Blog 18.08.23

Ocean Heat Content, The Unsung Climate Indicator

A broiling summer on Earth has brought about the hottest June and July on record. Greece, Italy, Spain, and Britain have endured extreme temperatures on the heels of 2022 when heat took the lives of more than 61,000 across Europe. Canada has seen a staggering 25 million acres burn due to wildfires. Heat in Hawaii wrought extreme devastation to Lahaina, where, as of the publication of this blog, more than 100 have died from wildfire. The circumstances on land are a concrete manifestation — experiential learning — of too much CO2 in the atmosphere.

The proof in the pudding isn’t only staring at us from headlines or the thermometer at the kitchen window. The ocean is telling us plenty, too. Big messages about climate change lie beneath the surface, and they’re less commonly known. Sticking your toe in surf won’t paint the picture either. However, delving a little into the science about ocean heat can explain a lot about this feature that covers 70% of the planet.

Studies of Ocean Heat Content

What is the ocean telling us? According to those who study ocean heat, a continuously increasing and non-decaying global ocean warming has been observed by multiple techniques for many years. More heat has been absorbed in the subsurface and deeper ocean over the past 29 years in an upward trend. It’s the “non-decaying” part that should have us sitting up.

Though the sheer size of the ocean is important, we need to pay attention to its behavior, and the ocean behaves differently than air. Air and saltwater obviously have different characteristics — particularly when it comes to heat transference. Ocean warmth doesn’t dissipate as quickly as heat in the atmosphere. That’s the case even if we reduce greenhouse gas (GHG) emissions. Due to past GHG emissions, many changes to the ocean, including melting ice sheets and rising sea levels, are already irreversible or would take centuries to millennia to unwind.

“The response of the oceans to increased atmospheric greenhouse gases has been slow and lagging, and impacts such as ocean warming from past carbon emissions will persist for several hundred years,” according to a new study of ocean heat content in the journal Remote Sensing. “If humans achieve net-zero CO2 emissions soon, some of the impacts of climate change may be reversible. However, changes in the oceans, ice caps, and global sea levels will be irreversible for thousands of years.”

Historical Ocean Data

Our understanding of the ocean comes from years of historical data gathered by increasingly sophisticated observation methods. Sailors and scientists have been sampling ocean temperatures for more than a century. Buckets dropped to the sea surface contributed to nearly half of all sea surface temperature measurements up until the late 1970s. We’ve even taken samples from pinnipeds (seals) by humanely supplying them with what we might think of as a smart watch, so that when they dive, they collect information for us. Technology has advanced to buoys, satellites, and other more precise methods, which support today’s research and establish more robust baseline data over consecutive years.

Temperature observations today are categorized by ocean depth. Each quarter, NOAA analyzes ocean heat content (OHC) and reports out, giving insights to surface temperatures and those in much deeper parts of the ocean. These reports may be the most under-appreciated climate updates in terms of the public; they receive little fanfare or attention. Unfortunately, rising heat in the ocean isn’t showing any signs of abating. Global OHC for January-March 2023 was the highest January-March on record, dating back to 1955.

Two side-by-side line charts showing the precipitous rise in ocean heat at 700 meters and to depth of 2000 meters between the 1960s through March 2023.

Each quarter, a new OHC report is released by the National Oceanic and Atmospheric Administration (NOAA), reporting on heat in the top 700 meters of the ocean and to a depth of 2,000 meters.

Researching the Ocean’s Future

Using this data, researchers have come closer to explaining how ocean and atmospheric heat are connected. Scientists call it an “Earth energy imbalance,” or EEI, in which the ocean steps in to take the punch of excess heat trapped in the atmosphere (thanks, ocean!). As a carbon sink, the ocean absorbs more than 90% of the heat in the atmosphere that has nowhere else to go.

Unfortunately, once absorbed, we must cope with the fact that it also stays there a lot longer than is helpful. Understanding these phenomena has researchers scrutinizing what ocean heat means for our future. Resources are being prioritized, especially for those on the frontlines, such as coastal communities. Knowing where we stand, toes in or out of the water, could make a difference.

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