I haven’t done a paper debunk for a while, but a reader got in touch to ask about a recent paper by Hans Rolf-Dübal and Fritz Vahrenholt, so I thought I would have a quick look. The paper is Radiative Energy Flux Variation from 2001–2020 and is in the open-access journal Atmosphere. The paper doesn’t actually draw any strong conclusions, but it does stress that [t]he declining TOA SW (out) is the major heating cause (+1.42 W/m2 from 2001 to 2020), where TOA stands for top-of-atmosphere, and SW is short-wavelength.
What the paper is essentially claiming is that most of the warming over the period 2001-2020 is due to a reduction in cloud albedo which then leads to more absorbed solar radiation. This has then been used by some to claim that [c]hange in clouds likely cause of warming in the past 20 years.
I haven’t worked through the details in the paper to really know if the results they present are correct, but I don’t think the results presented in the paper are necessarily all that surprising. I think it is mostly related to what I was highlighting in this post.
There can sometimes be a rather simplistic idea about how global warming actually happens. The simple view is that adding greenhouse gases to the atmosphere reduces the outgoing longwavelength flux, producing a planetary energy imbalance, which then causes the system to warm until it returns to energy balance. What actually happens is – unsurprisingly – a little more complicated.
It is correct that adding greenhouse gases to the atmosphere does reduce the outgoing longwavelength flux and does produce a planetary energy imbalance. However, as pointed out in this paper, as the system warms, there are then feedback responses that can further enhance the reduction in outgoing longwavelength radiation (water vapour). However, there are also other responses that can counter-act this (clouds), and others (also clouds) that can lead to a reduction in albedo and, consequently, an increase in the amount of absorbed solar radiation.
As this paper then illustrates, when energy accumulates in the climate system due to an enhancement in atmospheric greenhouse gas concentrations, this accumulation is primarily due to an increase in absorbed solar radiation, rather than simply being due to an imbalance in the long-wavelength fluxes. However, this doesn’t somehow contradict that adding greenhouse gases to the atmosphere will cause the system to warm, or suggest that changes in clouds are causing most of the warming.
So, although I haven’t work through the Dübal and Vahrenholt paper in detail, the basic result they present seems broadly consistent with what is expected. That they find that most of the warming over the 2001-2020 period was due to a reduction in cloud albedo doesn’t really contradict our understanding of greenhouse warming and doesn’t suggest that most of the warming over this period was due to changes in clouds.
Most of the warming is almost certainly due to the human emission of greenhouse gases into the atmosphere. How clouds then respond to the subsequent warming then leads to most of the accumulated energy being due to an increase in absorbed solar radiation. If anything, as highlighted in the video in this post, this might actually be suggesting that equilibrium climate sensitivity is well above 2oC, rather than highlighting some major challenge to our understanding of greenhouse warming.
Links:
Outgoing longwave radiation – post I wrote explaining why most of the accumulated energy is due to increased absorded solar radiation.
Global warming due to increasing absorbed solar radiation – paper by Trenberth and Fasullo.
Shortwave and longwave radiative contributions to global warming under increasing CO2 – paper by Donohue et al.