This is part 2 of a detailed look at the factors that helped make 2023 the warmest year on record, and arguably one of the weirdest too. For part 1, go here.

Last year offered perhaps the clearest warning we’ve ever received that we need to stop polluting the atmosphere with carbon dioxide and other climate-altering greenhouse gases.

As I mentioned in part 1 of this series, 2023 shattered the previous record for warmest year in observations dating back to the 1800s. Every month from June through December 2023 came in as the hottest month on record, with July ranking as the hottest month ever recorded, according to NASA. There was also outright weirdness — including a deranged jet stream with a wavy, loopy and swirling pattern that helped lock brutal heat domes in place.

The long-term driver has been our emissions of greenhouse gases, with the still ongoing El Niño episode helping to push temperatures up even more. But there’s more to the story of 2023, as the next series of visuals will show.

A Very High Oceanic Fever

Visual #5

As the graph above shows, sea surface temperatures have been rising since the 1970s. No surprise there. But if you look to the extreme upper right of the graph, you’ll see something quite unusual: 2023 blew away the record for warmest sea surface temperatures globally.

Drilling down from the overall global picture of sizzling seas, the next graphic illustrates another extraordinary aspect of 2023: marine heat waves.

Visual #6

These are periods of persistent abnormally warm ocean temperatures in particular areas of the globe. The heat waves harm marine life as well as coastal communities and economies.

The map above shows large areas of extraordinary warmth in the North Atlantic and North Pacific on July 30, 2023. Overall, marine heatwaves were unusually common and severe during the year just past.

And this brings us back to the long-term driver of climate change: human activities.

Lucky for us, over the long run the oceans have been absorbing about 90 percent of the heat resulting from our emissions of greenhouse gases. The following graph of ocean heat content over time shows the result.

Visual #7

Last year saw a significant increase in the amount of heat stored in the upper 2,000 meters of oceans. According to a recently published study, the increase amounted to 15 zettajoules in 2023 compared with that stored in 2022.

To understand just how much energy that is, consider that 1 nutritional calorie equals 4,184 joules. And 1,000,000,000,000,000,000,000 joules equals one zettajoule.

If your head is spinning, here’s another way to look at it: Through our burning of fossil fuels, we humans consumed 0.6 zettajoules of energy in all of 2022. That’s just 4 percent of the increase in heat experienced by the oceans last year. Chalk it up to the enormous power of greenhouse gases to keep incoming solar energy from being reradiated out into space.

As I mentioned, we’ve been lucky that the oceans have been absorbing so much heat. Otherwise, global warming and climate change would be staggeringly worse. But here’s the thing: Everything that goes in to the ocean doesn’t just stay in the ocean. As we learned from the animation in part 1 of this series showing the large, rising blob of warm water in the Pacific, some of the ocean’s deep heat ultimately is emitted at the surface into the atmosphere. And this is another factor that contributed to 2023’s record high temperatures.

Pollution Declines — With an Unfortunate Side Effect

While we still haven’t managed to drive global emissions of greenhouse gases down just yet, the world has been succeeding in scrubbing the skies of another form of pollution: aerosols. There are many sources of these tiny particles, which together can shade Earth’s surface like an umbrella, reducing solar heating and thereby helping to offset some of the warming impact of our greenhouse gas emissions. This brings us to…

Visual #8

Regulation of air pollution has helped reduce aerosol pollution. And in 2020, new international rules aimed at reducing the crud spewing from ship smokestacks — specifically, sulfur emissions from burning fuel — took effect.

These rules abruptly reduced the emissions from large ships by about 85 percent, according to a report from Berkeley Earth, an independent non-profit organization focused on environmental data science and analysis. “This change was made to preserve human health, due to the toxic nature of sulfur aerosols,” the report noted. “However, such aerosols also reflect sunlight, and as a result have a cooling effect.”

So with less of an aerosol umbrella, the oceans were less shaded, and temperatures should have gotten a boost as a result — particularly in the heavily trafficked northern shipping lanes, where one analysis showed an increase of about 0.2 degrees C, or 0.36 F.

This is potentially significant for that region. But globally the impact is small. “The reduction in marine sulfur aerosols from shipping, though regionally significant in areas with high shipping volumes, has likely only added a few hundredths of a degree to the global average temperature,” according to the Berkeley Earth report.

A Staggering Volcanic Eruption

The huge eruption of the Hunga Tonga volcano in the southern Pacific Ocean in early 2022 also could have affected the climate this past year. Large, explosive volcanic eruptions can spew massive amounts sulfur into the atmosphere, where they combine with water to create reflective aerosol particles. Once again, the umbrella effect typically cools the planet somewhat. But the Hunga Tonga eruption was atypical.

You can get a sense of the enormous power of the eruption in this amazing animation of satellite images:

Visual #9

Hunga Tonga is a seamount — a volcano under the water. Its eruption on Jan. 15, 2022 produced the biggest atmospheric explosion ever recorded by modern instruments — much more powerful than nuclear bombs tested after World War II, according to the BBC. It blasted some 351 billion cubic feet of rock, ash, sediment and other materials high into the atmosphere. There’s even evidence that the plume made it all the way into space.

The plumes from large volcanic explosions typically contain a lot of sulfur — the same culprit that spews from ship smokestacks. When it interacts with water, the sulfur form reflective, climate-cooling aerosols. But unlike most volcanoes, Hunga Tonga’s eruption was relatively rich in water vapor while low in sulfur.

In fact, Hunga Tonga injected 150 million tons of water vapor into the stratosphere, boosting its concentration there by an impressive 15 percent, according to Berkeley Earth. Water vapor is a very potent greenhouse gas, so it could have contributed to 2023’s warmth — and might continue to do so for several years.

But to what degree are the eruption’s cooling sulfur aerosols counteracting any warming, and what is the net effect?

Answering those questions has been scientifically challenging. But whatever the answer, the effect is likely to be small compared to our own climate impact. Even so, there is evidence that over the next few years the eruption will make it somewhat more likely that Earth will cross an important threshold: the 1.5 degrees C of warming over pre-industrial times that nearly all nations of the world have agreed to avoid as part of the Paris climate agreement.

What About the Sun?

Visual #10

Last but not least, we come to the Sun — our planet’s primary life-giving source of energy.

“The Sun may seem to shine at a constant rate, but it is a seething, churning ball of plasma whose radiating energy changes over many different time scales,” says Michael Wysession, Professor of Earth, Environmental, and Planetary Sciences at Washington University, writing in The Conversation.

The time scale most relevant to us is an ~11-year-cycle during which the Sun’s activity waxes and wanes. Since the minimum point in 2020 of the last solar cycle, the Sun’s activity has been on the upswing. Recent evidence shows that the rise has been quicker than expected, and that the Sun is heading toward a higher peak of activity, sooner than previously predicted.

When the Sun’s activity naturally increases like this, it emanates more energy towards Earth, and that in turn helps to warm the planet. How much could this have contributed to 2023’s record-breaking warmth and arresting weirdness?

During a solar maximum, the Earth warms by 0.05 degrees C, or 0.9 F. That’s roughly a third of the impact of a strong El Niño, according to Wysession. And while that’s something scientists must take into account, it’s quite small compared to our own influence on the climate.

The Final Take-Away

As we’ve seen, quite a few factors interacting in complex ways can help send global temperatures into record-breaking territory during any particular year. And for about half of last year, El Niño started pushing temperatures up. But its most significant impact is yet to come. Other factors, like volcanic eruptions, reductions in aerosol pollution, and an increase in solar activity, also have their impacts. But they are relatively small — compared to us.

Over the long run, there’s no denying it (even though many people still will): We dominate all the other factors.

Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, puts it this way:

“We are very interested in the weather and extremes of any particular year because those are the things that impact us. But the key difference between this decade and the ones before is that the temperatures keep rising because of our activities, principally the burning of fossil fuels.”

Until we stop doing that, temperatures will continue to rise, and global weirding will get even stranger.