Day 1 Question 2: Water resource issues with climate change

Question 2:

First, describe some of the anticipated changes in the distribution and amounts of snowfall and rainfall under a CO2-enriched atmosphere. Second, identify two locations where you might anticipate (or presently observe) large changes in municipal water supplies derived from melting snow and ice. What kind of a signal might be apparent in stream gauge data to indicate these changes? (Assume you have a near perfect data set with information about daily, monthly, and annual discharges from the past 50 years). Are all urban areas that are dependent upon meltwater going to have similar signals? How might this signal vary geographically and why?

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Introduction

An abundance of recent evidence suggests that 20th-century warming was unprecedented in the late Holocene and 21st-century warming is projected to be much greater than the natural variability exhibited by the climate system over the past 1000 years (Dyurgerov and Meier, 2000; IPCC, 2001, Thompson, 2000). Observations have identified a general thinning and retreat of glaciers globally (e.g. Dyurgerov and Meier, 2000; Oerlemans, 1994; Thompson et al., 2000). The retreat of the termini of alpine glaciers worldwide contributes to the growing evidence of rapid global climate change (Thompson et al., 1993). In this warming climate, with the rapid disintegration of glaciers, freshwater availability is expected to become a forefront issue for many regions.

Anticipated changes in precipitation in a warming climate

In a warming climate, as predicted by General Circulation Models (IPCC, 2001), the hydrological cycle will be enhanced, and the current distribution of precipitation will change significantly in regional distribution and seasonality (Benniston, 2003). Model results suggest higher evaporation rates and greater proportions of liquid precipitation in the coming decades. These physical and seasonal differences will affect soil moisture, groundwater reserves, and the frequency and intensity of droughts and floods. The most likely precipitation-related change with an enhanced greenhouse effect is more intense precipitation events happening over many regions (IPCC, 2001).

The increased evaporation rate, coupled with an expected increased water holding capacity throughout the atmosphere will also lead to an increase in the water vapor concentration throughout the atmosphere. This is important as water vapor is the most important and strongest of all greenhouse gasses, and may lead to intensified warming. As some of this water vapor condenses into clouds, the net effect on Earth’s radiation balance is unknown. High, thin clouds are transparent to solar radiation, but trap in Earth’s thermal radiation, and have a warming effect like other greenhouse gasses. On the other hand, low, thick clouds reflect solar radiation and cool the atmosphere.

Generally, precipitation is expected to increase over the globe, although this increase will not be uniformly distributed spatially or temporally; a significant portion will be caught by the oceans. While there has been a statistically significant 2% increase in precipitation over land in the last 100 years, precipitation over the U.S. has increased by 5 to 10%. In the last 50 years, however, China’s precipitation totals have decreased (IPCC, 2001).

Bhaskharan and Mitchell (1998) found that in a warming climate, the south Asian monsoon region will change, bringing about increased precipitation in the eastern regions and decreased precipitation in the west. This has tremendous implications for the western China, where semi-arid conditions already push the limits of the available water.

These changes come about in a climate of expanding human population and activities. With our growth and industrialization, we have become increasingly reliant upon water, including for the discharge of our wastes. A recent decline in water quality and availability has driven recent concerns for future water availability (Shiklomanov, 2001).

As mountains are the source of over half the world’s rivers, precipitation changes in a single watershed are not only likely to affect the mountain regions where the precipitation falls and is stored in snow and ice, but the more populated lowland areas that critically rely on the watershed as a resource will also be greatly affected (Benniston, 2003). In arid and semi-arid areas, mountains provide up to 100% of the available freshwater to the surrounding lowlands (Meybeck et al., 2001). As important as the precipitation regimes in mountains are, many fear that global climate models are too coarse to capture the orographic complexity of mountain regions. Others argue that the overall regional patterns of precipitation in the models will still likely hold in mountain regions.

Snow and ice are important components of the hydrological cycle in mountain regions, particularly the seasonality and amount of runoff (Haeberli and Hoelzle, 1995). Changes in regional snowfall are expected with global warming. In temperate regions, where the snow-pack is close to melting conditions, a small increase in temperature may have a significant change on the local mass-balance, and therefore the meltwater runoff. With enhanced warming, temperate regions will experience an increase in the frequency of rain, and the snowline will rise by an expected 150 m/̊C of warming (Beniston, 2003).

Two locations which might experience large changes in water supply from melting snow and ice

Glaciers are often the most the most important freshwater resources in arid regions. For example, glaciers are critically important sources of freshwater in the hyper-arid coastal regions of Peru in the tropics, and the semi-arid regions of western China in the mid-latitudes. These two areas may be the most important in terms of the hydrological response to glacier retreat, and are currently the focus of new studies regarding the connections between glacier melt and catchment runoff (Evans, 2001).

The meltwater runoff from a glacier depends on the glacier’s mass and energy balances at the surface. The mass balance of a glacier depends on mass gained, “accumulation”, through snow and deposition, and mass lost, “ablation”, through melting, iceberg calving, and sublimation. The amount of melt at the surface is determined by the energy balance. One of the key factors of the energy balance is the reflectivity, “albedo”, of the surface. The amount of sublimation greatly depends on the humidity of the air, an often overlooked variable in glacier mass-balance studies.

In a warming climate, the expected increases in precipitation are not expected to outweigh the increased melting predicted on glaciers except, possibly, in Antarctica (IPCC, 2001). Precipitation is important to a glacier’s mass balance, not just through the direct addition of mass, but indirectly through the enhancing of the surface albedo. Fresh snow has the highest albedo of any natural surface (Paterson, 1994). Adding fresh snow to a glacier surface can temporarily shut down melt as excess energy is reflected away. This effect can be up to ten times more important than the effect of the mass directly added by precipitation. Since the albedo of the surface does not depend on the thickness of the snow layer, this albedo effect depends more on the timing and frequency of snowfall, rather than the total amount. In contrast, as a greater balance of liquid precipitation is expected on temperate glaciers, the rainfall tends to add tremendous energy to the glacier surface, and in the case of rain on snow events, can significantly reduce the albedo.

Peru

Coastal Peru is a hyper-arid desert, yet it sustains a population of approximately 28 million people (CIA World Fact Book, 2005). This is possible only through the heavy reliance on glacier meltwater originating high in the Andes. Peru has more than its share of glaciers, being home to 70% of the world’s tropical glaciers (Kaser et al., 1996).

Being located in the tropics holds special importance for the glaciers of Peru. The tropics comprise nearly half the surface area of the Earth, are home to 70% of its population, and receives the bulk of solar insolation. Excess solar energy is then transported to the rest of the globe. Accordingly, characterization of climate change in the tropics is of extreme importance.

Furthermore, temperature in the tropics varies less horizontally than outside the tropics because the Coriolis effect is weakest there. In the tropics, pressure differences caused by temperature variations are eliminated by redistribution of mass that the Coriolis effect tends to limit outside the tropics. Although temperature is not horizontally homogeneous, it is much more uniform than any of the other meteorological variables which effect tropical glacier mass balance, including precipitation, humidity, and cloudiness. Accordingly, a secular retreat of tropical glaciers suggests a shift in temperature, rather than in some other atmospheric characteristic.

Strong retreat rates of tropical glaciers have been reported for recent decades from the high Andes and elsewhere in the tropics (Kaser, 1999; Peterson and Peterson, 1994; Ramirez et al., 2001; Thompson et al., 2002). In fact, all small glaciers and ice caps outside of the polar regions are expected to be completely absent by the end of this century (IPCC, 2001). The melting of these glaciers may account for as much as half of the expected rise in sea level over the next 100 years, the rest of which will be caused by thermal expansion (IPCC, 2001). Since glaciers act as natural reservoirs that provide stored water to people in the region for hydroelectricity and agriculture during the dry season (Paterson, 1994; Thompson, 2000), the demise of these ice caps has the potential to create significant social and environmental repercussions. The glaciers of Peru provide critical water resources to the hyper-arid coastal lowlands and high altiplano. They are also the source of glacier-related avalanches and floods that have leveled towns and caused over 35,000 recorded deaths (Williams and Ferrigno, 1999).

Although there is significantly more ice in high latitudes, tropical ice fields are temperate, generally existing closer to melt-threshold conditions and, accordingly, relatively small climate changes may significantly affect their mass balance (Martinson et al., 1998). The tropics host a number of climatic phenomena that greatly affect humans, including the El Niño - Southern Oscillation, the Inter-Tropical Convergence Zone, and the Asian monsoon, are characterized by relatively homogeneous thermal conditions, and are not significantly affected by migratory synoptic disturbances. Accordingly, fluctuations on tropical glaciers may be more directly related to secular climate changes (Kaser and Georges, 1999). These factors make tropical glaciers exceptional indicators of recent environmental changes and the effects of these changes on natural systems (Oerlemans and Fortuin, 1992).

Tropical glaciers have received much recent attention as their importance as water resources and as early indicators of climate change has recently been recognized (Georges, 2004). These glaciers are also of tremendous concern on a regional scale, as Peru exhibits a long, hyper-arid annual dry season during which time melt runoff from glaciers is a critical resource for the economic stability of the country. Peru relies almost entirely on this meltwater for its electricity production, which is over 90% hydroelectric, its drinking water, and for its agriculture, which is their primary economic activity.

Like the people and animals in Peru, the glaciers there are at the absolute threshold of existence. Accordingly, they are one of the earliest and most sensitive indicators of climate change. Melting glaciers not only pose a threat to water resources, but many rapidly melting glaciers form dangerous proglacial lakes that are prone to sudden outburst floods, especially in the steep, tectonically active terrain found in the Andes (Mark, 2002). Worse yet, the political and economic instability there, and a lack of institutional support precludes any possibility of large-scale engineering solutions.

The present contribution of meltwater to the regional runoff is large and well-documented. Monthly measurements of discharge from the Cordillera Blanca, Peru’s most heavily glaciated area, exhibit maxima in Austral Spring, prior to the maximum in precipitation. This suggests a large contribution from glaciers as the net radiation is high, surface albedo is low, and perhaps most importantly, relative humidity is high. Humidity is an often overlooked variable in glacier mass balance. In a dry environment, much of the excess energy receipt at a glacier surface will be utilized for sublimation, which is 8.5 times less effective as melting.

All other things equal, with global warming, we expect the contribution of glacier meltwater to initially increase. This has been witnessed over the last few decades, and has allowed Peru’s population and economy to grow. Presently, Peru is one of the fastest growing nations in the world. As warming continues, however, the rate of meltwater production will decrease as glaciers diminish in size, placing intense strain on an already limited system.

China

In China, the direction of water resources there is less well known. One study by Mirza (1997) suggests that changes in sub-basin drainage in the Ganges could range from 27 to 116% with a doubling of CO2 concentrations.

Of the changes in precipitation expected in a changing climate, few would be as significant as a shift in the Asian monsoon system. Throughout the Holocene, the Asian monsoon has faltered in times when the North Atlantic climate was cooler (Gupta et al., 2003). These changes have tended to occur abruptly, often coinciding with Dansgaard-Oeschger events. If glaciers continue to melt, driving freshwater into the North Atlantic, the thermohaline circulation (THC) is expected to slow, resulting in a cooler climate for the North Atlantic. This could result in more frequent failure of the Asian monsoon, which would be devastating to much of the world’s population.

There has been a widespread retreat of Himalayan glaciers. Since 1955, glaciers in the Tien Shan mountains have retreated by over 22% (Meier et al., 2003). Interestingly, however, glaciers in the Qilian Shan also retreated while temperatures there decreased and precipitation increased (Liu et al., 2003). This retreat is likely due to changes in the timing and variability of temperatures and precipitation events. This highlights an important aspect of glaciers, namely that glaciers thrive in stable conditions and tend to retreat in more variable conditions. As increased variability is predicted to accompany global warming, glaciers may be doomed even where precipitation increases.

Precipitation in the Qilian Shan is regulated by the Asian monsoon, westerly winds, intense storms in the summer, periodic rains in the spring and fall, and rare events in the winter (Liu et al., 2003). Meltwater from glaciers in the region account for 39 to 56% of the total runoff, and therefore holds great social and economic importance.

Future changes in the timing, type, and amount of precipitation falling in the Andes and in China will have a tremendous impact on the seasonal availability of water to the surrounding lowland populations, and therefore to the economic stability of those nations. As increased pressure on water resources in politically unstable regions can lead to upheaval and armed disputes, climate change has the potential to not only put increased economic pressure on a nation, but can ultimately lead to war.

Geographic variability

The consequences of a changed hydrological cycle vary from place to place, depending on the degree of reliance on runoff as a water resource, the seasonality and type of precipitation relied upon, and the changes these regions experience in their precipitation distribution, seasonality, and type.

The changes in glacier melt runoff in a warming climate do not vary linearly with climate. Instead, they depend also on the size of the glacier, the rate of temperature rise, and other factors which influence the glacier mass balance, including changes in precipitation rates and type with elevation, changes in the elevation temperature profile, changes in the seasonality of temperature and precipitation, and the mass balance distribution with elevation. Typically, however, as climate warms and glaciers retreat, the runoff tends to increase at first, but then decrease as the size of the retreating glacier diminishes (Ye et al., 2003). Larger glaciers tend to retreat faster, while smaller glaciers tend to lose area faster than length. For all glaciers, however, volume exhibits the most significant change in a warming scenario. Smaller glaciers with higher runoff peaks tend to exhibit the most variability in runoff and retreat more quickly than larger glaciers. Also, the faster the temperature rises, the runoff peaks become earlier and higher (Ye et al., 2003).

Water resources in some regions are highly sensitive to the frequency, intensity, and trajectory of tropical cyclones, monsoon systems, and other traveling synoptic patterns. For example, the water supply in southern and tropical Asia, where nearly half of the world’s population live, is greatly influenced by tropical cyclones (Benniston, 2003). These areas may ultimately become more highly dependent on seasonal storms and rainfall patterns in a time when they are predicted to become more variable.

Not all regions dependant on glacial melt will experience water shortages, but that appears to be the prevailing tendency. Even in northern Sweeden, where temperatures, like Antarctica, are low to begin with, studies by Schneeberger et al. (2001) of the Storglaciären in northern Sweeden suggest that increased ablation there will greatly outweigh the expected increases in precipitation.

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