Last weekend, at the very easternmost edge of England, tens of thousands of people of all ages gathered at a beach festival in Lowestoft to celebrate the sea joyously. To dance to trance music and listen to Linton Kwesi Johnson recite his poetry, and to hear marine scientists explain to seven-year-olds exactly why the sea smells the way it does. It was an idyllic scene. From dusk to dawn and back again, everyone was drawn to the vast and glorious element that connects us to the rest of the world. But then, amid the revelry, a solemn procession appeared. Two dozen festivalgoers carried a series of blown-up photographs into the sea. They were portraits by the artist Gideon Mendel of people, many of them from the global south, standing amid the floods that had overturned their lives. Suddenly, in the face of their fates, the sea seemed not so benign after all. It was a reminder that sea levels are rising around the world; and that here in the UK we face our own potential disaster – the drastic sudden warming of the sea off Britain and Ireland. It is one of this year’s nastier surprises. Sea temperatures are up to 5C higher than usual, in part due to human-induced global heating, and in part as a result of this year’s El Niño effect. It is a mortal reminder of how the sea dominates our entire planet – that a cycle of warm water in the distant Pacific should impact so severely on our local shores. As water temperatures reach an all-time high, the direct effect on wildlife – from seagrass to oysters and fish – will be devastating, the way “wildfires take out huge areas of forest”, as one scientist says. The very definition of our coastal sea is its chill. That’s the whole point for those of us who swim in it year-round. That sharp intake of breath as it reaches mid-thigh. The endorphin rush as you get out. But what will happen if that energising chill turns to a soupy stew? When the sea is filled with blooms of jellyfish and sea gooseberries prompted by the warm currents, as it has been these past few weeks? Scientific projections indicate that the fragile balance of the sea’s manifold ecosystems will be overturned. From the seagrass beds of Studland in Dorset, which act as vital “blue carbon” sinks, fixing carbon dioxide as they photosynthesise – as well as being safe nurseries for seahorses – to the deep cold-water coral reefs off western Scotland, with stunning colours and delicate structures to rival any Caribbean equivalent, the effects will be severe. Fish will be driven further north in search of cooler temperatures where their food sources can grow. Commercial fisheries will find their catches depleted. Since warmer water holds less oxygen – due to reduced oxygenating phytoplankton – the whole food web will be disrupted. Stalwart brent geese, which arrive each autumn on my local beach, choosing to migrate here all the way from Siberia, will find the seagrass they need to survive thin on the ground. The whole subtle evolutionary web of the shallow shore and the deep water beyond will change as the sea starts to behave in unpredictable ways. Scientists warn that even a slight 0.8C (1.5F) increase in global ocean temperatures represents “an enormous amount of heat, large enough to transform marine biodiversity, change ocean chemistry, raise sea levels and fuel extreme weather”. Warmer water itself expands. Half the sea level rise in the past 25 years is due to “oceans simply occupying more space”. Those images of flood victims taken by Mendel are directly linked to hotter oceans. The sea has already saved our skins by physically absorbing the heat we have released into the atmosphere. The more drastic effects of the climate crisis have been kept from us by the great cold oceanic reservoir. The UN calls the ocean “the world’s greatest ally against climate change”. Today, that seems like a hollow description. The breaking point has been reached. The sea is the home of the greatest biomass of life on Earth. Yet our ignorance of what lies beneath the “ocean’s skin”, as Herman Melville called it, has allowed us to pollute it and over-fish it and generally mistreat it in our illusory notion of biblical dominion. Up until the 1960s we were dumping radioactive waste into the Irish Sea. The North Sea is so full of PCBs and pesticides leaking from landfills and intensive agriculture that our resident orcas have not given birth to a healthy calf for 10 years. And yet, as the ocean explorer, Jacques Cousteau, said: “The sea, the great unifier, is man’s only hope.” We still look out to its horizon as though it were the beginning and the end of our world, as if nothing we do could change it. We were wrong. But we still have the chance to put that right, if we choose. We have much to gain, and a lot to lose. Philip Hoare is the author of several books, including Leviathan, The Sea Inside and Albert and the Whale