Enlarge /. The Gulf Stream as pictured from space.
The main currents in the Atlantic help control the climate by moving warm surface water north and south from the equator, with colder, deeper water pushing back from the poles to the equator. The presence of this warm surface water plays a key role in climate mitigation in the North Atlantic, giving places like Britain a far more temperate climate than its location – the equivalent of northern Ontario – would otherwise dictate.
However, the temperature differentials that drive this river are expected to subside as our climate continues to warm. A little over a decade ago, measurements of the currents appeared to suggest that temperatures were falling, suggesting that these predictions might come true. However, a few years later it became clear that there were just too many differences from year to year for us to assess.
However, over time, researchers have found ways to get indirect measurements of the currents, using material that is affected by the strengths of the water flow. These measures have now given us a look back at the behavior of the current over the past centuries. And the results confirm that the strength of the currents has decreased dramatically over the past century.
On the conveyor belt
The most famous of the currents in question is probably the Gulf Stream, which flows along the east coast of the United States and Canada and carries warm water from the tropics to Europe. However, the Gulf Stream is only part of a much larger ocean transport system that redistributes heat in all major ocean basins outside the Arctic. And while its range is global, much of the force that powers the system develops in the polar regions. There the surface waters cool, increase in density, sink to the sea floor and begin to flow south. It is this process that helps draw warmer water north to replace what has sunk.
The density of the cold, salty water is the key to the entire process – and this is where climate change can intervene to slow down or stop water turnover. The Arctic is warming faster than any other area on earth, which means that surface waters take longer to cool. Arctic warming is also melting much of the ice, both on land and in the floating ice sheets that have typically covered the Arctic Ocean. This process can create a layer of fresher water over the surface of the nearby ocean that, even after cooling, is not as dense as the salt water below.
When this process begins, we should be able to identify it by measuring the strength of the currents flowing north. But that turned out to be less informative than we might want. While we have seen significant declines in a few years, they have often been countered by sharp increases in other years. This internal variability in the system is so great that it would take decades for a trend to reach the point of statistical significance.
The alternative would be to extend our records back in time. Since we can't place buoys retrospectively in the North Atlantic at the beginning of the last century, researchers need to find other ways to find out how strong the water flow was before we had accurate measurements.
Currently through proxy
The entire research community has identified a number of ways to find out what was going on in the oceans in the past. Some are pretty straight forward. For example, stronger ocean currents can allow larger sediment particles to flow longer in the water. So studying the average particle size deposited in sediments on the ocean floor can tell us something about the currents that flow past this point. Other measures are a little less direct, such as nitrogen isotope ratios in corals, which tell us something about the productivity of the ocean in this area.
In total, there are about half a dozen different ways to understand the previous marine conditions used in the new study. Each has a different level of uncertainty, and many do not provide an accurate measure of conditions in a single year, but rather give an idea of what the average conditions were like over a period of several decades.
To make matters worse, the measures do not all come from the same locations. Samples from deeper waters record the equatorial cold water flow, while shallow locations provide data on the warm water flowing north. The Gulf Stream also breaks up into several individual currents in the North Atlantic, so that some locations only cover a small part of the overall picture.
Given all of this, it is not possible to form a complete picture of past Atlantic currents. However, if enough websites are covered, the overlap between the various data sets can be used to get a feel for whether there have been any general changes at any point in the past 1,600 years.
To identify major transitions, a research team performed a change point analysis, essentially looking for points in history where the mean behavior before and after is significantly different. They found two change points that appear consistently across multiple proxies' data. One occurred in the late 19th century and the second around 1960 when the current warming phase really began.
Of the 11 different records examined in the researchers' work, 10 show that the lowest level of current was in the last century. And that identification is statistically significant in nine of them. "Together, these data consistently show that the modern (current) slowdown is unprecedented in over a thousand years," conclude the paper's authors.
Of course, we want to create better records that better capture the dynamics of what is happening and, if possible, allow us to measure the actual strengths of the currents more directly. It is also important to emphasize that this does not necessarily mean a sudden, radical change to an entirely new climate. In Europe, ocean currents may warm a little less, but due to rising atmospheric temperatures, it will also warm much more. However, the decline in this current will have far-reaching effects on both the land surrounding the North Atlantic and the ecosystems it contains. So getting more data should be a high priority.
Nature Geoscience, 2021. DOI: 10.1038 / s41561-021-00699-z (About DOIs).