2024 Global Mean Surface Temperature

Most of the GMST temperature data is in for 2024, and as most predicted, it was the warmest year on record, though perhaps by a larger margin than expected. El Niño conditions developed in 2023, and typically the following year becomes the warmest year on record. When large El Niños developed in 1997 and 2015, it was the following year that became record years, and this pattern continued again. 

Above, here's how 2024 looks in several major datasets. I put the 2-sigma uncertainties for HadCRUT5 in dotted lines. This graph is set to the 1951-1980 mean (left scale), and this allows you to see that most of the variability between the datasets occurs in the late 19th century. The right scale is offset to show the warming above the 1850-1900 baseline for HadCRUT5 with the 1.5°C target as a dotted horizontal line. Below I set the same data to the 1850-1900 baseline and plotted centered 30-year means from HadCRUT5; this has the effect of better showing warming for each dataset above this baseline (though variability between the datasets artificially shows up in recent decades). I then used the most recent 30-year trend to estimate the current anomaly for the 30-year mean from HadCRUT5. NASA, JRA-3Q and ERA5 don't go back to 1850, but I found the proper baseline adjustment to align them to the other three. As you can see, all these datasets show that the 2024 anomaly came in between 1.47°C and 1.62°C. The average anomaly is 1.54°C.

Here's how 2024 compares to previous years in Berkeley Earth. Here you can see the clear separation between 2023 and previous years, with no overlap in the uncertainties with previous years. This comes from Zeke Hausfather and the Berkeley Earth dataset.

From Zeke Hausfather on X

And here's all the major datasets, including those in which data doesn't go back to 1850. The anomaly adjustments to make these comparable to HadCRUT, BEST, and NOAA were provided by Zeke Hausfather on X. HadCRUT5 is still right near the middle, which is why I continue to favor this one over the others.

These values for a single year are a poor indicator of how close we are to the IPCC's targets, since those depend on long-term averages. I also took 30-year means and used the current 30-year trend to "project" the current long-term anomaly from the middle of the last 30-year mean. There may be a better way to do this, but using this method the three datasets that go back to 1850 average 1.31°C warmer than the 1850-1900 mean. At the current rate of 0.23°C/decade, we have ~7 years until we hit the +1.5°C target.

There appears to be acceleration in 30-year trends as well. The trends I calculate are going to be affected by ENSO, which explains a good deal of the waviness in the overall upward trend in GMST warming trends. This shows that while many claim that there are "pauses" in global warming, that perception is really just an artifact of ENSO. Not only is global warming continuing, it's acceleration. That said, the acceleration may be due the fact that we're cleaning up aerosol pollution, de-masking the accumulated effects of GHGs.

This can also be seen by looking at the trends of El Niño years, La Niña years, and neutral years. All appear to be warming at about the same rate. I have not calculated the uncertainties for these trends, but I suspect they overlap - they're probably statistically identical.

Despite loud objections to the contrary, observations are well within the expectations of CMIP6 model projections, especially when filtered to show results from models calculating ECS to be near 3°C.. We've had about 1.2°C warming in this model-observation comparison.

In my post a year ago, I shared about the current thoughts about why the anomaly for 2023 was so high. With respect to 2022, the warmth of 2023-2024 does seem a bit surprising, but when compared to CMP6 models, it actually doesn't look so unexpected. But I think most of the weirdness of 2023 came from the last half of the year (and now the first half of 2024). I shared some preliminary thoughts about why the last half of 2023 was so warm, and I'd like to update this here. While perhaps answers are not definitive yet, we have a better idea what's going on.

1. Global Warming Trends. Given an overall trend of 0.23°C/decade global warming would add on average only ~0.02°C to the previous year's anomaly. So AGW doesn't explain much of the variability from year to yea. It's the long-term trend on which other sources of variability operate.
2. ENSO. Natural variability from ENSO can account for ~0.2°C variability in either direction, with El Niño years averaging much warmer than La Niña years. El Niño explains som e fractions of the 2023-2024 warmth, but probably not all of it. A recent study showed that there's a higher chance of large spikes in temperatures following three successive La Niñas, so ENSO could be the most important factor.[1]
3. Solar Variability. The Sun's 11-year cycle has been on the up-tick. It contributes virtually nothing to global warming trends (since it's a near decadal cycle), but it can be a small factor in annual variability. Changes in TSI have a 2-3 year delayed impact on GMST, and may contribute as much as ~0.03°C to a single-year's anomaly.
4. Hunga-Tonga Eruption. Since the Tonga eruption expelled large quantities of water vapor in the stratosphere, there has been some speculation that the Tonga eruption was the reason for the extreme warmth of 2023-2024. However, that's not likely at all. The effect of the Tonga eruption was likely small, and more likely a cooling influence than a warming influence.
5. Aerosol Pollution in Shipping Lanes. New legislation has resulted int he reduction of marine aerosol pollution along major shipping lanes, and this reduction of aerosol pollution can unmask warming from GHGs emissions and increase warming rates. It seems to me this needs more study, but so far it looks like this is a small influence on global temperatures, perhaps ~0.05 - 0.06°C, though some suggest a larger influence.[2]
   
6. Decreasing Cloud Cover. Scientists have observed an increase in absorbed solar radiation, some of which is associated with decreasing low cloud cover, which in turn decreases albedo. This may be involve some mix of being an emerging cloud feedback (1 above), or linked to decreases in aerosols (5 above) or simply internal variability, but I think it's worth giving it some attention.[3]

From Zeke Hausfather on X

To illustrate the long-term effect of carbon dioxide on global mean surface temperature, I also updated my plot of the correlation between GMST and CO2 forcings, with the usual caveats that need to be acknowledged. There are other forcings at play here, particularly the forcings of other GHGs and aerosol pollution. These forcings historically have roughly canceled each other out, but if we continue to clean up air pollution while emitting methane and other GHGs, this will be less the case going forward. And of course, correlation doesn't prove causation. Causation can be and has been determined by other means that go beyond mere correlation.

All in all, 2023-2024 was exceptional, and my guess is that the events in these two years will continue to fuel productive research in quantifying the effects of volcanic eruptions and aerosol pollution. It's not likely that 2025 will be as warm as the last two years, but at the same time, it's a stark reminder that warming isn't going to stop until we get our emissions under control.

References:

[1] Raghuraman, S. P., Soden, B., Clement, A., Vecchi, G., Menemenlis, S., and Yang, W.: The 2023 global warming spike was driven by the El Niño–Southern Oscillation, Atmos. Chem. Phys., 24, 11275–11283, https://doi.org/10.5194/acp-24-11275-2024, 2024.

[2] Min, SK. Human influence can explain the widespread exceptional warmth in 2023. Commun Earth Environ 5, 215 (2024). https://doi.org/10.1038/s43247-024-01391-x

[3] Helge F. Goessling et al. ,Recent global temperature surge intensified by record-low planetary albedo.Science0,eadq7280DOI:10.1126/science.adq7280. Full version here: https://epic.awi.de/id/eprint/59831/1/adq7280_Merged_AcceptedVersion_v20241206.pdf

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