Image 1 Caption: The up-to-date Keeling curve (atmospheric carbon dioxide (CO_2) abundance versus time graph) since inception in 1958 from Mauna Loa Observatory, Mauna Loa, Hawaii Island (the Big Island), Hawaii.
Image 2 Caption: The same for the last 5 years.
The red curve is the monthly-average curve and the black curve is a running-average curve (see detailed explanation below).
Features:
One can also say that the Keeling curve from Mauna Loa is one for an average location on Earth rather than an average of Keeling curves for many locations on Earth.
The bottom graph is the recent CO_2 measurements.
Prior to the beginning of the Industrial Revolution (starting ∼ 1700 or 1750 depending on your point of view), the CO_2 abundance was ∼ 280 ppm since ∼ 9000 BCE (i.e., since almost the beginning of Neolithic and Holocene) (see Wikipedia: Keeling curve: Mauna Loa measurements).
The Northern Hemisphere seasons dominate the season cycle because it has more land mass than the Southern Hemisphere (see Wikipedia: Keeling curve: Mauna Loa measurements).
The running-average curve will probably NOT fall below 400 ppm again for centuries.
The monthly-average curve did NOT fall below 400 ppm in 2016 September when it minimized: 401 ppm was the minimum. So the neither monthly-average curve nor the running-average curve will probably NOT fall below 400 ppm again for centuries.
Since 2011, the slope has been ∼ 2.2 ppm/year.
If this linear growth continues, CO_2 will reach 450 ppm in ∼ year 2038.
International discussion often cites stabilization at 450 ppm as goal. Climate modeling suggests that stabilization at 450 ppm will keep global warming at under or about 2° C over the ∼ year 1880 global average temperature which is also a discussed international goal (see Wikipedia: United Nations Framework Convention on Climate Change).
Yours truly finds it hard to believe we will stabilize at 450 ppm, but we might be able to delay it beyond 2038.
If the linear growth in CO_2 continues longer, CO_2 will reach 560 ppm (about twice the pre-industrial abundance of ∼ 280 ppm) in ∼ year 2088. It has been estimated that 560 ppm would lead to ∼ 3° C increase over year 1880.
It should be noted that climate modeling predictions have large uncertainties.
By the by, if CO_2 reaches 560 ppm, we all become werewolves---but don't let anyone know.
Year 1880 was ∼ 0.25° below the fiducial global temperature and as of 2022 the fiducial global temperature is ∼ 0.9° above (see Wikipedia: Instrumental temperature record; Wikimedia commons: Global average temperature change).
If the Earth were a perfect blackbody radiator except that it has its actual albedo (diffuse reflectivity) of ∼ 0.7, its temperature would be ∼ -18° C (see Wikipedia: Greenhouse effect).
The greenhouse effect raises the Earth's surface temperature above these hypothetical chilly values. So the greenhouse effect is good, in fact. But as global warming shows, you can have too much of a good thing.