CO2 saturation effect

[Clive Elsworth]

All

 

In yesterday’s NOAC meeting I mentioned CO2 saturation but didn’t know what it was. The following from Gemini is clearly explained and seems plausible to me:

 

In the context of climate change, the CO2 saturation effect (sometimes called the "Saturation Logic") refers to the diminishing marginal impact of adding more carbon dioxide to the atmosphere once certain concentrations are reached.

 

It is a common point of discussion because, while it is based on a real physical principle, it is often misinterpreted to mean that additional CO2 won't cause further warming.

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1. The Physical Basis: The Logarithmic Effect

The ability of CO2 to absorb infrared radiation is not linear; it is logarithmic.

 

In simple terms, the first 100 parts per million (ppm) of CO2 added to a "clean" atmosphere have a massive impact on trapping heat. However, as the concentration increases, the specific wavelengths of energy that CO2 absorbs become "saturated." Most of the heat trying to escape at those specific frequencies is already being captured.

 

Think of it like putting a black curtain over a window:

·         The first curtain blocks most of the light.

·         A second curtain makes it even darker, but the change isn't as dramatic as the first.

·         By the tenth curtain, you are adding more material, but the room is already dark.

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2. Why "Saturation" Doesn't Stop Global Warming

If the atmosphere were a single thin layer, warming might indeed stop once those wavelengths were fully absorbed. However, the Earth’s atmosphere is thick and layered, which leads to two critical reasons why adding CO2 continues to warm the planet:

 

A. The "Bucket" Effect (Altitude Shifting)

As we add more CO2, the "layer" of the atmosphere where heat finally escapes into space moves to a higher altitude.

·         The air at higher altitudes is thinner and colder.

·         Because colder air radiates less energy than warm air, the Earth becomes less efficient at shedding heat.

·         To restore balance, the entire atmosphere (including the surface) must warm up until the top layer is hot enough to radiate the necessary energy back into space.

 

B. Band Wing Expansion

While the "peak" absorption frequencies for CO2 are mostly saturated, the "wings" (the edges of the absorption bands) are not. As CO2 concentrations rise, these edges begin to soak up more energy in frequencies that were previously escaping into space.

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3. Current Scientific Consensus

The climate is nowhere near a point where CO2 ceases to be a driver of temperature. Scientists use the concept of Climate Sensitivity to measure this, generally estimating that every doubling of CO2 concentrations results in a 2.5°C to 4°C increase in global average temperature.

While the rate of warming per molecule of CO2 decreases as we add more, the total amount of energy trapped continues to rise, pushing the global climate system further out of equilibrium.

 

My only beef is that I prefer to trust Jim Hansen’s climate sensitivity of 4 to 5°C.

 

Anyone got anything else to add or change about CO2 saturation?

 

Clive

[Sev Clarke]

The logarithmic saturation effect means that GHG emissions reduction and Removal are now less effective means of reversing global warming than are the better SRM or TRM ones, with the possible exception of ISA nanoparticulates to remove tropospheric methane, ozone, etc. 

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[robert...@gmail.com]

Sev

Being picky, it isn't that emissions reductions and removal are now less effective than SRM.  They are equally effective in the sense that 1Wm^-2 reduced forcing from either would deliver a similar amount of cooling.  The central point is that because of CO₂'s low radiative efficiency the absolute amount of it that has to be removed from the atmosphere is infeasibly huge.  With increased ppm over the last few decades, this problem has gradually been exacerbated but it was always a problem.  The reason it's now such a big problem is that so much more warming (from both reduced outgoing longwave radiation and increased absorbed solar radiation) has occurred in recent decades that the decarbonisation only route numbers no longer add up.

Regards

Robert

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[rob...@rtulip.net]

Hi Clive

 

I raised the discussion of saturation yesterday in my response to your citing of the climate denier Mark Mills on Energy and Climate who discussed saturation in the talk you linked.

 

Mills falsely stated “Climate science is getting better. The later IPCC forecasts are ramping down the apocalyptic temperatures. They're talking about, you know, saturation.”

 

This is completely untrue.  The saturation point is explained at https://skepticalscience.com/co2-not-saturated-basic.html.   CO₂’s greenhouse effect is not near saturation at current atmospheric levels. The IPCC still treats CO₂ forcing as increasing approximately logarithmically with concentration, and assesses the effective radiative forcing from a doubling of CO₂ at about 3.93 W/m².  Of course Hansen calculates that sensitivity is even higher.  We are nowhere near a regime where adding more CO₂ has no further radiative effect as Mills wrongly misinforms.

 

Some individual CO₂ absorption lines are strong at their centres, but that does not mean the overall greenhouse effect has maxed out. Additional CO₂ still increases warming because it broadens absorption into the line wings and raises the effective altitude from which Earth emits infrared radiation to space. That is why the forcing curve flattens somewhat, but does not stop; it becomes logarithmic, not zero.   Current CO₂ additional increments still add radiative forcing, though with diminishing marginal effect per ppm. NOAA’s AGGI also shows that greenhouse-gas radiative forcing kept rising through 2024, with CO₂ still the dominant contributor.

 

Regards

 

Robert Tulip

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[Sev Clarke]

Robert,

I applaud your pickiness. Yes, SRM/TRM/albedo has had to become our primary and quickest acting set of defences against global warming because we have let GHG atmospheric concentrations become unmanageably high over the past several decades.

Consequent of this

AI Overview

The highest atmospheric carbon dioxide (\(\text{CO}_{2}\)) concentrations that mammals (including their direct synapsid ancestors) have experienced are estimated to have occurred during the Triassic period(roughly 252 to 201 million years ago), with levels potentially reaching 2,000–3,000 ppm. [1, 2, 3, 4]

While some generalized studies suggest CO2 was even higher early in Earth's history, the peak levels relevant to the earliest recognizable mammal ancestors (therapsids) and true mammals in the early Mesozoic Era consistently indicate elevated greenhouse conditions, often several times higher than today's \(\approx 420 \text{ ppm}\) levels. [1, 2, 3]

Key Details on Atmospheric History for Mammals:

• The Triassic Period (252-201 Ma): The earliest ancestors of mammals and the first true mammals evolved under high \(\text{CO}_{2}\) conditions, estimated to be up to 5 times higher than present levels.
• Thermal Maximum (56 Ma): During the Paleocene–Eocene Thermal Maximum (PETM), CO2 levels were again high, contributing to rapid global heating.
• Late Eocene (51 Ma): Reconstructed records suggest a "hothouse" peak of approximately 1600 ppm.
• Oxygen Levels: While \(\text{CO}_{2}\) was high, atmospheric oxygen levels in the early Triassic may have been significantly lower than today (possibly as low as 10-15%), challenging for modern mammals but adapted to by early therapsids. [1, 2, 3, 4, 5, 6]

While modern humans have only existed during lower-\(\text{CO}_{2}\) "icehouse" conditions (roughly \(180\text{--}300 \text{ ppm}\)), early mammals and their ancestors thrived in a much warmer, high-carbon

we might well do quite well at CO2 levels up to 1,000ppm (more than 1,500ppm becomes a health hazard for humans) - provided that our direct cooling methods bring us down to pre-industrial temperatures by 2050, and that there is enough primary production and oxygenation to prevent ocean acidification in surface waters.

Regards,

Sev

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[Tom Goreau]

It’s worth noting that while our ancestors in hyper-Greenhouse days had to be adapted to extreme high temperatures, the highest temperature-adapted fauna and flora largely went extinct during the severe cooling that caused the Ice Ages, while extreme cold-adapted species survived.

 

So while modern corals, and humans, had ancestors that thrived before the Ice Ages when it was much warmer, genetic capability to re-evolve tolerance to extreme heat again may have been lost during the last 3 million mostly frigid years.

 

It takes time for high temperature tolerance to re-evolve, for coral reefs around 3-4 million years after the major hyperthermal mass extinctions.

 

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