Unless you are a Creationist, in which case the following will be anathema to you, here is a broad brush history of the Earth's climate since the formation of Earth >4 billion years ago;
When the Earths crust solidified comets crashing into the Earth delivered water, CO2 and methane. Earth's primordial atmosphere was essentially devoid of oxygen. The first life that formed (mould-like slimes) lived in hot water and fed on methane. These evolved into forms that lived out of water (lichen, etc.). These evolved into plants and then trees. This was a tipping point, because plants and trees live on CO2 and exhale O2 by photosynthesis. Plants and trees flourished and were so successful they consumed a great proportion of the CO2, to the extent that the Earth's atmosphere contained 40% or more O2. This was hardly paradise on Earth so to speak. The high O2 content meant that a lightning strike typically caused near explosive bushfires. Then along came animals, who breath O2 and exhale CO2, symbiotic with trees. The animals consumed sufficient O2 (around 20%) so as to reduce the scale and severity of bushfires. Because plants and trees no longer burned as much they just died off and formed the vast coal deposits we now mine. Earth's early seas were not just salty as we know them but had very high levels of Fe (iron) dissolved in them. Lots of the oxygen first produced by the trees was absorbed into the seawater, converting the Fe to FeO and laying it down as the vast iron ore deposits we know mine. Thus on the billions of years scale Earth's climate has changed.
Many of our earliest civilisations were wiped out by Climate Change, e.g. the Anasazi Indians.
Moving on to recent history ... The Vikings were able to populate Greenland because large swathes of southern Greenland were ice free and could sustain agriculture, not so now. The Romans were able to grow grapes in England, not so now, although there is an attempt to re-establish cold climate grape growing in southernmost England.
Are you prepared to say that Earth's climate has evolved to its optimum just in time for your personal benefit, and only mankind is altering it? In which case you would be in a state of egotistical denial!
