Yes, the Earth has seen other periods of global warming. Still, we should worry about this "human effected" period of global warming. We weren't there to worry about the other periods.
The main reason why we should worry about this one is the fact that we are here to be affected by it.
The Earth survived it's other periods of global warming, but we weren't there to worry about it.
Back then there were no ski resorts lacking snow. There was no Columbia River hydroelectric plants worrying about lack of water from a low snow pack. There was no stock market to drop on talk of crop failures, property damage and coastal flooding.
We weren't there to worry.
If a tree falls in the forest and no one is there, does it make a sound?
We are here now, with our fragile economic needs. That's why we should worry.
In the past, there were significant periods of global warming that the Earth survived. The Earth is always changing, but our economy rides Earth like she's a good horse. When Earth starts changing, she's like a bucking bronco. We don't want to slap her because we don't want her bucking while we're riding. We're slapping her with our carbon emissions.
We are riding during the short period of evolution that we are here.
Millions of years ago, Earth saw huge lava flows pour out onto the surface. Many of these flows released carbon dioxide into the atmosphere leading to significant global warming. I recently heard a radio interview with some geologist about this topic. They described palm trees growing above the arctic circle.
Here in the Bellingham area, there are fossils of palm leaf in Chuckanut sandstone. Evidence of warmer eras in the past.
Much of the eastern part of this state is covered with lava flows. Flows that created the Columbia Plateau, often called the Columbia Basalt. Many of these flows crept out during the so called Miocene Era, millions of years ago.
Layers of basalt rock are visible in this coulee near Grand Coulee Dam and the Banks Reservoir. Seen on my 2005 bike tour.
We weren't around to worry the global warming back then and the Earth survived just fine. Maybe plants and animals suffered, but I am not hearing their cries.
The main reason why we need to worry about climate change now, is the fact that we are here. Our stock market doesn't want to take too big of a hit. Our economy is a fragile rider, so we hope that Earth remains a steady horse, at least for this moment in geological history.