<p>It is a well known fact that the carbon dioxide concentration in the atmosphere 3 million years ago was similar to what it is today (~420 ppm): <a href="https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide" rel="nofollow noreferrer">1</a>, <a href="https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#Concentrations_in_the_geologic_past" rel="nofollow noreferrer">2</a>, <a href="https://www.climate.gov/news-features/climate-qa/how-much-will-earth-warm-if-carbon-dioxide-doubles-pre-industrial-levels" rel="nofollow noreferrer">3</a>. All the of the links I added there also state that this time period and its climate can thus be treated as a proxy for the climate outcomes of the modern day.</p>
<p>My question is why aren't the current climate outcomes <em>already</em> like those 3 million years ago? In particular, why is our current average temperature only about 1.5 Celsius above the pre-industrial average, rather than the 4 - 5 Celsius from 3 million years ago? None of the 3 links I mentioned provide such an (convincing, or any) explanation (not even the IPCC report linked to on Wikipedia).</p>
<p>The latest explanation I've heard for this is that it takes about 30 years for the &quot;full warming effects&quot; of carbon dioxide to be felt in the climate; since about half of all carbon dioxide we have ever emitted has been emitted since 1990, this would explain the substantially lower average temperature increase. The person I heard this from is <a href="https://hu.linkedin.com/in/soulbp" rel="nofollow noreferrer">this man</a>, in e.g. <a href="https://www.youtube.com/watch?v=jvNCk-PUvC0&amp;ab_channel=Fenntarthat%C3%B3visszafejl%C5%91d%C3%A9s" rel="nofollow noreferrer">this presentation</a>. Unfortunately, he is, and the presentation is in, Hungarian, so most people on this site won't understand much.</p>
<p>I have thus far not been able to find any scientific publications to support (or contradict) this assertion, so I'm wondering if it has any basis, or if it is simply an explanation offered up as to why our current climate is not like that from 3 million years ago.</p>

CC BY-SA 4.0
2024-09-10T04:47:03.793Z
@Universal_learner What is "Panama's istme" and why did it close? Is the rest of your answer explaining why the Earth was warmer 3 million years ago or why the Earth is current cooler? It seems to me that what you're saying would imply a cooler Earth. Furthermore, are you then claiming that the "30 years" argument holds no merit?
Borealis
CC BY-SA 4.0
2024-09-10T05:48:42.910Z
@Universal_learner That's interesting; do you have any sources/references I can read for further information? You said you heard this in university; do you still know the textbook you used?
Borealis
CC BY-SA 4.0
2024-09-10T07:18:08.460Z
@Universal_learner comment are not for answers.
planetmaker
<climate-change><atmosphere><climatology><paleoclimatology>
Why is the Earth's average temperature lower now than 3 million years ago, even though the carbon dioxide concentration is the same?
Borealis
# Why is the Earth's average temperature lower now than 3 million years ago, even though the carbon dioxide concentration is the same?

(Asked by: username_0 on 2024-09-10)

It is a well known fact that the carbon dioxide concentration in the atmosphere 3 million years ago was similar to what it is today (~420 ppm): [1](https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide), [2](https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere#Concentrations_in_the_geologic_past), [3](https://www.climate.gov/news-features/climate-qa/how-much-will-earth-warm-if-carbon-dioxide-doubles-pre-industrial-levels). All the of the links I added there also state that this time period and its climate can thus be treated as a proxy for the climate outcomes of the modern day.

My question is why aren't the current climate outcomes *already* like those 3 million years ago? In particular, why is our current average temperature only about 1.5 Celsius above the pre-industrial average, rather than the 4 - 5 Celsius from 3 million years ago? None of the 3 links I mentioned provide such an (convincing, or any) explanation (not even the IPCC report linked to on Wikipedia).

The latest explanation I've heard for this is that it takes about 30 years for the "full warming effects" of carbon dioxide to be felt in the climate; since about half of all carbon dioxide we have ever emitted has been emitted since 1990, this would explain the substantially lower average temperature increase. The person I heard this from is [this man](https://hu.linkedin.com/in/soulbp), in e.g. [this presentation](https://www.youtube.com/watch?v=jvNCk-PUvC0&ab_channel=Fenntarthat%C3%B3visszafejl%C5%91d%C3%A9s). Unfortunately, he is, and the presentation is in, Hungarian, so most people on this site won't understand much.

I have thus far not been able to find any scientific publications to support (or contradict) this assertion, so I'm wondering if it has any basis, or if it is simply an explanation offered up as to why our current climate is not like that from 3 million years ago.

**Tags:** climate-change, atmosphere, climatology, paleoclimatology

> **Comments:**
> **username_0**: @Universal_learner What is "Panama's istme" and why did it close? Is the rest of your answer explaining why the Earth was warmer 3 million years ago or why the Earth is current cooler? It seems to me that what you're saying would imply a cooler Earth. Furthermore, are you then claiming that the "30 years" argument holds no merit?  
> **username_0**: @Universal_learner That's interesting; do you have any sources/references I can read for further information? You said you heard this in university; do you still know the textbook you used?  
> **username_1**: @Universal_learner comment are not for answers.  

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