It is expected that by the end of the 21st century, if the present trend continues, the CO2 concentration can reach up to the level of

If emissions keep rising, carbon dioxide is projected to approach about six hundred parts per million by the century's end.

Four hundred fifty parts per million is a near-term threshold already close in trend terms, not the century-end figure.

Five hundred parts per million falls short of the projected end-century level.

A thousand parts per million exceeds the level expected under the present trend.

The six hundred mark reflects business-as-usual growth in fossil fuel use.

Pre-industrial carbon dioxide stood near two hundred eighty parts per million.