It's not rare for a year to break record temperatures. But it's now happened two years in a row — and 2015 was "very, very clearly the warmest year by a long chalk," says Gavin Schmidt, director of NASA's Goddard Institute for Space Studies.
NASA is presenting the annual review of global average temperatures in conjunction with NOAA, which says that not only did 2015 finish as the warmest year on record, but it did so by the widest margin ever — nearly a third of 1 degree Fahrenheit warmer than 2014's average.
In 2015, the average temperature on land and ocean surfaces around the world was "1.62° F (0.90° C) above the 20th century average," according to NOAA.
That makes 2015 the hottest since instrument records began being kept in 1880, beating the record set in 2014 by 0.29° F (0.16° C).
The Northern Hemisphere saw the biggest rise in land temperatures, finishing 2.59° F hotter than the 20th century average.
As for the United States, NOAA released that data last week, saying that for the 19th consecutive year, the annual average temperature for the continental U.S. was hotter than the 20th century average.
The agency reported, "The last year with a below-average temperature was 1996."
Globally, 10 months in 2015 tied or broke monthly temperature records, culminating in a December that was more than half a degree Fahrenheit warmer than its predecessor in 2014 — a record margin, NOAA says.
"This record year really is just emphasizing the fact that there is a very, very strong long-term trend in temperature that we have associated very strongly with the human emissions of greenhouse gases," Schmidt says.
Concerns over Earth's changing climate resulted in nearly 200 nations adopting a global climate agreement last month, which sets a goal of holding the world's rise in average temperature to "well below 2 degrees Celsius [3.6° F] above pre-industrial levels."
Most of the numbers we've cited above represent world averages. Separately, NOAA says, land and ocean temperatures also easily set new records in 2015.
On land, average surface temperature was 2.39° F (1.33° C) above the 20th century average — surpassing the previous record set in 2007 by 0.45° F (0.25° C).
"This is the largest margin by which the annual global land temperature has been broken," the agency says.
The globally averaged sea surface temperature was 1.33° F (0.74° C) above the 20th century average — a rise of more than 1 degree from last year's record rise by 0.20° F (0.11° C).
While a warming El Nino trend likely played a role in 2015 setting the new record, Schmidt says, "it is the cumulative effect of the long-term trend that has resulted in the record warming that we are seeing."
Also, Schmidt reminds us, even incremental changes of one degree can have an impact on weather and ecosystems, noting that in many places around the world, glaciers are retreating.
The warming trend is likely to continue, he says: "We actually anticipate that 2016 might also be a record-warm year, and that will be the first time that we've had three record-warm years in a row in the whole record."
Offering a recent historical context for climate change, Schmidt says:
"When NASA first started talking about global warming, it was in 1988. 1988 was also a record year. But with the record in now for 2015, 1988 is not even in the top 20 of warmest years."