The Pause in the Stratosphere

stratosphereThe Intergovernmental Panel on Climate Change (IPCC) is awfully certain that solar activity doesn’t meaningfully impact climate. In fact, their 2014 Synthesis Report Summary for Policymakers offers only one passing reference to solar activity in the entire document. On page 10, the report notes a curious caveat regarding predicted surface temperatures from 2016‚Äì2035: “This assumes that there will be no major volcanic eruptions …or unexpected changes in total solar irradiance.”

Other than that, solar variability is essentially dismissed in the IPCC worldview. Carbon dioxide (CO2) is the primary driver of climate change, and that’s that.

But mankind has recognized the sun’s primal importance for eons. So how did the IPCC arrive at such certainty in discarding the impact of solar activity? After all, the sun’s variability appears to track rather closely with climate on millennial, centennial, and decadal timescales. For example, the Roman, Medieval, and Modern Warm periods all correspond with heightened solar activity. And the Little Ice Age suffered the cooling effects of not one, but three, solar minimums.

The IPCC’s aversion to solar irradiance apparently stems from two concerns.

First, there’s the divergence in trajectories over the past 30 years of measurements for temperature and solar activity.

The argument goes: Since solar activity stopped climbing around 1985 (and essentially peaked at that point), while surface temperatures kept climbing, this divergence proves that the sun isn’t driving temperatures.

But this is a mischaracterization. The Modern Solar Maximum peaked twice, first in the late 1950s and again in the mid 1980s. Starting around 1985, solar activity plateaued—but at levels higher than anything seen in the last 1,000 to 2,000 years. In essence, the sun remained at roughly full throttle until the mid-1990s—close to the starting point for the now infamous “pause” in surface temperatures.

Second, there’s the observation that, during the period ranging from about 1960 to 1995, stratospheric temperatures showed a net cooling. This decline occurred at the same time that surface temperatures were rising (driven, presumably, by increasing concentrations of CO2.) The absence of a parallel rise in stratospheric temperatures is thereby seen as negating the possibility of a solar connection.

A November 2013 statement from the World Meteorological Organization says: “…if warming had been caused by a more active sun, scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere and a warming at the surface and in the lower parts of the atmosphere.”

The stratosphere is a funny bird, though. Unlike the troposphere, the stratosphere is a cold and inhospitable place. Not only is it riddled with ozone and ultraviolet radiation, but it displays the curious phenomenon of temperatures increasing with altitude (the inverse of what occurs in the troposphere.)

Significantly, ozone is the primary “greenhouse gas” of the stratosphere. As NASA explains it, “Ozone is both a major absorber of incoming ultraviolet in the stratosphere (leading to stratospheric heating) and a strong emitter in the thermal infrared spectrum.” Simply put, stratospheric temperature is maintained by concentrations of ozone. If ozone levels decline, for instance, temperatures in the stratosphere will fall.

This direct relationship between ozone and temperature became apparent during the mid-twentieth century. Ozone suffered an existential threat as the continued release of chlorofluorocarbons (CFCs) ate away at its concentrations. Stratospheric ozone levels tumbled steadily from the late 1950s onward, creating a serious, and recognized, international problem. It wasn’t until the implementation of the Montr√©al Protocol in 1989 that real action to reduce CFCs took effect. Revisions to the Protocol subsequently led to a more complete ban on CFC production in 1996.

As NOAA’s Ozone Depleting Gas Index demonstrates, the Montreal Protocol has succeeded in gradually restoring ozone concentrations in the stratosphere. Specifically, CFC levels in the stratosphere continually rose until roughly 1996, the point at which the effects of the Montreal Protocol began to fully register. Starting in 1996, stratospheric CFC levels actually began to decline.

This action to restore ozone shows a remarkable correlation with recent stratospheric temperatures. Whereas cooling in the stratosphere was continually evident from 1960-1996, and tracked closely with falling ozone levels, temperatures have subsequently leveled off. Specifically, net temperatures in the stratosphere have remained essentially unchanged since the late 1990s.

It appears that the progressive cooling in the stratosphere during the twentieth century was forced by declining ozone, even as increased solar activity was driving a rise in surface temperatures.

The question, then, is whether the IPCC has considered this variable when citing stratospheric cooling as an invalidator of the solar activity thesis.

Certainly, the IPCC recognizes the connection between declining ozone and stratospheric cooling. Various reports establish this link, including a 2005 report, ‘Safeguarding the Ozone Layer and the Global Climate System,’ which notes: “Stratospheric ozone depletion has led to a cooling of the stratosphere. A significant annual-mean cooling of the lower stratosphere over the past two decades (of approximately 0.6 K per decade) has been found over the mid-latitudes of both hemispheres.”

One has to wonder why the IPCC has chosen to deny solar variability on the basis of cooling stratospheric temperatures. The evidence seems fairly clear that a decline in stratospheric temperatures was driven by declining ozone, a factor wholly unrelated to solar variability.

What’s rather striking is that the flat-lining of stratospheric temperatures since roughly 1998 corresponds quite remarkably with the current “pause” in surface temperatures. This prompts a question: Could the stabilization of ozone levels in the stratosphere help to explain the subsequent ‘pause?’

If so, would the IPCC wish to promote this fact? Such a correlation would finally solve a vexing, recent climate mystery. But it would also establish a more concrete solar connection to temperature variability.

The evidence is compelling, and the subject deserves further scrutiny.

About the author: For more than a decade, Mr. Capozzola’s work has focused on industrial trade policy. He has served as media director at both the Alliance for American Manufacturing (AAM) and the U.S. Business & Industry Council (USBIC.)