Danish waters warm by up to 2 degrees in 40 years

Sea temperatures around Denmark have risen by 1.5–2 degrees over the past 40 years, according to measurements from the Danish Meteorological Institute (DMI).

While the change may go unnoticed by swimmers, it has significant consequences above and below the surface, says Jacob L. Høyer, senior researcher and head of satellite data and Arctic studies at DMI’s National Centre for Climate Research.

“There can of course be warm years and cold years, but over a longer period, it is clear that the water in Danish waters is getting slightly warmer year by year,” Høyer states. He adds that researchers agree the warming is caused by greenhouse gases in the atmosphere, meaning it is human-induced climate change.

The warmer seas increase the risk of extreme weather, including more tropical nights and cloudbursts, as the ocean releases heat into the atmosphere. Warmer water also leads to greater evaporation, which can contribute to more intense rainfall.

Below the surface, cod populations are struggling. The fish once sought deeper, cooler waters when temperatures rose, but with warmer conditions reducing oxygen levels at the bottom, they now have fewer places to go, according to Karen Timmermann, professor of coastal ecology at DTU Aqua. The decline of cod, a predator, has allowed other species like crabs to thrive, disrupting the ecosystem.

Warmer water also fosters bacterial growth, particularly Vibrio and Shewanella, which can cause infections in open wounds or in people with weakened immune systems. Steen Ethelberg, professor and section head at the Statens Serum Institut, warns that these bacteria multiply rapidly in water above 20 degrees.

Additionally, rising temperatures reduce oxygen levels in the water, worsening the effects of agricultural runoff and increasing the risk of oxygen depletion. This harms eelgrass, a vital habitat for fish and small marine life, and promotes algal blooms, especially in fjords and areas with limited water exchange.