What if the water cycle changed?

The Sun warms the sea. Some water floats up into the air. It makes clouds and falls as rain.

What if the world were a bit warmer? Would there be more rain?

Water is always on the move. The Sun's energy turns liquid water into invisible water vapor, which rises into the air. It cools, forms clouds and falls as rain and snow. Gravity pulls it downhill in rivers and back to the sea.

This never-ending loop is the water cycle. So what happens to it in a warmer world? Does the whole cycle speed up? In this lab you can warm the planet in a model of the water cycle and watch the flows change.

The global water cycle moves water between pools, mainly ocean, atmosphere and land, through evaporation, transpiration, precipitation and runoff, driven by solar energy and gravity (USGS).

Temperature controls how much vapor air can hold. This lab asks how much the cycle's yearly flows would change in a world 2 °C warmer, and why two plausible rules, the vapor rule and the climate-model result, give quite different answers.

Most of Earth's water is in the salty ocean.

Plants put water into the air too. A big oak tree gives off lots of water from its leaves.

The ocean stores about 96 percent of all the water on Earth. Water also rises from land: plants pull it up from the soil and release it from their leaves, which is called transpiration. A large oak tree can give off about 151,000 liters of water a year.

In the model, about 486 thousand cubic kilometers of water evaporate every year, mostly from the ocean, and the same amount falls back as rain and snow. Rivers carry about 40 thousand cubic kilometers from land back to the sea.

USGS notes that oceans hold 96 percent of Earth's water and that evapotranspiration is the main route from the surface to the atmosphere. The model's present-day flows follow Trenberth et al. (2007): ocean evaporation 413, ocean precipitation 373, land precipitation 113, land evapotranspiration 73 and runoff 40 thousand km³ per year.

The model holds 12.9 thousand km³ of vapor in the air, so in the model water spends on average about 9.7 days in the atmosphere.

In our pretend lab, Olly makes the world 2 degrees warmer.

More water rises into the air. More rain falls. The water cycle spins faster!

Set the world 2 °C warmer. Warm air can hold more water vapor, about 7 percent more per degree, so the model's air holds about 14.8 thousand cubic kilometers instead of 12.9.

If evaporation and rain grew by that same 7 percent per degree, they would rise about 14 percent, to about 556 thousand cubic kilometers a year. But climate models find global rain rises only about 1 to 3 percent per degree. Choose about 2 percent and the increase is about 4 percent, to about 506.

With ΔT = 2 °C, vapor rises to 12.9 × 1.07² = 14.8 thousand km³. At r = 7%/°C all flows scale by 1.145: evaporation 556, land precipitation 129, runoff 45.8 thousand km³/yr, residence time unchanged at 9.7 days.

At r = 2%/°C flows grow only 4.0 percent (evaporation 506) while vapor still rises 14.5 percent, so residence time lengthens to 10.7 days. The model scales every flow uniformly; it has no regions, seasons, droughts or floods.

Warm air is like a bigger cup. It can hold more water.

So in a warm world, more water goes up and more comes down.

Warmer air can hold more water vapor before it forms clouds and rain. That is a basic rule of physics. Because the air can hold more, more water evaporates from warm oceans, and the heaviest rainstorms can drop more water.

But rain is also limited by energy. When water vapor turns back into cloud drops, it gives off heat into the air, and the air can only send that heat away to space a little faster as it warms. So climate models find that average rain around the world grows more slowly than the air's ability to hold water, about 1 to 3 percent per degree. The cycle speeds up, but gently.

The Clausius–Clapeyron relation sets low-altitude specific humidity rising about 7% per °C at constant relative humidity (IPCC AR6 WG1, Chapter 8). Extreme daily precipitation is projected to intensify at about that rate (SPM B.2.4). Global mean precipitation and evaporation, by contrast, are constrained by the balance of energy fluxes in the atmosphere and at the surface: the slow, temperature-driven part of the response depends mainly on how fast the warming atmosphere's net radiative cooling grows, since that cooling must balance the latent heat released when vapor condenses (Ch. 8, §8.2.1). The very likely range is 1–3% per °C.

The model's limits: a single global box, all flows scaled by one factor, land evaporation not limited by soil moisture, and no regional pattern. Its ice-melt output is illustrative only and is not used in this lab.

Scientists watch rain, rivers and clouds very carefully.

Towns use what they learn. Rain gardens let rainwater soak into the ground, and good plans save water for dry times.

The IPCC reports that heavy rain events have become more frequent and more intense over most land areas where there are enough measurements, since the 1950s. Scientists expect the water cycle to keep intensifying with more warming, with wet seasons and events getting wetter on average.

The changes are uneven: some places may get more rain and others less. Communities are preparing, for example with rain gardens that let rainwater soak into the ground, better-draining pavements, and plans for dry spells.

IPCC AR6 (WG1 SPM, A.3.2) finds that heavy precipitation has increased in frequency and intensity since the 1950s over most land with sufficient data, with human-induced climate change likely the main driver. B.3 projects further intensification of the global water cycle, including its variability and the severity of wet and dry events.

Chapter 8 notes that a warmer climate increases moisture transport into weather systems, making wet seasons and events wetter on average. NASA lists planning for drought and managing land among adaptation measures.