As discussed in our previous post, ocean temperatures today are higher than they have been in thousands of years, and potentially even higher than in several million years. This is catastrophic for many marine ecosystems, such as coral reefs. When corals are exposed to excessive warmth for prolonged periods, they bleach and die. Coral reefs are often referred to as the "tropical rainforests of the oceans" due to their high biodiversity. Their deterioration not only threatens ocean biodiversity, but also the sustenance of over one billion people who rely on them for food.
So, why are the oceans so warm today? There are two well-understood reasons for this record-breaking ocean warmth, and one intriguing hypothesis that some scientists are currently floating as an additional contributor to the warming. Let's go through each one.
Greenhouse Gases
The first reason, of course, is the buildup of greenhouse gases in the Earth's atmosphere, brought on by the continuously increasing burning of fossil fuels since the industrial revolution. Carbon dioxide is the most well-known greenhouse gas and is responsible for most warming since the start of the industrial revolution, but it's not the only one.
Other major greenhouse gases include methane (released from agriculture, particularly livestock, and fugitive emissions from petrochemical drilling), nitrous oxide (also from agriculture), and halocarbons (used as refrigerants and for other industrial applications). Land use changes have also released a lot of extra carbon dioxide into the atmosphere: as forests are turned into farmland, biomass is broken down and released into the atmosphere as CO2. The buildup of greenhouse gases in the atmosphere has caused a steady increase in global temperatures, warming the planet by 1.5°C (3.2°F) over the past century. However, the temperature increase has not been consistent due to the counteracting effect of industrial aerosols, which cool the planet.
Natural Variability
The second reason that the oceans are especially warm this year is because of something Earth system scientists call natural variability. If you look at the temperature of the Earth over time, you'll notice that it's not a smooth line, but a rough one. There are a lot of year-to-year ups and downs in the temperature record. This volatility is not due to year-to-year changes in greenhouse gases or changes in the brightness of the sun. Rather, it is due to ups and downs generated within the Earth system itself.
Some of these ups and downs are due to sudden physical events. Volcanic eruptions, for example, are the primary type of sudden geological event that can cause multi-year bouts of cooling in the Earth's climate system, since large volcanoes can inject reflective dust particles into the stratosphere (known as sulfate aerosols) which effectively block out some of the energy from the sun for a couple of years, thereby cooling the ocean surface. Cooler than average ocean temperatures due to a major volcanic eruption, such as Mt. Pinatubo in 1991, can persist for up to a decade.
However, other periods of multi-year warming or cooling occur due to 'internal modes of variability' within the Earth system. These are interactions between the atmosphere and ocean that cause year-to-year fluctuations, independent of external factors. One of these internal modes of variability is El Niño, known formally as ENSO, the El Niño Southern Oscillation. Interactions between the atmosphere and ocean in the tropical Pacific can cause a huge section of the tropical East Pacific to warm up for several years, disrupting fisheries off the coast of Peru, and sending giant Rossby waves through the atmosphere that give rise to large-scale teleconnections around the globe. Sometimes, the tropical Pacific is colder than normal for several years, which is known as La Niña. Just like El Niño, La Niña also impacts weather around the globe through large-scale teleconnections. The seesaw between El Niño and La Niña conditions is known as ENSO, which is the most important mode of variability in the Earth system.
This year has been strikingly hot, not just because greenhouse gases have built up in the atmosphere, but because we've had what's known as a Super El Niño. From 2019 to early 2023, the tropical Pacific was cooler than normal, and we were experiencing La Niña conditions. To some extent, this La Niña masked some of the warming due to extra greenhouse gases – we didn't really have any record-breaking warm years during that time, even though greenhouse gases were still increasing over that period. But with the Super El Niño that's developed over the past year, it's not just the tropical Pacific Ocean that's warm, but also the North and South Pacific Oceans, and even the Atlantic Ocean. The last time that we've seen such a Super El Niño event was in 1997, when we also had a record warm year (for that time; note that it wouldn't be a record hot year by today's standards, thanks to the extra buildup of greenhouse gases over the last 30 years leading to today). This year, the Super El Niño unmasks greenhouse gas warming hidden by La Niña, and significantly adds to it through El Niño-associated warming over the global oceans. No wonder we're seeing record-breaking temperatures in our oceans and around the world *.
Sulfate Emissions Reduction
Finally, we get to the most intriguing (and least certain) reason why global ocean temperatures are so warm today. In 2020, a major piece of international legislation by the International Maritime Organization (IMO) came into effect, which decreased the amount of sulfur permitted in ship fuel from 3.5% to 0.5%. Consequently, there has been an approximately 80% reduction in sulfate emissions from shipping. This decrease in sulfate emissions has lowered the reflectivity of shipping tracks – the bright cloud trails formed by the massive container ships traversing the world's oceans.
Source: Carbon Brief
For decades, scientists had been studying the effects of sulfate aerosols created from combustion of fuels and other industrial processes. These sulfate aerosols are known to cool the planet, but it's not clear exactly how much they cool the planet. The drop in sulfate aerosols from container ships is a real-world 'experiment' that can provide some insight into this aerosol-driven cooling. While many of our Earth system models suggest that cooling due to sulfate aerosols from shipping emissions is very small (< 0.1°C), some eminent scientists contend that the effects are much larger and may be a significant contributor to the record-breaking warm ocean temperatures we're seeing this year. Ironically, even as we clean our air of industrial aerosol emissions, which are known to lead to a variety of health maladies, decreased quality of life, and shorter lifespans, we may also be contributing to even more rapid warming of our planet.
Over the last year, the planet's oceans have certainly put on a show – temperatures warmer than anything we've seen before, since humans have started measuring, and very likely warmer than anything that the planet has experienced for millennia. As we transition to this warmer, more volatile world, we have two crucial tasks to commit to: decreasing emissions to stabilize global temperatures, and devising ways to adapt to this new world.
