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Someone I know posted a question about climate change that I do not know how to answer. While I do believe (for lack of better word) in climate change, I do not know how to answer this persons question. The tl;dr of the question is:

If climate change is not a natural phenomenon, then how did the last ice age end?

The person is implying that warming/cooling reoccurs periodically on the Earth, and that the current climate change threat is overblown.

I have searched online for answers, but cannot find anything concise on this specific subject. According to this wikipedia article,

There is evidence that greenhouse gas levels fell at the start of ice ages and rose during the retreat of the ice sheets, but it is difficult to establish cause and effect (see the notes above on the role of weathering). Greenhouse gas levels may also have been affected by other factors which have been proposed as causes of ice ages, such as the movement of continents and volcanism.

In short, how and why did the last ice ages end? I am assuming that the carbon dioxide concentration during these ice ages is much lower than the current estimate of ~410 ppm. Aside from the difference in carbon dioxide levels, what evidence suggests that the climate change of today is different than the climate change that ended the ice age(s)?

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  • $\begingroup$ Just to be clear, when you write "ice age" you're asking about the last glaciation rather than the current ice age, i.e., popular vs scientific use of the phrase? $\endgroup$ – Deditos Oct 4 at 9:49
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    $\begingroup$ @Deditos I am referring to the last glaciation (ie, not the current one). To my understanding (please correct me if I am incorrect), the current ice age would be dramatically worse/colder without human-induced climate change. $\endgroup$ – allthemikeysaretaken Oct 4 at 10:43
  • $\begingroup$ I think the answers have interpreted it as you intended, so that's good. Note that the last glaciation (ended ~10 ky ago) is a period within the current ice age (started 2.5 My ago and ongoing). We're currently in an interglacial period rather than a glaciation, but whether the impact of humans is dramatic enough geologically is still being debated. $\endgroup$ – Deditos Oct 8 at 11:14
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Difference #1: Speed. The warming at the end of the ice ages went much, much slower than what we are currently experiencing.

Estimates of the amount of warming since the beginning of the industrial revolution are in the order or 1 degree Celsius* - that's in 100-150 years time.

* Page 7

When the Earth moved out of ice ages over the past million years, the global temperature rose a total of 4 to 7 degrees Celsius over about 5,000 years.

(NASA Source: How is Today’s Warming Different from the Past?)

The article Earth is warming 50x faster than when it comes out of an ice age sums it up nicely:

What humans are in the process of doing to the climate makes the transition out of the last ice age look like a casual stroll through the park. We’re already warming the Earth about 20 times faster than during the ice age transition, and over the next century that rate could increase to 50 times faster or more. We’re in the process of destabilizing the global climate far more quickly than happens even in some of the most severe natural climate change events.

That rapid climate destabilization is what has climate scientists worried. It’s faster than many species can adapt to, and could therefore cause widespread extinctions, among other dangerous climate change consequences. Coastal flooding in places like Florida has already become much more common than it was just 50 years ago, and sea level rise is expected to keep accelerating.

Difference #2: Civilization. At the end of the ice ages we did not have 3 billion people living within 200 km from a coast line
That is not an difference in the cause of the warming, but definitely in the effect.

One could argue that that same civilization will make us able to deal with the consequences, but looking at thing that already exists several centuries without change, surely raises doubts to our ability to deal with the current rate of climate change. Think (1) unequal global wealth distribution or (2) the psychology of our behavior (denial, politics).


BTW The question asked to you

If climate change is not a natural phenomenon, then how did the last ice age end?

is based on a false premise - it's a bit of a word game.
The current warming is still a natural phenomenon, but it's now primarily driven by our input, and not other 'natural' phenomena.

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  • $\begingroup$ Comment to the community regarding my last statement: IIRC there was a Q/A on this site quoting research to the contribution of different factors (like brightness of the sun, orbital variations etc), nicely showing graphs for each factor and all combined. But I could not find that to link to in that last sentence. $\endgroup$ – Jan Doggen Oct 4 at 9:22
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There is a major difference. The climate changes that caused the end of the last Ice Age (and the start/end of previous ones) were basically the result of changes in the Earth's orbital eccentricity and axial tilt. These changes are called the Milankovich Cycles, and you can read more about them here: https://en.wikipedia.org/wiki/Milankovitch_cycles

The current global warming that is often called "climate change" is the result of humans burning fossil fuels. This adds more CO2 to the atmosphere. The addition is measured, e.g. https://en.wikipedia.org/wiki/Keeling_Curve and correlates well to the amount of coal, oil, and natural gas burned. That atmospheric CO2 traps solar radiation*, causing the Earth to warm up.

*OK, it's a little more complex than that. What happens is that the solar radiation (light) passes through the atmosphere and warms the ground. The warm ground emits infrared radiation, which the CO2 blocks. It's rather like putting on an extra sweater when you're cold. If you want a detailed explanation, Weart's "The Discovery of Global Warming" is a good on-line introduction: https://history.aip.org/climate/index.htm

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  • $\begingroup$ Regarding the first point, I was under the impression that the eccentricity of the Earth’s orbit was very close to zero (nearly circular orbit); has the eccentricity and axial tilt changed considerably since the last ice age, and if not, then why has its impact varied from then to today? To quote from the answer posted here, "Note that Milankovitch cycles' effect on long-term climate is not clear". $\endgroup$ – allthemikeysaretaken Oct 4 at 2:40
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    $\begingroup$ @allthemikeysaretaken: A bit of cherry picking, it seems :-) The full quote at that link is "(Note that Milankovitch cycles' effect on long-term climate is not clear. This quote is pulled, out of context, from 1982, 37 years ago. We've gotten better at climate science since.)" If you read the Wikepedia link and/or the Weart book, you will know about as much as I do, and find that it's much better written than I could manage. $\endgroup$ – jamesqf Oct 4 at 17:29

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