June 2015 global temperatures. NOAA.

In June, in a bombshell study in Science, federal researchers with the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information (NCEI) took away climate skeptics’ number one favorite argument — the notion that in recent years, global warming had slowed down or hit a “pause.”

[Federal scientists say there never was any global warming ‘pause’]

Applying a number of corrections and adjustments to their dataset of global temperatures — one of the world’s most influential and widely used — so as to correct for lingering biases, the NOAA researchers pronounced that the “newly corrected and updated global surface temperature data…do not support the notion of a global warming ‘hiatus.’”

Not everyone, though, is convinced. And this week in Science, climate scientist Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Co, counters by once again making the case that there really was a global warming “hiatus” from about 1998 to 2013.

“The perception of whether or not there was a hiatus depends on how the temperature record is partitioned,” Trenberth writes. The NOAA scientists, Trenberth argues, picked 1950 as a starting year for their analysis, which happens to have been right in the middle of — that’s right — another “pause.” This, Trenberth says, reduced the trend from 1950 to 1999, and thus would have made 1998-2013 seem more comparable to it.

Not that any of this is good news for skeptics — Trenberth says that the warming trend seems to have since resumed, that temperatures rose in 2014, and that they are spiking even more this year. He expects that global warming advances “more like a rising staircase than a monotonic rise” — and has previously suggested that we’re in the middle of a “jump” in temperatures.

The debate matters, though, because Trenberth wants people to see that there really are major natural wobbles that will always prevent the global temperature rise from simply being a linear one.

The main wobble at issue here is called the Pacific Decadal Oscillation — or PDO — in which the planet’s biggest ocean goes through a cycle of burying heat and then releasing it, a phenomenon with global weather and temperature reverberations. An index of the PDO, kept by the National Oceanic and Atmospheric Administration, suggests that we’ve just swung back into its positive phase — one in which that pent up heat gets released to the atmosphere and across the globe:


The Pacific Decadal Oscillation. National Centers for Environmental Information/NOAA

“The main pacemaker of variability in rates of [global mean surface temperature] increase appears to be the PDO,” Trenberth writes, noting a “strong switch in the sign of the PDO since early 2014.”

Certainly, the recent temperature evidence is consistent with Trenberth’s interpretation. We’ve had a record warm temperature year in 2014, and 2015 appears poised to blow that one away. Indeed, we’ve had by far the warmest start to any year on record. Or as our own Capital Weather Gang’s Jason Samenow recently put it:

The first six months of 2015 marked the warmest first half of the year on record for the globe, 1.53 degrees (0.85 Celsius) above the 20th century average, 0.16 degrees (0.09 Celsius) ahead of 2010, which had the next warmest opening to a year.

Moreover, with a potentially strong El Nino here, many climate watchers are pretty convinced that 2015 will likely surpass 2014’s warmth. NOAA’s Deke Arndt, who heads the monitoring branch at NCEI, even wrote in June that “I think 2015 is going to be the warmest year on record. There, I said it.”

So while it’s certainly possible to interpret Trenberth’s article as a way of re-opening the “pause” debate that had appeared to be finally closed, it’s very important to bear in mind that going forward, there’s not much disagreement over where things are headed.

Namely, the planet’s going to get hotter. It’s just that while others would put less emphasis on the significance of temporary temperature stalls, Trenberth sees the warming proceeding a bit more like the ascent of a staircase.

“There will be fluctuations in rates of warming and big regional variations associated with natural variability,” Trenberth concludes. “It is important to expect these and plan for them.”