The hydrological cycle consists of water vapor rising into the atmosphere, with some escaping Earth’s atmosphere but most returning as precipitation, intergalactic water brought in by comets, and water losses in areas of subduction of the tectonic plates spanning the planet. Overall, there is about the same amount of water on Earth as three billion years ago, with more than 90% of it salt water. The majority of the freshwater, which accounts for 3% in total, exists as ice, two-thirds of it in the polar ice caps. Liquid freshwater – on which we depend – makes up a mere 1% of the water on Earth, but more than enough to sustain life on the planet. Almost a year ago, U.N. Secretary-General Antonio Gutteres described how the unsustainable use of the planet has “broken” the water cycle, destroyed ecosystems, and contaminated our water resources. According to studies, 52% of the planet’s lakes and reservoirs have disappeared, 50% of constant watercourses have become intermittent, and a number of intermittent watercourses have gone dry. How have we done that in such a short span(on the geological scale), given that Homo sapiens came on the scene only some 190,000 years?
Water consumption
We have been draining ancient sources of freshwater faster than they can be restored. According to NASA, one-third (21) of the planet’s major groundwater basins have reached the tipping point and it would take 2500 years of rain to restore the depleted resources. Depletion of the planet’s water resources has caused the Earth’s axis to tilt 80 cm (approx. 31”) to the east, thereby changing the planet’s rotation and reducing soil moisture and fertility in areas subjected to higher temperatures. The global warming stemming from human activity has increased the amount of water vapor trapped in the atmosphere and thus the greenhouse gas effect that triggers more warming, so we pump more water in response, thus setting off a vicious cycle.
Given the cyclical nature of evaporation and precipitation, we have long thought of water as a renewable resource, but it actually isn’t. It is a finite resource. As already pointed out, the freshwater on the surface and underground has sustained the needs of the planet’s plants and animals for billions of years. However, anthropogenic changes due to urbanization, transport, industry, and agriculture (impermeable coatings, compacted soils, and the destruction of soil life due to pesticides and herbicides) have reduced the soil’s ability to take up water, increased and accelerated runoff, and made the ground less able to cope with the more erratic periods of heavy rainfall and changes in airstream movements and humidity that have accompanied global warming (a 1 °C increase in air temperature leads to a 7% increase in storm strength due to increased water droplet suspension in the air).
A rain shower 100 years ago, for example, would have fallen on cooler, less compacted soil. As a result, two-thirds of the rain would have gone into the atmosphere, 28% would have run off the land (carrying silt, sediment, and some organic matter into the watercourses), and 8% would have seeped into the water table and eventually fed the rivers and streams with good quality water. Today, not only is the proportion of runoff greater, but runoff, surface water and groundwater carry more pollutants (pesticides, herbicides, heavy metals, plastic, microplastics, trace chemicals, drug metabolites, etc.), fertilizers, and organic matter. That increases water treatment costs on the whole and leads to influxes of poor quality water in our surface and groundwater. In addition, the dams built to regulate flooding and provide drinking and irrigation water have decreased sediment transport, thereby reducing ecosystem fertility in the deltas and thus fish populations and food production.
Vegetation and the water cycle
Trees metabolize the water that they pump up through their roots and release it through evapotranspiration as water vapor. Organoleptic particles in the air nucleate the water vapor to form water droplets that eventually fall back to earth as rain and snow. In the Northern Hemisphere, the prevailing westerlies carry water from the Atlantic to Northern Africa and Eurasia and from the Pacific to the Americas. Below the Equator, the prevailing easterlies play the same role in the opposite direction. On land, extensive forests work like biotic pumps. They pump groundwater and capture atmospheric water, taking up some 1000 L (or 264 U.S. gallons) all told per tree in the Amazon, a portion of which is then released through evapotranspiration to create “flying rivers” of moisture that distribute the water vapor deeper inland. Deforestation in the Congo and Amazon River Basins thus reduces the water vapor emissions and downwind distribution into the continental heartland. So, the Amazon is not just losing trees, but is also becoming drier, jeopardizing the rainforest’s rich biological diversity.
Climate change and the water cycle
The rising mean temperatures that have attended rising greenhouse gas levels affect the water cycle in several ways. Air masses pick up more moisture as they pass over the warmer oceans, so that they release their burdens over land more frequently as torrential downpours. In addition, the decrease in the temperature difference between the Equator and the poles is changing global air mass dynamics, with more sluggish circumnavigation of the globe and broader north-south swings. As a result, weather conditions are tending to stagnate more and cold polar air is reaching farther south (in the Northern Hemisphere). Rising temperatures lead to more water vapor being trapped in the atmosphere, which likewise contributes to the greenhouse effect and global warming. As already mentioned, rising temperatures increase storm intensity as well as evapotranspiration and plants and animals’ water needs, giving rise to more extreme weather events and increased runoff. Melting polar ice caps, rising sea levels, and coastal subsidence augur major problems for islands, coastlines, and the planet’s major coastal cities. However, the combination of higher temperatures and deforestation has another, more insidious, effect, for plants – and thus trees – play a major role in sequestering CO2 and thus combating global warming. The caveat is that this requires six water molecules for each carbon atom taken out of the cycle, which brings us back to water’s crucial role in the various compartments of life on earth.
The stakes
As we can see, the stakes riding on the water cycle are planetary, as temperatures rise, global air mass dynamics and precipitation patterns change, and freshwater becomes less available and more expensive. Flooding, drought, irregular precipitation and overexploitation of ancient water resources are the ever-more-obvious consequences around the world: China is facing the perils of overexploitation due to rice production, which is pumping the water table to the point where some aquifers are expected to dry up in ten years; in the U.S., California supplies 80% of the food on Americans’ plates but the sector’s profitability is decreasing as water resources dwindle; while in Libya, the water table is being pumped at a rate equal to 6000 times the volume of H20 in the atmosphere. Overall, thirty-three countries are expected to be facing extreme water crises by 2040 and, according to the International Water Resources Association, at least fifty cities will become “dry” in the next thirty years.
This dearth of a vital resource is expected to lead to increased competition for water resources, either directly or indirectly (food and fashion, for example), and thus higher prices for water and everything that depends on it. Water security (“the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development, for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability” – UN-Water) and social justice are at stake. The solution? Sobriety, sustainability, inventiveness, and behavior change: protecting ecosystems and water sources, economizing and reusing water (drip irrigation, rainwater tanks, and dry toilets), and adapting zoning laws to reality (grass on the pampas, palms in Hawaii, and cactus in the desert!). Awareness is the first step.
Based on the August 30, 2023, episode of “Déclic: le Tournant,” a Belgian French-speaking radio (RTBF) podcast on climate change presented by Arnaud Ruyssen.