There has been just as much CO2 in the atmosphere before and it worked out fine!

Natural increases in carbon dioxide concentrations have helped warm the Earth's temperature for millions of years. Increases in carbon dioxide concentrations could only occur when there was an increase in sunlight. An increase in sunlight occurs when there is a small wobble in the Earth's rotational axis, or a change in the Earth's orbit around the Sun.

The last time we had this much CO₂ in the atmosphere was more than 3 million years ago. At that time, temperatures were about 1°C higher than now, and sea levels were 15–25 meters higher than today.

If we look at the last million years, the temperature of the Earth has changed according to the concentration of CO2 in the atmosphere, as well as changes in the Earth's axis around the sun. More or less sunlight has, in the short term, little effect on the global climate, because the oceans adjust their carbon stores according to how much sunlight penetrates the atmosphere. But when the changes persist, the ocean will represent a self-reinforcing effect on temperature changes. And then it will either get very cold or very hot!

read more about carbon dioxide in the atmosphere.