Can Changes in Cloud Cover Drive Global Warming?

Ned Nikolov and Karl Zeller have published a new paper (NZ24) in the quasi-predatory MDPI journal geomatics [1] which claims to rule out the effects of greenhouse gases as a cause for the increase in GMST in recent decades. Their paper concludes,

Our analysis revealed that the observed decrease of planetary albedo along with reported variations of the Total Solar Irradiance (TSI) explain 100% of the global warming trend and 83% of the GSAT interannual variability as documented by six satellite- and ground-based monitoring systems over the past 24 years. Changes in Earth’s cloud albedo emerged as the dominant driver of GSAT, while TSI only played a marginal role.

They produce a graph that they believe supports their claim, which sure enough shows a decrease in the Earth's albedo over the last 24 years and a corresponding increase in absorbed solar radiation (ASR).

Let me be clear here at the beginning that there absolutely has been both a decrease in albedo and a corresponding in crease in ASR. This has been documented in other studies[2][4] as well. Where NZ24 goes wrong is in the attribution of this trend. NZ24 asserts that the driver of recent warming has been the changes in cloud cover that increase ASR, and not increases in GHGs (NZ deny that there is a greenhouse effect). The attribution of what's driving recent warming is what I believe is seriously misguided.

Background

Earlier this year, Norman Loeb published a paper in a legitimate journal assessing the changes in the Earth's energy imbalance since 2000. The paper was published on May 7, 2024, and NZ24 was submitted for publication in the MDPI journal on July 2, 2024.. I find it interesting and suspicious that NZ submitted this paper 2 months after Loeb's paper was published using similar data and the same start date. Nevertheless, Loeb's paper makes some interesting conclusions. In the abstract, they write:

Satellite observations from the Clouds and the Earth’s Radiant Energy System show that Earth’s energy imbalance has doubled from 0.5 ± 0.2 Wm−2 during the first 10 years of this century to 1.0 ± 0.2 Wm−2 during the past decade. The increase is the result of a 0.9 ± 0.3 Wm−2 increase absorbed solar radiation (ASR) that is partially offset by a 0.4 ± 0.25 Wm−2 increase in outgoing longwave radiation (OLR). Despite marked differences in ASR and OLR trends during the hiatus (2000–2010), transition-to-El Niño (2010–2016) and post-El Niño (2016–2022) periods, trends in net top-of-atmosphere flux (NET) remain within 0.1 Wm−2 per decade of one another, implying a steady acceleration of climate warming.
To summarize, the Earth's energy imbalance (EEI) has doubled in the last 24 years and now exceeds 1 W/m^2. The increase in EEI is explained by a 0.9 W/m^2 increase in ASR offset by 0.4 W/m^2 increase in outgoing longwave radiation (OLR). Clearly Loeb is indicating that the increase in ASR has had a large impact on the current EEI. But Loeb's conclusions are very different from NZ24. For Loeb, clouds are not a driver of recent warming but a feedback amplifying warming: "Changes in cloud cover in response to SST increases imply a feedback to climate change yet a contribution from radiative forcing or internal variability cannot be ruled out." Who's right?

Explanation

What NZ apparently don't understand is that cloud cover is and only can be a feedback; it cannot be a driver of recent warming. Drivers are external forcings - that is, forcings that originate from outside the climate system, such as carbon emissions from fossil fuels (carbon that has been sequestered in the Earth are added to the climate system when burned), volcanic activity, aerosol pollution, or changes in solar activity (TSI). These are forcings that act on or enter the climate system from outside, so they are candidates for drivers of global warming. Cloud cover, on the other hand, is an internal response within the climate system; it can change in response to external forcings or with internal variability. In other words, it's clear that NZ24 are wrong to suggest that changes in cloud cover can drive current warming, and Loeb is correct that changes in cloud cover imply either a result of internal variability or a feedback to increases in SSTs from global warming.

Loeb 2024 Showing ASR, -OLR and Net Change in EEI

When the Earth's temperature is stable, ASR = OLR - that is, the incoming and outgoing energy flux are equal. An imbalance can be created by changes in ASR or OLR or both. Loeb observed that an increase in ASR of 0.9 W//m^2 and increase of OLR of 0.4 W/m^2 since 2000 resulted in a change in EEI from 0.5 W/m^2 to 1.0 W/m^2. But if the increase in GMST is being driven by increases in GHGs and not by cloud cover, why is the increase in ASR larger than the increase in OLR? I think this is the question that is causing confusion among those that are following the NZ24 paper. 

To answer this question, it's important to recognize that external forcings can create an energy imbalance by altering either the ASR and OLR side of the equation, and feedbacks can similarly impact either or both sides of the equation. GHGs are an external forcing that change OLR. Changes in solar or aerosol forcings cause changes in ASR. Cloud feedbacks can work on either side of the equation. Changes in low cloud cover affect albedo and therefore ASR. Changes in high altitude cloud cover affect the greenhouse effect and therefore OLR. But on the OLR side of the equation, over time changes in GHG concentrations and change in temperature cancel each other out. That is, if you reduce OLR by increasing GHGs, GMST will warm until that imbalance reduces to zero. Since GHGs are not added instantaneously, increasing temperature is always reducing EEI on the OLR side of the equation. There is nothing that has the same effect on the ASR side of the equation. So if ASR increases, temperature increases (increasing OLR) to close that imbalance. ASR can increase by more than OLR even though warming is driven by increases in GHGs (partially masked by aerosol pollution). The imbalance created by increases in GHGs is always going to be at keast partly compensated for by increases in GMST, and so OLR changes may well be much smaller than changes in ASR.

So Loeb is correct that warming is driven by GHG forcings partially masked by aerosols, and positive feedbacks to that warming can cause additional changes in ASR and/or OLR, with cloud cover being possible culprits affecting either side of the equation. Global warming (particularly SSTs) can have strong impacts on cloud cover. If warming increases high altitude clouds, then that would function as a positive feedback enhancing the greenhouse effect. If warming decreases low cloud cover, then that would also function as a positive feedback by decreasing planetary albedo and increasing ASR. Decreases in aerosol forcings (such as those decreases caused by enhanced shipping regulations) would also decrease albedo and increase ASR. In effect, decreasing anthropogenic aerosol pollution reduces the masking effect of aerosols on GHG-induced warming. 

Conclusion

This has always been a somewhat weird objection to climate science, imho, but it's been popularized (I think) in recent years because CERES satellite data has been able to give us some really great information that have been published in the literature, including several by Normal Loeb. Contrarians can easily misrepresent these papers and use cool graphs that make their misrepresentations seem plausible. But the truth is that all this new information from satellite data is actually giving us a lot more information about the role of clouds in feedback mechanisms within the climate system. And while we may not yet be able to parse our the fractional roles of feedbacks, aerosols, and internal variability, there is nothing here that invalidates the greenhouse effect. Recent papers[3] have been able to "constrain global cloud feedback to 0.43 ± 0.35 W⋅m−2⋅K−1 (90% confidence), implying a robustly amplifying effect of clouds on global warming and only a 0.5% chance of ECS below 2 K.” This robustly amplifying effect of the cloud feedback is consistent with Loeb's findings this year. Rather than falsifying GHG warming, Loeb's paper confirms that the cloud feedback is likely a net positive feedback.

Even more recently, a Gössling et al 2024[4] evaluated the causes of the warming spike in 2023-2024 (discussed at Inside Climate News and RealClimate) and showed that the most likely explanation for the 0.17 C jump in temperature during 2023 was due to decreases in low cloud cover. Inside Climate News reports, "The findings, Gössling said, suggest that the sharp drop of low-elevation cloud cover over some ocean regions could account for most of the sudden spike of global temperatures in 2023, when the Earth’s fever jumped 0.17 degrees Celsius (0.3 degrees Fahrenheit) above the previous temperature record set in 2016." The methodology used Gössling et al 2024 cannot quantify how much of this is due to cloud feedbacks, but they highlight the possibility that this decrease in low cloud cover may be an emerging positive feedback. From Inside Climate News:
If the drop in the proportion of solar radiation being reflected back to space—called albedo—is due to feedbacks between global warming and low clouds, “we should expect rather intense warming in the future,” Gössling said. “We could see global long-term climate warming exceeding 1.5 degrees Celsius sooner than expected.”
I'm not going to say anything about the likelihood of this being the case. Even the paper admits, "Disentangling contributions to the low-cloud trend from internal variability, indirect aerosol effects, and a possibly emerging low-cloud feedback remains challenging." But what should be clear is that the observed increase in ASR in recent years is not an argument against GHG-induced warming. If anything, it's confirmation that cloud feedbacks are to some extent net positive enhancing warming caused by GHGs.


References:

[1] Nikolov, Ned, and Karl F. Zeller. 2024. "Roles of Earth’s Albedo Variations and Top-of-the-Atmosphere Energy Imbalance in Recent Warming: New Insights from Satellite and Surface Observations" Geomatics 4, no. 3: 311-341. https://doi.org/10.3390/geomatics4030017

[2] Loeb, N.G., Ham, SH., Allan, R.P. et al. Observational Assessment of Changes in Earth’s Energy Imbalance Since 2000. Surv Geophys (2024). https://doi.org/10.1007/s10712-024-09838-8
https://link.springer.com/article/10.1007/s10712-024-09838-8

[3] Paulo Ceppi, Peer Nowack. Observational evidence that cloud feedback amplifies global warming. Proceedings of the National Academy of Sciences Jul 2021, 118 (30) e2026290118; DOI: 10.1073/pnas.2026290118 https://www.pnas.org/content/118/30/e2026290118

[4] 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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