New Paper (Fails To Adequately) Explain Global Warming Spike of 2023 In Terms Of Planetary Albedo

OK, serious post now for today; seriously boring some readers might think! Because it’s a ‘nuts and bolts’ science post looking in depth at a paper which has just been published. It’s necessary because the paper is not quite what some people think it is and it’s important IMO to analyse exactly what it does say, and even more importantly, what is does not say. It’s caused quite a stir in the climate alarmist vs. climate sceptic arena. Basically, the gist is that the authors have claimed to identify the proximate cause of the 2023 warming spike - namely a global but regionally varying decline in low level cloud cover, thus allowing more solar energy to reach the surface of the planet. So I’m just going to go through it methodically, explaining what I think the authors are saying, and what the implications are. Here is the Abstract (emphasis my own):

In 2023, the global mean temperature soared to 1.48K above the pre-industrial level, surpassing the previous record by 0.17K. Previous best-guess estimates of known drivers including anthropogenic warming and the El Niño onset fall short by about 0.2K in explaining the temperature rise. Utilizing satellite and reanalysis data, we identify a record-low planetary albedo as the primary factor bridging this gap. The decline is caused largely by a reduced low cloud cover in the northern mid-latitudes and tropics, in continuation of a multi-annual trend. Understanding how much of the low-cloud trend is due to internal variability, reduced aerosol concentrations, or a possibly emerging low-cloud feedback will be crucial for assessing the current and expected future warming.

The authors identify the reduction in ship fuel aerosols, the solar cycle maximum and Hunga Tonga volcano as being possible contributors to the huge spike in global mean surface temperature which began in May 2023. Then they rule them out as being major contributors to that spike.

Besides the onset of El Niño and the expected long-term warming due to anthropogenic greenhouse gases, there are several factors that may have contributed to the anomalous global mean temperatures in 2023 (Schmidt2024). The 11-year solar cycle is approaching its intensity maximum (Kopp2016); the submarine volcano Hunga Tonga—Hunga Ha’apai has released large amounts of water vapor into the stratosphere (Schoeberl2024); and new ship fuel regulations, aimed at reducing sulfur emissions, were implemented in three phases, in 2010, 2015 and 2020 (Hansen2024). While these regulations may be associated with a spatial pattern that is roughly consistent with the pronounced warming of the traffic-heavy North Atlantic and despite further evidence for recent warming due to reduced aerosols (Manshausen2022,Hansen2024,Hodnebrog2024), it has been estimated that the combined global effect of all three factors is below 0.1K and that an unexplained warming of about 0.2K remains (Schmidt2024).

By looking at CERES satellite data (absorbed and emitted global long wave and short wave radiation) and ERA5 reanalysis data, for the period 2013-22, plus 2023 alone, the authors conclude:

Note: the authors identify the reduction in low cloud cover from December 2020, and the resultant increase in short wave solar radiation over that period (two and a half years until June 2023) for the global warming spike beginning in June 2023 and lasting basically right up to the present (November 2024). But they don’t mention the continuation of the warming for another year nearly to the end of 2024, when El Nino faded very rapidly in the spring and could not have contributed much to the ongoing warmth. So, did low cloud cover continue to boost temperatures for the whole period? That question is not addressed by this latest study.

What is also not explained adequately is why global mean surface temperature rose so suddenly and unexpectedly beginning May/June 2023 (the North Atlantic warmed first in the spring). Was it a case that cloud cover was declining slowly since December 2020 and then suddenly that decline accelerated over the north Atlantic and the tropics in the first part of 2023? AFAICS, there is no answer to this question in the authors’ text other than the observation that the ASR (absorbed solar radiation) anomaly in 2023 reached a record maximum. Yes, but why? Why would cloud cover so suddenly decrease? What were the relative contributions to the temperature record in 2023 from the gradual decline beginning Dec 2020 and the sharp decline in 2023? No answer as far as I can see.

Here’s what climate sceptics are getting excited about:

Since 2000, there has been an increasing positive trend in earth’s energy imbalance which reached a record maximum in 2023, but what’s really notable is that, according to data from CERES, that positive trend has not been due to a decrease of outgoing long wave (infrared) radiation (as one would expect from the greenhouse gas due to accumulating carbon dioxide and methane etc.) but due to an increase in shortwave solar radiation (which the authors identify as being due to regional reductions in low level cloud cover).

So, yes, on the face of it, this is bad news for the climate crisis cultists because basically the entire global warming period from 2000 onwards, reaching a peak in 2023, has been due to more solar radiation reaching the surface, not because of so-called ‘heat-trapping gases’ (Boris Johnson’s “tea cosy in the sky”)! Wow. But it still doesn’t really explain why 2023 was so hot and why 2024 will probably be even hotter still (which no doubt the scientifically challenged morons at the BBC and Guardian will blame on the ‘accumulation of heat trapping gases’ - if they haven’t already).

One thing which is interesting is that the Sun itself appears to have become slightly more energetic in 2023, which did in fact contribute a not insignificant amount to the total absorbed solar radiation flux:

What was so different about the 2023 approaching solar maximum vs. the previous solar maximum? No explanation forthcoming from our authors.

Then comes the clue that the multiannual decrease in albedo beginning 2000 may be a consequence of internal multidecadal variability. The authors point out that the 2023 negative planetary albedo anomaly may have been the lowest since at least 1940. Planetary albedo in the 1940s and 50s was also quite low compared to the latter half of the 20th century.

Not looking at all good for the climate crisis loons.

This should also finally put the ‘weird El Nino’ afficionados to bed as regards explanations for the 2023 warmth because El Nino contributed just 5% to the positive ASR anomaly of 2023.

Having said that, the El Nino of 2023 does largely explain the observed regional difference in the pattern of ASR anomalies in 2023 vs. the 2013-2022 trend in the tropics, including in the eastern Indian ocean and tropical Pacific. Note in particular the strong El Nino-like signature of heat in the eastern Pacific in 2023 vs. the cold La Nino-like pattern evident in the long term surface temperature trend, a pattern which is also strongly reflected in the low level cloud cover anomalies.

Now, here’s something interesting. As one would expect, changes in cloud cover (albedo) are closely related to absorbed incoming short wave solar radiation, but, there has been a growing mismatch between the total cloud cover radiative effect and ASR, particularly since 2020, when the authors identify a reduction in low cloud cover. This is explained as follows:

What could be causing this mismatch? According to the authors, it’s either an increase in cloud reflectivity or an increase in clear sky absorbed solar radiation (due to aerosols), or both.

The difference is explained by relatively greater reductions in low level cloud vs. total cloud cover.

Reductions in low level cloud cover in 2023 over the eastern north Atlantic were a major driver of the warmth in 2023 according to the authors, but this reduction was just a continuation of a longer term trend:

This invites the question: If low level cloud over the eastern north Atlantic had been declining for a decade prior to 2023, why did it suddenly decline more sharply beginning spring 2023? Nobody knows. If the declining LCC over this region was due to internal multidecadal variability, why did that variability accelerate markedly in 2023? Nobody knows.

But what about aerosols? Maybe reductions in aerosols (Saharan dust or shipping emissions) played a significant role in the 2023 warm spike? Not very likely. Reduced aerosols from shipping or reduced Saharan dust may explain some of the clear-sky ASR anomaly and trend in the north Atlantic region but they are an order of magnitude smaller than the all-sky ASR effect and even then, their contribution remains unclear.

Now here’s the kicker. There are three fundamental mechanisms whereby low level cloud might have decreased in 2023 and over the longer term:

Aerosols and aerosol-cloud feedbacks can basically be ruled out, which leaves only internal variability and an emerging (unanticipated) cloud feedback in response to warming:

So, in the final paragraph of the paper the authors suggest, if the declining low level cloud is not due to internal variability, or only partially explained by internal variability, then the cloud feedback response to global warming must be underestimated by the models. You see what they did there? OMG, we’re back to the “Uncharted territory” explanation for the sudden onset 2023 record warming!

Summary

This new study demonstrates that most warming since 2000, plus the warming spike in 2023, is due to a low planetary albedo associated with regionally variable declining low level cloud cover. It doesn’t really explain how or why the multiannual trend in low level cloud cover suddenly accelerated in 2023. It doesn’t explain how or why that trend has continued into 2024. It suggests that either multidecadal internal variability or an underestimated cloud feedback response to warming explains the long term trend and the short term warming in 2023, but it fails to explain how either of these two mechanisms could have suddenly accelerated in 2023. Finally, the authors raise the spectre of Schmidt’s Uncharted Territory once again.