Current state of glaciers in the tropical Andesa multi-century perspective on glacier evolution and climate change

A glaciologist in Bolivia in the early 1970s was drilling into ice and logging mass balance numbers. He was recording something that looked almost manageable: minus 0.2 meters of water equivalent per year. The glacier was losing ice, but slowly, at a rate that had persisted for a decade. Then something changed. By the late 1970s, that number had more than tripled. The ice wasn't just retreating anymore; it had shifted into a fundamentally different regime. Understanding what flipped that switch and what it means for the mountains and the millions of people below them is what this research is about. Tropical glaciers are unusual almost by definition. They sit near the equator, where the seasons barely register in temperature — no cold winter, no warm summer, just a steady, high-altitude chill punctuated by wet and dry seasons. That stability makes them exquisitely sensitive to even small shifts in mean temperature or moisture. Rabatel and colleagues, synthesizing decades of research across the tropical Andes, documented that near-surface air temperature in the region rose at about 0.10 degrees Celsius per decade over the last 70 years — an overall increase of 0.68 degrees Celsius since 1939. Borehole temperature measurements near the summit of Illimani glacier captured this from the inside: englacial data document a mean atmospheric warming of 1.1 plus or minus 0.2 degrees over the 20th century. The tropical Andes split into two climate regimes that shape how glaciers respond. In the inner tropics — straddling the equator through Colombia and northern Ecuador — the freezing level sits close to the glacier equilibrium line altitude, which is the elevation where accumulation and melt balance out. Small shifts in that freezing level translate directly into year-round changes in ablation, the technical term for mass lost through melting and sublimation. In the outer tropics — southern Ecuador, Peru, and Bolivia — the equilibrium line sits above the freezing level, so moisture availability matters more alongside temperature. Reanalysis data show the freezing level rose about 60 meters in the inner tropics and about 160 meters in the outer tropics over the past 55 years. A sensitivity study on Zongo Glacier in Bolivia found that a one-degree air temperature rise pushes the equilibrium line altitude up by 150 plus or minus 30 meters. That relationship becomes alarming when you look at the projections, but first, the historical record. To understand how dramatic the current retreat is, you need to go back to the Little Ice Age — the period of maximum glacier extent centered in the mid-17th to early 18th century. Lichenometric dating and lake sediment records show the peak in Peru around 1630, and in Bolivia between 1657 and 1686. Ecuador's story splits by altitude: glaciers with summits above 5,700 meters reached their maximum around 1730, while lower ones peaked around 1830. After those maxima, a slow withdrawal played out through the 18th and much of the 19th century, with a clearer acceleration after about 1840 and stronger losses into the early 20th century. Bolivian glaciers retreated on the order of a thousand meters in length from the mid-17th to the late 19th century. That sounds like a lot, but what happened next makes it look modest. The country-level numbers from recent decades are stark. Peru holds 71 percent of all tropical glaciers, and the Cordillera Blanca — the country's most glaciated range — declined from 723 square kilometers in the 1960s to 527 square kilometers in the 2000s, a 27 percent loss. The Cordillera Vilcanota lost 32 percent between 1962 and 2006. In Ecuador, Chimborazo's glacier area fell from 27.7 to 11.8 square kilometers between 1962 and 1997 — a 57 percent loss, at about 1.6 percent per year. Colombia's total glacier area dropped from 89.3 square kilometers in the 1950s to 43.8 square kilometers by the mid-2000s, a 51 percent reduction, with loss rates four times greater in the later period than the earlier one. This is what unprecedented looks like in practice. Now back to that break point in the late 1970s, because this is where the analysis gets precise. Mass balance is the net gain or loss of ice on a glacier, measured in meters of water equivalent per year. Across Colombia, Ecuador, Peru, and Bolivia, using field measurements, hydrological records, geodetic surveys, and remote sensing, Rabatel and colleagues identify the shift clearly: mean annual mass balance from 1964 to 1975 averaged minus 0.2 meters of water equivalent per year. From 1976 to 2010, it dropped to minus 0.76. That's not a gradual slide but a step change. The glaciers most at risk are the small, low-altitude ones that lack a permanent accumulation zone. Glaciers with maximum elevations above 5,400 meters showed an average loss trend of minus 0.6 meters of water equivalent per year, with large interannual swings. Glaciers with maximum elevations below 5,400 meters averaged minus 1.2 meters per year — and at that rate, many will completely disappear within one or two decades. Chacaltaya Glacier in Bolivia is not a projection; it's already gone, having disappeared by 2010. The tropical Andes average deficit over this period is also more negative than the global average for monitored glaciers, and the acceleration arrived roughly 15 years earlier than in mid- and high-latitude glaciers, which didn't show their major uptick until the 1990s. So what caused the late-1970s break? This is where the attribution gets interesting, because the obvious answer — less rain — turns out to be wrong. Precipitation across the tropical Andes shows no consistent long-term downward trend through the 20th century. Vuille and colleagues analyzed station records and found that decadal variability, mostly driven by the El Niño–Southern Oscillation, or ENSO, is larger than any multi-decadal signal. Some stations north of eleven degrees south show increasing precipitation, while stations in southern Peru and the Bolivian Altiplano show decreases. The pattern is spatially mixed. Precipitation is not the driver. Temperature is. And more specifically, the behavior of the tropical Pacific Ocean. Monthly mass balance anomalies on inner-tropical glaciers correlate closely with Niño-3.4 sea surface temperature anomalies, with the ocean leading the ice by about three months. For outer-tropical glaciers, the best match is with Niño-1 plus 2 anomalies at about a four-month lag. The mechanism runs through the surface energy balance: El Niño events bring warmer, drier conditions to parts of the Andes — in Bolivia, El Niño raises near-surface summer temperature by roughly 0.7 to 1.3 degrees Celsius relative to La Niña years and reduces precipitation by 10 to 30 percent. Less snowfall means lower surface albedo — albedo being how reflective a surface is. Lower albedo means more shortwave solar radiation absorbed at the surface. More absorbed radiation means more melt. The causal chain is tight. What Rabatel and colleagues argue is that the post-1976 increase in tropical Pacific sea surface temperatures, combined with a change in the frequency and character of El Niño events — including a shift toward central-Pacific events — altered conditions over the Andes systematically. Superimposed on an already-warming troposphere, this combination explains, in their words, "much of the recent dramatic shrinkage" of tropical Andean glaciers. Looking forward, the picture is stark. Bradley and colleagues, synthesizing eight climate models from the Intergovernmental Panel on Climate Change Fourth Assessment under the high-emission scenario A2, project mean annual free-air temperature increases of 4 to 5 degrees Celsius at elevations above 4,000 meters by the end of this century. Apply Zongo Glacier's sensitivity of 150 meters of equilibrium line rise per degree of warming, and you get an equilibrium line altitude shift of roughly 480 to 900 meters from its current position near 5,150 meters above sea level. That pushes the line into the upper reaches of most glaciers in the Cordillera Real and likely eliminates much of what remains. The authors are careful to call these preliminary results, noting they need expanded tests and longer time horizons. What’s not preliminary is the practical stakes. Cities and communities across the Andes depend on glacierized catchments for water, especially during dry seasons when glacier melt is the primary source of streamflow. As glaciers retreat, lakes form in the depressions left behind, and those lakes can produce catastrophic outburst floods. The research identifies key gaps: better glaciohydrological modeling, locally grounded water resource studies, and longer monitoring records to resolve exactly how large-scale Pacific variability translates to ice loss at the surface. The tropical Andes are telling us something in the language of disappearing ice, and the work now is making sure we can read it precisely enough to act. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.