An apparent weakening of one of the world’s most important ocean currents is probably a measurement error caused by a shift in the geomagnetic field, a new study proposes. If correct, it would mean we face less danger from one of the most alarming, but also most uncertain, consequences of global heating.
The Atlantic Meridional Overturning Circulation (AMOC) helps bring warm waters from the equator to the North Atlantic, contributing to Europe’s benign climate for its latitude. It’s also thought to be the initiator of the global thermohaline circulation, which plays a key role in stabilizing temperatures worldwide. There are fears that rising global temperatures could disrupt the AMOC, probably through the release of more meltwater from Greenland. However, unlike many other aspects of the science of climate change, where relevant experts are overwhelmingly in agreement, this threat remains very contentious.
The Florida Current is a key part of the AMOC, taking water disgorged by the Gulf of Mexico Loop Current and conveying it north along the US east coast. It’s the fastest current in the entire Gulf Stream system and one of the fastest in the world. Since 1982 the strength of the Florida Current has been measured using an approach not available for many counterparts. Submarine cables between Florida and the Bahamas provide an indication of the current’s strength using the voltage their flow induces in the cables.
Previous analysis of these cables has been considered evidence the Florida Current is weakening, a bad sign for the AMOC as a whole.
However, the Earth’s geomagnetic field also affects the voltage in these cables. According to the authors of a new study, failing to take account of changes in the field strength has led to misreading of the current’s strength. Once these changes are taken into account, they report, the weakening of the Florida Current measured by the cables drops to insignificance.
The Florida Current is so speedy (up to 2 meters per second or 4.5 miles per hour) partly because it flows through relatively shallow waters, seldom more than 800 meters (2,600 feet) deep. That makes the cables a more useful measure than in a current traveling through much deeper waters.
The movement of salty (and therefore electrically conductive) water in a magnetic field induces voltage differences, which affect the cables. These differences, averaging 1.25 volts, have been measured on a minute-by-minute basis since 1982, with a 17-month hiatus around the year 2000.
Based on this data, the Florida Current appeared to be carrying 30,000 cubic meters per second (0.3 sverdrups) less each decade. That’s at the border of statistical significance, given uncertainties and fluctuations, but since 2000 the trend has been -0.7 sverdrups per decade, which would be both statistically significant and alarming.
However, over this time the Earth’s magnetic poles have been moving, and this has reduced the vertical component of the electric field in the Florida region. Allowing for this, the reduction in the strength of the current becomes 0.1 sverdrups, well within the margin of error from no change.
The telephone cable between Florida and Grand Bahama develops a voltage across it thanks to the moving charges in a magnetic field.
Image credit: NOAA Atlantic Oceanographic and Meteorological Laboratory
The Florida Current’s movements have also been tracked using measurements from ships and floats, as other currents are. These have generally shown no significant trend in current strength in either direction. However, these measurements often use proxies such as the temperature of the water, whose reliability has been questioned.
Because the Florida Current is only part of the AMOC, the work does not prove the system as a whole is ok. “This study does not refute the potential slowdown of AMOC, it shows that the Florida Current, one of the key components of the AMOC in the subtropical North Atlantic, has remained steady over the more than 40 years of observations,” said Dr Denis Volkov of the University of Miami Rosenstiel School in a statement.
“With the corrected and updated Florida Current transport time series, the negative tendency in the AMOC transport is indeed reduced, but it is not gone completely. The existing observational record is just starting to resolve interdecadal variability, and we need many more years of sustained monitoring to confirm if a long-term AMOC decline is happening.”
Observations of the AMOC generally have produced widely divergent results depending on the methods used and the portion studied. Some report potentially catastrophic slowing, while others appear to record a statistically insignificant strengthening from 2009-2018; other findings fall between these.
Human-induced climate change will have terrible consequences that are well understood by science, but a major weakening of the AMOC is one of a catalog of amplifiers that will make it far worse if they occur. Other examples include conversion of the Amazon rainforest to savanna and largescale release of the methane from the northern permafrost. Any one of these would be catastrophic, but for the moment the likelihood of them occurring in the near future remains unknown.
This research implies that for the AMOC, the uncertainty is even greater than had been recognized.
The study is published open access Nature Communications.
