Will global temperatures exceed 1.5C in 2024?
With a growing El Nino event next year is shaping up to be a record-breaker. But by how much?
Next year is shaping up to be a scorcher, and is very likely to be the warmest year since global temperature records began in the late 1800s. Here we provide the first formal projections of where 2024 might end up depending on how strong an El Nino event we have. We find that while there is a real chance that the world exceeds 1.5C above preindustrial levels in 2024 in the Berkeley Earth record, it is still more likely than not that 2024 temperatures come in below that level.
Global temperatures have been exceptionally warm over the past decade, with the last eight years clocking in as the eight warmest years since records began in the mid-1800s. However, despite being quite warm, 2016 remains the warmest year on record to-date (though 2020 was effectively tied in some datasets).
Why? It turns out that while human emissions of greenhouse gases drives the long-term warming trend, year-to-year variability is dominated by El Nino and La Nina events. These natural cycles redistribute heat between the oceans and the atmosphere, and result in elevated global surface temperatures during El Nino events and lower temperatures during La Nina ones. However, these are just redistributing heat, not adding new heat to the system, and there is a roughly seven year cycle between El Nino or La Nina events.
We saw a super-El Nino in 2016, one of the largest recorded on record. This added close to 0.2C of additional warmth to the year beyond what we would have expected due to human-caused climate change alone, causing it to shatter the prior record set in 2015. However, during the past few years we have seen an unusual triple dip La Nina event, resulting in a cooler 2021, 2022, and start to the year 2023.
Next year is shaping up to be a scorcher, with La Nina ended and a rapidly developing El Nino event. Put on top of the rapid global warming trend we’ve seen over the past 50 years, it is very likely that 2024 will be the warmest year on record. But will it be the first year to be 1.5C above preindustrial levels?
What would it mean to pass 1.5C?
In the 2016 Paris Agreement the world set an aspirational to “keep global temperatures well below 2C above pre-industrial times while pursuing means to limit the increase to 1.5C.” However, these targets have created a fair bit of confusion, as they refer to long-term average global temperatures rather than short-term year-to-year variability.
A single year above 1.5C (or 2C for that matter) would not mean that the world has passed that temperature target. Given the warming rate of 0.2C per decade since 1970, and the fact that a super El Nino event can add up to 0.2C to a specific year’s global temperatures through natural variability, it is possible for us to see a single year exceeding 1.5C a full decade before the long-term average warming driven by human emissions of greenhouse gases does.
Because of this, we should be a bit cautious about how we interpret the first year passing 1.5C (whether it happens in 2024 or not)! We don’t expect the longer term average to pass 1.5C until the early-to-mid 2030s, but we do expect many individual years to pass 1.5C before then. In fact, back in 2022 the UK Met Office gave a 50 percent chance for a single year to pass 1.5C during the period from 2022-2026.
Estimating the effects of El Nino and La Nina
There are a number of different ways to estimate what global temperatures might be in 2024. Here we take the approach of separately assessing the effect of the long term warming trend and a potential El Nino event, in order to test the sensitivity of 2024 projections to different El Nino event magnitudes.
To start with, we need to remove the effect of El Nino and La Nina (collectively referred to as ENSO) from the global temperature record to see what would have happened in the absence of these events. The figure below shows the standard Berkeley Earth global temperature record (that I help produce) in red, along with an estimate of what it would look like with both El Nino and La Nina events removed.
Here we remove the effect of El Nino and La Nina (ENSO) events using the Foster and Rahmstorf (2011) approach, which finds an approximately three month lag between conditions in the ENSO 3.4 region of the tropical Pacific and the peak ENSO influence on global temperatures. We us a simple regression between monthly temperatures and a 3-month-lagged ENSO 3.4 index to estimate the effect of ENSO on each month. The estimated effect of ENSO on each year using this approach is seen in the figure below. A super El Nino event could cause ~0.15C or more additional annual warming, a strong El Nino event around 0.1C, and a moderate El Nino event around 0.05C (with roughly the same magnitude of cooling for La Nina).
Once we remove the effect of ENSO, we see that the resulting record is much smoother. With ENSO removed 2020 would have been the warmest year on record, followed by 2022 and 2016. There is still some residual year-to-year varaibility, the largest of which is associated with volcanic eruptions such as El Chichón in 1982 and Pinatubo in 1991. As an aside, its much easier to see the effect of volcanic eruptions on the temperature record with ENSO removed, as there is a strange tendency to have a big El Nino event after a big volcanic eruption in the historical record.
The figure below shows where we would estimate 2024 temperatures to end up, along with its uncertainty, in the absence of an ENSO events. Based on the long-term warming trend and without an El Nino event, we would expect 2024 to be around 1.33C above preindustrial levels, with a range of 1.22C to 1.45C.
In store for an exceptionally warm 2024
However, almost everyone expects we will end up with an El Nino event that develops in the coming months. The figure below shows the projections as of mid-April from a number of different modeling groups around the world. Most range from a moderate (~1C Nino 3.4 SST anomaly) to strong (~1.5C), but few are currently projecting super El Nino conditions like we saw in 1998 or 2016 (~2C to 2.5C).
While the El Nino event will develop in mid-2023, its largest effects will be felt in 2024 (recall the ~3-month lag between ENSO 3.4 region anomalies and global surface temperature response). To estimate the effect of this El Nino event on top of the long-term warming trend, we add the following to our earlier projection to simulate different ENSO scenarios:
Moderate El Nino: +0.05C
Strong El Nino: +0.1C
Super El Nino: +0.15C
In addition to El Nino, there has been a lot of recent attention around the effects of phasing in low-sulfur fuel for maritime shipping. Human SO2 emissions have a strong global and regional cooling effect, and probably dampen the amount of global warming the world has experienced by around 0.5C compared to what we’d expect from greenhouse gas emissions alone.
While scientists are still figuring out the magnitude of the effect of the low-sulfur fuel transition on global surface temperatures, lets make a rough estimate that it reduces global SO2 emissions by approximately 10%, resulting in a warming effect of +0.05C. This lets us generate an upper bound scenario that includes both a super El Nino and the sulfur phaseout:
Super El Nino plus: +0.2C
The figure below shows what we’d estimate global 2024 temperatures to be if we take our prior no-El-Nino estimate and combine it with these four El Nino (and sulfur phaseout) scenarios. In all cases 2024 is the odds-on favorite to be the warmest year on record.
Here even if we end up with a super El Nino (which is generally not predicted by models at the moment), the world is more likely than not to stay below 1.5C in 2024. This is also true if we estimate the combined effect of the sulfur phaseout and a strong El Nino. Its only when we combine the estimated effect of the sulfur phaseout with a super El Nino that we end up with a central estimate above 1.5C.
However, El Nino is not the only uncertainty in the climate system! As we say above, even when ENSO is removed there is still some year-to-year variation in global surface temperatures. If we combine these El Nino estimates with the uncertainty in our no-El-Nino projection earlier, we get the figure below.
What this tells us is that with a moderate El Nino (and no effect from sulfur removal), the world is very likely (~95%+ chance) to remain below 1.5C in 2024. However, in the other scenarios there is a real chance that the year will be warmer than 1.5C. Its too early to tell where 2024 will end up, and while I would give better than even odds that it will remain below 1.5C, I probably wouldn’t be willing to bet too much money on it.
That said, this analysis is done using the Berkeley Earth dataset, which has the highest current global temperatures due to its lower estimated global temperatures prior to 1900 (where uncertainties between the groups are larger). If we use HadCRUT5 all of these estimates would be around 0.06C lower, while NASA’s GISTEMP would be ~0.15C lower. If 2024 does end up passing 1.5C in some records, it may well not in others!
thank you for taking on this task!
Glad Zeke (and Andrew) have joined the climate scrum on Substack. I sure hope reporters and editors read this post before the flood of overheated headlines asserting the world has crossed into climate doom as the global average touches 1.5 when the next #ElNino comes atop CO2 heating. More on Twitter: https://twitter.com/Revkin/status/1653097497198227456