Monitoring of Geoengineering Effects and their Natural and Anthropogenic Analogues

Abstract

A number of climate intervention concepts, referred to as "geoengineering," are being considered as a potential additional approach (beyond mitigation of greenhouse gas emissions) to manage climate change. However, before governments go down the path of attempting deliberate climate intervention including precursor field-experiments, it is essential that the scientific community take the necessary steps to validate our understanding that underpins any of the proposed intervention concepts in order to understand all likely consequences and put in place the necessary strategies for monitoring the expected and unintended consequences of such intervention. The Keck Institute for Space Studies (KISS) has sponsored a project to identify specific priorities for improved scientific understanding and focused efforts to address selected priorities. This project does not advocate the deployment of geoengineering, outdoor geoengineering experiments, or monitoring systems for such proposed geoengineering field experiments, but is rather a precautionary study with the following goals: 1) enumeration of where major gaps in our understanding exist in solar radiation management (SRM) approaches, 2) identification of the research that would be required to improve understanding of such impacts including modeling and observation of natural and anthropogenic analogues to geoengineering, and 3) a preliminary assessment of where gaps exist in observations of relevance to SRM and what is needed to fill such gaps. This project focuses primarily on SRM rather than other proposed geoengineering techniques such as carbon dioxide removal from the atmosphere because there exist a number of analogues to the SRM methods that currently operate on Earth that provide a unique opportunity to assess our understanding of the response of the climate system to associated changes in solar radiation. Additionally, the processes related to these analogues are also fundamental to understanding climate change itself being of central relevance to how climate is forced by aerosol and respond through clouds, among other influences. In other words, this research has likely powerful co-benefits for climate science writ large. The study phase of the project was executed in 2011 and consisted of two workshops at Caltech (May 23-26 and November 15-18) as well as several smaller meetings and telecons. Participants in the study included individuals with an established track record of geoengineering research (primarily modeling studies), experts in the theory and observation of related physical processes, as well as engineers with expertise in risk management and systems analysis. Graduate students and post-doctoral fellows were active participants in the study. Four major topics that were identified during the workshops as priorities for subsequent research and development, particularly in regards to addressing related observational gaps: 1. Volcanoes as analogues of geoengineering with stratospheric aerosols 2. Ship tracks and cloud/aerosol interactions in general as analogues of geoengineering with marine-cloud brightening 3. Studying more targeted geoengineering interventions to counteract specific consequences of climate change, and 4. Identifying the satellite-based albedo monitoring needs that would be required for monitoring either a geoengineering test or its natural and anthropogenic analogues. Major volcanic eruptions that inject sulfate aerosol into the stratosphere cool the planet and are one of the motivating examples behind geoengineering. Much more could be learned about the intentional introduction of stratospheric aerosols through a combination of more thorough analysis of existing data, and development of a rapid-response observing strategy to maximize what we can learn from a future large eruption. Gaps in our knowledge include the evolution of aerosol size, the interaction with cirrus, water vapor, and ozone, and tropospheric chemistry more broadly. There are also attribution challenges that need to be understood, as the conditions following volcanic eruptions are not the same as those due to SRM (e.g. the presence of ash, or the discrete vs continual injection). The second main concept put forth for geoengineering is to introduce aerosols (e.g. salt) to change the optical depth of marine clouds; the current analog for this effect is ship tracks and other cloud/aerosol interactions. There is potential for further analysis of existing data to better understand these interactions and assess the science behind this SRM approach. The sensitivities of cloud albedo to specific processes and parameters are poorly understood. There are also observational gaps, such as the entrainment rate, or direct measurement of albedo, that limit our current ability to assess this approach. Third, it is important to understand what the actual goals for a possible eventual implementation of SRM might be, since SRM would quite possibly be deployed in response to a particular concern, rather than a generic desire to restore the overall climate. The highest priority identified during the study program was to focus on the high risk, high impact potential for a "tipping point" associated with Arctic permafrost melt, and the potential for geoengineering to reverse this. Other tipping points involving Arctic sea-ice and the Greenland and Antarctic ice-sheets may also warrant targeted intervention studies. Finally, one of the specific gaps in our observational capability is the ability to monitor albedo accurately enough to measure and attribute changes, with sufficient spatial, spectral, and temporal resolution. This capability is needed for all of first three SRM topics.

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