A city hits "climate departure" when the average temperature of its coolest year from then on is projected to be warmer than the average temperature of its hottest year between 1960 and 2005. For example, let's say the climate departure point for D.C. is 2047 (which it is). After 2047, even D.C.'s coldest year will still be hotter than any year from before 2005. Put another way, every single year after 2047 will be hotter than D.C.'s hottest year on record from 1860 to 2005. It's the moment when the old "normal" is really gone.

A big study, just published in the scientific journal Nature, projected that the Earth, overall, passes climate departure in 2047. The study also projects the year of climate departure in dozens of specific cities. Here, from The Post's graphics team, is a map of their findings:

The cities marked by dark red dots are projected to hit climate departure really, really soon. Bad news: Many of these are the most vulnerable to the effects of climate change. Lagos, Africa's largest city, with a population 21 million and rising, is already vulnerable to flooding. It's got only 16 years before it hits climate departure. Also vulnerable are Caribbean cities such as Kingston, Jamaica, which passes the tipping point in 2023.

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The light red cities have a bit more time but are some of the most worrying cases, including megacities in China and India, not to mention the major urban centers of the Middle East. Food insecurity and drought are difficult issues in many of these areas. The fact that these cities pass climate departure so soon is a scary reminder of how rapidly they're going to feel the effects of climate change.

Temperate cities in Europe and the United States look a bit better, but we're talking about a difference of maybe 20 years separating Western capitals from Kingston or Lagos. In the long run, 20 years is not much of a difference. The study published in Nature projects 2047 for Washington, D.C., and New York City -- just 34 years from now. Los Angeles will hit the mark the next year and San Francisco the year after. Even the best-off cities, such as Moscow and Oslo, have just 50 years before passing the milestone. That feels like a long time right now, but in historical terms it's not.

As Christopher Field, who directs the Department of Global Ecology at the Carnegie Institution for Science and was part of this study's research team, told my colleague Lenny Bernstein, "The boundary of passing from the climate of the past to the climate of the future really happens surprisingly soon."

The good news is that, while it's too late to stop the world or any of its cities from passing the point of climate departure, we can slow the process -- and thus significantly mitigate the effects of climate change. Here's what that map would look like, according to the Nature study's projections, if the world can substantially bring down carbon dioxide emissions: