I would highly recommend a couple background, approachable videos on the topic from mainstream experts:
* 11 min, Glen Peters, climate scientist: https://www.youtube.com/watch?v=529Og_YR0IU
* 53 min, Kevin Anderson, climate scientist: https://www.youtube.com/watch?v=ZF1zNpzf8RM
To answer your specific question: paleoclimate has varied for many reasons, from solar and ecological changes to ongoing natural processes. CO2 is absorbed by the biosphere - by acidifying oceans, by plants that capture carbon then die and don't decay it all back into the air/water. The problem is this cycle is far too slow and low in capacity in human-relevant periods of time, relative to our present emission levels (or politically achievable fractions thereof). The biosphere is already absorbing about half the CO2 we emit, and there's evidence that share may be dropping as we overload its capacity in myriad ways (see today's permafrost bubble story as an example of a carbon sink turning upside down).
Just as a final ironic point - where do you think all the CO2 we're emitting is coming from? From carbon sequestered naturally over millions of years into oil, natural(!) gas and shale/tar sands/etc. The natural processes that put it there are what resulted in the climate we have today. We're literally digging up the carbon that millions of years ago kept the planet warm enough for dinosaurs and pumping it back up into the atmosphere. That climate is the mean to which we will return.
For the majority of Solar existence, we have been a ball of incandescent star dust.
For the vast majority of earths existence, it has not been supportive to complex life.
Or do you want to go back to those halcyon days of the preCambrian period
In all seriousness, why? Have you not been able to look it up yourself? Will any sort of argument or evidence persuade you?
We know two things at this point about your stance.
1) Your gut is a valid data source.
2) You haven't looked up any of the plentiful information on this topic and prefer to ask people here.
This suggests that you're asking disingenuously. Please comment on how you will use the information we share with you (perhaps to supersede your gut?) before asking us to spend time constructing sourced, cogent responses.
BTW - No one brought up the spectre of Trump, you invoked it. People are, rightly, criticizing your gut feeling that flies in the face of our best understanding of multiple hard scientific fields and the tons of works and dedication by tens of thousands of researchers around the globe.
CO2 levels have been this high, and much higher, in the distant past. But the climate has also varied enormously in the distant past as well. Roughly 55 million years ago, the planet had nearly no ice. A bit over two billion years ago, it's believed that the planet was almost entirely covered in ice.
So no, it's not really a reasonable issue. CO2 levels have varied a great deal, as has the climate. Nothing in the Earth's climate past suggests that the current dramatic rise in CO2 levels wouldn't also result in dramatic changes to the climate.
This is, of course, a very simplified explanation. As one goes back hundreds of millions to billions of years, other large scale changes like plate tectonics, atmospheric composition/mass, sun brightness, continental weathering, etc. will cause the average surface temperature to vary considerably.
Theres tons of data out there, literally an entire field of work in science.
And that entire field of work is regularly debunked and debated with such old adages as - "scientists have an incentive to get grants, so they keep putting out material which the < evil illuminati types > want to hear".
And I've seen such arguments on HN.
I think there isn't any convincing left to be done, at most link people to sources and if they are motivated they'll cover it themselves.
Whats left is collective action.
Well then, in the court of public opinion at least, you've already lost.
Nobody is going to listen to the hobo screaming about how the government is spying on us in our own houses with our TVs. It doesn't matter if he's right if everyone ignores him, in fact the opposite, since people will associate the argument with screaming hobos.
Don't be a scientifically literate screaming hobo. It hurts the cause.
That's where climate science was taken and rendered impotent, and more lasting damage done to it and to expertise itself, that we haven't recovered.
A long time ago, a particular news network helped climate cranks gain public recognition
The scientists had ignored these cranks, rightly inferring that engaging with crazy people makes them seem more credible.
The media group spun this reticence into a conspiracy by liberal/ivory tower/money stealing/hippies; evil experts crushing the lone voice of reason.
Simultaneously, the political party best served by this audience, developed their language and communication playbook to convert the media attention into a political force.
When scientists realized that they had to engage if they wanted to have any hope, they accepted tv shows or debates. The hope was that if they engaged, if facts and science as presented, people would understand what was at stake and would act in good faith.
And as you can guess, this was naive. The point was never to have an informed debate, it was to have a spectacle. Scientists and science lost.
This system works so well, that within the past few years, it's been expanded to target something as fundamental as evolution.
Frankly I'm in awe of it, and take notes. more than tech, communication and marketing are America's strengths.
In short I'm not a scientific literate screaming hobo. I'm an old fart, who's seen the hope in this fight die out several times. I've even been at the point where I said "human beings won't let this happen. It's just too obvious and important." Twice.
I no longer believe that there is another "side" to reach out to, because the message will never get through.
Instead it's time to just connect with people who are already acting, focus on ideas which move the needle, invest in communication and mobilization.
If you don't want to be a reasonable, level headed and compassionate fellow human who wishes to gently lead in the direction of truth, rather be silent so that someone else can.