Looking Back on 2015: The Hottest Year in Recorded History

Jesse Farmer Climate Change, Politics

The inexorable rise in global temperatures combined with a strong El Niño puts 2015 in the record books

2015 is officially the hottest year in recorded history, according to independent analyses released yesterday by NASA and NOAA scientists. If this sounds like something you’ve heard before, it is. This time last year, 2014 was announced as the hottest year in recorded history. But unlike 2014, which squeaked by the previous record (2010) by less than five-hundredths of a degree, 2015 blew the doors off the 2014 record by a full 0.16°C. It’s the largest-ever margin by which a previous yearly temperature record was broken.


Monthly Temperatures 1880 to 2015
Global temperature anomalies plotted by month from 1880 to 2015. Each year is represented by a single line. Blue lines are the five coldest years, and orange lines are the second- through fifth-warmest years. The warmest year—2015—is shown in red. Data from NOAA.


Globally, all but two months (January and April) were the warmest of those months in recorded history (see above). And this warmth wasn’t limited to some random, remote area in the middle of the ocean or Siberia. In the United States, every single state east of the Mississippi River just had its warmest December on record:


12/2015 US Temp Records
Average temperatures for December 2015 by state, ranked from coldest ever month (1) to warmest ever month (121). From NOAA.


RIP to the nonexistent “hiatus”

2015’s record temperature is the latest nail in the coffin for an unfortunately popular misconception about climate change. Called the “hiatus” or “pause,” this oft-cited talking point has it that global temperatures peaked in 1998, and have not risen since.

Temperatures were, in fact, unusually warm in 1998, thanks in part to a strong El Niño event that year. During El Niño events, warmer-than-usual ocean water covers the equatorial Pacific Ocean, typically resulting in warmer-than-average global temperatures. 2015 saw the start of the strongest El Niño event since 1998, and so we would expect warm temperatures in 2015, all else being equal. But, as the plot below shows, all else is not equal. El Niño events have been getting warmer over the past 30 years, with the 2015 El Niño just the latest, and warmest, example:


ENSO-Global Temperature
Monthly averaged global temperatures from 1980 to 2015, with red colors indicating El Niño months, blue colored months indicating La Niña months, and gray colored months are neutral months. From NASA’s “Annual Global Analysis for 2015” press release.


The “hiatus” is based on comparing the strong 1998 El Niño event to following years, which were at best weak El Niño events. 2015 was a strong El Niño, as well, making a comparison to 1998 more reasonable. Still, any comparison of a single year to another year is not only pretty silly, it is scientifically and statistically wrong. Earth’s climate is a complicated beast, and many different factors—a lot of which are still poorly understood—cause year-to-year swings in global temperatures. If we look at temperature records over longer time periods, though, the trend becomes obvious: global temperatures are increasing. This would be true regardless of whether 2015 was the warmest year on record or not.


Global T 1880-2015
Yearly averaged global temperatures from 1880 to 2015. Global temperatures have been above the 20th century average every year since 1976. Data from NOAA.


2015 in longer context

What does a 0.9°C warmer world actually mean? To get context, we can step back to the last time when scientists know global temperatures were quite different from today: the peak of the last ice age, around 20,000 years ago. Even though thermometers were only invented 300 years ago, scientists can infer temperatures at the last ice age by measuring biological and geochemical features that are closely related to temperature. A recent compilation of hundreds of these measurements by the Intergovernmental Panel on Climate Change suggest the ice age world was, on average, 4.4 ± 1.3°C colder than the 20th century:


LGM Temperatures
Reconstructed temperatures at the peak of the last ice age, around 20,000 years ago. Each circle and diamond represents a single measurement of last ice age temperature. Modified from the 2013 IPCC Report, Working Group 1, Chapter 5: Figure Box 5.1-1.


In other words, the temperature difference from average last year (+0.9°C) was up to 30% as large as the temperature difference at the peak of the last ice age, when ice sheets up to a mile thick covered North America down to New York, global sea level was about 450 feet lower—equivalent to the height of Great Pyramid of Giza—and woolly mammoths roamed across the high latitudes.

This isn’t an entirely fair comparison; there were undoubtedly warm and cold years during the last ice age. But the bigger point is this: Earth’s history is marked by massive physical and biological changes associated with relatively small changes in global temperature. A year that was 0.9°C warmer than average, on top of now 39 straight warmer-than-average years, is a change that simply cannot be ignored. Fortunately, the recent COP21 agreement in Paris is a key first step to potentially limit the consequences of climate change in the future, when the record warmth of 2015 will be forgotten under the weight of increasingly extreme years.


Read More:
NOAA’s 2015 State of the Climate Report: Global Analysis and U.S. Analysis