A possible answer to your “how much” question can be found in the latest IPCC report
. From the “Summary for Policymakers”: “A.1.3 The likely
range of total human-caused global surface temperature increase from 1850–1900 to 2010–2019 is 0.8°C to 1.3°C, with a best estimate of 1.07°C. It is likely
that well-mixed GHG
s contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by –0.1°C to 0.1°C, and internal variability changed it by –0.2°C to 0.2°C. It is very likely
that well-mixed GHG
s were the main driver of tropospheric warming since 1979, and extremely likely
that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s.” (Or, to put it simply: probably all of it.)
In your first video with Ivar Giæver, much of the physics is correct, but at 1:45 the “interviewer” makes the claim that for the last 18 years, temperatures have been flat. The rise in measured surface temperature was a little slower between 2001 and 2015 (which, to be fair, seems to be when the video was recorded), but saying “flat for 18 years” is basically a blatant lie, at best based on extremely cherry-picked data (I’ve previously mentioned an example of the kind of cherrypicking deniers use in my article “World of bullshit”), and therefore most of the conclusions drawn in this video are also incorrect. Giæver does mention that water vapor is a strong climate gas. That is true, but water doesn’t linger in the atmosphere in the same way that CO₂ does. Long-term atmospheric water concentration will thus only change because of other climate drivers (basically, the amount of water vapor that can be in the air depends on the temperature, among other things). But when it changes, it does have an effect, which means that when we emit more greenhouse gases, water vapor actually amplifies the effect, and makes it much worse. For this reason, CO₂ is considered a cause of climate change, while H₂O is only considered an amplifier (but an important one).
Sure, there are many people out there who might claim the physics is “not settled”. For the most important questions, that’s not really true (at least not in the sense they’d like you to think), but even if it was, why should it matter? If it’s “not settled”, it means that there are two options: everything is fine, or we could all die, and we don’t yet know which. So even if it’s “not settled”, there’s a risk. If there’s a chance your house could burn down, but you’re not sure, do you not still buy insurance for it? Do you not want a fire department nearby? Why would we not want to take action even if it was the case that the physics is “not settled”? There’s even less excuse now, because for all practical purposes, it is in fact settled. (Some details are still to be investigated, of course, but there is no doubt which way we’re heading.) But some people (even scientists in some cases) just don’t want to believe and will do anything they can to sow doubt (like in that other video you posted, in which the guy pretty much says nothing substantial about climate change, beyond perhaps trying to create doubt).
“The optimum CO₂ concentration” probably depends a little bit on what we want from the climate, but I’d probably be fine with what the concentration was around the year 1900.
And yes, it absolutely matters a whole lot if the average temperature rises 2 degrees over a century or two, because that’s much faster than natural evolution could keep up. It’s especially bad if it’s due to CO₂, because too much CO₂ doesn’t just cause warming, it has other bad effects as well. In particular, CO₂ is an acid when dissolved in water (“carbonic acid”, in Norwegian known as “kullsyre”), and too much of it kills life in the oceans. It also matters on land; while it’s possible it wouldn’t be too dangerous exactly where Giæver lives, there are large areas of Earth which will become effectively uninhabitable because it would become too common for summer temperatures to exceed what most life could survive. Warming would also cause sea level rise (remember, water expands as it heats, when above 4°C) and cause a lot of current coastal land to be flooded. The increased amounts of energy in the atmosphere (remember, heat is energy) would cause more severe weather, such as stronger hurricanes. All in all, given how quickly the climate is changing because of us, much of life on Earth would not be able to adapt quickly enough and would die, and with it, much of humanity, perhaps all, would die as well.
[Edit 1: added links and minor clarifications]
[Edit 2] I thought it might also be interesting to check just how much energy a 2 degree change in the atmosphere is. A quick calculation shows it to be roughly 10²² J. That is about 600 million Hiroshima bombs. Also, it seems to be the total energy usage worldwide for about 17 years. Interesting. And this calculation only includes the air, not the oceans.
[Edit 3] I’ve written another article, “Vi må tørre å tenke nytt”.