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Nature Climate Change (2024)
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Marine cloud brightening (MCB) is a geoengineering proposal to cool atmospheric temperatures and reduce climate change impacts. As large-scale approaches to stabilize global mean temperatures pose governance challenges, regional interventions may be more attractive near term. Here we investigate the efficacy of regional MCB in the North Pacific to mitigate extreme heat in the Western United States. Under present-day conditions, we find MCB in the remote mid-latitudes or proximate subtropics reduces the relative risk of dangerous summer heat exposure by 55% and 16%, respectively. However, the same interventions under mid-century warming minimally reduce or even increase heat stress in the Western United States and across the world. This loss of efficacy may arise from a state-dependent response of the Atlantic Meridional Overturning Circulation to both anthropogenic warming and regional MCB. Our result demonstrates a risk in assuming that interventions effective under certain conditions will remain effective as the climate continues to change.
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This work resulted from support from the National Center for Atmospheric Research (NCAR) Early Career Faculty Innovator Program Cooperative Agreement number 1755088 (J.S.W. and K.R.). This work was supported by the NCAR which is a major facility sponsored by the National Science Foundation (NSF) under Cooperative Agreement number 1852977. The CESM project is supported primarily by NSF. We acknowledge the high-performance computing support from Cheyenne (https://doi.org/10.5065/D6RX99HX) provided by NSF NCAR’s Computational Information Systems Laboratory (project numbers UCOR0057 and UCSD0040). J.S.W. acknowledges the support of the National Defense Science and Engineering Graduate Fellowship Program and the Achievement Rewards for College Scientists Foundation. M.T.L. acknowledges the support of National Aeronautics and Space Association Future Investigators in NASA Earth and Space Science and Technology Fellowship 80NSSC22K1528. We also acknowledge the CESM2 Large Ensemble Community Project and supercomputing resources provided by the IBS Center for Climate Physics in South Korea. Hersbach et al. (2023) was downloaded from the Copernicus Climate Change Service (C3S) Climate Data Store (2023). We thank J. Moore for guidance on calculating apparent temperature and P. Polonik, D. Watson-Parris and S.-P. Xie for helpful discussions.
Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA
Jessica S. Wan, Matthew T. Luongo & Katharine Ricke
Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
Chih-Chieh Jack Chen & Jadwiga H. Richter
Atmospheric Chemistry, Observations, and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA
Simone Tilmes
School of Global Policy and Strategy, University of California San Diego, La Jolla, CA, USA
Katharine Ricke
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J.S.W. and K.R. conceived of the study. J.S.W., C.-C.J.C., S.T., J.H.R. and K.R. designed the experiments and developed the methodology. C.-C.J.C. developed the CESM source code modifications. J.S.W. and S.T. ran the simulations. J.S.W., S.T. and M.T.L. analysed the output. J.S.W. and K.R. developed the visualizations of the results. J.S.W. wrote the original draft of the paper and all co-authors contributed to review and editing.
Correspondence to Jessica S. Wan or Katharine Ricke.
The authors declare no competing interests.
Nature Climate Change thanks Yuanchao Fan and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Change in 2010 cloud droplet number concentration at the nearest pressure level (~857 hPa) to boundary layer height (a, e), low cloud cover (b, f), net top-of-atmosphere radiative forcing (c, g), and reference height air temperature (d, h) to mid-latitude (a–d) and subtropical (e–h) MCB. The mean cloud brightening region is denoted by the magenta and green contours for mid-latitude (a–d) and subtropical (e–h) MCB, respectively. Values shown are within 95% confidence from a two-sided t-test on related samples and the top right value is the area-weighted annual global mean anomaly. Averages over the last 30 years of the simulation.
a, Mean 1984–2009 observed low cloud fraction from March to November from the International Satellite Cloud Climatology Project (ISCCP) (see ‘International Satellite Cloud Climatology Project (ISCCP) dataset’ in Methods). The magenta (mid-latitude) and green (subtropical) contours show the average cloud brightening regions from March to November (includes grid cells brightened in at least half of the seeding months). The black contour shows the Western U.S. target region. b, 1984–2009 ensemble mean low cloud fraction from March to November from the CESM2 Large Ensemble (LENS2) ensemble mean. c, d, Reference height temperature response normalized by the absolute global mean forcing due to (c) 2010 mid-latitude and (d) subtropical MCB. Top-right values show the area-weighted global mean.
2010 mid-latitude MCB annual mean changes in reference height air temperature overlaid with changes in near-surface winds (~993 hPa; arrows) (a, c, e, g) and sea level pressure (b, d, f, h) for the first 4 years of the simulation.
The schematic diagram begins with (0) the marine cloud brightening perturbation in the mid-latitude region causing a negative temperature anomaly within the seeding region. Then (1) mean wind in the North Pacific High advects the cool temperatures southward along the coast of North America, which develops (2) a high sea-level pressure anomaly. Increased SLP in this region (3) strengthens the trade winds which amplifies cooling throughout the subtropical Pacific basin through evaporative cooling. Perturbed trade winds alter equatorial convective processes, strengthening the Northern Hemisphere Hadley cell and shifting the Intertropical Convergence Zone southward (not shown). (4) Increased subtropical subsidence, further strengthens the high SLP anomaly and increases lower troposphere stability which is conducive toward surface cooling and low cloud formation. The shading shows the 95% confidence mean air temperature anomalies at 2 m averaged over the 30-year analysis period for the 2010 mid-latitude MCB case. Arrows not drawn to scale.
2010 subtropical MCB annual mean changes in reference height air temperature overlaid with changes in near-surface winds (~993 hPa; arrows) (a, c, e, g) and sea level pressure (b, d, f, h) for the first 4 years of the simulation.
Change in 2050 cloud droplet number concentration at the nearest pressure level (~857 hPa) to boundary layer height (a, e), low cloud cover (b, f), net top-of-atmosphere radiative forcing (c, g), and reference height air temperature (d, h) to mid-latitude (a–d) and subtropical (e–h) MCB. The mean cloud brightening region is denoted by the magenta and green contours for mid-latitude (a–d) and subtropical (e–h) MCB, respectively. Values shown are within 95% confidence from a two-sided t-test on related samples and the top right value is the area-weighted annual global mean anomaly. Averages over the last 30 years of the simulation.
a, Change in bias corrected summer (JJA) apparent temperature [see supplementary materials, equation 1] and (b) annual mean precipitation under 2010 subtropical MCB compared to no MCB. c, the same as (a) and (d) the same as (b) except for the responses under 2050 conditions. Ocean values are masked out and insignificant (p>0.05) values from two-sided t-tests are stippled.
Change in Atlantic Meridional Overturning Circulation (AMOC) to mid-latitude MCB under 2010 conditions (a) and 2050 conditions (b). c, Change in AMOC due to warming under SSP2-4.5 without MCB. d, Change in AMOC between the 2050 mid-latitude MCB and 2010 no MCB cases, where near-zero values indicate a restoration of present-day conditions from MCB. The x-axis shows the latitude of the zonal mean, the y-axis shows depth from the sea surface, and the shading shows the AMOC strength in Sverdrups (Sv). 30-year difference from monthly mean output is plotted. Note the different color bar scale to Fig. 4a.
Change in Atlantic Meridional Overturning Circulation (AMOC) to subtropical MCB under 2010 conditions (a) and 2050 conditions (b). c, Change in AMOC due to warming under SSP2-4.5 without MCB. d, Change in AMOC between the 2050 subtropical MCB and 2010 no MCB cases, where near-zero values indicate a restoration of present-day conditions from MCB. The x-axis shows the latitude of the zonal mean, the y-axis shows depth from the sea surface, and the shading shows the AMOC strength in Sverdrups (Sv). 30-year difference from monthly mean output is plotted. Note the different color bar scale to Fig. 4a.
a, c, Change in annual mean 2m air temperature overlaid with climatological mean winds. b, d, Change in surface heat flux to mid-latitude MCB (a, b) and subtropical MCB (c, d) under 2050 conditions. Insignificant (<95% CI) values are masked out and the top right value in each panel is the area-weighted global mean. The magenta and green polygons show the annual mean midlatitude and subtropical cloud brightening regions respectively.
Supplementary Figs. 1–11 and Tables 1–4.
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Wan, J.S., Chen, CC.J., Tilmes, S. et al. Diminished efficacy of regional marine cloud brightening in a warmer world. Nat. Clim. Chang. (2024). https://doi.org/10.1038/s41558-024-02046-7
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