What exactly is amusing? Large asteroids have the capability of introducing a sufficient large temporary disturbance of the worlds climate so that an extinction cascade might be triggered. It doesn't have to kill every single animal, just to disrupt enough food chains.
Which is exactly the worst-case with the current, human caused greenhouse effect.
You’re right that it always changes, but we don’t have record of it changing this quickly without it being caused by some traumatic geological event or something. We’re trending warmer way, way faster than evidence shows we normally do.
Nobody disputes that earth's climate changes naturally over time. People are arguing that human behavior (pollution) is causing climate change at a much faster pace than would otherwise occur.
Of course, as anyone who has a bit of scientific knowledge knows, it isn't a myth. It is an effect on top of the natural climate change caused by raising the CO2 content by 50%, mostly in the last 50 years. This causes a very quick warming at a speed far quicker than natural climate change. It is this speed which makes the climate change so dangerous. Which is where we are back at the asteroids. Beyond the direct impact damage, they cause a sudden climate shift.
3. That things heated by visible light radiate much of that heat back as infrared light?
4. That CO2 levels in the atmosphere are rapidly rising?
5. That raising CO2 levels will cause warming (see #1, #2, #3)?
6. That there are different isotopes of carbon, with different half lives?
7. That living things regularly exchange carbon with atmospheric carbon?
8. That because of that exchange the isotope ratios in carbon from living things matches the atmosphere carbon isotope ratios?
9. That when living things die they stop breathing?
10. That because of #6-10 we can tell from isotope ratios how long ago something died?
11. That as the CO2 levels have risen dramatically over the last couple hundred years, the isotope ratios of atmospheric carbon have changed?
12. That from these ratios we can tell that most of the increase in CO2 comes from sources that have not been exchanging carbon with the atmosphere for a very long time?
13. That fossil fuels come from long dead things, and have isotope ratios that match whatever provided most of that increased atmospheric CO2?
14. That the amount of CO2 that we are putting in the atmosphere from burning fossil fuels matches up closely with the amount of extra carbon we know must have come from sources that have not exchanged carbon with the atmosphere for a long time?
15. That we have satellites that can measure incoming and outgoing radiation and see that we are out of equilibrium, by an amount that fits with the amount that we would expect from that extra CO2?
If the climate change is caused by humans, that is more likely due to deforestation and destruction to nature.
CO2 is used by plants, and plants produce oxygen. If you kill more plants and build concrete cities over where they were...you could get your global warming.
Deforestation and destruction of nature could change CO2 levels, but they don't change the isotope mix of the carbon in atmospheric CO2. We know the increase is mostly coming from something whose carbon has a different isotope mix.
Which is exactly the worst-case with the current, human caused greenhouse effect.