365 Days of Climate Awareness 96 – Ocean Surface Salinity Changes

Salinity is one of the basic aspects of the ocean which we track, for a number of reasons. Salt content affects the water's density, which affects its behavior in the thermohaline (density) current system. In some cases it can affect ecosystems. And, being a function primarily of evaporation and precipitation, it's a useful measure in climate change. Increased precipitation (rain and snow) lowers surface salinity; evaporation increases it.

Salt content in the ocean is effectively constant, since it does not evaporate and the input from rivers is insignificant by comparison. It's estimated that there are 5x1016 (50 quadrillion) kilograms of salt in the ocean,while rivers discharge about 3.6x1012 kg of dissolved solids (i.e. salts) into the ocean each year. At that rate, it will take rivers nearly 14,000 years to double the salt content of the ocean.

We assume also that salt is well-mixed,meaning that it is evenly distributed around the global ocean. Local differences in salinity are contained within water parcels which either move or remain in one location, and exchange salt with their surroundings at their boundaries. There are no major point sources, as there are for pollutants like ozone and sulfur dioxide, which cause large-scale gradients.

We infer meteorological causes for the salinity gradients we do see, and over the longer term (>25 years), climatic causes. The IPCC has concluded in AR6 V1 that anthropogenic climate change has changed global precipitation patterns, on balance increasing overland rain and snow rates, and,with less confidence, over the ocean as well.

The plot shows this. In regions of the ocean already known for heavy rain, such as the equatorial band across the Pacific and in the northeastern Pacific near Alaska and Canada, precipitation rates have increased over the past seven decades. Meanwhile, east of major land masses—i.e. in the continental rain shadows—precipitation has decreased, due to the greater rainfall over land.

Large-scale weather pattern changes like this are, along with mean temperature increases and sea level rise, one of the top-tier changes effected by global warming. We will examine many aspects of weather pattern change over the remainder of the series.

Tomorrow: global precipitation anomalies.

Be brave, and be well.