Glaciers have shaped the landscapes and mindscapes of much of the Earth and its inhabitants, ever since parts of the planet were cool and wet enough to start forming ice masses. But the Earth’s current Late Cenozoic collection of glaciers (which first started forming on Antarctica around 34 million years ago) are melting, and nowhere is this more pronounced than in the “Tropics.”
The term “Tropical Glacier” may seem somewhat counterintuitive to those unfamiliar with their presence, but they exist because temperature decreases substantially with altitude, allowing precipitation to fall as snow, which accumulates and pressurizes over time as ice. However, as temperatures continue to rise due to climate change, the melting of Tropical Glaciers is due to worsen over the coming decades. This reality threatens aquatic and terrestrial ecosystems as well as the food and water security for tens to hundreds of millions of people.
Glacial melt water accounts for a significant portion of downstream river flow, especially during dry seasons in semi-arid areas at tropical and subtropical latitudes. This in turn is critical for ecosystems, and for fresh drinking water, agriculture, hydroelectricity and varied industries, both in rural and urban settings. The impacts of the recession and disappearance of glaciers will also extend far beyond the identifiable losses of precious water resources; the cultural and spiritual significance of glaciers and their meltwater for many traditional peoples is profound, and their disappearance will have deep consequences for these people and their territories.
Additionally, glaciers are extremely important tools for scientific research that aims to understand past, present and future climate change. Tropical Glaciers in particular are both extremely vulnerable to climate change, and some of the best indicators of yearly changes that are taking place globally and regionally with the climate, from the distant past to the present and projected future. Tropical Glaciers are often referred to by glaciologists and paleoclimatologists as a kind of “global thermometer” due to the ability to study their melt in relation to global warming.
The Rapid Recession of Tropical Glaciers: Causes and Projections
Since the last glacial maximum, known as the “little ice age,” which peaked in the mid 18th century and ended in the mid 19th century, there has been a trend of global glacial recession. This trend has increased markedly since the 1970’s in the low-latitude tropics (glacial retreat for many mid-to high latitude mountains began rapidly increasing in the early 1990’s, with the glaciers of the Tibetan Plateau being of particular concern), where 40 to 50% of Tropical Glacier mass and volume has been lost in less than four decades. The smallest glaciers are the most vulnerable, and many have already disappeared or are at risk of disappearing before the middle of this century.
The homogeneity of fast recession of small glaciers in the tropics is explained best by the steady rise in atmospheric temperature over the past decades. Since 1940, there had been a temperature increase in the Tropical Andes of initially 0.1 degrees celsius (0.18 degrees fahrenheit) per decade up until the mid 1970’s, which has now tripled to approximately 0.3 degrees celsius (0.54 degrees fahrenheit) increase in atmospheric temperature per decade, over the past three-four decades.
Tropical Glaciers naturally exist closer to their melting point than most mid-to-high latitude glaciers, making them especially sensitive to increases in atmospheric temperature. Due to a complex interplay between heat exchange, solar radiation and absorption, energy balance, ablation rate, rising altitude of the equilibrium line (space between ablation melt and the accumulation zone of snow) and the zero degree isotherm (the freezing level, or altitude where it is 0 degrees celsius, or 32 degrees fahrenheit), and importantly the increased proportion of the day where melting can occur, a seemingly unstoppable trend towards more melting (and sublimation, which is the direct conversion of ice to water vapor) is now upon us, driven by atmospheric temperature rise. If these trends persist (and there is little reason to expect they won’t, due to insufficient or complete inaction for decades on the parts of nations, corporations and consumers to tackle global climate change), all small tropical glaciers at lower altitudes (below 5,400 meters, or 17,700 feet) could be gone before the middle of the 21st century. Larger and higher altitude tropical mountain glaciers will face continued and increasing recession on a massive scale as well, as will middle-to-high latitude mountain glaciers and ice caps, such as those in the Himalayas, Rocky Mountains, Alps, Southern Andes and at the poles.
More than 40% of the world’s population lives within the “tropics,” a latitudinal band between approximately 24 degrees North and South of the equator, defined by the Tropic of Cancer to the North and the Tropic of Capricorn to the South. This percentage is projected to rise dramatically as well, with parts of Africa, India and South America experiencing significant population growth. The vast majority of the planet’s terrestrial biodiversity also resides in the tropics (between 70-80%). Within this range there are many high altitude Tropical Glaciers that tens of millions of some of the world’s most vulnerable populations and ecosystems directly depend upon. It is estimated that more than 50 million people rely on “small” Tropical Glaciers (usually defined as less than 0.5 square kilometers, or 0.19 square miles, in glaciated area), which are the most likely glaciers to disappear completely in the coming decades, for fresh drinking water, agriculture and energy generation.
As mentioned above, glaciers form at tropical latitudes because temperature decreases with altitude, at approximately six to seven degrees celsius per kilometer of altitude gained, or around three degrees fahrenheit decrease for every 1,000 feet in altitude gain. In the tropics, warm air rises and then expands and cools, which is only partially offset by the heat released from the condensation of water vapor. The formation and survival of Tropical Glaciers subsequently depends on precipitation, which at cool temperatures, in high altitudes, falls on mountains as snow. Over time and under the weight and increased pressure of more fresh snow, the snow becomes ice.
There is great seasonality in terms of precipitation in the tropics, with characteristic wet and dry seasons, while generally the temperature remains relatively uniform across the entire zone. In the Andes, these wet and dry seasons are further pronounced in the “outer tropics,” which includes Peru and Bolivia (where around 90% of the Tropical Andean Glaciers are). In the “inner tropics,” which includes the glaciers of Ecuador and Colombia, the horizontal homogeneity of temperature at high altitude is particularly consistent. This means that the rapid recession of glaciers in the inner tropics of the Andes, such as the six remaining mountain glaciers in Colombia, are almost entirely driven by changes in atmospheric temperature. Though rising atmospheric temperatures are the main driving force in glacial recession in the outer tropics of the Andes as well, changes in precipitation patterns may be a pronounced additional factor as these glaciers depend on wet seasons for fresh snow accumulation.
The Andes of South America, the world’s second largest mountain range after the Himalayas, accounts for more than 95% of the world’s Tropical Glacier ice mass, with some recent figures putting the number as high as 99%. Approximately 70% of Tropical Andean Glaciers are in Peru, 20% are in Bolivia, and the remaining glaciers are divided between Ecuador, Colombia and Venezuela. Mt. Kilamanjaro and Mt. Kenya (between Kenya and Tanzania in Eastern Africa), and the mountains of Irian Jaya (West Papua), Indonesia, account for the other remaining glaciers that are defined as “tropical,” and they are all receding rapidly. In total, it is estimated that only 1,400-1,600 square kilometers (around 540 – 620 square miles) of glaciers in the tropics remain. This estimation is down from nearly 3,000 square kilometers (1,160 square miles) in the mid 1970’s, and 1,900 square kilometers (730 square miles) in the year 2000. The world’s largest Tropical Glacier (44 square kilometers, or 17 square miles) is the Quelccaya Ice Cap (5600 meters in altitude, or 18,400 feet) of the Peruvian Andes, and it is rapidly deglaciating by approximately 60 meters (200 feet) per year in glaciated surface area.
Ablation rate describes the balance between snowfall and glacial ice melt, and it has been rising rapidly for essentially all tropical glaciers over the past 30-40 years. Because tropical glaciers are receding universally, and temperature is not variable at these latitudes as it is in higher latitudes, it is a strong indication that temperature rise due to climate change is primarily responsible for Tropical Glacier recession. In fact, global temperature changes are more pronounced at higher altitudes (and at high latitudes, like the North and South Pole), and it is expected that there will be 4.5 – 5.5 degree celsius rise in the high altitude tropical andes (above 4,000 meters altitude) by the end of the century, while the planet’s global surface temperature will rise between two and three degrees celsius.
Additional and Indirect Factors Impacting Tropical Glacier Recession
There are many factors at play that can both exacerbate and inhibit the underlying trend of temperature-driven glacial recession, and effect the energy balance of glaciers.
One example is intensified and more frequent El Nino Southern Oscillation events (ENSO, or commonly referred to simply as “El Nino”) that often raise temperatures and decrease precipitation at high altitudes in the Andes (La Nina events tend to do the opposite). The impacts of ENSO events are not homogenous, and are what account for the majority of the variability in the data taken from glacial ice cores in recent years. There have also been detected changes to the patterns of Surazos, or South American monsoons, which generate precipitation during the dry seasons, and can have impacts on accumulation and ablation in the mountains.
Another factor potentially driving Tropical Glacier recession is declining Albedo due to increased melting of fresh white snow and changes in the seasonal distribution of snowfall within the glacier’s accumulation zone. This results in increased absorption of solar radiation along the glacier due to lowered reflectivity. Radiation influx from the sun is known to be exceptionally strong at high altitudes in equatorial regions, which in turn causes glaciers to melt at higher rates, especially when there is decreased reflectivity.
Additionally, with rising atmospheric temperatures, the snow-rain limit is moving to higher elevations, so even if precipitation stays the same or increases, for lower altitude glaciers, this precipitation may begin to fall as rain instead of snow. The effect is especially pronounced in the equatorial mountains of Colombia and Ecuador, where there is essentially no seasonal variability in temperature. It is estimated that in these mountains, a temperature increase of 1 degree celsius (1.8 degrees fahrenheit) can move the snow-rain limit approximately 150 meters (500 feet) up the glacier.
On the eastern slopes of the Peruvian Andes, changes in precipitation patterns due to a combination of climate change and land use changes (principally deforestation throughout the Amazon rainforest), will also have impacts on the amount of snow that falls on the Tropical Glaciers of Peru.
Some models do indeed predict that certain parts of the tropics will have increased precipitation due to more water vapor in the atmosphere, which would increase snowfall and raise albedo for certain Tropical Glaciers that are at higher altitude, and potentially slow glacial recession to some degree, but the underlying temperature-driven mechanisms moving towards glacial loss will continue.
Despite these additional factors and the occasional “sporadic” positive mass balance (glacial growth) on certain Tropical Glaciers over the past decades, the general trend has remained consistent across the board over decadal, or multi-decadal time frames: climate change and rising temperatures are driving the recession of Tropical Glaciers.
References and Further Reading
IDEAM. “Informe del Estado de los Glaciares Colombianos.” 2018.
New Scientist. “The History of Ice on Earth.” May 2010.
