A Warming Atmosphere Means A Warming Ocean

A study published this month in Advances in Atmospheric Sciences states that the world’s oceans set new record high temperatures last year. Why is this happening? Because heat-trapping greenhouse gasses (GHGs) emitted from human sources have raised the atmosphere’s mean temperature by 1.1 Celsius (almost 2 Fahrenheit) degrees since the Industrial Revolution, and there appears to be no end in sight to this rise. And because more than 70% of the Earth’s surface is the ocean, the heat increase in the atmosphere is bound to alter the temperature of the ocean surface.

You can test this hypothesis yourself on warm days next to bodies of water where the shallows warm rapidly because of the interaction between warmer air and surface water. How much of the GHGs heat-trapping gets absorbed by the oceans of the world? The number is an astounding 90% and since 1958, the year the first comprehensive studies began, ocean temperatures have been rising. And since 1990, the rise is accelerating.

When ocean surface waters get warmer, a number of phenomena occur.

1. Ocean heat contributes to extreme weather event increases. Hence more intense tropical storms and rain contribute to flooding (see California over the past few weeks).
2. Ocean heat causes thermal expansion which raises sea levels contributing to increases in surface flooding and seawater intrusion into inland aquifers in low-lying coastal areas.

The study notes that there is “an energy imbalance in the earth’s climate system” brought on by anthropogenic GHGs. It goes on to describe the ocean-atmosphere interaction creating “thermal inertia to sea surface temperatures” that exert “considerable control over the world’s weather.” The end result is more water vapour in the atmosphere. That means changes to precipitation patterns, salinity and surface water density.

The more ocean water warms the more evaporation occurs. More evaporation means increases in ocean salt content. At the same time, more evaporation causes rainfall which dilutes surface water making it less saline. These more and less saline anomalies are becoming more noticeable, and although ocean circulation tends to even things out, nonetheless, the salinity variability in ocean surface waters is on the increase.

The combination of warmer temperatures and increased salinity is impacting the natural stratification pattern within the ocean water column that runs from the surface to the seafloor. The vertical movement of water from the surface to the sea bottom provides a critical exchange for carbon, oxygen, food, and other important factors contributing to the health of ocean ecosystems and the stabilizing of Earth’s climate. The study reports that since 1958 the upper 2,000 metres (approximately 6,500 feet) of the ocean has seen a significant rise in temperature.

There are regional hotspots.

• The Indian Ocean has been showing rapid warming since 2000.
• The tropical Atlantic Ocean where hurricanes originate is also warming faster than the global average.
• The same is true of the North Atlantic since the late 1950s.
• The Blob, a recent phenomenon that occurred in the North Pacific Ocean is an indicator of large-scale persistent warming leading in recent years to intensive heat waves in Western North America. The Blob has also caused areas of the North Pacific to suffer from deoxygenation in the water column threatening marine life.
• Even the Southern Ocean that rings Antarctica has exhibited a warming trend since the 1950s.

In its conclusions, the study states, “The global long-term warming trend is so steady and robust that annual records continue to be set with each new year.” The rise is creating further imbalances in the world’s climate let alone stressing ocean health through changes in salinity, stratification imbalances, and oxygen exchange with implications for sea life.

This is an unprecedented study with contributions from researchers at universities and within the national administrative bodies of their respective countries responsible for studying the ocean and atmosphere. Four nations contributed to the data gathering and analysis, China, the United States, New Zealand, and Italy. Data collected came from the World Ocean Database supported by the Argo network, SeaDataNet, and remote sensing satellites.