Estimating the potential cooling effect of cirrus thinning achieved via the seeding approach

• Main Authors: J. Liu, X. Shi
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/10609/2021/acp-21-10609-2021.pdf

<p>Cirrus thinning is a newly emerging geoengineering approach to mitigate global warming. To sufficiently exploit the potential cooling effect of cirrus thinning with the seeding approach, a flexible seeding method is used to calculate the optimal seeding number concentration, which is just enough to prevent homogeneous ice nucleation from occurring. A simulation using the Community Atmosphere Model version 5 (CAM5) with the flexible seeding method shows a global cooling effect of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1.36</mn><mo>±</mo><mn mathvariant="normal">0.18</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="b8cdd28dcfd60765569f784fad31f311"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10609-2021-ie00001.svg" width="64pt" height="10pt" src="acp-21-10609-2021-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">W m⁻²</span>, which is approximately two-thirds of that from artificially turning off homogeneous nucleation (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1.98</mn><mo>±</mo><mn mathvariant="normal">0.26</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="fe7a8cd09bcac493042c0e55c50c4ed1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10609-2021-ie00002.svg" width="64pt" height="10pt" src="acp-21-10609-2021-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">W m⁻²</span>). However, simulations with fixed seeding ice nuclei particle number concentrations of 20 and 200 <span class="inline-formula">L⁻¹</span> show a weak cooling effect of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.27</mn><mo>±</mo><mn mathvariant="normal">0.26</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="1ddc38e443d1a5dbf4096ec9fc912034"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-10609-2021-ie00003.svg" width="64pt" height="10pt" src="acp-21-10609-2021-ie00003.png"/></svg:svg></span></span> <span class="inline-formula">W m⁻²</span> and warming effect of <span class="inline-formula">0.35±0.28</span> <span class="inline-formula">W m⁻²</span>, respectively. Further analysis shows that cirrus seeding leads to a significant warming effect of liquid and mixed-phase clouds, which counteracts the cooling effect of cirrus clouds. This counteraction is more prominent at low latitudes and leads to a pronounced net warming effect over some low-latitude regions. The sensitivity experiment shows that cirrus seeding carried out at latitudes with solar noon zenith angles greater than 12<span class="inline-formula">^∘</span> could yield a stronger global cooling effect of <span class="inline-formula">−2.00</span> <span class="inline-formula">±</span> 0.25 <span class="inline-formula">W m⁻²</span>. Overall, the potential cooling effect of cirrus thinning is considerable, and the flexible seeding method is essential.</p>