The human suffering inflicted by war is so terrible that we may naturally overlook the environmental damage which large-scale conflict causes. Sometimes, of course, it is all too obvious, as with the case of Agent Orange, the highly toxic defoliant sprayed by the Americans over 12,000 square miles of forest during the Vietnam war, or with the mass oil pollution from the Sea Island terminal in Kuwait during the Gulf war in 1991. But on the whole, and certainly with major conflicts, damage to the natural world is a secondary consideration.

There were at least 100,000 individuals on Skomer in 1934. But in 1963, the numbers had dropped to 4,856 birds

It is wholly understandable: in the course of the second world war, the deadliest conflict in human history, there is wide agreement that about 60 million people died (and possibly millions more), which was about 3% of the world population of 2.3 billion in 1940. Try to visualise it and your head swims; the individual grief involved is beyond comprehension. No surprise, therefore, that there might be no room in our consideration for damage to the environment caused by the second world war, even if it were large scale and long term. No one has ever really looked for it.

And yet it occurred, an intriguing new piece of research suggests. It occurred invisibly, but its effects are with us still. It came from the amount of shipping sunk in the battle of the Atlantic, when German U-boats desperately tried to cut Britain’s strategic lifeline to the US: a battle which was once presented to me, specifically, as having no environmental cost.

Guillemots on Skomer Island, off the coast of Pembrokeshire in west Wales. Photograph: Dimitris Legakis/D Legakis Photography/Athena

That happened during another battle, the battle of Brent Spar. Remember? Brent Spar was a huge North Sea oil storage buoy, a gigantic floating tank. In 1995 Shell, its operator, announced plans to dispose of it by sinking it in water more than 7,000 feet deep, 160 miles west of Scotland. But Greenpeace organised an astonishingly successful campaign against the move, and Shell was forced to back down and break it up onshore – even though its executives still thought sinking it in deep water was the best and safest option. Someone closely involved said to me at the time: “Look. Between 1940 and 1943 the U-boats of Grand Admiral Dönitz sank the equivalent of several thousand Brent Spars, and no one has ever suggested that they caused a major marine pollution problem.”

Well, that’s not quite accurate. The 3,500 merchant vessels and 175 allied warships sunk in the battle of the Atlantic weighed about 15m tonnes, and Brent Spar weighed 66,000 tonnes, so purely in terms of weight, the equivalent is about 250 Brent Spars, rather than thousands. But no, no one has ever suggested that they did present a major marine pollution problem. Until now.

In a paper in the current issue of the journal British Birds, under a title so anodyne it might seem of interest only to the most narrowly focused of specialists – “Changes in the number of Common Guillemots on Skomer since the 1930s” – Tim Birkhead, a professor at Sheffield University, makes the revolutionary suggestion that the battle of the Atlantic actually had a devastating effect on marine life. He does so by looking at the breeding population of guillemots, penguin-like seabirds which are actually auks, on Skomer island off the coast of west Wales, before and after the second world war.

A specialist in animal behaviour, Birkhead has carried out much research on Skomer’s guillemots over recent decades, and he is the British expert on the species. His originality is to reconstruct the prewar population on the island – never counted – by detailed analysis of old photographs which have recently come to light of the cliff ledges on which the birds breed. He concludes that there were “at least 100,000 individuals in 1934”. But in 1963, when the first proper count was done, the numbers had dropped by a staggering 95%, to 4,856 birds. (They have slowly climbed back up to the present level of 23,746). He adduces other evidence to show that the steepest decline was between 1940 and 1946, and that oil pollution from the ships sunk off the western coasts of Britain during the war was the principal cause.

He remarks: “The magnitude of the effect of second world war activities on guillemots (and presumably other marine wildlife) has not previously been appreciated.” It does not weigh in the balance for us, does it, with the 60 million people? It cannot do. And yet it happened, and it carries a lesson for us: that the natural world cannot take the punishment it receives at the hands of human society without consequences, and that even the ocean, which we have long presumed is able to absorb anything we throw in it, is far less resilient than in our arrogance we might care to think.