In seven years, inhabitants of New Guinea could be living in an unfamiliar world, one with a wholly different climate. A new analysis published today in the journal Nature finds that by 2020, New Guinea's climate will permanently enter a state never seen before, outside of the bounds of historical variability and short-term extremes.

To put it simply: The coldest year in New Guinea after 2020 will be warmer than the hottest year anyone there has ever experienced.

The global analysis also predicts that if greenhouse gases continue to be emitted at a "business as usual" rate, New York City and Washington, D.C., will experience radically altered climates in 2047 (plus or minus about five years for a margin of error). So in about 35 years, even the coldest monthly dips in temperature on the eastern seaboard will be warmer than any time in the past 150 years.

"We're providing a new metric on when ongoing climate change will lead to environments like we have never seen before," lead researcher Camilo Mora of the University of Hawaii told reporters, "when the coldest year of the future will be warmer than the hottest year in the past."

The study's authors refer to this new metric as a "climate departure."

By combining data from 39 different climate models, Mora and his team built a timetable of these climate departures for any given location on Earth. Along with their study, Mora and his colleagues published an interactive map that allows users to find the year of climate departure anywhere on the globe.

Identifying the Date of Climate Departure

The index uses temperature records from 1860 to 2005 to set the historical bounds of climate variability, then crunches projections from the 39 climate models to identify a year, specific to any point on a global map, when the temperatures will shift outside of those historical bounds for good.

The study actually makes two projections: one "optimistic" scenario that, in Mora's words, "reflects a strong and concerted reduction in carbon dioxide emissions"; and one "business as usual" scenario in which emissions rise unimpeded by international climate agreements or strong domestic policies in the developed world. (The dates given so far all refer to this "business as usual" scenario.)

The average date of climate departure globally is 2047, according to the more pessimistic model (putting New York City and Washington, D.C.—as well as Ankara, Turkey, and Kampala, Uganda—right at the global mean). If greenhouse gases are stabilized at what Mora refers to as an "optimistic" level (538 parts-per-million of atmospheric carbon dioxide), the global average pushes back to 2069, but inhabitants of New Guinea still would experience the climate departure in 2025.

Either way, say the study's authors, climate departure at some point in the relatively near future is inevitable. "We hope that this analysis will bring home the clear message that change is on its way, and that it will occur soon," said co-author Abby Frazier.

(Read "Rising Seas" in National Geographic magazine.)

Oceans Already Departed

In addition to the temperature records, the scientists generated climate departure timetables for other variables, including surface evaporation, precipitation, ocean surface temperature, and the acidity of the sea surface.

The calculations for ocean pH were stunning. Mora and his team found that ocean acidity in 2008 (give or take three years) already exceeded historic bounds. (A separate study, conducted by researchers based at Oxford University, recently reported that the oceans' rate of acidification is the fastest in 300 million years.)

Lower Latitudes First to Leave the Norm

Climate scientists often refer to the polar regions as the "canary in the coal mine" for climate change, as the physical changes in the upper latitudes are most dramatic and the temperature spikes the greatest.

But according to Mora, this focus on the startling absolute changes at the poles paints an incomplete picture. "In fact," said Mora, "our study shows that the tropics, not the poles, will be experiencing unprecedented climates first." Why is this? Mora explained that tropical regions have relatively little variability from year to year, whereas the Arctic and Antarctic are subject to much broader ranges of extremes.

Mora referred to this as a sort of "double jeopardy" of climate change. "The largest absolute changes are happening at higher latitudes," he said, but those dramatic temperature increases don't fully break out of the range of historical extremes. "Unprecedented climate is happening more rapidly at the tropics."

On average, tropical locations will reach their climate departures 15 years earlier than the rest of the world. All of the locations where the impacts will occur earliest—for instance, under the baseline scenario, New Guinea in 2020, Jamaica in 2023, Equatorial Guinea in 2024—sit at lower latitudes.

Mora said that this is particularly troubling for a couple of reasons. First, the tropics are "home to the greatest diversity of species on the planet," he said. "These species are adapted to a stable climate, and thus it's very easy for these small changes to exceed what a species can tolerate." Past studies have already shown that tropical species like coral are pushing up against their environmental limits.

Second, because the world's population is disproportionately concentrated in the tropics, unprecedented climate conditions will impact a larger percentage of the world's population.