National Academies Press: OpenBook

Climate Intervention: Reflecting Sunlight to Cool Earth (2015)

Chapter: Appendix D: Volcanic Eruptions as Analogues for Albedo Modification

« Previous: Appendix C: Planned Weather Modification
Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×

APPENDIX D

Volcanic Eruptions as Analogues for Albedo Modification

Considerable progress has been made in understanding the volcanic response problem, but the attempts to reconcile simulations with observations underscore clearly that the present capability for simulating stratospheric aerosols and the climate response to the associated radiative forcing is in a relatively primitive state. As discussed in Chapter 5, the current understanding of albedo modification is insufficient to permit accurate assessment of the likely effects of climate intervention by deliberate alteration of stratospheric aerosols, let alone to plan for deployment. This section highlights some recent work on understanding the climate’s response to volcanic eruptions and discusses prospects for future research directions.

OBSERVATION AND SIMULATION OF RESPONSE TO VOLCANIC ERUPTIONS: PAST STUDIES

There are many different approaches to simulation of volcanic response, which can be used to shed light on the processes involved. The approaches differ in the choice of what is calculated in the model versus what is imposed as boundary conditions based on observations. At the extreme end of the spectrum of forcing models with observations, one can specify the sea surface temperature and sea ice patterns and impose observed volcanic radiative perturbations to the atmosphere, and then see how well the observed changes in land surface temperature and atmospheric circulation patterns can be simulated (as in Graf et al., 1993). As a variant on this approach, different sea surface temperature patterns (e.g., El Niño vs La Niña) or initial circulation states of the stratosphere can be imposed in order to assess which aspects of the observed posteruption climate are due to the aerosol-related radiative forcing versus natural variability which may or may not have been influenced by the eruption (Kirchner et al., 1999; Stenchikov et al., 2004; Thomas et al., 2009a, b). If one is interested primarily in testing aerosol chemistry and microphysics, one can instead impose the observed stratospheric temperature and circulation pattern and see how well the observed aerosol properties can be modeled. At the opposite limit of simulation approaches, models can be driven by estimates of the observed injection of volcanic sulfur dioxide and other substances; both the resulting aerosol and ozone distribution and the

Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×

ocean-atmosphere circulation and associated sea ice changes are simulated using a fully coupled model. This approach requires a coupled ocean-atmosphere model with a full representation of stratospheric dynamics and chemistry and is very demanding. It is the kind of simulation that most closely mimics what would be required for assessment of climate intervention actions, but very few simulations of this type have so far been conducted in the context of volcanic response. Various intermediate combinations of the approaches have appeared in the literature.

The complexity of the atmosphere’s response to volcanic eruptions serves as a stark reminder of the challenges confronting any attempt to engineer the climate through deliberate modification of stratospheric aerosols. Aerosol characteristics and the length of time the aerosols remain in the atmosphere depend on the latitude at which the volcanic sulfur dioxide is injected. The aerosols absorb incoming solar infrared and thermal infrared upwelling from below, in addition to keeping some sunlight from reaching the surface, and the infrared effects lead to stratospheric heating that warms the stratosphere. This heating affects stratospheric circulations, which via a range of complex fluid mechanical processes affect the climate of the lower parts of the atmosphere, including surface temperature. The character of the response to the aerosol-induced stratospheric heating is sensitive to interannual variations in the state of the stratosphere at the time the injection occurs, in particular to the state of the Quasi-Biennial Oscillation (Stenchikov et al., 2004; Thomas et al., 2009a). Most attempts to simulate the effects of stratosphere-based climate intervention crudely represent the effect of the engineered aerosols by simply reducing the amount of solar energy hitting the top of the atmosphere; simulations of this sort do not represent the important dynamical and chemical effects of the aerosol-induced stratospheric heating and can lead to severe distortions of the climate response (Tilmes et al., 2009).

As a result, the volcanic response is not a simple cooling of the planet. Large eruptions lead to severe reductions in rainfall over land, especially in the tropics (Trenberth and Dai, 2007). Furthermore, though eruptions cool the following summers, the first winter following an eruption exhibits pronounced high-latitude warming (Robock and Mao, 1992). This winter warming, as well as many other regional aspects of the volcanic response, cannot be accounted for as a response to the blocking of sunlight but instead results as an indirect effect of stratospheric heating; it requires accurate calculation of the aerosol and radiative processes leading to the heating, a well-resolved stratosphere, and a good representation of the interaction between the stratosphere and the lower parts of the atmosphere. Models that incorporate stratospheric heating, either by calculation or by imposing it from observations, can yield a winter warming pattern that has some resemblance to observations, but accurately reproducing the magnitude of the response has proved problematic.

Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×

The discussion in Chapter 3 (“Observations and Field Experiments of Relevance to SAAM”) summarizes a recent assessment of the ability of coupled ocean-atmosphere models to reproduce the winter volcanic response as found in the study by Driscoll et al. (2012); see Figure 3.11. There have also been a number of simulation studies aimed at testing models of aerosol evolution rather than climate response (English et al., 2013; Kravitz et al., 2010, 2011b), and these highlight the considerable remaining difficulties both in observing and modeling aerosol properties. Arfeuille et al. (2013) argued that even with accurate observationally based specification of aerosol properties, existing radiative transfer codes could not accurately reproduce the stratospheric heating.

VOLCANIC RESPONSE IS FAR FROM AN EXACT ANALOGY FOR CLIMATE INTERVENTION BY STRATOSPHERIC AEROSOL MODIFICATION

It has been argued that the climate response to engineered stratospheric aerosol modification would have much in common with that from volcanic eruptions, but the volcanic response should nonetheless not be taken as an exact analogue for climate intervention (Robock et al., 2010, 2013). From a microphysical standpoint, the key difference is that eruptions inject sulfur dioxide into a relatively clean stratosphere, whereas engineered injections would add sulfur dioxide to a stratosphere that already has a considerable burden of aerosols. This changes various aspects of the physics determining droplet size growth and coalescence of smaller droplets to form larger ones, both of which affect the residence time of aerosols and their effects on albedo. Engineered injection may also involve a different range of altitudes, and the latitudinal distribution would probably also be different; it is generally assumed that climate intervention would produce a more spatially uniform distribution of aerosols than point-source volcanic eruptions, but it is not yet known how well the actual distribution of aerosols can be controlled. Furthermore, volcanic eruptions inject a range of substances, such as ash, that would not be present in an engineered injection.

From the standpoint of climate response, the chief difference between volcanic and engineered injection is that volcanic eruptions give rise to a short-lived radiative forcing perturbation (at most a few years), which is sufficient to yield a strong climate response over land in the case of large eruptions but does not last long enough for the ocean temperature to be much affected, and insofar as the ocean is affected at all it is only the uppermost layers of the ocean that are involved; sustained aerosol forcing due to climate intervention action would involve a considerably deeper part of the ocean, and a larger ocean response. The probable difference in land-sea temperature contrast between engineered and volcanic stratospheric aerosol injection has impli-

Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×

cations for all atmospheric circulations driven by land-sea thermal contrast, notably monsoons and diversions of the midlatitude jet streams. Response of sea ice is sensitive to subtle changes in the ocean circulation, and probably cannot be adequately tested by examination of volcanic response. This is a particular concern, since there are indications that multiple closely spaced eruptions—a rare occurrence such as happened at the time of the Little Ice Age—which approximate the sustained cooling resulting from engineered aerosol modification, can switch the North Atlantic over into an icy mode that can persist for centuries (Miller et al., 2012).

Despite these shortcomings of the volcanic analogue vis-à-vis engineered modification of stratospheric aerosols, the volcanic response engages almost all of the same aspects of atmospheric chemistry, physics, and dynamics as does the climate intervention problem and, therefore, serves as a useful test of the simulation capabilities that would be needed to assess the effects of deployment of climate intervention schemes involving stratospheric aerosol modification.

Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×
Page 235
Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×
Page 236
Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×
Page 237
Suggested Citation:"Appendix D: Volcanic Eruptions as Analogues for Albedo Modification." National Research Council. 2015. Climate Intervention: Reflecting Sunlight to Cool Earth. Washington, DC: The National Academies Press. doi: 10.17226/18988.
×
Page 238
Next: Appendix E: Discussion of Feasibility of Albedo Modification Technologies »
Climate Intervention: Reflecting Sunlight to Cool Earth Get This Book
×
Buy Paperback | $65.00 Buy Ebook | $54.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The growing problem of changing environmental conditions caused by climate destabilization is well recognized as one of the defining issues of our time. The root problem is greenhouse gas emissions, and the fundamental solution is curbing those emissions. Climate geoengineering has often been considered to be a "last-ditch" response to climate change, to be used only if climate change damage should produce extreme hardship. Although the likelihood of eventually needing to resort to these efforts grows with every year of inaction on emissions control, there is a lack of information on these ways of potentially intervening in the climate system.

As one of a two-book report, this volume of Climate Intervention discusses albedo modification - changing the fraction of incoming solar radiation that reaches the surface. This approach would deliberately modify the energy budget of Earth to produce a cooling designed to compensate for some of the effects of warming associated with greenhouse gas increases. The prospect of large-scale albedo modification raises political and governance issues at national and global levels, as well as ethical concerns. Climate Intervention: Reflecting Sunlight to Cool Earth discusses some of the social, political, and legal issues surrounding these proposed techniques.

It is far easier to modify Earth's albedo than to determine whether it should be done or what the consequences might be of such an action. One serious concern is that such an action could be unilaterally undertaken by a small nation or smaller entity for its own benefit without international sanction and regardless of international consequences. Transparency in discussing this subject is critical. In the spirit of that transparency, Climate Intervention: Reflecting Sunlight to Cool Earth was based on peer-reviewed literature and the judgments of the authoring committee; no new research was done as part of this study and all data and information used are from entirely open sources. By helping to bring light to this topic area, this book will help leaders to be far more knowledgeable about the consequences of albedo modification approaches before they face a decision whether or not to use them.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!