Ocean salinity is not a topic people often think about when it comes to climate change. After all why in the world would the salinity of the oceans change just because human produced greenhouse gases like carbon dioxide are accumulating in the atmosphere and the ocean? There’s a simple answer to this question though and I explained it in a detailed blog post about six years ago. You can read it at your leisure ( https://waterblogger.org/general-information/ocean-salinity-climate-change/  ). The short answer for the sake of brevity in this post is that greenhouse gases are causing the atmosphere to warm and warm air holds more moisture resulting in increased rainfall. If the amount of rain exceeds the amount of evaporation then a body of water becomes fresher and if there’s more evaporation than rain the water becomes more saline.

 This is all measurable and scientists have been measuring atmospheric water vapor since the 1970’s. It is raining more over the Pacific Ocean where there is already a lot of warmth with accompanying atmospheric moisture and it is raining less in the mid latitudes of the Atlantic Ocean. The Pacific Ocean is becoming less salty and the Atlantic Ocean saltier. Scientists in fact have been measuring the very dickens out of the oceans saline content. There are measurements through the Soil Moisture and Ocean Salinity (SMOS) satellite system and ARGO’s 2000 plus ocean based floating instruments. The latest 2021 International Panel on Climate Change report entitled Climate Change: The Physical Science Basis indicates that between 1950 and 2019 trends in near surface ocean waters have shown strengthening of the contrast between high and low salinity areas of the ocean. The report quantifies the increase in contrast as 0.14 parts per thousand. This is up slightly from the 0.13 parts per thousand reported in the 2013 IPCC report of the same name.

So what if some parts of the ocean are getting saltier and others are getting fresher? Who cares? Well scientists are concerned that changes in salinity will affect the density of ocean water as higher saline content results in denser water while fresher water is less dense. Ocean density is already being impacted by temperature changes. Cold water is denser than warm water. Certain deep water circulation patterns in the oceans are dependent on dense surface water sinking in the polar regions, for example the Atlantic Meridional Overturning Circulation (AMOC). AMOC brings warm surface water from the equatorial region of the Atlantic Ocean to the northern Atlantic where it cools, sinks and is transported back towards the equator. It’s like a big conveyor belt in the ocean. Scientists say that AMOC impacts the climate of the Northern Hemisphere but they seem quite uncertain as to how. They suspect it keeps conditions a lot warmer in the Northern Hemisphere than it would otherwise be. But all they really know is what is measurable and what is measurable is that AMOC is weakening. This makes sense if polar waters are warming and denser saline waters in the middle latitudes of the Atlantic are not being transported to the poles breaking the chain of the Atlantic Ocean circulation. If you live on the east coast of the United States you are probably familiar with AMOC as it includes the Gulf Stream which moves warm water from the Gulf of Mexico along the coast of the United States and Canada then on up to the Northern Scandinavian countries. Scientists are also uncertain as to what exactly is going to be the result of a weakening AMOC. There is of course a lot of speculation by researchers often based on what occurred in the geologic past when AMOC weakened. The IPCC report though does not take a position on the veracity of the various studies purporting to know exactly what happened when AMOC weakened in the past nor are they even a hundred percent sure if the weakening of AMOC is being caused by climate change. The IPCC report says that the record is too short to determine if weakening of AMOC is natural variability or driven by climate change. So I won’t speculate either, I’ll leave that to the popular press.