2023 was the warmest in Earth’s recorded history, and perhaps in the past 100,000 years, breaking the previous record set in 2016 by 0.27°C (0.49°F). According to recent data from NOAA’s National Center for Environmental Information, 2024 is likely to be warmer than 2023.
Scientists are sounding the alarm because this warming is shockingly large, greater than we had expected given the long-term warming trend caused by climate change driven by fossil fuels. But why were 2023 and 2024 so warm?
The warming temperatures in 2016 were driven by a strong El Niño – a naturally occurring cycle in the Earth’s climate system – which typically leads to a rise in the global average temperature. That year, the El Niño phenomenon increased global warming caused by the accumulation of greenhouse emissions in the atmosphere, sending temperatures to record levels.
That’s why 2023 and 2024 are so worrying: El Niño was only moderately strong (contributing a small amount to warming) in 2023 and was neutral for most of 2024 (contributing almost no warming), so we can’t To attribute the record breaking heat in 2023 and 2024 to the El Niño phenomenon as we did in 2016, we certainly should not break heat records under the condition El Niño neutral. In other words, 2023 and 2024 were hotter than scientists expected.
What is a possible explanation for the record-breaking warmth? This question was the focus at the 2024 American Geophysical Union (AGU) Annual Meeting in Washington, D.C., where more than 30,000 scientists gathered to present their latest research. The two main theories to explain the record warming are a decrease in small particles in the atmosphere called aerosols due to shipping fuel regulations that reduced sulfur oxide (SOx) emissions, or a decrease in cloud cover. Before we get to these two potential culprits, let’s talk about whiteness.
What is whiteness?
An important concept in both theories is planetary albedo. Albedo is the total reflection of solar radiation coming from the Earth. This reflection is caused partly by light-colored surfaces such as ice sheets, ice shelves, clouds and deserts, and also by aerosols. Think about walking outside on a sunny day after a snowstorm or in the desert; Sunlight reflects off the surface of the light, making it more difficult to see. These solar rays are usually reflected back into space.
The planet typically reflects about 30% of incoming solar radiation, but this number can change slightly depending on the amount of snow or ice cover present, on the amount of cloud cover, or on the number of aerosols present in the atmosphere (remember, which are small atmospheric particles that reflect… light). Humans have a direct effect on albedo through the emission of artificial aerosols such as sulfates, which accumulate in the atmosphere due to the burning of fossil fuels.
You may be wondering: “If burning fossil fuels increases the Earth’s albedo due to additional aerosols in the atmosphere, shouldn’t this offset any effect from increased greenhouse emissions such as carbon dioxide?” It’s a great question, but the warming effect of heat-trapping gases far outweighs the cooling effect of synthetic aerosols.
Reduced aerosol results in decreased albedo
One popular theory that could explain why temperatures have risen in the past two years relates to a change in aerosol emissions due to new regulations on shipping fuels that are intended to help tackle pollution that harms health and the environment. In 2020, the International Maritime Organization (IMO) lowered the maximum amount of sulfur in bunker oil, which subsequently led to a decrease in sulfur dioxide and emissions of particulate matter that forms aerosols into the atmosphere.
According to two recent studies, this decrease in aerosols may have led to a rise in the average global temperature. how? As artificial aerosols decreased due to this new regulation, especially over the North Atlantic, the planet’s albedo decreased slightly, meaning that the planet absorbed more incoming solar radiation rather than reflecting it.
However, another study found that the impact of additional artificial aerosols in the atmosphere would only affect the average global temperature by a few hundred degrees, instead of the 0.27°C recorded in 2023.
Of course, continuing to strengthen public health-based standards to reduce harmful air pollution caused by burning fossil fuels, including sulfur dioxide and particulate matter, is essential and life-saving. More scientific work is being done to help advance our understanding of how and to what extent this contributes to changes in synthetic aerosols and how this could affect the rate of warming. At the same time, sharply reducing our use of fossil fuels is the best way to reduce carbon dioxide (CO2).2) Emissions, the main driver of climate change.
Decreasing cloud cover due to rising temperature creates more warming
A study released last month, and another preprint of a study presented at the Arabian Gulf University, offer a different explanation for the rise in global average temperatures: clouds. In this case, the authors of both papers argue that decreased cloud cover led to decreased planetary albedo.
Over the past few decades, there has been a marked decline in total planetary cloud cover, especially over the North Atlantic Ocean off the northeastern coast of the United States. Here, low-level cloud cover has declined dramatically, much of it associated with increased ocean surface temperature.
The decline in low-level cloud cover due to warming ocean surfaces is particularly worrying because the process could be a manifestation of a feedback associated with global warming: as the oceans warm, low-level cloud cover decreases, causing the planet’s albedo to decrease . Hence, the world’s temperature is rising faster.
It is also possible that ocean surface temperatures in the North Atlantic could become much warmer due to a weakening of the Atlantic Overturning Circulation (AMOC), which I blogged about in November. While ocean surface temperatures are increasing globally due to climate change caused by fossil fuels, they are rising faster off the northeastern coast of the United States, which may be a result of the slowing of the AMOC and the accumulation of warm water in the region.
Important questions remain to be resolved
Climate scientists are still trying to figure out why 2023 and 2024 will be so warm. We have discussed some possible reasons that could explain the significant rise in temperatures, but the details have not yet been resolved.
What’s interesting is that the sudden warming may also be due to a combination of the two theories.
Did you know that for water droplets in the atmosphere to form clouds, they need a small particle to condense on? These small particles are called cloud condensate nuclei (CCN), and one type of CCN is industrial aerosols such as sulfates. After the IMO reduced sulfur in fuel oil shipping, a reduction in sulfate aerosols in the atmosphere could have reduced the CCN available for cloud droplet formation, leading to a lower planetary albedo.
And perhaps to add another layer to everything, during my doctoral research I studied how different patterns of ocean surface temperature affect the rate of global warming. For example, if the western Pacific warms more than the rest of the world, the planet will actually warm at a slower rate than if that warming were distributed evenly across the ocean surface.
We find that the most likely pattern of ocean surface warming in the near future, currently developing, will result in the planet warming rapidly. This may be part of the puzzle as to why 2023 and 2024 will be so warm.
Scientists continue to sound the alarm
Additional warming in 2023 and 2024 adds a layer of complexity to climate change caused by fossil fuels (not the kind of complexity we want, given that the planet appears to be warming faster than ever before). Both of these ideas from the scientific community are still being debated, and it will take some time to understand the exact reason behind this rise in average global temperatures.
One thing is certain: the planet’s warming is mainly due to increased greenhouse gas emissions into the Earth’s atmosphere from the burning of fossil fuels. The only way to slow global warming is to reduce said emissions through a rapid and equitable transition to clean, renewable energy.
