2023 Global Mean Surface Temperatures

Most of the GMST temperature data is in for 2023, and it was a doozy. Around June 1, GMST headed into unchartered territory, and they stayed there for the rest of the year, with only a few days not record temperatures for the instrumental record. Now with the the availability of daily reanalysis data, it's been possible to track this on a near daily basis. 

ClimateReanalyzer data for 2023 from ERA5

At the end of 2022, even with the likelihood that El Niño conditions would develop, most thought that there would be a low probability that 2023 would be a record year, beating 2016/2020, as you can see from this graph by Zeke Hausfather. 2023 came in significantly warmer than the range expected at the beginning of the year. When large El Niños developed in 1997 and 2015, it was the following year that became record years, and there's good reason to think this pattern would continue, though since scientists don't know exactly why 2023 was so warm, we should perhaps take that expectation with a larger uncertainty.

The Surprising Result of 2023

Here's how 2023 looks in the four major datasets. I put the 2-sigma uncertainties for HadCRUT5 at the bottom of the graph. This graph is set to the 1951-1980 mean, and this allows you to see that most of the variability between the datasets occurs in the late 19th century.

GMST Datasets with HadCRUT5 Uncertainties

Here's how 2023 compares to previous years. 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.

Berkeley Earth

I set the three datasets that cover 1850-1900 to that baseline and calculated the anomalies for each. They range between 1.35°C and 1.54°C, due to variability between the datasets during the baseline period. But the average is 1.45°C, and HadCRUT5 appears to strike the middle ground between the other. These values are a poor indicator of how close we are to the IPCC's targets, since those depend on long-term averages. So I also took 30-year means and used the current 30-year trend to "project" the current 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 average 1.27°C warmer than the 1850-1900 mean. At current rates at 0.22°C/decade, we have ~10 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 accelerating (though the attribution of the acceleration may be due the fact that we're cleaning up aerosol pollution, demasking the 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. We've had about 1°C warming in this dataset.

The most interesting question about 2023 is, why was the anomaly so unexpectedly high? Part of me wonders if we should have found 2023 surprising. Afterall, 2020 statistically tied 2016 for the warmest year on record without the benefit of it being an El Niño year. Since 2023 was an El Niño year, perhaps in hindsight it shouldn't be that surprising, especially since it falls well within model projections. But the last half of 2023 was exceptional. Nearly every day broke records, and I don't think we've ever seen another year do this. There have been several proposed factors that could explain at least a portion of why 2023 was so warm, which appears to have beat 2016 by 0.17°C.

1. Global Warming Trends. At an overall trend of 0.22°C/decade global warming would add only about ~0.02°C to last year's anomaly. It doesn't explain much of variability year-to-year. In the 7 years since 2016, though, we'd expect ~0.14°C should all things be equal between 2016 and 2023. But they weren't at all equal. 2016 began with El Niño in full swing, whereas 2023 did not; it would be more comparable to 2015, and 2023 was ~0.3°C warmer than 2015.
2. ENSO. Natural variability from ENSO can account for ~0.25°C variability in either direction, with El Niño years averaging much warmer than La Niña years. The influence of ENSO would likely have increased throughout the year as El Niño conditions developed, but the effects of this are delayed by months, and they are likely to affect 2024 more than 2023.
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 ~0.02°C to a single-year's anomaly.
4. Hunga-Tonga Eruption. Since the HT eruption expelled large quantities of water vapor in the stratosphere, there has been some news coverage that the HT eruption may contribute to a significant fraction of the 2023 anomaly. Another ClimateBrink post covers this quite well, so I won't cover the details here. The HT eruption was unique in the amount of water vapor it put in the stratosphere, so there have been several studies investigating the impact. Jenkins et al 2023  concluded that water vapor is likely to cause ~0.03 C warming over the next few years and slightly increase our odds of crossing the +1.5 target. However, two other studies, Zhang et al 2023 and Zhu et al 2023 estimated that, considering both aerosol cooling and water vapor warming, the HT eruption is likely to result in a net cooling impact. I think at best it's premature to conclude that any of the current anomaly is due to the HT eruption.
5. Aerosol Pollution on 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. Forcings from aerosol pollution still have large uncertainties. Current estimates show the impact of aerosol pollution to have a real but small impact on the 2023 anomaly. It's enticing to consider that we have underestimated the impact of aerosol forcings and further study may show that aerosols cause more cooling than we have thought. If so, it would explain more of the current anomaly and increase ECS estimates by a significant amount, and a recent paper by Hansen suggests that this may be the case. However, the evidence does not yet warrant this, and the reduction of aerosols along shipping lanes is likely only good for ~0.05 - 0.06 C.
   
   
   

All in all, it was an exceptional year, and my guess is that the events in 2023 will lead to some productive research in quantifying the effects of volcanic eruptions and aerosol pollution. But at the same time, it's a stark reminder that warming isn't going to stop until we get our emissions under control.

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