This is very bad (albeit completely unsurprising) news, but I really don't see that the word "threshold" is appropriate. Is there some special bad thing that happens once 410ppm is reached, that's qualitatively different from 400ppm? Nope.
The article itself is quite clear about this: e.g., it quotes one climate scientist as saying "These milestones are just numbers, but they give us an opportunity to pause and take stock and act as useful yard sticks for comparisons to the geological record." I suspect it's the usual story: journalist writes reasonable article, sub-editor puts needlessly overexcited headline on it.
The article itself is quite clear about this: e.g., it quotes one climate scientist as saying "These milestones are just numbers, but they give us an opportunity to pause and take stock and act as useful yard sticks for comparisons to the geological record." I suspect it's the usual story: journalist writes reasonable article, sub-editor puts needlessly overexcited headline on it.