estimated total annual ice content retained in the near-surface firn in areas with positive surface mass balance west of the ice divide in Greenland reached a maximum of 74 ± 25 Gt in 2012, compared to the 1958–1999 average of 13 ± 2 Gt, while the percolation zone area more than doubled between 2003 and 2012. Increased melt and column densification resulted in surface lowering averaging −0.80 ± 0.39 m yr−1 between 1800 and 2800 m in the accumulation zone of western Greenland. Since 2007, modeled annual melt and refreezing rates in the percolation zone at elevations below 2100 m surpass the annual snowfall from the previous year, implying that mass gain in the region is retained after melt in the form of refrozen meltwater. If current melt trends over high elevation regions continue, subsequent changes in firn structure will have implications for the hydrology of the ice sheet and related abrupt seasonal densification could become increasingly significant for altimetry-derived ice sheet mass balance estimates.
Surface roughness, defined as the standard deviation of small‐scale elevation fluctuations from the linear trend over 0.5 km, can be estimated from high‐resolution airborne laser altimetry. Here we ...present results for the northern half of the Greenland Ice Sheet using laser data collected in May 1995. Roughness is smallest in the central region straddling the ice divide, increases in amplitude toward the coast, and appears to be correlated with slope of the ice surface. For most of the study region surface roughness is 8 cm or less (<2.5 cm water equivalent). In smaller regions associated with fast flow, larger values are found. Comparison of the size of small‐scale topographic disturbances with the spatial noise estimated from five closely spaced ice cores drilled in northwest Greenland shows good agreement. Similar correspondence was found earlier using nine ice cores from the Summit region. These results indicate that the airborne laser altimeter provides an efficient platform for characterizing the statistical nature of the snow surface over large areas of the polar ice sheets.
A new compilation of annually resolved time series of atmospheric trace gas concentrations, solar irradiance, tropospheric aerosol optical depth, and stratospheric (volcanic) aerosol optical depth is ...presented for use in climate modeling studies of the period 1500 to 1999 A.D. Atmospheric CO2, CH4, and N2O concentrations over this period are well established on the basis of fossil air trapped in ice cores and instrumental measurements over the last few decades. Estimates of solar irradiance, ranging between 1364.2 and 1368.2 W/m2, are presented using calibrated historical observations of the Sun back to 1610, along with cosmogenic isotope variations extending back to 1500. Tropospheric aerosol distributions are calculated by scaling the modern distribution of sulfate and carbonaceous aerosol optical depths back to 1860 using reconstructed regional CO2 emissions; prior to 1860 the anthropogenic tropospheric aerosol optical depths are assumed to be zero. Finally, the first continuous, annually dated record of zonally averaged stratospheric (volcanic) optical depths back to 1500 is constructed using sulfate flux data from multiple ice cores from both Greenland and Antarctica, in conjunction with historical and instrumental (satellite and pyrheliometric) observations. The climate forcings generated here are currently being used as input to a suite of transient (time dependent) paleoclimate model simulations of the past 500 years. These forcings are also available for comparison with instrumental and proxy paleoclimate data of the same period.
A sulfate record covering the period A.D. 1000–1997 from the Dasuopu glacier in the Himalayas reveals that this site is sensitive to anthropogenic activity originating in southern Asia. Prior to 1870 ...atmospheric sulfate concentrations were relatively low and constant, but thereafter concentrations have increased and since 1930 the rate of increase has accelerated rapidly. This accelerating trend in sulfate deposition is paralleled by growing SO2 emissions over southern Asia resulting from the increased energy demand. The concentration of sulfate deposited in the last 50 years exceeds that for any prior 50‐year period in the last millennium. Unlike the Greenland ice core‐derived sulfate concentrations that have declined since the 1970s, sulfate concentrations deposited on the Himalayan ice fields continue to increase, having nearly doubled since 1970. This reflects regional differences between Europe and Asia in source strength and transport pathways for atmospheric sulfate, as well as differing degrees of environmental regulation.
Data collected between 1974 and 2016 from snow pits and core samples from two Peruvian ice fields demonstrate the effect of the recent warming over the tropical Andes, augmented by El Niño, on the ...preservation of the climate record. As the 0°C isotherm is approaching the summit of the Quelccaya ice cap in the Andes of southern Peru (5,670 meters above sea level (masl)), the distinctive seasonal δ18O oscillations in the fresh snow deposited within each thermal year are attenuated at depth due to melting and percolation through the firn. This has become increasingly pronounced over 43 years. In the Andes of northern Peru, the ice field on the col of Nevado Huascarán (6050 masl) has retained its seasonal δ18O variations at depth due to its higher elevation. During the 2015/2016 El Niño, snow on Quelccaya and Huascarán was isotopically (δ18O) enriched and the net sum of accumulation over the previous year (NSA) was below the mean for non–El Niño years, particularly on Quelccaya (up to 64% below the mean) which was more pronounced than the NSA decrease during the comparable 1982/1983 El Niño. Interannual large‐scale oceanic and middle to upper‐level atmospheric temperatures influence δ18O in precipitation on both ice fields, although the influences are variably affected by strong El Niño–Southern Oscillation events, especially on Quelccaya. The rate of ice wastage along Quelccaya's margin was dramatically higher during 2015/2016 compared with that of the previous 15 years, suggesting that warming from future El Niños may accelerate mass loss on Peruvian glaciers.
Key Points
Midtropospheric warming in the Peruvian Andes is destroying climate signals preserved in glaciers and driving glacier retreat
The impact of the 2015/2016 El Niño on Quelccaya ice cap was more pronounced than for previous events in the last four decades
Peruvian Andes snow δ18O is linked to tropical Pacific SSTs, regional 500 mb temperatures, and convection
PDF
