Imagine it’s autumn 2022. The earth has warmed by 1.2°C since the preindustrial era. Our planet’s water system—the network that connects rivers, lakes, and oceans as well as glaciers, soil, air, and all living things—is changing. Rising temperatures make this system more volatile.
If we think of the planet as a human body, increasing global temperatures are like a rise in our internal temperature: At 37°C (98.6°F), we are comfortable and able to thrive, but a sustained increase of even one or two degrees can be harmful. The planet’s water cycle is like our circulatory system—an intricate network carrying a steady supply of a crucial substance that sustains all forms of life, including our own. Disruptions in this network can pose grave danger.
Clouds are made of droplets of water that each contain trillions of H2O molecules, yet are still light enough to stay suspended in air. Imagine an H2O molecule in the atmosphere joining a nearby droplet, making it just heavy enough to fall from the sky as a raindrop.
This drop splashes onto a road about four minutes after beginning its descent. The asphalt is not porous, and the water needs to go somewhere. The engineers who designed the road planned for rainy days, so the road is higher in the center and lower on the sides, allowing gravity to pull water toward the curb. Our molecule joins rushing water in the gutter and tumbles through a grate into a storm sewer.
Engineers designed this storm sewer for a range of precipitation, but today it’s raining more heavily than the designs anticipated. Water has nearly filled the storm sewer, and the river it flows into is rising dangerously. If this heavy rain continues, the sewer will exceed its capacity, and subsequent rain will stay on the surface, following the contours of roads and valleys until it floods the low-lying parts of the city. For now, the system is working. Our molecule becomes part of the swiftly moving river, and within an hour, it is in the ocean.
When the ocean and atmosphere are warmer, water evaporates more readily. Warm air acts like a sponge—if water is available, the air pulls it from the earth’s surface, until it reaches a saturation point.Our molecule now spends less time in the warmer ocean before it separates from the swirl of other water molecules, evaporates back into the atmosphere, and is swept away by the wind.
Our molecule travels a long distance as vapor, carried by the winds and passing over a forest. Trees transpire water from their leaves and needles, which moistens and cools the air around them. Our molecule collides with recently transpired molecules, and they condense into a new water droplet in a cloud. Other molecules condense quickly, adding weight, and our molecule is once again in a falling raindrop, this one landing on the crown of a tree. The tree’s leaves and grooves in its bark channel water toward the trunk and down to the soil.
The Clausius-Clapeyron Relation
Over the following weeks, our molecule passes through many small organisms living in the soil before it’s absorbed by the tree’s roots. Tubes within the trunk pull it up to the branches and out to the leaves. The undersides of the leaves exhale oxygen and water vapor. Our molecule is airborne again.
Above the forest, our molecule is swept farther by the prevailing winds. After a few days, it joins other molecules in a group of clouds above a mountain range. Twenty years ago, at this time of year, snow would have been typical at this altitude. Our molecule would have connected with others in a crystalline pattern to form a snowflake where it would have stayed all winter.
In the spring or summer, snow would melt into liquid and make its way gradually down to the valley below, providing gentle, steady water supply for the village and farms. Because the air is warmer, however, our molecule’s droplet grows bigger and heavier. Eventually the atmospheric “sponge” reaches its maximum capacity, and our molecule falls in a deluge.
Local patterns of precipitation have been increasingly erratic as the atmosphere has warmed, and this valley has been in a drought for a couple of years. One might think the villagers would welcome the rain, but the soil isn’t ready for it, having become dry and hard, and much of the vegetation has withered—an increasingly bad match for heavy rainfall.
Each fat drop loosens more of the parched surface, and soon a mudslide is underway, tumbling and surging into the riverbed below. Our molecule becomes part of a turbulent slurry of topsoil, plants, and debris. The river banks widen, eroding parts of the landscape with the current.
Downstream is an artificial lake where engineers dammed the river to generate hydropower. The water is welcome, but the silt and debris in the water are not. They raise the lakebed, reducing the lake’s capacity, clogging the dam’s machinery, and threatening the local water supply in a way that gradual snowmelt in the past did not.
Our molecule squeezes through the dam’s turbines and flows toward the delta where the river meets the sea. Local people have cut channels to irrigate farmland in this region, and our molecule travels to a field. Rice is the main crop there, and farmers grow it by submerging the land in knee-deep water. Our molecule circulates in the field for a while, but instead of becoming part of a rice plant, the warm atmosphere quickly evaporates it.
This constant movement and change from liquid to vapor and back again is a dramatic, frenetic cycle for our molecule. Every bit of additional warming adds energy, volatility, and uncertainty to the entire water system, raising the risk that the stable patterns we’ve built our lives upon could disappear.
Illustrations by Berke Yazicioglu