No, the answer is much simpler than that. Think how smart you'll feel when you know it! Like Socrates unbound.

Well, the pop-sci story that I've seen is that at the beginning of the Carboniferous period the climate was warm and humid, but during the later part of the period, as carbon was sequestered from the atmosphere, both the CO2 concentration and the temperature declined to approximately present levels.

Right. The Carboniferous period is very suitable for showing that CO2 concentration and temperature track. It was like our world backwards. Life changed the climate by storing CO2.

The Carboniferous period was characterized by dense, widely distributed forests which consumed much of the atmospheric carbon (CO2). Over millions of years the trees fell layer on layer and did not rot -- re-releasing CO2 that had been sequestered. as the timbers sank, were covered, compressed, and heated the plant matter was converted into coal. As the levels of CO2 declined, so did the temperature, and with the temperature declines, the formation of polar ice caps and glaciation.

Mammals, by the way, weren't around in the Carboniferous period -- they appeared around the time the dinosaurs appeared (within 10 million years).

Somewhere along the line I read that the reason the dead plant matter in the carboniferous period didn't rot was that the organisms that are capable of digesting wood (lignin, etc.) like fungi, hadn't evolved yet, or hadn't evolved far enough. So the wood didn't rot (and thus release the captured CO2 back into the atmosphere).

As the levels of CO2 declined, so did the temperature, and with the temperature declines, the formation of polar ice caps and glaciation. — Bitter Crank

Correct. Glaciers have a significant impact on the mean temperature. The world was unglaciated through much of the Carboniferous. We would expect the average temperature over the whole period to reflect that. (Get it? Reflect that)

Mammals, by the way, weren't around in the Carboniferous period -- they appeared around the time the dinosaurs appeared (within 10 million years). — Bitter Crank

That's correct professor. The ancestors of both were on the scene during the Carboniferous.

I'm all fascinated by the emergence of mammals these days, so I came across this odd piece of information about the Carboniferous period: atmospheric CO2 concentration was around 800 ppm (twice the present level, but down from 7000 ppm earlier in the evolution of life). Yet the mean surface temperature was 14C. It's now 14C.

Anybody know why this is? — frank

The Sun is a yellow dwarf main sequence star. Main sequence stars grow brighter over time. The Sun was thus several percents dimmer several hundreds of years ago. This explains why the main surface temperature wasn't much higher, it at all, during the Carboniferous in spite of the higher CO2 concentration. (Also, CO2 forcing is a logarithmic function of concentration rather than a linear function).

Are you sure you're thinking of the Carboniferous? That was only 300 million years ago.

Also, CO2 forcing is a logarithmic function of concentration rather than a linear function). — Pierre-Normand

Would you say this is a more significant factor than the impact of glaciation?

"Average global temperatures in the Early Carboniferous Period were hot- approximately 20° C (68° F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12° C (54° F). As shown on the chart below, this is comparable to the average global temperature on Earth today!

Similarly, atmospheric concentrations of carbon dioxide (CO2) in the Early Carboniferous Period were approximately 1500 ppm (parts per million), but by the Middle Carboniferous had declined to about 350 ppm -- comparable to average CO2 concentrations today!

Earth's atmosphere today contains about 380 ppm CO2 (0.038%). Compared to former geologic times, our present atmosphere, like the Late Carboniferous atmosphere, is CO2- impoverished! In the last 600 million years of Earth's history only the Carboniferous Period and our present age, the Quaternary Period, have witnessed CO2 levels less than 400 ppm."

From here: https://www.geocraft.com/WVFossils/Carboniferous_climate.html

When I saw your OP I immediately wondered whether 800 ppm could have been the CO2 concentration in the atmosphere over the whole approximately 60,000,000 year period that is called the Carboniferous Period; such a thing seemed unlikely.

The article cited, if correct, seems to substantiate that doubt.

Right. The figures I gave are averages. If the average temp was 14, why wasn't the average CO2 concentration 400 ppm?

Three answers have been given. There are probably more.

So you're talking about the average temperature and the average CO2 concentration over a 60,000,000 year period? What's the point in that?

Would you say this is a more significant factor than the impact of glaciation? — frank

Over large timescales, glaciation is an effect rather than a cause. Snow and ice albedo functions as a feedback. It's the sum of the forcings (mainly greenhouse gas forcing and solar forcing) that is the independent variable and that determines whether or not glaciation is supported. When glaciation is supported by a low enough total forcing, glaciation ensues and the snow/ice albedo feedback lowers the temperature even further.

If they tracked proportionally, that would work.

So why don't they track proportionally? Think about how glaciers form. It's a case of positive feedback.

Also, you'd be one of those people asking Socrates: what is the point of asking what clouds are .

Over large timescales, glaciation is an effect rather than a cause. Snow and ice albedo functions as a feedback. It's the sum of the forcings (mainly greenhouse gas forcing and solar forcing) that is the independent variable and that determines whether or not glaciation is supported. When glaciation is supported by a low enough total forcing, glaciation ensues and the snow/ice albedo feedback lowers the temperature even further. — Pierre-Normand

It's hard to see how solar forcing would be a significant factor in large scale ice ages, which come and go. We're in one now, obviously.

Does solar luminosity vary significantly over time?

But yes, glaciation has to be triggered.

Are you sure you're thinking of the Carboniferous? That was only 300 million years ago. — frank

The Sun is only four and a half billion years old. 300 million years ago is about 7% of its age. One estimate that I've seen is that the total solar irradiance increased by about 4% over the lase 400 million years ago. That would translate into a forcing change of 6.75 W/'m^2 over the last 300 million years. This is just about the same as the effect from a fourfold increase of atmospheric CO2 concentration. Hence, other things being equal (e.g. same continental mass distribution), an atmospheric concentration of 1200ppm, 300 million years ago, would have yielded the same surface temperature as the recent pre-industrial era (300ppm).

Right. The figures I gave are averages. If the average temp was 14, why wasn't the average CO2 concentration 400 ppm? — frank

Because of co2 dissolving in water.

One estimate that I've seen is that the total solar irradiance increased by about 4% over the lase 400 million years ago. — Pierre-Normand

Is that considering the faint young sun paradox? If so, I'm curious how they're solving it.

It's hard to see how solar forcing would be a significant factor in large scale ice ages, which come and go. We're in one now, obviously. — frank

One mustn't confuse the glacial/interglacial periods that are occurring within the current ice age with major ice ages. The former is governed by the Milankovitch cycles and is modulated by the ice albedo and carbon cycle feedbacks.

Does solar luminosity vary significantly over time?

Indeed it does. As I pointed out above, the variation over the last 300 million years is equivalent to a fourfold decrease in CO2 concentration.

One mustn't confuse the glacial/interglacial periods that are occurring within the current ice age with major ice ages. The former is governed by the Milankovitch cycles and is modulated by the ice albedo and carbon cycle feedbacks. — Pierre-Normand

I'm not. That's why I specified "large scale ice ages" to avoid confusion. They do come and go and we are in one now.

Indeed it does. As I pointed out above, the variation over the last 300 million years is equivalent to a fourfold decrease in CO2 concentration. — Pierre-Normand

It would have to actually oscillate to track large scale ice ages.

s I pointed out above, the variation over the last 300 million years is equivalent to a fourfold decrease in CO2 concentration. — Pierre-Normand

thats actually the entire problem of averages in one sentence, because before man-generated co2 since the atmosphere was first cooled down by plants consuming co2 and generating oxygen, sun radiation has been a larger varying factor, as well as, of course, cloud cover, which is almost entrely unkowable.

It would have to actually oscillate to track large scale ice ages. — frank

The total forcing (solar + greenhouse) and the continental mass distribution effect on albedo feedback and ocean circulation tack large scale ice ages; and not any one single factor in isolation.

and not any one single factor in isolation. — Pierre-Normand

Of course. And about the faint young sun paradox?

Im glad to find the topic is settled and we can return to thinking about philosophy then

thats actually the entire problem of averages in one sentence, because before man-generated co2 since the atmosphere was first cooled down by plants consuming co2 and generating oxygen, sun radiation has been a larger varying factor, as well as, of course, cloud cover, which is almost entrely unkowable. — ernestm

In recent times solar variations have provided very small forcing variation compared with the enhanced greenhouse gas forcing. See the second and third graphs in this web page.

The magnitude of the cloud feedback is the main source of uncertainty regarding climate sensitivity to radiative forcing. But it's a feedback, so it merely amplifies or mitigates climate change, whatever its cause.

Of course. And about the faint young sun paradox? — frank

The problem in the OP stemmed from only considering CO2 variation and ignoring solar variations. The faint young sun paradox stemmed from only considering solar variations and ignoring CO2 variation. Taking into account both solar and CO2 forcing solves both problems.

Also, you'd be one of those people asking Socrates: what is the point of asking what clouds are . — frank

Socrates was asking because he wanted to see if the people talking knew what they were talking about. Most often they did not. So that's the point to Socrates asking what clouds are, to demonstrate that the people talking about clouds didn't really know what they were talking about, because they couldn't even say what clouds are.

Another factor: Did it make any difference that the distribution of land was much different during the carboniferous? The continents were not distributed, but were clumped together:

The Carboniferous was marked by the progressive formation of the supercontinent Pangea. The present day Northern Hemisphere landmasses moved towards the equator to form Laurasia and to join the large Southern Hemisphere landmass Gondwana. The collision between Siberia and Eastern Europe created the Ural Mountains, and China was formed with the collision of several microcontinents and Siberia. The collision between Gondwana and Laurasia led to the formation of the Appalachian belt in North America and the Hercynian Mountains in Europe. Gondwana also shifted towards the equator while the continents moved from east to west.

[Tectonics and Paleoclimate ----> http://www.ucmp.berkeley.edu/carboniferous/carbtect.html ]

The problem in the OP stemmed from only considering CO2 variation and ignoring solar variations. The faint young sum paradox stemmed from only considering solar variations and ignoring CO2 variation. Taking into account both solar and CO2 forcing solves both problems. — Pierre-Normand

The faint young sun paradox is not actually solved. The quantity of CO2 that would be required to account for the young climate would have left a mineral behind that is absent from the young rocks. Plus it doesn't explain why there was liquid water on Mars at the time. A larger early sun that blew off mass in a large solar wind is one possibility on the table.

Another factor: Did it make any difference that the distribution of land was much different during the carboniferous? The continents were not distributed, but were clumped together: — Bitter Crank

I don't know. It would have affected the ocean currents and so heat conveyors. I'll look into it.

Socrates was asking because he wanted to see if the people talking knew what they were talking about. Most often they did not. So that's the point to Socrates asking what clouds are, to demonstrate that the people talking about clouds didn't really know what they were talking about, because they couldn't even say what clouds are. — Metaphysician Undercover

Not according to Aristophanes. He just wondered what they were.