All of Earth's water in a single sphere (2019)
An infographic from the USGS visualizing all of Earth’s water as a single sphere prompts surprise at how thin the planet’s oceans and freshwater reserves are relative to its size. Commenters unpack the math behind the visualization, compare the water sphere to bodies like the asteroid Ceres and human biomass, and note that our intuitions about volume and scale are often misleading. Others point out that the graphic omits vast amounts of water locked deep in the mantle, while still seeing it as an effective way to highlight how limited accessible freshwater is and why it should be treated as a constrained resource.
Visualization, Scale, and Intuition
- Many find the spheres visually striking but hard to intuit; the 860‑mile water sphere looks “small” next to Earth despite reaching far into space.
- Several note that oceans are just a very thin film on a large rock: average ocean depth (~few km) vs Earth’s ~12,700 km diameter.
- Some argue the graphic is inherently misleading because it compares volume to planetary volume; others say it effectively conveys the thinness and preciousness of Earth’s water.
- Suggestions include overlaying “equivalent depth” over the U.S., using cubes instead of spheres, or comparing to “accessible crust” volume rather than the whole planet.
Freshwater vs Total Water
- The smallest sphere (lakes and rivers) and the medium sphere (all liquid freshwater including groundwater and swamps) are highlighted as surprisingly small.
- Back‑of‑envelope calculations show large per‑person allocations if all freshwater were evenly divided, but commenters stress distribution, accessibility, and pollution matter more.
Water in the Mantle and Definitions of “Water”
- Multiple comments note newer research suggesting the mantle may contain as much H₂O (bound in minerals) as all oceans combined.
- The USGS numbers are based on a 1993 source and exclude this mantle water.
- Debate over whether mantle H₂O should “count” as water: some say it’s hydroxyl in minerals, effectively inaccessible; others still see it as part of Earth’s total water inventory.
Physics, Shapes, and Math Pedantry
- Discussion on whether a Ceres‑sized water sphere would stay intact in space: gravity would form a sphere, but stability depends on temperature (freezing vs boiling/sublimation).
- Repeated riffs on how humans misjudge curved volumes; spheres and small radius changes drastically affect volume.
- Lengthy side thread on “polynomial vs exponential” growth and the colloquial misuse of “exponential.”
Comparisons and Thought Experiments
- Comparisons to Ceres, icy moons (Europa, Ganymede), and Jupiter’s liquid hydrogen “ocean.”
- Speculation about crashing Ceres or Europa into Mars to terraform it, with pushback on catastrophic side effects and timescales.
- Related analogies: all mined gold as a ~22 m cube, all humans as a ~1 km “meat sphere,” atmosphere as a sphere comparable in size to the freshwater one.