Temporary cooling of part of the Atlantic does not question the existence or severity of global warming

At the end of August 2024, numerous disinformation social media accounts shared misinformation intended to deny the severity of climate change, as we show in this article. Most of these posts shared an article published in the New Scientist, entitled “The Atlantic is cooling at record speed and nobody knows why”. For example, Steve Milloy, who has no background in science and publisher of the conspiracy website junkscience.com, states in a short video on X on August 25: “[…] now scientists say the Atlantic ocean is cooling, and they don’t know why. ”

RAPID COOLING RECORDED FOR SEVERAL WEEKS AT THE EQUATOR

Arthur Prigent, researcher in physical oceanography at the International Centre for Theoretical Physics, commented on a similar claim made in French for Science Feedback: 

The Atlantic cold records in August 2024 relates to two localized phenomena: a cooling of the waters off Nova Scotia and a cooling of the equatorial eastern Atlantic.

So we can see right away that the claim of a cooling of the Atlantic is inaccurate; it’s not the entire Atlantic ocean that has cooled at the end of summer, but only two regions representing a small fraction of it.

The New Scientist article follows an August 14, 2024 article on the climate.gov website by physical oceanography researcher Franz Philip Tuchen (Cooperative Institute for Marine and Atmospheric Studies). In it, Tuchen describes a temporary and localized cooling of the Atlantic: “Much of the North Atlantic has been extremely warm so far this year. In contrast, since the beginning of June, sea surface temperature (SST) in the central equatorial Atlantic has been 0.5-1.0 degrees Celsius (0.9-1.8 degrees Fahrenheit) colder than average for this time of the year.” As stated above, contrary to what Steve Milloy and the title of the New Scientist article claim, it’s not the entire Atlantic Ocean that’s cooling, but mostly a localized region (the central equatorial Atlantic).

Following the spread of misinformation on the subject, Franz Philip Tuchen and his colleagues have published a second article on the climate.gov website. They clarify the location of this “cold pocket”: in the eastern equatorial region of the ocean, as illustrated in Figure 1. They also underline the reduction of cooling during August. “There has been no Atlantic-wide cooling trend in the past couple of months”, the scientists point out on climate.gov.

IN EQUATORIAL ATLANTIC REGION, A CLIMATE NATURALLY DEFINED BY HOT AND COLD SEASONS

Contrary to the claim that “no one knows why”, this temporary cold pocket does not surprise the scientific community, and the phenomenon is well known. The equatorial Atlantic naturally has different seasons. “This part of the Atlantic is following a distinct seasonal cycle with the warmest temperatures observed during March-April (28-29°C) and coldest temperatures during July-August (24.5-25°C)”, Franz Philip Tuchen explained to Science Feedback, “So there is always some natural cooling between April and July.”

And in some years, these seasons are even more marked, which we refer to as Atlantic Niño or Niña. This natural variability in the climate of the equatorial Atlantic has been described in several scientific articles since the 1980s^[2-4]. Specifically, as soon as temperatures in this zone exceed the average by 0.5°C for three months, an Atlantic Niño is declared. Conversely, when temperatures fall by more than 0.5°C below the average for three months, we speak of an Atlantic Niña. In 2024, an Atlantic Niño was observed in spring (see Figure 2). As for this summer’s cooling, it is “less likely this event will make the official cut, given the August weakening”, the scientists wrote.

The “record speed” mentioned in the press refers to the observation of Franz Philip Tuchen: “2024 began with extremely warm sea surface temperatures in the eastern equatorial Atlantic during February to March, when temperatures exceeded 30 degrees Celsius (86 degrees Fahrenheit). This was the strongest warm event since 1982. Equally remarkable was the rapid transition from warm to cold SST [sea surface temperatures] anomalies. Never before in the observed record has the eastern equatorial Atlantic swung so quickly from one to another extreme event.”

Tuchen told Science Feedback that “research continues to understand why the equatorial Atlantic cooled so rapidly between April and July.” Arthur Prigent added: “2024 is exceptional for the equatorial eastern Atlantic […]. However, this is part of natural variability and is neither the first nor the last occurrence of a cold event in this region.”

SUMMER COOLING AT THE EQUATOR DOES NOT QUESTION THE LONG-TERM EFFECTS OF CLIMATE CHANGE

It’s important to note the difference between short-term observations, over a few months, and long-term observations, over a few decades. In a given region, climatic variables (temperature, wind, humidity, etc.) vary naturally from one year to the next (short-term), which is known as natural climate variability. Figure 3 shows this variability for the equatorial Atlantic.

Local and short-term observations are therefore insufficient to determine global trends. A large amount of both long-term and large-scale data demonstrates recent global warming linked to human activities^[5]. It is therefore misleading to rely on the cooling observed this summer in the equatorial Atlantic to argue about global warming. 

Franz Philip Tuchen explained:

The rapid cooling that we observe in the equatorial Atlantic is not at all questioning the overall long-term warming in the Atlantic that is due to global warming. You can think of global warming as a long-term relatively steady increase of ocean temperatures, while the equatorial Atlantic cooling event this summer is only a short-term event.

THE IMPACT OF CLIMATE CHANGE ON THE ATLANTIC OCEAN 

A large amount of data shows the impact of climate change on the oceans. According to the latest report by the Intergovernmental Panel on Climate Change (IPCC)^[5], the global average surface temperature of the oceans has risen by an average of 0.88°C since the beginning of the 20th century. IPCC projections agree that this warming will continue, increasing by 0.8°C to 2.9°C by 2100 (compared with the period 1995-2014), depending on future greenhouse gas emissions (see Figure 4). 

The Atlantic Ocean in particular is also experiencing a rise in surface temperature. It warmed by around 0.5 to 0.75°C per century over the period 1854-2019^[6]. Only the northern part of the Atlantic has cooled over the same period, by around 0.5 to 1°C per century (see Figure 5). This cool patch, known as the ‘cold blob’, is linked to the weakening – caused by climate change – of an ocean current in this region, therefore limiting the flow of warm waters northwards^[7]. The IPCC states that by the end of the century, at least 83% of the ocean surface will be warmer (if GHG emissions are sharply reduced), or even 98% if GHG emissions continue at current rates^[5].

Added to this are numerous other indicators of the effects of climate change in the Atlantic. For example, scientists are measuring a rise in sea level throughout the Atlantic Ocean (Figure 6) due to melting ice and the expansion of the ocean as it warms – a phenomenon known as thermal expansion.

The oxygen content of the Atlantic has been declining over the past 50 years, particularly at the Equator and to the south, due in part to warming ocean waters^[8]. Lastly, the pH of Atlantic waters is decreasing – the ocean is becoming more acidic – as a result of rising CO₂ concentrations due to human activities.

CONCLUSION

The equatorial Atlantic Ocean cooled by around 6°C between March and July 2024. This cooling is in line with the climate of this region of the Atlantic (warm in spring and cold in summer). Due to a natural variability well documented by scientific studies, warmer (Atlantic Niño) and colder (Atlantic Niña) seasons than average are regularly recorded. Scientists are currently establishing the reasons for the record cooling speed in the summer of 2024. This localized and temporary cooling does question the effects of climate change on the Atlantic, as some posts on social media suggest. Over the long term, numerous indicators unequivocally show the impact of climate change on the Atlantic: warming of surface waters, acidification of waters, rising mean sea level and falling oxygen content.

References:

• 1 – Johnson et al. (2024) State of the climate in 2023. Global Oceans. Special Online Supplement to the Bulletin of the American Meteorological Society.
• 2 – Zebiak (1993) Air-sea interaction in the equatorial Atlantic region. Journal of Climate.
• 3 –  Merle et al. (1980) Annual signal and interannual anomalies of sea surface temperature in the eastern equatorial Atlantic ocean. Oceanography and Surface Layer Meteorology in the B/C Scale.
• 4 – Servain et al. (1982) Evidence of remote forcing in the equatorial Atlantic Ocean. Journal Of Physical Oceanography.
• 5 – IPCC (2021) Sixth Assessment report, Chapter 3.
• 6 – Garcia-Soto et al. (2021) An Overview of Ocean Climate Change Indicators: Sea Surface Temperature, Ocean Heat Content, Ocean pH, Dissolved Oxygen Concentration, Arctic Sea Ice Extent, Thickness and Volume, Sea Level and Strength of the AMOC (Atlantic Meridional Overturning Circulation). Frontiers in Marine Science.
• 7 – Caesar et al. (2018) Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature. 
• 8 – Schmidtko et al. (2017) Decline in global oceanic oxygen content during the past five decades. Nature. 
• 9 – Lübbecke et al. (2018) Equatorial Atlantic variability—Modes, mechanisms, and global teleconnections. WIREs Climate change.
• 10 – Keenlyside and Latif (2007) Understanding Equatorial Atlantic Interannual Variability. Journal of Climate.