Global temperature anomaly for 2015 compared to the 1951-1980 average. Image: Berkeley Earth

During the next week, the official climate agencies around the world that are responsible for tracking the planet's average temperatures will almost certainly come to the same conclusion: 2015 was the warmest year on record. This would mean that 2015 would beat the previous warmest year, which occurred in 2014 — remember that?


The combination of a record strong El Niño event plus the highest amount of greenhouse gases in the atmosphere at any time in human history have given the climate system the equivalent of a Power Bar plus a shot of espresso. On Wednesday, one unofficial temperature tracking group, known as Berkeley Earth, revealed its determination that 2015 was by far the planet's warmest year, both on land and sea.

There's one especially important about fact about this group's determination: It was set up in early 2010 as an independent fact check of other surface temperature data sets, and led by a physicist — Richard Muller — who had previously been quite skeptical of mainstream climate science findings. Instead of proving surface data from government agencies like NASA and the National Oceanic and Atmospheric Administration (NOAA) wrong, the Berkeley group has consistently reaffirmed their data.

The Berkeley Earth group said in a release on Wednesday that "2015 was unambiguously the hottest year on record." More importantly, the group found that for the first time in recorded history, the Earth’s temperature is clearly more than 1 degree Celsius (1.8 degrees Fahrenheit) above the 1850-1900 average, and halfway to world leaders' climate target of limiting global warming to under 2 degrees Celsius (3.6 degrees Fahrenheit) above average.

“At the recent rate of warming may begin to cross that threshold in about 50 years,"-Robert Rohde, a scientist with the Berkeley Earth team, said in the release.

At the Paris Climate Agreement, agreed to in December by every country in the world, leaders pledged to hold global warming to "well below 2 degrees Celsius", or 3.6 degrees Fahrenheit, above preindustrial levels and "to pursue efforts to limit the temperature increase to 1.5 degrees Celsius", or 2.7 degrees Fahrenheit, above preindustrial levels through 2100. Warming greater than 2 degrees would increase the risk of dangerous impacts of global warming on low-lying island nations, as sea levels rise, and have more dramatic effects on other vulnerable nations, such as Bangladesh and the nations of Sub-Saharan Africa.

In the Berkeley record, 2014 tied with 2005 and 2010 as the world's hottest year on record, whereas NOAA and other agencies had ranked it as the warmest. The Berkeley Earth team found that 2015 set the record with "99.996% confidence."

Berkeley Earth has taken a "cautious approach to announcing hottest years," according to the group's cofounder, Elizabeth Muller. "A year ago, we announced that 2014 was not a clear record, but only in a statistical tie with 2005 and 2010. Now, however, it is clear that 2015 is the hottest year on record by a significant margin,” Muller added in a statement.

Year-to-date extremes

Year-to-date extremes in temperatures, showing the many areas that saw record warmth in 2015.

“This new high temperature record confirms our previous interpretation that the pause was temporary and that global warming has not slowed," said Richard Muller, scientific director of Berkeley Earth, referencing the much-debated but largely debunked "pause" in global warming since about 1998.

In total, Berkeley Earth estimates that 16.9% of Earth's surface and 16.4% of its land surface set record high annual averages in 2015. There were record highs in much of South America and the Middle East, and parts of the U.S., Europe, and Asia.

The team found that both land and ocean temperatures separately set record highs.

Berkeley Earth’s analysis over land is based on temperature observations from more than 40,000 weather stations, including 20,755 stations reporting in 2015. This is combined with ocean surface temperature data from the Hadley Center in the UK.

Annual Time Series

Annual time series of global average surface temperatures, showing the spike in 2014 and 2015. Image: Berkeley Earth

Many studies have shown a buildup of heat in the oceans during recent years, particularly since 1998. Some scientists, including Kevin Trenberth of the National Center for Atmospheric Research in Colorado, contend that that heat is now being added to the atmosphere, leading to faster rates of warming. Such bursts and relative slowdowns in global warming are related to the interaction between manmade emissions of greenhouse gases and natural climate cycles, including El Niño, which tends to boost air and ocean temperatures.

Another cycle, known as the Pacific Decadal Oscillation, can also transfer more heat energy from the sea to the air, accelerating warming.

The NOAA, NASA, Hadley Center and Japan Meteorological Agency (JMA) are all expected to make their annual temperature announcements by Jan. 21. The JMA has published a preliminary determination on its website, stating that 2015 was, by far, the warmest year on record according to its database.

Each center uses different methods of analyzing global temperature readings, such as by using techniques to estimate temperatures in data sparse regions, like the Arctic.

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