Warmer seas are undermining Denmark’s environmental efforts

A paradox: Discharges of nitrogen and phosphorus into Danish waters have fallen sharply since the 1980s, when oxygen depletion first emerged as a widespread environmental concern. Yet oxygen depletion has never been as extensive as it is today.

According to marine ecology professors Erik Kristensen and Mogens Flindt, the explanation is that temperature has been overlooked. Danish waters have warmed by 2.2°C since 1980—a shift with a major impact on the formation of oxygen depletion.

“Warmer seawater allows algae to grow faster and spread more widely. At the same time, decomposition at the seabed accelerates by around 25%, which amplifies oxygen depletion,” says Erik Kristensen, “Nutrients are also released more quickly and recycled back into the system. This means that the same amount of nutrients causes greater damage today than it did in the past.”

Nitrogen remains the lever we can still adjust

In a review article in Marine Pollution Bulletin, the researchers warn of a potential “death spiral,” where oxygen depletion could push Danish marine ecosystems beyond recovery unless action is intensified.

Since the first water environment plan in 1987, Denmark has cut phosphorus discharges by 80% and nitrogen by 45%. The goal was to curb the oxygen depletion that began plaguing coastal waters in the 1980s.

But climate change has shifted the baseline. While rising temperatures cannot be reversed in the short term, nutrient inputs can still be reduced. Phosphorus levels are already so low that further gains are limited. Nitrogen, however, remains a key lever. Annual nitrogen discharges have hovered around 60,000 tonnes since 2005.

Current agreement is not enough

Under the current green tripartite agreement, Denmark aims to cut this by 14,000 tonnes per year.

“That is not enough,” says Kristensen. “It will help in many areas, but fjords will remain heavily impacted. In my view, we need cuts that are twice as large—and possibly three times as large as temperatures keep rising in the coming decades.”

So where should the cuts come from?

“Primarily agriculture, which accounts for about 70% of nitrogen load,” he says.

Wastewater treatment plants also contribute, but according to the researchers, their share is comparatively small—around 10%.

Fewer pigs may help Denmark’s seas

The issue is closely linked to agricultural policy. A recent government plan suggests that Denmark’s pig production should be scaled back to focus mainly on domestic consumption and value-added exports.

Such a shift could have significant environmental implications. Denmark currently produces about 30 million pigs annually, exporting roughly 17 million of them. Fewer pigs would mean less manure—and potentially less nitrogen runoff. But the impact is uncertain.

“If fewer pigs lead to less manure being spread and less intensive feed production, the effect could be significant. But if crop production continues unchanged, the gains may be limited,” Kristensen notes.

Climate change may also bring more frequent storms, which can temporarily oxygenate marine waters.

“Storms can provide short-term relief,” Kristensen says. “But they cannot solve the problem. To make a real difference, we would need a summer storm every week from July to October—and that is simply not going to happen.”