Stratospheric aerosols play an important role in controlling Earth’s radiative balance, particularly after volcanic eruptions. Although there have been no major volcanic eruptions since the 1991 Mt. Pinatubo blast in the Philippines, the role of minor eruptions during the past decade has been of interest as part of the explanation of the rate of global warming during that period. New satellite and ground-based capabilities have given us important new insights into measuring and characterizing the stratospheric aerosol loading, as summarized in the recent Reviews of Geophysics paper “Stratospheric aerosol – Observations, processes, and impact on climate” by Kremser et al. The team of 34 authors who wrote this paper include virtually all the experts on this topic from around the world, and the paper clearly describes the state of current technology as well as existing gaps that can be filled. As we now work to produce a stratospheric aerosol data set for use in the upcoming CMIP6 climate model simulations, the results of this paper, and many of the authors, will play a crucial role. AGU asked the authors of the article to highlight the important results that have emerged from their research and some of the important questions that remain.

Why is this topic timely and important?

The stratospheric aerosol layer is a key component of the climate system as it directly affects how the incoming solar radiation is absorbed and scattered in the Earth’s atmosphere. Stratospheric aerosol plays a crucial role in ozone chemistry through chemical reactions that occur on the surface of aerosol (called heterogeneous reactions). From 1960 to 2000, most stratospheric aerosol observations occurred during a period that was dominated by a few large volcanic eruptions followed by a decade of slow recovery toward background levels. In the early 2000s, aerosol reached the lowest levels observed by modern instruments, followed by a general increase primarily due to a series of small volcanic events. This relatively clean period is of significant interest, since it has previously been difficult to infer the role that non-volcanic sources, natural and human-derived, play in maintaining the stratospheric aerosol levels. The last 15 years provide an important window in which to infer the impact on stratospheric aerosol concentrations of emissions from the ocean, the biosphere, and human activities. The subtle changes to the stratospheric aerosol levels throughout the last 15 years may have played a subtle role in modifying climate, but the scale of these small but not insignificant effects remains difficult to assess.

What recent advances in particular are leading to a new understanding or synthesis?

In the previous comprehensive review on stratospheric aerosol in 2006, a major challenge was the discrepancy between in situ and space-based inferences of aerosol properties, particularly during low aerosol loading periods. These differences have been substantially closed due to improved understanding of both the in situ and space-based measurements. In addition, there have been major advances in the ability of models to reproduce observed surface temperatures following major volcanic events like the 1991 eruption of Mount Pinatubo. Some climate models now contain aerosol modules which produce, transport, and remove aerosol internally, rather than depend on prescribed data sets. While there is much refinement to do, these developments point to an improved understanding of how stratospheric aerosol affects climate, and soon to an understanding of how climate impacts aerosol. Current estimates of the non-volcanic source of aerosol and its gas precursors to the stratosphere are about 50% higher than estimated in 2006 mostly due to an improved understanding of the emissions of key sulfur containing gases. There is also a new appreciation for the importance of non-sulfate aerosol, such as organics and meteoritic material, in the stratosphere.

What are the societal implications of the new understanding?

Improvements in our ability to measure and to model stratospheric aerosol provides a new found capability to fully account for stratospheric aerosol in global climate simulations. Accounting for stratospheric aerosol is part of understanding global warming and communicating the complexities of the climate system to the public. In the event of another major, potentially catastrophic, volcanic eruption national and international agencies will need a rapid assessment of the immediate impact on surface temperatures and the longer term climate. Advancements in climate modelling means better assessments of impacts on temperature, rainfall, ocean temperatures, and ultimately on agriculture and human life. The Tambora eruption of 1815 leading to the “year without a summer” in 1816 and worldwide crop failures and famine, demonstrated clearly that volcanism can impact human livelihood on a global scale. Recognition of this potential has made intentionally increasing stratospheric aerosol, to counteract a warming climate, a favored candidate for geoengineering. Without fully understanding the impact of changes in stratospheric aerosol levels on surface climate and stratospheric chemistry, the impact and side effects of artificially enhanced stratospheric aerosol levels cannot be fully assessed.

What are the major unsolved or unresolved questions and where are additional data or modeling efforts needed?

The role of man-made sulfur dioxide (SO 2 ) in contributing to maintaining and modifying the stratospheric aerosol layer remains an unknown. The research to discern human and natural contributions to SO 2 is complicated by the dearth of reliable measurements of SO 2 , particularly at the low concentrations required. Measurements in the tropical upper troposphere and lower stratosphere at background levels for SO 2 (<10 ppt) are needed to understand the flux of SO 2 into the stratosphere. More robust modelling of the pathways for SO 2 , particularly from Asia would help to illuminate the contribution of human activities on SO 2 reaching the stratosphere. In addition, not much is currently known about how climate change, in turn, affects the production of stratospheric aerosol precursors like carbonyl sulfide (OCS) and dimethyl sulfide (DMS).

Maintaining a continuous observational record of stratospheric aerosol is essential to detect modest changes in aerosol levels due to changes in natural and anthropogenic emissions and to maintain a test bed for testing of future climate models. Maintaining continuity has proven to be challenging due to changes in instrumentation and measurement approaches over the past decade. Unlike many measurements, stratospheric aerosol is primarily measured by a few optical properties from which more detailed properties are inferred. Changes in the measurement techniques which occurred in the middle of the last decade from primarily solar occultation to space-based lidar and limb scatter observations, means that the primary measurements, each with their own strengths and limitations, changed fairly abruptly and represent a significant challenge to the continuity of the aerosol measurement record. We can expect further changes on measurements techniques. How stratospheric aerosol will be measured from space past 2020 is not presently known. Space measurements are also crucially dependent on long-term in situ measurements to provide context to the aerosol properties, which cannot be directly inferred from optical measurements. Funding limitations make the maintenance of crucial long term records difficult.

The mechanism of how trace gases, including sulfur and sulfur compounds, cross the tropopause into the stratosphere is still a major research topic. It is suspected that the Pacific warm pool, as the gateway to the stratosphere, plays an important role. A number of measurement campaigns and a large coordinated project have been initiated this year to perform targeted measurements of sulfur compounds in the tropical region to address this key question.

—Alan Robock, Editor, Reviews of Geophysics; email: [email protected]