The climate has changed before, but its change (due to natural causes) has been mostly slow, giving ecosystems and species time to adapt. When it has been abrupt, it has caused extinctions and changes in ecosystems: https://www.skepticalscience.com/climate-change-little-ice-age-medieval-warm-period.htm
You can see more information here:
Is the Current Climate Change Unusual Compared to Earlier Changes in Earth’s History?
Climate has changed on all time scales throughout Earth’s history. Some aspects of the current climate change are not unusual, but others are. The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years, and has done so at an exceptionally fast rate. Current global temperatures are warmer than they have ever been during at least the past five centuries, probably even for more than a millennium. If warming continues unabated, the resulting climate change within this century would be extremely unusual in geological terms. Another unusual aspect of recent climate change is its cause: past climate changes were natural in origin (see FAQ 6.1), whereas most of the warming of the past 50 years is attributable to human activities.
What Caused the Ice Ages and Other Important Climate Changes Before the Industrial Era? https://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-6-1.html
You can find more info in the FAQs here:
or visiting any of the sections here: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/contents.html