Is the Red Sea Drying Up? The Science of Water Loss

The Red Sea is not drying up. Despite losing enormous volumes of water to evaporation every year and receiving virtually no rainfall or river input, the Red Sea maintains its water level through a continuous exchange with the Indian Ocean via the narrow Bab el Mandab Strait at its southern end. In fact, satellite measurements show the sea’s surface has been rising, not falling, at roughly 4 mm per year over the past three decades. The real story of the Red Sea is not one of disappearance but of a surprisingly dynamic water body whose extreme conditions are shaped by geography, climate, and a delicate connection to the wider ocean.

Why People Assume the Red Sea Should Be Shrinking

The Red Sea sits in one of the hottest, driest corridors on Earth. Temperatures along its shores regularly exceed 40 °C, and rainfall is negligible along most of its length. No permanent rivers feed into it. Under those conditions, evaporation is relentless, stripping away an estimated two meters of water from the surface each year. If the Red Sea were a closed basin with no outlet and no replenishment, it would indeed shrink and eventually dry into a salt flat, much the way the Aral Sea contracted when its feeder rivers were diverted.

That comparison is what makes the question intuitive, but the Red Sea is fundamentally different. It is not a lake. It is an arm of the Indian Ocean, connected at the south through the Bab el Mandab Strait and at the north through the Suez Canal (though the canal’s contribution to the water budget is tiny). The evaporation that draws water off the surface actually drives a self-sustaining circulation loop that pulls replacement water in from the Gulf of Aden. The hotter and drier it gets, the more vigorously that inflow responds.

The Bab El Mandab Strait and the Red Sea’s Lifeline

The Bab el Mandab, Arabic for “Gate of Grief,” is a strait only about 30 km wide at its narrowest point and roughly 160 meters deep at its shallowest sill. It is the Red Sea’s primary connection to the open ocean. Direct measurements of the flow through this strait, collected using moored instruments over a 10-month period in 1995–1996, revealed a layered exchange: relatively fresh, cooler water from the Gulf of Aden flows in near the surface, while saltier, denser Red Sea water flows out along the bottom.¹ This two-layer flow operates year-round, though its character shifts with the seasons.

The annual mean transport of the Red Sea’s outflow is relatively small, less than 0.4 Sverdrups (a unit oceanographers use for massive water flows), but its high salinity has a disproportionate influence on Indian Ocean water properties at depth.² As this dense, salty water spills over the sill and descends into the Gulf of Aden, it mixes with surrounding water and gets diluted by a factor of roughly two and a half before settling at its equilibrium depth. Meanwhile, the lighter Indian Ocean water streaming in at the surface replaces what evaporation has removed. The system balances itself: evaporation lowers the surface and increases salinity, which drives the density contrast that keeps the exchange going.

A Seasonal Breathing Pattern

The Red Sea does not exchange water at a constant rate. Its circulation follows a seasonal rhythm largely governed by the monsoon winds over the Arabian Sea and the Gulf of Aden. During winter, a strong surface inflow from the Gulf of Aden feeds the Red Sea, and a pronounced deep outflow of very salty water (salinities reaching 40 or higher) pushes south along the bottom of the strait. In summer, the pattern shifts: the surface layer reverses in parts of the southern Red Sea, and a distinctive mid-depth intrusion of cooler, fresher Gulf of Aden water penetrates into the basin.¹

Modeling studies covering nearly two decades of this exchange have confirmed that the flow varies from year to year as well, responding to shifts in monsoon intensity and broader climate patterns.³ The winter monsoon also drives biological productivity: the strength of the winter Arabian monsoon controls how much nutrient-rich water gets pulled horizontally from the Indian Ocean into the Red Sea, regulating phytoplankton blooms and, by extension, the entire food web.⁴ So the strait is not just a water pipe; it is the Red Sea’s primary source of both hydration and nutrition.

What Happens Inside the Basin

Evaporation does not just remove water from the Red Sea; it concentrates the salt left behind. As surface water grows saltier and colder during winter, it becomes dense enough to sink. This process, called deep-water formation, happens primarily in the far northern Red Sea, inside the Gulfs of Aqaba and Suez. Observations from those gulfs show that dense water capable of reaching the very bottom layers of the Red Sea is regularly produced there.⁵ During unusually cold winters, deep-water formation can also occur over coastal areas just outside those gulfs.⁶

This sinking of dense water ventilates the deep basin, cycling oxygen downward and preventing the abyss from becoming stagnant. At intermediate depths, a separate overturning cell forms when strong atmospheric events spawn cyclonic eddies that force episodic convection.⁷ The Red Sea’s internal circulation is surprisingly vigorous for a semi-enclosed basin, and that vigor is a direct consequence of evaporation. Far from being a symptom of the sea drying up, the high evaporation rate is the engine that drives the whole system.

When the Lifeline Almost Closed

If you want to see what happens when the Bab el Mandab connection weakens, the geological record offers a vivid case study. During the last ice age, global sea levels dropped so dramatically that the shallow sill of the Bab el Mandab was nearly exposed. The exchange with the Indian Ocean slowed to a trickle. With evaporation continuing but replacement water barely trickling in, the Red Sea became hypersaline, reaching an estimated maximum salinity of around 57 practical salinity units, roughly 60 percent saltier than typical ocean water.⁸ At those levels, most marine life would have been pushed to the brink or forced into whatever pockets of less extreme water remained.

Going back even further, between roughly 10 and 5 million years ago, the Red Sea basin accumulated massive evaporite deposits, layers of salt that in the southern Red Sea originally reached about 2 km in thickness. These formed during a period when the connection to the Indian Ocean was restricted or repeatedly cut off, allowing seawater to evaporate and leave behind enormous salt accumulations.⁹ When the Indian Ocean eventually flooded back through the strait, the Red Sea refilled. These episodes show that the Red Sea can lose water catastrophically when its outlet is blocked, but also that it recovers when the connection reopens. Today, with sea levels relatively high and the Bab el Mandab comfortably submerged, that particular scenario is not on the table.

Sea Level Is Rising, Not Falling

Satellite altimetry data from 1993 to 2021 tell a story that runs exactly counter to the “drying up” narrative. The average sea level trend in the Red Sea over that period was about 4.17 mm per year, roughly in line with or slightly above the global average.¹⁰ About 40 percent of that rise comes from thermal expansion, meaning the water itself is getting warmer and therefore taking up more volume. The remaining rise is attributed to other factors including changes in ocean mass and circulation patterns. An abrupt acceleration was observed after 2008, with post-2008 trends significantly steeper than those in the preceding period.¹⁰

So the Red Sea’s water level is climbing, not dropping. The sea is getting bigger, not smaller. The real concern for the region’s coasts is not water loss but water gain, with implications for low-lying infrastructure, coastal habitats, and the millions of people living along its shores.

The Actual Threat Is Warming

While the Red Sea is not drying up, it is heating up faster than almost anywhere else in the ocean. The overall warming rate for the Red Sea has been measured at about 0.17 °C per decade, which already exceeds the global average. But the northern Red Sea is warming two to three times faster still, at 0.40 to 0.45 °C per decade.¹¹ A separate analysis covering 1982–2016 found the Red Sea’s warming trend to be roughly 2.6 times higher than the global ocean warming trend, and showed that the rate has intensified over time, with the most recent period (2005–2016) warming faster than the two preceding periods.¹²

This warming has consequences that ripple through the entire ecosystem. Higher water temperatures can alter evaporation rates and circulation patterns, shift the distribution of marine species, and stress coral reefs. For a body of water already operating near thermal extremes in its central and southern reaches, even modest additional warming can push organisms past their tolerance limits.

The Northern Red Sea as a Coral Refuge

The Red Sea harbors some of the world’s most heat-resilient coral reefs, and the reasons for that resilience are part of what makes this region so scientifically interesting. In the central Red Sea, corals already live close to their maximum temperature tolerance. Heat stress experiments comparing corals from the northern and central Red Sea confirmed that central corals are more sensitive to thermal anomalies than their northern counterparts.¹³ Corals in the northern Red Sea, by contrast, live well below their bleaching thresholds, meaning they can tolerate significantly more warming before they start to bleach. Researchers have proposed that the northern Red Sea represents a thermal refuge of global importance for reef-building corals.

One factor contributing to this heat tolerance appears to be the Red Sea’s high salinity itself. Laboratory experiments with a coral model organism showed that higher salinity can convey increased thermotolerance, though the effect depends on the particular host strain and its associated symbiotic algae.¹⁴ This finding may partly explain why corals in the Red Sea and the Persian Gulf, both exceptionally salty environments, can survive temperatures that would trigger bleaching in less saline waters. Corals at the northern Red Sea’s highest-latitude fringing reefs have been observed enduring water temperatures that exceed established bleaching thresholds for extended periods, suggesting they have developed physiological mechanisms to cope.¹⁵

Desert dust, carried by winds from the surrounding Saharan and Arabian deserts, also plays an unexpected role in keeping these reefs productive. Dust deposition delivers iron, manganese, zinc, and other trace elements that are otherwise scarce in Red Sea waters. Experiments on the reef coral Stylophora pistillata showed that dust-supplied metals enhanced chlorophyll concentration and photosynthesis in the coral’s symbiotic algae, suggesting that primary production on Red Sea reefs is partly limited by metal availability and that dust storms effectively fertilize the reef ecosystem.¹⁶

Human Pressures on the Coast

Even though the Red Sea itself is not shrinking, human activity along its shores is creating localized water-quality problems that can mimic the effects of water loss in specific areas. Desalination plants, which are essential for freshwater supply in the arid nations bordering the Red Sea, discharge concentrated brine back into the sea. A critical review of studies on desalination plant impacts found that the greatest ecological damage occurs around older plants discharging into poorly flushed water bodies. In those settings, brine discharge can raise local salinity and temperature, and concentrate metals and toxic anti-fouling compounds in the receiving waters. Effects on nearby ecosystems range from minimal to widespread alterations in the structure of seagrass beds, coral reefs, and soft-sediment communities.¹⁷

Coastal development is another pressure point. Rapid, often unplanned expansion of tourism infrastructure and urban areas along the Red Sea coast has led to measurable ecological degradation, including loss of coral reefs and mangroves, alongside economic inefficiencies driven by speculative land use.¹⁸ Mangroves serve as nursery habitat for reef fish and buffer the coast against storm damage, so their loss compounds the stress on an already pressured system. These localized degradations are far more immediately damaging to Red Sea ecosystems than any hypothetical water loss.

A Growing Ocean, Not a Shrinking One

Beneath all the discussion of evaporation and salinity is a deeper geological reality that puts the “drying up” question in perspective. The Red Sea is not just maintaining itself; it is, on geological timescales, getting wider. The basin sits along an active tectonic rift where the African and Arabian plates are pulling apart. Recent geophysical work has confirmed that a new ocean basin is forming along this rift, one that is no longer in its infancy but has developed the characteristics of a true, albeit narrow, oceanic spreading center.¹⁹ The spreading rate is only a few centimeters per year, far too slow for anyone to notice in a human lifetime, but the direction of change is unambiguous. The Red Sea is growing.

This tectonic context also explains the Red Sea’s unusual depth. Despite being narrow, it plunges to over 2,500 meters in places, a depth profile created by the same rifting that is widening it. As the rift continues, the basin will gradually deepen and widen, accommodating more water rather than less. Millions of years from now, if the process continues, the Red Sea will look less like a narrow strait between continents and more like a young version of the Atlantic. The notion that it might dry up runs directly counter to the geological forces shaping it.