I recently came across a Climate Change debate on LinkedIn that shed light on the competing arguments regarding AGW. The debate was interesting because it was conducted by people who clearly knew the subject very well and were prepared to make their cases at length without descending into ad hominem attacks. Also, the detail provided, whilst scientific was not too technical. I was mostly able to follow the logic being presented. I have replicated the transcript here, together with a Glossary and some context explainers.
The starting point was Tom Harris presenting an article he had written. Link below. Tom is a firm believer in AGW. Simon Bevan, a member of the Royal Meteorological Society, provided constructive feedback on that article and it went from there.
https://drtomharris.substack.com/p/why-do-we-focus-on-carbon-dioxide.
An extract from Tom’s article:
‘Water capture is the number one greenhouse gas and its making things hotter but it’s not the cause of global warming. Fossil fuel emissions and land use change control the thermostat.’
Glossary:
IPCC: International Panel on Climate Change is a UN body that was created to find evidence of man-made Climate Change.
um: correctly known as ‘μm’: This is a measurement of wavelength known as ‘micrometre’. One ‘μm’ is one millionth of a metre. CO2 primarily absorbs infrared radiation in two main wavelength bands: a strong band centered at 15 μm and a secondary band centered at 4.26 μm (often referred to as the 4.3 μm band).
ERB: Earth’s Radiation Budget
HITRAN: High-resolution Transmission molecular absorption database.
CERES: The Clouds and the Earth’s Radiant Energy System project provides satellite-based observations of ERB and clouds.
EEI: Earth’s Energy Imbalance. It is the balance of incoming and outgoing radiation.
ECS: Equilibrium Climate Sensitivity is a metric in climate science that measures the long-term global temperature increase resulting from a doubling of atmospheric CO2 levels relative to pre-industrial levels.
TOA: Top Of Atmosphere. Refers to measurements far above the Earth.
Context:
• Tom Harris thinks that warming comes from CO2 trapping longwave radiation – i.e. ‘heat’ – that would otherwise escape into space, hence warming. Others think that reduced cloud cover or increased energy output from the sun cause warming. Simon Bevan is arguing the reduced cloud cover aspect as this phenomenon causes less solar radiation to be reflected back into space.
• CO2 mainly absorbs radiation – at a certain point of the infra-red spectrum whereas Water Vapour absorbs radiation all across the infra-red, hence an overlap exists. That overlap increases at lower altitudes.
• Incoming solar radiation is shortwave whereas reflected radiation, emitted to space is longwave.
• Increasing atmospheric CO2 reduces the amount of outgoing longwave radiation (OLR) that escapes directly to space, thereby trapping more heat in the climate system. Satellite measurements confirm this effect, showing a drop in outgoing radiation at CO2 absorption wavelengths (specifically around 15 micrometers).
• Water vapour absorbs both SW and LW radiation.
• Clouds reflect solar radiation. Less cloud cover contributes to higher temperatures.
• AGW proponents argue that reduced escape of LW radiation causes feedback loops that further increase temperatures – less snow/ice meaning less reflected solar radiation and more atmospheric water vapour, from warming oceans, meaning less escaping LW radiation.
Here is the thread. I have applied bold font to pertinent points:
Simon Bevan comment:
‘The article is repeating the standard IPCC-style explanation. The water vapour (WV) continuum adds a major layer of uncertainty. WV does not absorb infrared radiation only at sharp spectral lines, it also has a broad “continuum” absorption that spreads across wide parts of the IR (infra red) spectrum. In the real, humid atmosphere (i.e. tropics), this continuum overlaps heavily with CO2’s main 15 μm band. Lab measurements used in HITRAN significantly underestimate how much WV is already absorbing IR radiation in the real world. Adding extra CO2 produces less additional warming than standard models assume. CERES data strongly support this: recent EEI gain has
been dominated by +1 to +2 W/m² of extra absorbed solar radiation from declining cloud cover, not by any clear rise in LW [Long Wave] radiation from GHG [Greenhouse Gases]. The WV continuum is one of the largest remaining uncertainties in radiative transfer and is a key reason why effective CO2 forcing in the real atmosphere appears smaller than commonly claimed.’
Tom Harris to Simon Bevan:
So what your saying is the amplification factor of WV is even higher than modelled. This makes ECS [Equilibrium Climate Sensitivity]
even higher than hoped and
the need to reduce long lived GHG emissions even more
important.
AF: Tom is making the assumption that there is more water vapour due to warmer temperatures due to LW radiation trapping by CO2 (I.e. amplification factors).
Simon Bevan to Tom Harris:
‘No, that’s the opposite of what I’m saying. I’m saying the effective forcing from extra CO2 is likely weaker than models assume because of:
1) Strong water vapour
continuum absorption already blocking a lot of infrared in the real humid atmosphere.
2) Pressure broadening causing more overlap between CO₂ and water vapour bands [at lower altitudes].
3) The fragile ocean skin layer reducing longwave heat transfer into the oceans (>SS4 50-60% time).
These factors mean extra CO2 adds less additional warming than the standard lab-calibrated models predict. CERES data reinforces this: recent planetary heat gain has been dominated by +1 to +2 W/m² extra absorbed sunlight from declining clouds, not by rising longwave radiation from greenhouse gases.
If shortwave changes are doing most of the heavy lifting, this points to lower, not higher, effective climate sensitivity to CO2. The amplification argument only works if you assume CO₂ is the main driver. The observations suggest it is not.
Ron K enters the chat:
Ron K to Simon Bevan
‘Wrong again, Simon Bevan BSc, MBA, FRMetS. Declining clouds means warmer days, colder nights, equal temperatures. More CO2 means more energy, day and night. CO2 is the main driver.’
Simon Bevan to Ron Koenigs
‘No, that’s incorrect.
Declining cloud cover does not produce “equal temperatures.” It produces net warming.
CERES satellite data show declining clouds have caused a large +1 to +2 W/m² increase in absorbed solar radiation (daytime energy gain). This extra energy input far outweighs any extra nighttime longwave loss. The net result is a positive energy imbalance which is exactly what we observe.
More CO2 does add energy day and night – no one disputes the basic greenhouse mechanism. The question is how much it adds compared to other factors. CERES shows the recent planetary heat gain is dominated by the shortwave increase from fewer clouds, not by rising longwave from CO2.
Saying “CO2 is the main driver” while ignoring the largest observed term in the actual energy budget (shortwave) is not accurate. The data show shortwave/cloud changes have been the dominant driver of recent warming.’
Ron Koenigs to Simon Bevan:
“This extra energy input far
outweighs any extra nighttime longwave loss” because we caused 54% more CO2 that prevents nighttime longwave loss. Energy increase caused by CO2 increase is 2,29 W/m². The temperature increase, that needs time to reach that value, is 1,86 degrees. Science. Calculated with the scientific formula. So today’s 1,5 degrees increase is completely real. CO2 is the main driver. Stop burning fossil.’
Simon Bevan to Ron Koenigs
‘No – you’re mixing theoretical forcing with real-world observations. The 2.29 W/m² figure is the theoretical radiative forcing from CO2 increase based on lab models. It is not a direct measurement of the actual energy imbalance in the climate system.
CERES satellites measure the
real top-of-atmosphere
radiation budget globally. They
show the recent planetary
energy imbalance is
dominated by +1 to +2 W/m² extra absorbed solar radiation
from declining cloud cover not by reduced nighttime longwave loss from CO2. If shortwave changes (clouds/ albedo) are driving most of the observed energy gain, then CO2 is not the main driver as you claim. The data show shortwave dominance, not longwave dominance. The 1.5°C warming is real. The debate is about how much is due to CO2 versus natural cloud variability and other factors. CERES and the muted tropical hotspot strongly suggest your calculation overstates CO₂’s role.
Chris Wilke then addressed a few words to Simon the nature of which you can determine from Simon’s response.
Simon Bevan to Chris Wilke:
‘No, I understand the difference between forcing and feedback perfectly.
The issue is not whether water
vapour acts as a positive feedback – it does. The real debate is how strong that feedback actually is in the real world, and whether CO2 is the dominant primary forcing. When CERES satellites show the recent energy imbalance is dominated by +1 to +2 W/m² extra absorbed sunlight from declining clouds (shortwave), not by increased longwave trapping, it suggests the net feedback loop (including water vapour and clouds) is not behaving as strongly as high-sensitivity models assume.
If shortwave changes are doing most of the heavy lifting, then the effective sensitivity to CO2 is lower, not higher. That is the point. Dismissing this as “not understanding forcing vs feedback” is just evasion. The observations (CERES, muted hotspot, ocean skin layer) are the ones challenging the high-sensitivity narrative – not a misunderstanding of basic concepts.’
We hadn’t heard from Tom for a while…
Tom Harris to Simon Bevan:
‘In a balanced system an increase in shortwave absorption would be released and allowed to escape through long wave radiation. The non-escaping long wave from clear sky data shows it is being trapped. Long wave from cloudy areas is increasing as clouds warm despite the area drop. It’s not one thing it’s a combination but without CO2 none of it would be happening since the rest are feedbacks.’
Simon Bevan to Tom Harris
‘The net global energy budget tells the real story. In a balanced system, extra SW absorption should eventually be balanced by increased LW escape to space. But CERES data (global TOA) show recent EEI is dominated by the SW increase (+1 to +2 W/m² from declining cloud cover), not by a LW deficit.
I agree it’s a combination. IPCC treats this as a +ve feedback to CO₂ warming, so the scale and timing raise questions about how much is natural variability versus forced response. Because cloud (and ice) albedo changes are so powerful and uncertain (±124% uncertainty), they remain one of the biggest reasons why effective climate sensitivity to CO2 may be significantly lower than high-end claims.
So, if the SW [short-wave] term is the largest single contributor to the observed imbalance, treating it mainly as a CO2 feedback becomes circular reasoning. The data show SW / cloud changes have been doing most of the heavy lifting. This, to me, points to lower, not higher, effective sensitivity to CO₂.’
Tom Harris to Simon Bevan:
‘I agree, but if the cloud changes are a feedback of warming, which they seem to be, then that would push ECS upwards.’
Simon Bevan to Tom Harris
‘To me that is the standard assumption, but it’s circular.
Yes, if the large decline in cloud cover (with +1 to +2 W/ m² extra absorbed sunlight) is mainly a feedback to CO₂ warming, then ECS would indeed be higher. But CERES data show this shortwave increase is the dominant term in the recent EEI – larger than the incremental CO2 longwave forcing over the same period. Treating the biggest observed driver as purely a CO2 feedback is an assumption, not a proven fact.
If a substantial part of the cloud reduction is natural variability (or only weakly related to CO2), then the amount of warming that needs to be explained by CO₂ is smaller. That points to lower, not higher, effective sensitivity.
The muted tropical hotspot after 46 years and the ocean skin-layer limitations add further evidence that overall amplification is weaker than high-sensitivity models assume.
The data show strong shortwave dominance. Assuming it must all be a CO2 feedback is begging the
question.’
Ron Koenigs to SB
Wrong again, Simon Bevan BSc, MBA, FRMetS, I’m calculating measured increase of CO2 and temperature with scientific formula’s. CO2 is the main driver, your conclusions are only yours, non scientific. Temperature rise started when CO2 started to rise due to burning fossil. Repeating nonsense doesn’t make it real. Temperatures can’t drop at night with clear skies, because energy is blocked by CO2. Average temperature rises. Proved in time. 50 years ago night temperatures went much lower than today when skies were clear, stay the same with cloudy skies. As a result of higher temperatures clouds change place. Warmer air can contain more moisture, can, because there are also very dry hot places on earth. There are longer dry periods due to the higher temperatures, so less moisture, as a result of high CO2. Other places have more moisture, more clouds, more and heavier rain. On average there is no cloud change. CO2 remains the main driver of climate disruption, already since we burn fossil.
Simon Bevan To Ron Koenigs: ‘You’re still equating correlation with causation.
Yes, CO2 has risen and temperatures have risen. No one disputes that basic timeline. The question is how much of the warming is actually caused by CO₂ versus other factors.
You calculate theoretical CO₂ forcing (2.29 W/m²) and declare it the “main driver.” But CERES satellite data – the best global measurement of Earth’s actual energy budget ― show the recent planetary imbalance is dominated by +1 to +2 W/m² extra absorbed solar radiation from declining cloud cover, not by reduced longwave loss from CO2. If shortwave/cloud changes are the largest observed term, then claiming “CO2 is the main driver” is an assumption, not a proven fact. The data point to shortwave dominance playing a major role.
Night-time temperatures have risen, but that does not override the global radiation budget measurements. CERES is real-world observation, not theory.’
Ron Koenigs to Simon Bevan:
‘Wrong, Simon Bevan BSc, MBA, FRMetS, every denier comes up with causation and correlation. First CO2 goes up due to burning fossil, temperature goes up, climate changes, then, much later there are less low clouds and, due to airtraffic, much more high clouds and even more CO2 in high altitudes. Those 2 at high altitudes re-radiate more. More energy on earth surface due to more CO2,
more high clouds, higher temperatures. Climate change.
Simon Bevan To Ron Koenigs: ‘You’re still doing exactly what you accuse others of assuming your conclusion.
You claim: CO₂ rises →
temperature rises → clouds
change → more warming. That is textbook circular reasoning. You assume CO2 is the main driver, then use the resulting cloud changes to “prove” CO2 is the main driver. CERES satellites measure the actual global energy budget. They show the recent planetary imbalance is dominated by +1 to +2 W/m² extra absorbed solar radiation from declining cloud cover. This shortwave increase is the largest observed term – larger than the incremental CO₂ forcing over the same period.
If shortwave/cloud changes are doing most of the heavy lifting, then CO2 is not the main driver as you insist. The data point to lower, not higher, effective sensitivity to CO2.
Repeating “CO2 is the main driver” while ignoring the dominant shortwave signal in global observations is not
science. It’s narrative protection. I will leave it there – plain and simple.’
Conclusion
• I learnt a lot from this debate.
• This debate gets to the heart of the differences between tge two sides. Climate realists say that water vapour fluctuations; cloud cover and incoming energy have more impact on global temps than CO2. AGW proponents respond that all of those factors result from the impact of CO2. I find myself on the side of the Realists.
• Computer models under-estimate how much ‘heat’ is being absorbed by water vapour and over-estimate how much heat is absorbed by CO2.
• CERES has oberved +1 to +2 W/m² extra absorbed shortwave solar radiation from declining cloud cover not from reduced nighttime longwave loss from CO2.
• Increased shortwave energy suggests lower, not higher, effective climate sensitivity to CO2 because the EEI is coming from additional radiation input rather than reduced radiation output.







