No, the “hiatus” doesn’t disprove global warming

In fact, scientists at the National Oceanic and Atmospheric Administration say that the “hiatus”—a “pause” in global warming since 1998—may never have happened at all.

The paper, spearheaded by Thomas Karl and eight other NOAA researchers, was published in Science in late June. The authors found the “hiatus” in global warming might not be a real trend, but instead results from differences in ship and buoy measurements. Their findings have garnered the attention of other scientists, the media…and the US Congress.

The authors of this paper, and NOAA itself, were recently issued a subpoena by Lamar Smith, Chairman of the House Committee on Science, Space, and Technology. This subpoena seeks “all documents and communications referring or relating to corrections to sea temperature data from ships and buoys,” particularly emails between Karl and his collaborators.

“This subpoena appears to be furthering a fishing expedition,” Committee Ranking Member Eddie Bernice Johnson wrote in response to Smith. “Unfortunately, this is reflective of much of the Committee on Science, Space, and Technology’s ‘oversight’ work this Congress, and it is a disturbing trend for the legitimacy of this Committee.”

What is the global warming “hiatus”?

Global average surface temperature since 1951 has increased by 0.11 degrees Celsius per decade, but the warming trend since 1998 is less then half that — 0.04 degrees per decade.

This slowdown in global warming, dubbed the “hiatus,” has become a favorite argument for climate change skeptics.

“One-third of Man’s entire influence on climate since the Industrial Revolution has occurred since January 1997. Yet for 224 months since then there has been no global warming at all,” writes a popular climate skeptic blog, Watts Up With That. “On the evidence to date, therefore, there is no scientific basis for taking any action at all to mitigate CO2 emissions.”

These arguments ignore two key facts: 1) Though warming has slowed in the past decade, it is still occurring; 2) Natural climate variability can act as brakes—or accelerators—on larger warming trends.

What is natural climate variability? For years, scientists have acknowledged the effects of variations in ocean temperature on the rate of global warming. In particular, the Pacific Decadal Oscillation, a pattern of sea surface temperatures in the North Pacific, has helped the ocean absorb much of the heat that would otherwise accumulate in the atmosphere. In fact, the ocean absorbs a whopping 93 percent of heat trapped by greenhouse gases. The “hiatus” in warming refers to the tiny 2 percent of heat that remains in the atmosphere.

Where is global warming going?
The vast majority of the heat trapped by greenhouse gases doesn’t accumulate in the atmosphere, but in our oceans. (Figure from Skeptical Science)

These numbers are changing. In the past year, scientists have observed a change in the Pacific Decadal Oscillation that could throttle down the ocean’s heat uptake. Some of that excess heat will stay in the atmosphere, and could lead to accelerated warming in coming years.

So, a “hiatus” doesn’t “disprove” global warming—it’s evidence that climate sensitivity can change the rate of warming caused by greenhouse gases, but that warming is continuing nonetheless.

A “faux pause”?

Although climate variability has been used to explain the “hiatus”, “observational biases in global surface temperature data have not received similar attention,” write Thomas Karl and colleagues in their recent Science paper. “In particular, residual data biases in the modern era could well have muted recent warming.”

Data biases occur because measuring global temperature is more complicated than averaging every thermometer reading worldwide. For example, we have more temperature measurements in more populated and developed areas, which are warmed by heat exhaust and create temperatures that are hotter than their rural counterparts. Scientists identify these biases, compare them to nearby, unaffected temperature readings, and apply mathematical corrections to compensate for these biases.

Karl and his fellow NOAA researchers have discovered new biases in the temperature record. For example, ocean temperatures rely on measurements from commercial ships and surface buoys. In recent years, the amount of data collected from buoys has outstripped ship data—and the ship data is often warmer. Complicating matters even more, old ship data was collected with a “bucket seawater” method, and is now done with engine intake thermometers. Each method has certain biases.

To make useful comparisons between these different data sources, NOAA scientists had to correct these biases. They used overlapping buoy and ship measurements to determine the average difference between the two, and used this difference to correct the ship temperature data.

The new corrections may look awfully similar to the old ones, but they raise recent temperatures by a fraction of a degree. That’s enough to eliminate the tenuous “pause” in global warming.

“Newly corrected and updated global surface temperature data…do not support the notion of a global warming ‘hiatus’,” the NOAA scientists conclude.

The rate of warming during the “hiatus” is still slower than in previous decades, but not as slow as previously thought, and certainly not the “pause” that skeptics claim it to be.

The backlash

Rep. Lamar Smith (R-Texas), Chair of the House Committee on Science, Space, and Technology, hasn’t taken these findings well.

After the study was published, Smith ordered NOAA to provide “all data related to this study and the updated global datasets”—data that was already published and available—along with “all documents and communications referring or relating to corrections to sea temperature data from ships and buoys” and relating to “other global temperature datasets” and “satellite bulk atmospheric temperatures.”

NOAA has briefed the Committee repeatedly and provided staffers with all data, but Smith nevertheless issued a subpoena to NOAA in October.

Committee member Eddie Bernice Johnson (D-Texas) wrote a scathing response. “This subpoena appears to be furthering a fishing expedition,” she wrote to Smith. She noted that this investigation “appears to be adopting the discredited tactics of fossil fuel industry funded climate denier groups” and “seems more designed to harass climate scientists than to further any legitimate legislative purpose.”

Emails, unlike papers, statements, or other official documents, have not been vetted or proofed, which makes them easy to misconstrue. That possibility is well known to climate scientists. In 2009, the emails of many climate scientists were hacked and leaked. This incident, dubbed “Climategate” by skeptics, lifted quotes from their original context to create the appearance of a conspiracy. Investigations cleared all scientists of any misconduct, but the defamation, frivolous lawsuits, and even death threats continue today.

If the subpoena of NOAA is another quote-mining expedition, its sheer scale dwarfs “Climategate.” The “documents and communications” ordered by the subpoena refer to more than data and emails: they include letters, diaries, presentations, meeting notes, texts, phone calls—any form of communication—for each of NOAA’s 12,000 employees.

“NOAA, rightfully, has been reluctant to waste their time and resources, not to mention break confidence with their superb research scientists, by responding to this demand,” wrote Rep. Johnson.

At worst, NOAA’s research has become more ammunition in Congress’ battle between climate change skeptics and believers, a battle that scientists settled long ago. At best, the ongoing “hiatus” debate shows that the gap between scientists and policymakers needs a stronger bridge.