Editing IPCC:AR6/WGI/Chapter-8 (section)

=== 8.6.3 Abrupt Water Cycle Responses to Initiation or Termination of Solar Radiation Modification ===

<div id="h2-22-siblings" class="h2-siblings"></div>

Solar radiation modification (SRM) techniques seek to reduce the impacts of climate change by modifying the Earth’s radiation budget, either by reflecting incoming solar radiation or increasing the amount of heat lost to space. Note that, following SR1.5, the definition of SRM in this Report refers to changes in both solar and longwave radiation ( [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] and Glossary). A variety of methods have been proposed, including injection of aerosols or their precursors into the stratosphere, cloud brightening, and cirrus cloud thinning (Table 4.8). Since SRM alters the planetary energy balance, changes in the hydrological cycle are theoretically expected ( [[#8.2|Section 8.2]] ). These changes can be abrupt if the initial magnitude of SRM is large, rather than increased gradually. Since AR5, a diversity of SRM techniques have been tested using climate model simulations, with an increasing focus on consequences for regional water availability. Techniques targeting shortwave radiation (sulfate injection, surface albedo modification, cloud brightening) are ''likely'' to reduce global mean precipitation relative to future CO <sub>2</sub> -emissions scenarios (Bala et al. , 2008; A. Jones et al. , 2013; Tilmes et al. , 2013; Ferraro et al. , 2014; Crook et al. , 2015) . In contrast, cirrus cloud thinning, a longwave radiation technique, results in increased global precipitation as it causes enhanced radiative cooling in the troposphere ( ''medium confidence'' ) ( [[#Crook--2015|Crook et al., 2015]] ; [[#Kristjánsson--2015|Kristjánsson et al., 2015]] ; [[#Jackson--2016|Jackson et al., 2016]] ).

The magnitude of hydrological disruption for both the initiation and termination of SRM depends on the method used, as well as the strength and duration of its implementation ( [[#Ekholm--2016|Ekholm and Korhonen, 2016]] ; [[#Irvine--2019|Irvine et al., 2019]] ). Under abrupt SRM implementation, hydrological shifts are rapid, occurring within the first decade ( [[#Crook--2015|Crook et al., 2015]] ). Artificial enhancement of albedo in Northern Hemisphere desert regions causes a southward shift in the Hadley Cell and ITCZ, and extreme drying in the northern tropics ( [[#Crook--2015|Crook et al., 2015]] ). Uniform or tropical stratospheric sulfate injection weakens the African and Asian summer monsoons and causes drying in the Amazon ( [[#Robock--2008|Robock et al., 2008]] ; [[#Crook--2015|Crook et al., 2015]] ; [[#Dagon--2016|Dagon and Schrag, 2016]] ). Changes in evapotranspiration can produce large deficits or surpluses in soil moisture and runoff in different regions and seasons ( [[#Dagon--2016|Dagon and Schrag, 2016]] ).

Rapid changes (years to decades) in the hydrological cycle are also expected if SRM is terminated abruptly, either purposefully or because of technical failure or political disagreement. We reiterate the AR5 conclusion that if SRM ‘were terminated for any reason, there is ''high confidence'' that surface temperatures would increase rapidly (within a decade or two) to values consistent with the GHG forcing’. The additional global warming caused by SRM termination may result in a rapid increase in global mean precipitation ( ''medium confidence'' ) ( [[#Jones--2013|]] [[#Jones--2013|A. Jones et al., 2013]] ). Heterogenous regional and seasonal changes are also expected, but are model-dependent ( [[#Jones--2013|]] [[#Jones--2013|A. Jones et al., 2013]] ). As with SRM initiation, the impact of SRM termination is expected to be dependent on the technique deployed.

In summary, it is ''very likely'' that abrupt water cycle changes will occur if SRM is abruptly initiated or halted, especially in tropical regions. Further assessment of the potential side-effects of SRM is found in [[IPCC:Wg1:Chapter:Chapter-4#4.6.3.3|Section 4.6.3.3]] .

<div id="8.7" class="h1-container"></div>

<span id="final-remarks"></span>