Climate Intervention using marine cloud brightening (MCB) compared with stratospheric aerosol injection (SAI) in the UKESM1 climate
Geoengineering News
Abstract. The difficulties in using conventional mitigation techniques to maintain global mean temperatures well below 2 °C compared with preindustrial levels have been well documented, leading to so-called ‘climate intervention’ or ‘geoengineering’ research whereby the planetary albedo is increased to counterbalance global warming and ameliorate some impacts of climate change. In the scientific literature, the most prominent climate intervention proposal is that of stratospheric aerosol injection (SAI), although proposals for marine cloud brightening (MCB) have also received considerable attention. In this study, we design a new MCB experiment (G6MCB) for the UKESM1 Earth system model which follows the same baseline and cooling scenarios as the well-documented G6sulfur SAI scenario developed by the Geoengineering Model Intercomparison Project (GeoMIP) and compare the results from G6MCB with those from G6sulfur. The deployment strategy used in G6MCB injects sea-salt aerosol into four cloudy areas of the eastern Pacific. Despite MCB being intended as a technique to modify clouds, much of the radiative effect in G6MCB is found to derive from the direct interaction of the injected sea-salt aerosols with solar radiation. The results show that while G6MCB can achieve its target in terms of reducing high-end global warming to moderate levels, there are several side-effects. Some are common to SAI, including overcooling of the tropics, and residual warming of mid-and high latitudes. Others side effects specific to common choices of MCB regions include changes in monsoon precipitation, year-round increases in precipitation over Australia and the maritime continent and increased sea-level rise around western Australia and the maritime continent; these results are all consistent with a permanent and very strong La Niña-like response being induced in G6MCB. It should be stressed that the results are extremely dependent upon the strategy chosen for MCB deployment. As demonstrated by the development of SAI strategies which can achieve multiple temperature targets and ameliorate some of the residual impacts of climate change, much further work is required in multiple models to obtain a robust understanding of the practical scope, limitations, perils and pitfalls of any proposed MCB deployment.
Stephen Salter
Dear Jim, Andy, Anthony and Phil,
Thank you for your EGU sphere paper.
If you were planning a complicated car journey it might not be wise to lock the positions of the steering wheel and accelerator at the start of your trip. It would be better to install an intelligent vision system, able to respond to varying conditions along the road.
Twomey showed that is the fractional change in the number-concentration of successful nucleations which sets the change in reflectivity.
The total effect depends on
Initial solar input which depends on season and latitude and time of day.
High cloud fraction which might reduce the input.
Initial nuclei concentration
Depths of clouds and the turbulent marine boundary layer
Liquid water content of the air mass
Relative humidity of the air mass
Air updraft velocity
Cloud fraction
Subsequent life of injected nuclei.
The last two are likely to be inversely related
The blue graph in the figure below fromhttps://arxiv.org/ftp/arxiv/papers/1809/1809.08142.pdf shows an extremely wide range in nuclei concentrations with a peak up to 500/cm3 many times below 10/cm3 with large swings day by or even hour by hour.
See also Atmos. Chem. Phys., 21, 12757–12782, 2021
I am trying to do calculations of the range of susceptibility but I expect that it will be at least one order of magnitude. I am sure that you all could produce a more accurate result. If the climate models used in your paper had similar range then operations during the times of high concentration would be a waste of energy. It would be useful to know what that range was.
When spray vessels with hydrofoils are not spraying they can go much faster than the wind. They will steered by intelligent fleet controllers with real-time satellite data and multiple forecast computers to give them hourly forecasts several days ahead . This would allow predictions of the results of different tactical spray options. Spray regions can be placed in merit order so that they can cherry-pick the time and place to give results selected by regional Governments if only they can agree.
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