The news is full of record high temperatures. Phoenix broke 110 degrees for 31 straight days. July 3 was the hottest day on Earth since record keeping began, until July 4, then July 6, and then July as a whole. It is anomalously hot—at least if you look at air temperatures 2 meters (about 6 and 1/2 feet) above the surface, which is how it's usually reported by government agencies. But if you look a little more broadly, the heat is not an anomaly—and that's an even bigger problem.
As earth system scientists, we've learned it's sometimes more helpful to look at Earth as, well, a system. In this case, the system of the air and the oceans. Understanding how they interact is the key to understanding what is, and what isn’t, unusual about this very hot year.
First, the basics. As we burn fossil fuels to power our cars, our factories, and our lives, the amount of carbon dioxide in the air climbs. It’s up about 30% since we were born in the early 1970s. Carbon dioxide traps energy that would otherwise escape into space, so with each incremental increase in carbon dioxide, the Earth System warms incrementally, by about 2F since the industrial revolution.
But that’s just the air. It turns out that the bigger story is in the oceans, which have absorbed 90% of all that extra energy. And that’s a lot. If you add up all the energy human society has used since the 1950s—all the oil, gas, and coal burned; all the planes, trains, and automobiles, all the nuclear power plants, all the wind, solar, hydro-electricity, and all the biomass burned—that unimaginably vast amount of energy is still 10 times less than the amount our greenhouse gas emissions have captured and forced into the ocean during that same time.
All that energy has warmed the upper ocean waters by about 1.5F, causing ocean heat waves, coral death, and sea level rise (water expands when it warms). But despite these dramatic effects, we should all be very grateful for the energy oceans have absorbed. If all that energy went into heating the air, rather than into heating the oceans, the surface of the Earth would be hot enough to boil water.
But that ocean energy can come back to haunt us, and we’re getting a taste of that now. Every five years or so, during an El Niño, warm surface waters get trapped in the equatorial Pacific along the coast of South America and spread westward, releasing heat into the air and scorching marine ecosystems in their wake. The Atlantic Ocean has a similar phenomenon, but with a different cadence. This year those warming rhythms are in sync, so we’re getting a double warming whammy.
El Niño helped produce record temperatures in 1998, again in 2016. In 2016, ocean heat waves killed off about 20% of the corals on the Great Barrier Reef, which stretches along the coast of Australia for 1,400 miles (the same as San Diego to Vancouver Canada). One decimated coral around Palau, one of the most remote islands in the world. Our colleague tells of weeping into her scuba mask in the face of the underwater death scene. On land, the 2016 El Niño-driven drought and fire killed billions of trees across the worst-affected parts of the Amazon rainforest. Ocean heat can drive our climate system to extremes around the world.
But here's the scary part. The “hottest year on record” in 1998 wasn’t as hot as our cool years are now. 2023 is making 2016 look cool. And by 2040 this year’s record highs will feel like an anomalously cool year. That’s because the amount of energy trapped in the oceans and the air combined, is only going up, and up, and up. Whether the air or ocean has a bit more of that energy almost doesn’t matter in the grand scheme of things, though it matters for air temperatures in a given year.
There are other rhythms in the ocean that we need to pay attention to if we want to understand our warming world. Ocean currents act like giant conveyor belts that shuttle heat around Earth’s surface. Part of one such current, the Gulf Stream, moves warm surface water from the equatorial Atlantic north to northern Europe. This keeps the British Isles and northern Europe relatively mild, since those warm waters give off heat to the northern air. That’s why London, England is much warmer than Newfoundland, Canada even though London is further north. As the surface waters lose heat to the air, some cool, sink and return south at great depths in a “meridional overturning.”
This overturning circulation is also threatened by climate change. Fresh water pouring off melting Greenland glaciers makes it difficult for the water to sink, and recent work suggests we may be close to a tipping point where the giant Atlantic Ocean overturning slows down substantially. If that happens it will, paradoxically, slow or even temporarily stop warming in the North Atlantic, since that region will lose its source of ocean heat. More heat will be trapped at low latitudes, so warming will speed up there. Once again, our fate is intimately tied to the fate of our warming oceans.
So, is this year anomalously hot? The answer is a little bit more complicated than a simple “yes” or “no.” Yes, the air and surface ocean are both much hotter because of the rhythms of ocean circulation. But in another way, the whole Earth system has been warming steadily in proportion to the greenhouse gasses we put into the air. It will only stop getting warmer when we stop burning fossil fuels. Until then, we are rapidly warming ourselves into a climate catastrophe.
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