Worldwide, rates of melting are exceeding rates of new ice formation. Over the last two decades, melting has outpaced ice accumulation, leading to net losses of ice mass in Greenland, Antarctica, and alpine glaciers. From 2005 to 2009, the rate of loss was about three hundred cubic kilometers per year, contributing to a bit less than one millimeter per year of global sea-level rise.
Melting of continental ice has the potential to cause large amounts of sea level rise: for example, the quantity of ice on Greenland is sufficient to raise global sea levels by over seven meters; the ice on Antarctica represents about seventy meters of potential sea-level gain. And while much of Antarctica is too cold to be at serious risk of melting, the West Antarctic Peninsula, representing close to five meters of potential sea-level rise, is not.
Melting continental and sea ice also amplify warming by replacing a white, reflective surface with a dark surface that absorbs much of the incoming sunlight. The same principle explains why you stay cooler on a hot day by wearing a white, rather than black, shirt. The numbers are daunting: in 2012, the annual minimum in Arctic sea ice was about three million square kilometers fewer than the 1981–2010 average.
Although water availability is classically thought of in terms of quantity, water is useful (usable) only if it is of sufficient quality for its intended purpose. And water quality is critical regardless of its intended use, whether it be used by humans directly for consumption, recreation, sustaining fisheries, and irrigation, or by the broader eco – system to support aquatic life, for example. This broader context of water availability and water quality is directly linked to changes in climate via impacts on meteorological conditions, as alluded to above.
The link between climate and water quality is perhaps most poignantly illustrated through the lens of coastal and freshwater eutrophication: the delivery of excessive nutrients–nitrogen and phosphorus are typically the most concerning– to water bodies from agricultural production as well as from urbanization and other human activity.
The effects of eutrophication are many, but some of the most common and worrisome are harmful algal blooms by toxin-producing species of phytoplankton and widespread low-oxygen “dead zones”–in which the decomposition of organic matter consumes nearly all of the dissolved oxygen–that disrupt aquatic food chains and can lead to massive fish kills.
Hundreds of coastal and in – land water bodies globally are already routinely impacted by harmful algal blooms and hypoxia, including many in North America. A harmful algal bloom in Lake Erie in 2011 stretched across five thousand square kilometers, an area larger than the state of Rhode Island. The dead zone in the lake the very next year was estimated at close to nine thousand square kilometers, an area larger than the state of Delaware.In August 2014, a pileup of toxin producing cyanobacteria from that year’s algal bloom near the Toledo, Ohio, water intake shut down the city’s water supply for two days.
What is the link to climate? Although the excess nutrients nominally result from land management practices, their delivery to water bodies and the effects they engender once there are highly dependent on weather patterns, which are themselves evolving in response to climate change. Variations in precipitation, whether the amount of rain, its seasonality, or the intensity of storms, affect how much nitrogen and phosphorus are flushed into water ways. Temperatures control conditions in the water, including when the water is warm enough to sustain blooms and the degree of stratification, which prevents cold (heavy) water from being replenished with oxygen due to warm (light) water acting as a lid.
Wind affects stratification– with stronger winds helping to mix the water column–as well as water flow (and there fore nutrient transport) within water bodies. All of these interconnected proceses are changing with the climate. In the case of Lake Erie, extreme springtime precipitation in 2011 followed by warm and quiescent conditions helped supercharge the bloom. In 2012, an intense drought led to stagnant conditions that supercharged the dead zone.
The global energy system relies massively on water, either as a direct energy source (hydropower) or for cooling (electricity generation), irrigation (biofuels), or extraction (hydraulic fracturing). Preparing and using the water to support energy production–a process that includes collection, cleaning, transportation, storage, and disposal– it self involves massive amounts of energy. This interdependence has sometimes been referred to as the water-energy nexus. The interface between water and energy invariably also introduces a number of debates about alternative uses of water and impacts on water availability (quantity and quality).
As global energy demand continues to grow, and as the climate impacts of fossil fuel–based energy sources become untenable, increasing emphasis is being placed on renewable sources of energy. These sources of energy are rightfully considered more sustainable than energy that relies on nonrenewable energy sources. The sustainability of specific technologies, however, must be assessed within the context of their reliance and impact on water resources.
The need to assess the implications of alternative energy production for water is perhaps nowhere more poignant than in the case of biofuels. We are accustomed to thinking about the energy requirements of our vehicles in terms of miles per litre, a measure of fuel efficiency. The unit against which we measure efficiency is, of course, a gallon of petrol. But what if it were a gallon of water? The water requirements of corn-based or soybean-based biofuels translate to a fuel-efficiency value of less than 0.1 miles per gallon of water! The vast majority of this water is used for growing crops, rather than for converting them to biofuels.
High water demands, combined with the uncertainty surrounding future water availability due to changes in climate, point to the need to carefully consider the water implications of alternative energy choices. For example, the water requirements of wind and solar energy production are dramatically lower than those of biofuels, and lower also than even some “traditional” energy sources.
Globally, agriculture accounts for approximately 86 percent of consumptive water use. Rising populations and rising living standards combine to create rapid increases in global demand for food, especially food with a high land and water foot – print, such as meat. Ensuring a secure food supply is therefore inextricably linked to the availability of plentiful clean water for growing crops. Predictable water availability is critical both for rain-fed and irrigated agriculture, and uncertainty about water availability compounds uncertainty about future food security. Water quantity and quality are also integral to non-agricultural sources of food, such as fisheries.
Plants grow by using the energy from sunlight to convert carbon dioxide in the atmosphere into carbohydrate and, eventually, more plant. But plants on land cannot take up carbon dioxide without losing water. The pathway by which carbon dioxide enters and leaves is the same as the path by which water evaporates. The ratio of water loss to carbon dioxide uptake varies with carbon dioxide concentration and atmospheric humidity, as well as among plant species.
In most habitats, plants lose fifty to one hundred and fifty gallons of water through evaporation–a process called transpiration when the water comes from leaves–to make a single pound of new plant. This mechanism underlies a massive water footprint for food, whose size depends not only on the amount of water transpired per unit of plant growth but also on the fraction of the plant consumed as food or on the amount of plant required to produce each unit of consumable animal product.
Irrigation can substantially increase yields and year-to-year predictability. About 33 percent of the world’s crops come from the approximately 25 percent of crop-land that is irrigated worldwide. In areas that are sometimes wet enough for rain-fed agriculture, irrigation can enhance water availability through dry periods. Irrigation can also allow the extension of agriculture into areas that are otherwise too dry. But irrigation is viable only if there is excess wa ter to tap. Locally, this can mean ground – water that is recharged during wet periods; regionally, it can mean snowpack, rivers, streams, lakes, and reservoirs.
Agriculture not only uses water, but also has downstream impacts via the water that runs off the fields or seeps into the ground. The unprecedented growth in agricultural production of the last century has been enabled in large part by the use of fertilizers. In the last decade, the drive toward the pro duction of biofuels is putting further pressure on the agricultural system. When water leaves agricultural fields, whether through runoff, seepage, or drainage, it carries with it nutrients that have not been assimilated into the soil and crops. While the total acreage devoted to agriculture has changed little since the 1950s, the total amount of commercial fertilizer used on that land has more than doubled. This has led to increases in agricultural productivity, but also in the amount of nutrients washing off fields and into waterways. That nutrient run-off results not only in the contamination of coastal and inland water bodies, but can also lead to massive algal blooms and dead zones
The environmental interests of inland and coastal water bodies appear to be increasingly at odds with the interests of the agricultural system. That said, an antagonistic view of the situation is overly simplistic. Ultimately, neither farmers nor fish are interested in fertilizer ending up in lakes rather than in fields. Identifying remedies that recognize the central role of water in agriculture (both in terms of water supply for feeding crops and in terms of downstream vulnerabilities), as well as the complexity of nutrient delivery and impacts to waterways, will require a systems approach. Such an approach will need to recognize that each change made to one part of the system affects all other components, and future changes in management need to address not only the intended goals but also other, often unintended concurrent consequences. This will be especially important as demand for crops continues to grow and concerns about food security grow along with it.
Climate change is complicating the task of ensuring water availability: it decreases avail – able supplies, degrades storage in snow – pack and glaciers, and increases the fraction of precipitation that comes in the heaviest storms. Further, energy production puts huge demands on water availability. While many of the demands of the energy system, especially for cooling and hydroelectric power, return the water to the river, these uses still produce major environmental consequences. Consumptive uses for fossil fuel extraction generate large amounts of contaminated water that requires disposal. And the production of crops for biomass energy is a huge consumer of water.
Where does all of this leave the needs of nature? Over the last few decades, many of the high-profile conflicts over water have involved allocation disputes between consumptive uses and instream flows needed to sustain rare or endangered species. Instream flows, uncontaminated lakes, and watersheds also provide a wide range of valuable goods and services; thus, allocating water for nature is about more than just protecting fish. It is about protecting the viability of Earth’s life support system.
By Professor Ashoka Jahnavi Prasad