Solar geoengineering is the act of deliberately altering Earth's climate to mitigate global warming. Scientists estimate that it will cost just ~$36 billion over a period of 15 years to cool the planet.
But, is it safe?
With every year that passes by the carbon and climate problem grows even bigger. With every new scientific publication the consensus gets even stronger that climate change is not only real and man-made but its impacts are already upon us. The urgent need for unprecedented action means that very soon humanity would have to do anything and everything possible to address this issue and avoid the worst case scenarios. Solar geoengineering, which is the act of deliberately altering the Earth's climate system to counter man-made global warming, particularly solar radiation management is often cited as the last ditch effort to mitigate climate change.
Since the industrial revolution humans have emitted enough carbon dioxide and methane (both extremely potent greenhouse gases) into the atmosphere to increase the global average temperature by about 1°C. It is increasing further at the rate of 0.2°C per decade. This change is alarming because the enormous carbon emissions and subsequent warming are disrupting the natural cycles of the planet that humans and other species rely on, most importantly, the water cycle.
We are experiencing extreme precipitation, extreme droughts, super-charged storms and unpredictable weather conditions, among other effects. All the carbon that is in the atmosphere, most of it goes into the oceans which makes the ocean more acidic, combined with rising temperatures it is severely damaging marine life. The situation will only get worse if we continue on this path unabated.
Naturally, scientists and innovators across the globe are scratching their heads to come up with possible solutions to this precarious situation. While a vast majority of experts believe that reducing greenhouse gas emissions to zero should be our top priority, it is becoming increasingly apparent that this transition will be a long process with many hurdles along the way. Time is of the essence here.
We must come up with a 'Plan B' to deal with global warming.
One alternative approach that is proposed by many is solar radiation management (SRM). SRM methods work on the basic premise of reducing the total amount of solar radiation entering Earth's atmosphere by reflecting part of it back into space. Thereby, preventing warming by reducing the amount of radiation available for absorption by greenhouse gases in the first place.
Over the last decade, multiple potential techniques have been introduced by scientists to reflect incoming solar radiation. From placement of giant mirrors in space to injection of reflective aerosol particles into the stratosphere or brightening the marine clouds using small sea salt particles. Several studies have been published assessing the pros and cons of the different techniques in terms of technological, economic, societal and governance factors.
The leading approach among them is the injection of sulfate aerosols (tiny atmospheric particles) or their precursors at a height of ~20 km above the Earth's surface, into the lower stratosphere. These particles are known to reflect incoming solar radiation back into space and cool the planet. While there is no precedent of a large scale human experiment to actually prove this, there have been natural events which show that this is true.
For instance, in 1991, a giant volcano, Mount Pinatubo in the Phillipines, erupted, spewing out some 10 million tonnes of sulfur high into the air which was followed by a drop of ~0.6°C in global temperature for at least one year. Historically, it has been observed that major volcanic eruptions are followed by a dip in global temperatures, which has been attributed to the sulfur particles that are ejected into the atmosphere.
There are a couple of key aspects of SRM which make it a very attractive solution to mitigate climate change.
First and foremost, modeling studies show that the effect of SRM will be immediate, we will see the global temperatures drop as soon as we begin the program. Just like the climate response to the 1991 volcanic eruption was immediate.
Second, it is dirt-cheap (relatively speaking)! In almost all the studies so far, the estimated cost of solar geoengineering is only a small fraction of the costs associated with climate change impacts such as droughts, heatwaves, wildfires and super-storms.
Most recently, a study published in Environmental Research Letters  lays out a detailed 15 year plan for how we could use solar geoengineering in the future and how much it will cost. The total costs for this 15 year plan are estimated to be ~$36 billion. Where, ~$3.6 billion are the pre-deployment costs which include manufacturing costs of specially designed aircrafts that are capable of carrying millions of tonnes of sulfate aerosols and flying at ~20 km above the ground. Followed by ~$2 billion per year operational costs for running hundreds to thousands of flights which would loft increasing amounts of aerosols into the stratosphere each year, during the 15 year program.
These are peanuts compared to the costs of damages that would incur on the global economy if we take no action on climate change. 
While still in its early research stage, solar geoengineering appears to be a fast, cheap and technologically accessible solution to the greenhouse problem.
That begs the question, why is it not being used already?
Well, there is another side to the coin. Many experts fear that solar geoengineering, if implemented, will be another grand human experiment with mother nature (just like the burning of fossil fuels) and there could be unintended consequences. Some modeling studies show that SRM schemes deployed on a large scale could weaken the global water cycle.[5,6] This could lead to changes in regional rain- and snow-fall patterns around the world with consequences for freshwater availability and food production. Changing the stratospheric chemistry could also adversely impact the ozone layer which is essential for preventing harmful UV radiation from reaching the Earth's surface.[7,8]
Moreover, it is clear that solar geoengineering would at best be a 'quick fix' for climate change but not a real cure to the problem, which instead would be reducing greenhouse gas emissions to zero. Once initiated, it must scale up and continue indefinitely to counteract the growing greenhouse effect. If it is halted for some reason, the aerosol particles would eventually be washed out from the atmosphere in a couple of years and global temperatures will rise rapidly. Also, it does not help in any way in the problems that are directly related to excess carbon in the atmosphere. Air pollution and ocean acidification, for example, will continue to get worse with rising carbon emissions.
Considering all the open scientific, political, and societal, questions and concerns regarding SRM, research must continue in this area to better understand and evaluate this option. If at all SRM is to be considered seriously, it should be seen as a complimentary solution to reducing greenhouse gas emissions and not a stand-alone holy grail. It can certainly be useful in that it can buy us time to make the transition from fossil fuels to clean renewable energy. With the future climate change predictions getting more dire each year, it is an all-hands-on-deck situation. In the end, we might have to explore any and all possible solutions to tackle the most pressing issue of our time.
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