Measurement of a World-Wide Transfer Function for Marine Cloud Brightening

Special Article - Global Warming

Austin Environ Sci. 2021; 6(3): 1063.

Measurement of a World-Wide Transfer Function for Marine Cloud Brightening

Salter SH*

Institute for Energy Systems, School of Engineering, University of Edinburgh, Scotland

*Corresponding author: SH Salter, Institute for Energy Systems, School of Engineering, University of Edinburgh, Scotland

Received: July 06, 2021; Accepted: August 09, 2021; Published: August 16, 2021


Marine cloud brightening with a sub-micron spray of filtered sea water can exploit the Twomey effect to enhance planetary cooling. Several previous climate model results show that it can also affect precipitation in both directions in different places. Modulating the climate model settings for the concentration of cloud condensation nuclei with separate coded sequences in a number of spray regions round the world and correlating each sequence with the resulting weather patterns in observing stations round the world can give an everywhereto- everywhere transfer function of spray from each region to each observing station. The short life of spray allows the best choices of spray regions and seasons. Spray patterns can be modified tactically to suit real-time weather observations.

Keywords: Climate model; Cloud condensation nuclei; Pseudo-random sequence; Marine cloud brightening; Twomey effect; Albedo; Solar radiation management; Stratospheric Sulphur; GeoMIP


Most readers of this special issue will already be aware of the progress of our world leaders in the reduction of greenhouse gas emissions, the rate of Arctic ice loss and the resulting possibility of increased methane emissions. Many will be familiar with the physics of the Twomey effect on cloud brightness [1,2] and the proposal by Latham [3,4] for its exploitation to reduce the effects of global warming.

A key feature of marine cloud brightening is that it allows control of the magnitude, place and time of the spraying. Plans can be quickly adjusted with information from real-time observations giving the option of a tactical response. This paper suggests a way to use climate models to identify and quantify both beneficial and adverse side-effects of marine cloud brightening. We want to produce an everywhere-to-everywhere transfer-function of the relationship between spray quantity, place and time as they affect temperature, precipitation, polar ice, snow cover and vegetation, using several leading climate models in parallel. This should especially show the times and places at which spraying should NOT be done.

The technique involves changing model settings for the concentration of condensation nuclei at many spray regions round the world according to separate, individual coded sequences unique to each region and correlating each of these sequences with model results at observing stations anywhere.

A first test on a set of 16 artificial changes with different magnitudes to a real 20-year temperature record showed that the magnitude of each change could be detected to 1% or 2% of the standard deviation. This is better than many thermometers. Confidence has been increased by a PhD project carried out by Ben Parkes at Leeds, who has shown that the effects of marine cloud brightening on precipitation are bidirectional.

The technique may let us steer towards beneficial climate patterns if only the world community can agree what these are. The differences between climate models may point to general model improvements, for which there is plenty of room. As well as humanitarian benefits the project may lead to better understanding of atmospheric physics and teleconnections.

Previous Work on Side Effects

Elementary physics would predict that, because of the increase in vapour pressure of warmer sea surfaces, global warming would also increase evaporation and subsequent precipitation. Caldeira and Wood [5] showed that if geoengineering with tropospheric sulphur was used to cancel a thermal increase it would over-compensate the increase in precipitation.

The immediate effect of marine cloud brightening is the reduction of sea-surface temperatures and in turn a reduction in the rate of evaporation and an increase in condensation from vapour to liquid. However marine cloud brightening also reduces the size of drops of marine clouds. The production of rain is complicated but one of the requirements is large drops falling through and coalescing with smaller ones. If we reduce drop size the immediate effect is to reduce rainfall over the sea which would leave more to fall over land. Furthermore the higher temperature difference between land and sea will mean stronger monsoon winds to transport the air mass ashore. Stronger winds mean more spray from breaking waves and more air bubbles in foam.

Bala and Caldeira [6] studied the effect of widespread and continuous marine cloud brightening in the Indian sub-continent where changed precipitation can be a matter of life and death [7]. They found that it produced a smaller reduction in precipitation effect than stratospheric sulphur and that this was more than offset by a lower evaporation rate leaving an increase of river run-off.

One of the early attempts at the identification of side effects of marine cloud brightening was in 2009 by Jones, Haywood and Boucher of the Hadley Centre [7]. They picked three regions representing only 3.3% of the world ocean area and raised the concentration of cloud condensation nuclei to 375 per cubic centimeter from initial values of 50 to 300. The regions were off California, off Peru and off Angola/ Namibia. In Figure 1 top left these are labelled NP for North Pacific, SP for South Pacific and SA for South Atlantic. These areas usually have good conditions for cloud cover and solar input. Parts close to the coast have rather high nuclei concentrations. They are good but by no means the only suitable sites for cloud spraying. The increased nuclei concentration was held steady regardless of summer/winter, monsoons or the phase of the el Niño Southern oscillation. The resulting global cooling for the separate regions was 0.45, 0.52 and 0.34 watts per square meter giving a mean annual total of 1.31 watts per square meter. However if all of the regions were sprayed together all of the time the 3.3% of ocean area would cool a little less, 0.97 watts per square meter. Even the lower amount of cooling would be a substantial fraction of the widely-accepted increase of 1.6 watts per square meter since preindustrial times.