Icebergs are freshwater. They form from compacted snow that fell on land, accumulated over thousands of years, and eventually became glacial ice before breaking off into the ocean. When researchers have directly measured iceberg samples, the salinity reads zero, confirming that despite floating in saltwater for months or years, the ice itself contains no meaningful salt content.1Progress in Oceanography. Biogeochemical characteristics of brash sea ice and icebergs during summer and autumn in the Indian sector of the Southern Ocean That distinction between icebergs and the frozen ocean surface around them turns out to matter for everything from ocean circulation to ambitious engineering proposals.
How Icebergs Form and Why That Makes Them Fresh
The freshwater character of an iceberg is locked in long before it ever touches the sea. Snow falls on a glacier or ice sheet, gets buried under more snow, and gradually compresses into dense glacial ice. Because that snow started as evaporated water from the atmosphere, it carries essentially no dissolved salts. Over centuries and millennia, more layers pile on, squeezing out air pockets and transforming the snow into solid ice. When a chunk of that ice eventually calves off the edge of a glacier or ice shelf and drops into the ocean, it becomes an iceberg, but its chemistry still reflects its origin as precipitation. It is, in effect, a floating block of ancient rainwater and snowfall.
This process is fundamentally different from what happens when the ocean surface freezes. Sea ice forms directly from saltwater, and while some salt gets expelled during freezing, sea ice retains a measurable salinity. Icebergs, by contrast, never had salt to begin with. Their journey from snowflake to iceberg does not introduce any. Even after drifting through the Southern Ocean or the North Atlantic for years, the ice itself stays fresh.
Icebergs Versus Sea Ice
One of the most common points of confusion is the difference between icebergs and sea ice, and it matters because the two have very different salt profiles. Sea ice is frozen ocean. When seawater freezes, ice crystals form from relatively pure water, but pockets of concentrated brine get trapped between those crystals. Over time, gravity pulls some of that brine downward and out of the ice in a process called brine drainage, which injects salty, dense water into the ocean below.2The Royal Society Publishing. Sea-ice thermodynamics and brine drainage As a result, older sea ice (multi-year ice that has survived several summers) tends to be less salty than freshly formed first-year ice, because it has had more time to flush its brine out. But even old sea ice retains some salt.
Icebergs skip this entire process. Researchers sampling brash ice in the Indian sector of the Southern Ocean found that iceberg samples had a salinity of zero and low concentrations of dissolved nutrients, while sea ice samples showed a wide range of values for every parameter measured.1Progress in Oceanography. Biogeochemical characteristics of brash sea ice and icebergs during summer and autumn in the Indian sector of the Southern Ocean You can think of it as the difference between freezing a glass of salt water (which traps some salt) and freezing a glass of distilled water (which was never salty). Icebergs are the distilled-water version.
This distinction also has ecological implications. Sea ice hosts a rich community of cold-adapted microorganisms, including diatoms, bacteria, and protozoa, collectively known as the sea ice microbial community.3Annual Reviews. Poles apart: biodiversity and biogeography of sea ice bacteria Those organisms thrive in the brine channels within sea ice, the very feature that icebergs lack. Icebergs can support some surface-dwelling life and carry sediment scraped from the land, but their interiors are a relatively sterile environment compared to the biologically active lattice of sea ice.
What Is Actually Inside Iceberg Ice
If iceberg ice is essentially salt-free, what is in there besides frozen water? Mostly air. As snow compresses into glacial ice, tiny air bubbles get sealed off and preserved. These trapped bubbles are time capsules of ancient atmosphere, and scientists have used them extensively to reconstruct how concentrations of gases like carbon dioxide and methane have changed over hundreds of thousands of years.4PubMed Central. Gases in ice cores The bubbles also explain why glacial ice often appears white or pale blue: light scatters off the countless tiny air pockets, giving the ice its characteristic milky appearance.
The purity of iceberg ice is striking. Atmospheric deposition of nutrients and pollutants onto remote ice sheets is minimal, so even trace elements are present only at very low concentrations. That chemical cleanliness is part of what makes ice cores from glacial ice so valuable for climate research, and it is also what makes iceberg ice appealing to anyone thinking about it as a potential freshwater source.
Why Some Icebergs Are Green or Blue
Not all icebergs look the same, and the color differences hint at a more nuanced story about freshwater purity. Most icebergs appear white because of air bubbles, or blue because dense, bubble-free ice absorbs red light and transmits blue. But some Antarctic icebergs are strikingly green, and for decades the explanation for that color was debated.
The green icebergs turn out to come from a special type of ice called marine ice, which forms on the underside of floating ice shelves. In supercooled seawater beneath an ice shelf, ice crystals grow upward and attach to the shelf’s base, scavenging particles and dissolved organic matter from the water as they go. The resulting layer of marine ice can be roughly 100 meters thick. When a section of the ice shelf breaks off and flips over, that basal layer is exposed, and the iceberg appears green or jade-colored.5Journal of Geophysical Research: Oceans. Green Icebergs Revisited
Here is the interesting part for the freshwater question: marine ice, despite forming from seawater, is desalinated and bubble-free. Chemical and isotopic analysis of green iceberg samples confirmed that the ice consists of desalinated frozen seawater. The green color comes from marine-derived organic matter trapped in the ice, not from salt.6Journal of Geophysical Research: Oceans. Green icebergs formed by freezing of organic‐rich seawater to the base of Antarctic ice shelves So even the most unusual-looking icebergs are essentially freshwater. They contain trace organic compounds that change the color, but the salt has been excluded during the slow freezing process at the base of the ice shelf. Marine ice forms gradually enough that dissolved salts are rejected much more thoroughly than in the rapid surface freezing that creates ordinary sea ice.
What Happens When All That Freshwater Melts Into the Ocean
An iceberg drifting through the open ocean is, in a sense, a slow-release freshwater delivery system. As it melts from below and around its edges, it dumps low-density, salt-free water into the surrounding saltwater. That influx does not just dilute the local ocean; it changes the physical structure of the water column in ways that have cascading effects.
Research on giant icebergs has shown that basal melting erodes a layer of cold, salty “winter water” that normally sits between the surface and the deep ocean and acts as a barrier to vertical mixing. At the same time, the freshwater runoff from the iceberg increases stratification near the surface, creating a lighter cap of water on top.7Nature Geoscience. Giant iceberg meltwater increases upper-ocean stratification and vertical mixing The net effect is complex: the iceberg simultaneously removes one layer of stratification while creating another, and in doing so alters how heat and nutrients move between deep water and the surface. For the surrounding ecosystem, this reshuffling of the water column can be significant.
Icebergs also carry iron and other trace nutrients scraped from bedrock during the glacier’s passage over land. When that material is released into iron-starved waters, it can stimulate the growth of phytoplankton, the tiny photosynthetic organisms at the base of the marine food web. Modeling work has shown that iron from melting continental ice (glaciers and icebergs) and sea ice together can boost carbon export in the Southern Ocean by roughly 14% over the regional total, with continental ice contributing about a third of that increase and sea ice the remaining two-thirds.8Geophysical Research Letters. Continental and Sea Ice Iron Sources Fertilize the Southern Ocean in Synergy These two iron sources work in synergy: the fresh meltwater from icebergs can help keep the iron dissolved and available in surface waters where phytoplankton need it.
Do Melting Icebergs Raise Sea Levels
A common misconception is that melting icebergs cannot raise sea levels because they are already floating and therefore already displacing their weight in water. The physics of displacement does support this as a first approximation, but the full picture is more subtle. The melting of floating ice introduces cold, fresh water into warmer, saltier ocean water, and those two changes (cooling and diluting) have opposite effects on ocean density. Unless they exactly cancel each other out, the total ocean volume shifts slightly.9Geophysical Research Letters. Melting of floating ice and sea level rise
The practical effect is small for individual icebergs, but it is not zero. And there is a larger point here that often gets muddled in public discussions: icebergs themselves were once part of glaciers and ice sheets that sat on land. The sea level rise associated with them happened primarily at the moment they calved into the ocean, when the mass of ice that had been supported by rock was suddenly displacing seawater. The additional contribution from the density mismatch when that floating ice later melts is a secondary effect, but researchers have found it worth quantifying, especially when considering the total mass of ice shelves and icebergs currently afloat.
How Icebergs Break Apart
An iceberg’s life in the open ocean is a story of progressive decay. Warm water eats at the submerged surfaces, waves carve notches at the waterline, and wind and sun work on the exposed top. But there is another mechanism at play that is less intuitive. As an iceberg melts unevenly, its shape and weight distribution change, which can create internal stresses from buoyancy forces (the ice is trying to float in a new orientation but is held in place by its own geometry). These hydrostatic stresses can fracture the iceberg from the inside, splitting it into smaller pieces that then melt faster because they have more surface area relative to their volume.10Geophysical Research Letters. The “footloose” mechanism: Iceberg decay from hydrostatic stresses
This cascading breakup explains why icebergs do not simply shrink in place like an ice cube in a glass. They shatter, roll, and calve off smaller bergs in events that can be dramatic and dangerous for nearby ships. The freshwater release from these breakup events comes in pulses rather than as a steady trickle, which makes predicting their local impact on ocean conditions more complicated.
Towing Icebergs for Drinking Water
The fact that icebergs are enormous blocks of pure freshwater has not been lost on engineers and entrepreneurs in water-scarce regions. Proposals to tow icebergs to arid coastlines have circulated since at least the 1970s, and they periodically resurface when droughts hit. The basic logic is appealing: Antarctic icebergs contain vast amounts of freshwater, and some coastal cities are desperate for it.
Numerical modeling of the idea has explored what it would actually take. Simulations of towing icebergs to Cape Town, South Africa, found that an iceberg moved at roughly half a meter per second would need to be at least about 300 meters long and 200 meters thick at the time of capture to survive the journey. An unprotected iceberg that size would deliver around 2.4 million liters of water, which sounds like a lot but is a tiny fraction of a city’s needs. Wrapping the iceberg in insulating material to slow wave-driven erosion changes the math dramatically, potentially yielding about 4.5 billion liters from the same starting iceberg.11Scientific Reports. Towing icebergs to arid regions to reduce water scarcity That is closer to being meaningful as a municipal water source, though the logistics of wrapping an iceberg in insulation in the open Southern Ocean remain formidable.
A separate proposal by an Emirati businessman envisioned towing an Antarctic iceberg to the coast of the United Arab Emirates, a much longer and warmer route. Beyond the engineering challenges, that idea raised questions about compliance with the Antarctic Treaty System and international maritime law.12CEEOL. UNITED ARAB EMIRATES “ICEBERG PROJECT” – WOULD AN AMBITIOUS CONCEPT COMPLY WITH INTERNATIONAL LAW? Antarctica’s resources are governed by a web of international agreements, and whether an iceberg floating freely in the Southern Ocean counts as an “Antarctic resource” once it drifts past a certain latitude is not entirely settled. The legal ambiguity has so far been somewhat academic, since no one has actually completed a large-scale tow. But as water scarcity intensifies in parts of the Middle East and southern Africa, the conversation keeps coming back.
Why Freshwater Icebergs End Up in Saltwater
It might seem paradoxical that the world’s largest reserves of freshwater ice end up floating in salt oceans, but the geography makes it inevitable. The ice sheets that produce icebergs sit on land at the edges of continents or large islands, and gravity pulls glaciers downhill toward the coast. When the ice reaches the sea, pieces break off. In Antarctica, massive ice shelves extend out over the water before they calve tabular icebergs that can be the size of small countries. In Greenland, outlet glaciers channel ice through fjords and into the North Atlantic. In both cases, the ice has no choice but to enter the ocean.
Once afloat, icebergs are carried by ocean currents and wind. Antarctic icebergs tend to circle the continent in the Antarctic Circumpolar Current, gradually drifting northward and melting as they encounter warmer water. Arctic icebergs from Greenland can travel far south along the Labrador Current, occasionally reaching shipping lanes where they pose hazards. The iceberg that sank the Titanic likely calved from a glacier in western Greenland and drifted south for a year or more before the collision. Throughout that entire journey, the ice remained fresh.
The durability of icebergs in saltwater is itself a consequence of their freshwater composition. Freshwater ice is denser and more structurally coherent than sea ice of the same thickness, in part because it lacks the brine channels and pockets that weaken sea ice. A tabular Antarctic iceberg with vertical walls of solid glacial ice can survive for years in the open ocean, slowly releasing its ancient freshwater into the surrounding sea as it goes.