It isn't the fact that the earth has warmed and cooled through its 4 billion year history. It is the rate of warming that is the problem. We are seeing this increase in global average temperatures being measured in decades instead of thousands or tens of thousands of years barring a cataclysmic event like a super volcano eruption or a meteor impact of significant size. The global mechanisms that control the overall temperate environment of the planet cannot adapt and adjust fast enough to keep up with the warming of the planet. As the planet warms further, more and more of those mechanisms break down and you have a feedback loop that will worsen the situation as each protective element falls.

The Arctic Circle losing year around sea ice increases the amount of heat being absorbed by the oceans, increasing the ocean temperatures has a cascade effect on the plants and animals in the ocean including phytoplankton which produce most of the oxygen present in the atmosphere as well as destabilizing the thermohaline current which means warm water from the equator no longer moves up towards the poles. This is probably one of the first mechanisms that will fail and will lead to brutally cold winters in the northern and southern latitudes and will also probably create storm systems that human beings have never seen before. Humans are absolutely causing the planet to warm, atmospheric CO2 concentrations are the highest they have been in several hundred thousand years if I recall correctly and we've managed to raise it to that point within 150 to 160 years since the industrial revolution.