NASA describes how air bubbles long trapped in ice enable us to go back in time and see what the Earth’s atmosphere and climate have been like over time. The air bubbles reveal that levels of CO₂ in the atmosphere are higher now than they have been at any time in the past 400,000 years. CO₂ levels were around 200 ppm during ice ages, and during interglacial periods, CO₂ levels hovered around 280 ppm. Before the Industrial Revolution, CO₂ levels did not exceed 300 ppm.
Ninety-seven per cent of climate scientists agree that climate-warming trends over the past century are very likely due to human activities. We can testify that our sensors are showing an increasing level of CO₂. In 2013, CO₂ levels surpassed 400 ppm for the first time in recorded history. This relentless rise in CO₂ exposes the constant relationship between fossil-fuel burning and CO₂ levels. This can be accounted for by the premise that circa 50 per cent of fossil-fuel emissions stay in the atmosphere.
Scientists say that we are on the threshold of a new geologic era, one in which the climate is very different from the one our ancestors knew. If fossil-fuel burning continues at the same pace as now, so that humanity depletes the reserves over the next couple of centuries, CO₂ levels will continue to rise to levels of the order of 1,500 ppm. If that happens, the atmosphere would not be able to return to pre-industrial levels even tens of thousands of years into the future.
You can read more on global emissions in this 2018 UN report.
You can see a video depicting the history of atmospheric carbon dioxide from 800,000 years ago to 2016 here.
If you want to have a closer look at the monthly average CO2 level at Mauna Loa, visit https://www.esrl.noaa.gov/gmd/ccgg/trends/.