Given how Humankind keeps on spewing dozens of billion tonnes of carbon dioxide each year, this was expected : carbon dioxide concentration have reached 400 parts per million.
While this may sound mundane to most people, this is a real event, and one that could have dire consequences. Last time CO2 concentration were this high, temperatures were 8°C (or 14°F ) higher.
Additionally, sea level were significantly higher. Both higher temperatures and higher sea levels would be lethal threats to our civilization.