How it was reached?

For all of human history until about 200 years ago, our atmosphere contained 275 parts per million of carbon dioxide.

Beginning in the 18th century, humans began to burn coal and gas and oil to produce energy and goods. The amount of carbon in the atmosphere began to rise, at first slowly and now more quickly. The rapid industrialization did its bit in this and so did the cutting down of forests on a large scale. Many of the activities we do every day like turning the lights on, cooking food, or heating or cooling our homes rely on energy sources like coal and oil that emit carbon dioxide and other heat-trapping gases into the atmosphere. We're taking millions of years worth of carbon, stored beneath the earth as fossil fuels, and releasing it into the atmosphere. By now the planet has 385 parts per million CO2 – and this number is rising by about 2 parts per million every year.