A group of scientists from Britain, Germany and Ireland studying the Atlantic Meridional Overturning Circulation — that is, the circulation pattern that warms the North Atlantic — have sought to compare how it is behaving now with its recent past. Experts only began directly measuring the pattern in 2004, so they looked for clues in seafloor sediments and ocean temperature patterns, which suggested how the currents behaved before. The clues present a consistent picture: The circulation has weakened in a way that is unprecedented in the past 1,000 years, said Niamh Cahill, a statistician from Ireland’s Maynooth University.
The scientists believe the ultimate cause is global warming. The circulation occurs because warm tropical water cools and becomes saltier as it travels north, which makes it denser. This dense water eventually sinks to the bottom of the ocean, then travels south, where it is once again heated in another part of the cycle. Higher rainfall, lower amounts of sea ice and ice melting on the Greenland ice sheet are adding far more fresh water than usual to the system, making the water up north less salty and, thus, less dense and less prone to sink, undermining the circulation. This may account for a giant stretch of unusually cold water that has stubbornly persisted near Greenland and for unusually high water temperatures on the U.S. East Coast.
The study’s authors warn that climate change may further destabilize the Atlantic circulation over coming decades. The consequences are hard to predict with precision, in part because warming air might offset some of the cooling associated with slower circulation. But previous research suggests that the last time the circulation was severely destabilized, some 12,000 years ago, Europe was slammed with bitter winters and, because of how the circulation interacts with air, severe summer heat and droughts.
Climate change is not some isolated change in the air temperature. It encompasses sea-level rise, heavy storms, heat waves, droughts, wildfires, acidifying oceans and disruptions in the sensitive planetary rhythms on which human society developed. Scientists know some things for sure — the planet will warm because of greenhouse gas emissions, with a variety of negative results. But they have not catalogued all the consequences. Some are only just coming clearly into view, and some remain obscure. The longer we humans fail to adjust our behavior, the worse the consequences are likely to be.