Record-breaking ‘record-breaking’: The extraordinary heat of 2023-2024
Monthly global temperature records have been broken for twelve successive months from June 2023 to May 2024.
Globally in 2023, and the first five months of 2024 have been particularly warm.
Monthly global temperature records have been broken for twelve successive months from June 2023 to May 2024.
Almost a year ago today, July 6, 2023, was the warmest day on record for the entire planet with a global average temperature of 17.08 degrees Celsius (62.74°F); July 2023 was the warmest month on record globally. Averaging 1.8 degrees Celsius above the preindustrial average temperature, September 2023 was the globe’s most anomalously warm month.
If this wasn’t worrying enough, the magnitude of the jump in extreme temperatures was also a record. For several months the global temperature peak exceeded the 1.5 degree Celsius threshold compared to the pre-industrial average set as the target in the 2015 Paris Agreement for limiting warming.
It is not just that there was a new record global temperature – that was to be expected; a medium-strong El Niño supplemented by the decadal trends associated with the inexorable rise in greenhouse gases meant that we would expect new records for temperature for some months in 2023.
It is the margin by which the record was broken and broken month after month for a whole year. We can even track the monthly increment. It reached its highest value in September 2023 when the increase was more than 0.5 degrees Celsius above any previous September.
It would be as if, over the past fifty years the record for the men’s hundred-meter sprint had been broken by a tenth of a second every five or ten years. And a runner has now arrived who has broken the previous record by half a second.
What’s causing the rise?
There has to be a reason and a cause. What caused this jump in global temperatures?
Was the 2023 El Niño particularly intense? Maybe the pool of hot water stretched out across the equatorial Pacific was particularly deep, broad, or hot? Yet as measured by temperature or depth or width, El Niño does not seem particularly unusual.
Could it have been that giant submarine eruption in Tonga in January 2022 which injected 150 million tons of water vapor and sulfate aerosols into the middle stratosphere? The water vapor is a greenhouse gas expected to increase the downward infrared radiative flux, while the aerosols reduce solar radiation.
However, a set of models suggests that the effect of the aerosols should outweigh the water vapor. Another study suggests a modest increase in warming by 0.035°C over the next five years. The Tonga eruption does not look to provide the answer.
The culprit looks to be somewhere else completely: ships’ funnels and the outcome of the International Maritime Organization (IMO) regulations on sulfate emissions.
Shifting to low sulfur shipping fuels
Sulfate emissions are linked to the sulfur content of ship fuels, and heavy fuel oil (HFO) with a tar-like consistency has powered shipping for seventy or more years. Cheaper than other fuels, HFO, also known as bunker fuel or residual fuel oil, is the result or remnant of the petroleum distillation and cracking process.
The IMO aims to reduce sulfur dioxide emissions by 75 percent, and regulations have been progressively tightened, moving away from HFO with a maximum content of 3.5 percent to lower sulfur fuels such as marine gas, which consist solely of distillates.
The sulfur content of fuels was limited to one percent in North Atlantic coastal regions in 2010, then reduced to 0.1 percent in 2015. In 2020, a global limit was placed of 0.5 percent.
Sulfur dioxide emissions from fossil fuels have long contributed to severe health impacts by forming particulate matter known as PM2.5.
The previously high sulfur content of marine fuel – much higher than levels allowable on land, and concentrated sulfate emissions from busy shipping ports and routes in areas such as India and China have proved harmful to those living in neighboring communities.
A study in 2018 showed that before cleaner shipping fuels, ship-related health impacts included ~400,000 premature deaths from lung cancer and cardiovascular disease and ~ 14 million childhood asthma cases annually.
Less harmful emissions resulting from the switch away from HFO to lower sulfur fuels for shipping, hopefully result in less suffering in terms of health impacts for coastal communities.
So far, so good. So, why are sulfate emissions important when looking at record-rising temperatures? It is because sulfate emissions provide the nuclei for condensation and cloud formation, producing ‘whitetop’ clouds with higher albedo – able to reflect sunlight that produces a cooling effect.
The greatest impact from this is where there are the most ships – in the northern Atlantic and north Pacific. With the IMO regulations coming in, after 2015 ship smoke ‘tracks’ were found to be significantly reduced.
By 2020, ship tracks had more than halved, and sulfur dioxide emissions from shipping plummeted by around 80 percent. Such a rapid reduction in sulfur emissions must correspond with a rapid reduction in the cooling effect generated by the enhanced reflective qualities of the clouds.
One study showed that switching to low-sulfur fuels reduces the radiative cooling effect from ship aerosols by around 80 percent. This equates to around a three percent increase in current estimates of total anthropogenic forcing.
From January 2015 to December 2022 global absorbed solar energy had increased by 1.05 watts/m2 relative to the previous mean. This impact is much greater than had been expected.
However, now that sulfate aerosols have been reduced, can global temperatures be expected to rise faster than was previously anticipated?
The Intergovernmental Panel on Climate Change (IPCC) reports have converged on a climate sensitivity of three degrees Celsius for a doubling of carbon dioxide, alongside cloud feedbacks and potentially optimistic assumptions for how excess heat passes into the deep ocean.
However, studies of the Ice Ages and the link between global temperatures and carbon dioxide levels suggest a 4.8 degrees Celsius increase for a doubling of carbon dioxide implying that the contribution of reduced aerosol forcing is more negative than the best estimates of the IPCC.
This is bad news, implying that the rate of increase in temperatures seen through 2023 will become the new normal. No one seems too keen on restoring the polluting ship’s fuel.
That would mean our projections for 2050 become what happens by 2035, which is bad news for communities, businesses, farmers, and insurers viewing the impacts on crops and flash floods in the remote future.
Aerosol cloud interactions remain the largest area of uncertainty in projected temperatures.
However, the ships’ smokestacks have proved that raising atmospheric aerosols could one day become a principal means of slowing further temperature rises.
Robert Muir-Wood is chief research officer at Moody’s RMS.
The piece originally appeared on Moody’s blog and has been republished here with permission.