The Gulf Stream really is weakening, a new study confirms: a finding which has profound implications for one of the biggest weather systems on our planet. When the Gulf Stream changes, so does the climate.

Researchers from the Woods Hole Oceanographic Institution and the University of Miami looked at four decades of data from the Florida Straits, including measurements of the volume of seawater transported through the region in that time.

They found that Gulf Stream transport has decreased by about 4 percent over the past 40 years, which they describe as the first "conclusive, unambiguous observational evidence" of a slowing. Though the underlying reasons weren't assessed as part of the study, there's a 99 percent chance that this weakening isn't a random event, the researchers say.

Florida Straits
The study looked at current flow around Florida. (Piecuch et al., Geophysical Research Letters, 2023)

"This is the strongest, most definitive evidence we have of the weakening of this climatically-relevant ocean current," says physical oceanographer Chris Piecuch, from the Woods Hole Oceanographic Institution.

"While we can definitively say this weakening is happening, we are unable to say to what extent it is related to climate change or whether it is a natural variation."

The Florida Straits are a good showcase for the effects of the Gulf Stream, which flows out from the Gulf of Mexico, around Florida, up the east coast of the US, and then across the Atlantic. The current takes with it warmer water, which then affects temperatures, precipitation, sea level, hurricane activity and more. It also transports nutrients across the ocean, including carbon.

Gulf measurements
Some of the data used was gathered from field recordings. (Paloma Cartwright)

In this case, a complex Bayesian model – one which works out probabilities and uncertainties to high levels of precision – was applied to the data collected from satellite readings, undersea cables, and field recordings.

As well as being a worrying report on the changing state of Earth's climate, the study is also evidence of how important long-term ocean observations are in identifying trends that last over several decades or even longer.

It's now clear that the Gulf Stream is weakening – and that global warming is a likely cause – but we're less sure about what will happen next. The stream and its associated weather patterns are crucial components in the planet's climate, influencing extreme weather events as well as average temperatures and levels of rainfall.

What also needs to be considered is how shifts in climate are going to feed back on themselves, causing further disruption to weather systems. The researchers behind this current study are hopeful that the data analysis they've used can be applied to other regions of the ocean.

"The Gulf Stream is a vital artery of the ocean's circulation, and so the ramifications of its weakening are global," says oceanographer Lisa Beal, from the University of Miami.

"I think there is potential for this technique to extract other climate change signals from among the scattered observations we have in the ocean."

The research has been published in Geophysical Research Letters.