Wednesday, February 21, 2018

Solar Geo-Engineering Using Sulfate Aerosols To "Cool The Planet"? Unlikely!

Recall  albedo is that property of a planet which measures the fraction of solar radiation reflected back into space. The darker a planet's surface, the lower the reflectivity of that surface, so the lower the albedo and the higher the absorption of solar radiation. This has special implications for global warming, in particular the melting of the polar ice caps as the late Carl Sagan elucidated in his classic essay:, 'Ambush : The Warming of the World', in his book 'Billions and Billions: Thoughts on Life and Death at the Beginning of the Millennium':

"Melting of ice caps (already occurring) results in diminished albedo (reflection of solar radiation back into space), and a darker Earth surface - with more infrared radiation absorbed - reinforcing the tendency while enhancing the melting effect, leading to further darkening of the surface, reduced albedo and more melting."

On the planet's present course of enhanced CO2 production we are experiencing reduced albedo, more absorption of infrared (heat) radiation and (not surprisingly) more melting of the polar ice caps. Now consider a novel proposal to create an artificial albedo  effect by infusing the upper atmosphere with millions of tons of sulfate aerosols -  leading to trillions of tiny particles to reflect back more of the Sun's infrared radiation to enable a cooling of the planet.  Such a proposal appeared in a 2106 paper by two Harvard scientists:  David W. Keith and Peter J.  Irvine, see e.g.

According to a recent piece on the research (WSJ  Review, Feb. 17-18, p. C4, 'A Big Sky Plan To Cool The Planet')  "the intervention they describe would ramp up after a decade, to delivering perhaps a million tons of sulfate aerosols into the stratosphere each year. The impact would be unnoticeable to the naked eye, reflecting well under 1 percent of solar radiation back into space"

At the same time the WSJ authors   (Gernot Wagner and Martin L. Weitzman) admit that this plan cannot be a replacement for reducing carbon pollution but rather a "supplement to other efforts to combat climate change."   They estimate the start up costs would also be low, "perhaps too low"- based on simply letting a high flying plane "deliver aerosols to the stratosphere near the  equator". The biggest annual expense would likely be satellite monitoring, say for around $2 b a year. Easy peasy right? Not so fast.

One critical element in the satellite monitoring would be how the interjection of up to 0.2 gigatons of sulfate aerosols a year might affect cloud cover and specifically the albedo associated with it.  Of  particular relevance is earlier research  appearing in Eos Transactions: 'Can Earth’s Albedo and Surface Temperature Increase Together’ in Vol. 87, No. 4, Jan. 24, 2006, p. 37,  wherein the authors pointedly noted evidence that Earth’s albedo increased from 2000 to 2004 but that this had NOT led to a reversal in global warming. They also remarked on apparent temperature anomalies and divergence between differing altitudes but pointed to the differences between clouds at those altitudes.

The authors cited  the most up to date cloud data from the International Satellite Cloud Climatology Project (ISCCP). The data – from a range of meteorological satellites covering the entire Earth- disclosed the most likely reason for the anomaly was primarily in the redistribution of the clouds. As the authors observed:

"whereas low clouds have decreased during the most recent years, high clouds have increased to a larger extent leading to both an increase in cloud amount AND an increased trapping of infrared radiation.”

The question then becomes: can the Harvard researchers be  absolutely certain their injection of sulfate aerosols at that altitude actually produce higher albedo AND global cooling? 

Subsequently, research from  Mark D. Zelinka et al.   basically decomposed cloud feedbacks to better connect them to specific physical processes and outcomes.  One primary process is radiative transfer, especially in a plane parallel atmosphere. The basic layout of the problem (especially involving clouds) is well known but difficult. Thus one would wish to investigate  Rayleigh scattering in concert with standard gray atmosphere radiative transport. An equation of transfer that applies is: -dI/dt (1/k r ) = I – J

Where k is a mass scattering coefficient, r  is the molecular density (e.g. in cloud cover) and J is the vector source function for a specific intensity I. If the correct Stokes parameters (I, Q, U, V) which describe degree of polarization are included, and the right incidence angle of radiation occurs, we can expect the propagation of radiant energy from the S. hemisphere to the north very effectively. can't forget or omit diffusive reflection and re-transmission of radiation, say arising from particulates . Chandrasekhar in Radiative Transfer, (Dover Publications) shows that for angles of incidence in the range : 0.5 < i < 0.8 radian, diffusive reflection allows the radiation reflected normal to the incidence direction to actually have higher intensity than the original. Have Keith and Irvine properly factored this into their sulfate aerosol modeling?

For the Zelinka team, the approach entailed using so-called cloud radiative kernels along with detailed model cloud information provided by the International Satellite Cloud Climatology Project Simulator. They calculated the feedback due to 49 different types of clouds—which were divided into seven altitude categories and seven optical depth categories.  See e.g.

  The specific findings included:

1.  Upper level clouds rise to higher altitudes in all models, warming the planet by trapping more heat

2. Low-level cloud cover decreases in all models, warming the planet by reflecting less incoming sunlight back to space.

3.  Optical depth of low-level clouds increases in all models, cooling the planet by reflecting more sunlight

How then will the injected sulfate aerosols affect the optical depths of the clouds with which they interact at different altitudes , and how will this influence the albedo expected?   Given we have evidence that higher albedo in clouds doesn't necessarily affect global warming, how can they be sure these sulfate aerosols will?

What about reflecting 1 percent of solar radiation back into space? Can that magnitude of albedo change really be attained, and is it enough given the surface darkening forces already at work? (Including which have caused massive permafrost melting and release of methane - a greenhouse gas with an even larger forcing component than carbon dioxide. See e.g.

We note here that the solar insolation is  1360  W/ m2    so a 1 percent change translates to   13. 6 W/ m2         The planet is currently out of balance by 0.6 W/ m2  and this is almost entirely due to the annual rate of CO2 concentrations increasing. Further,  every increase in CO2 concentration by 2 ppm increases the radiative heating effect by 2 W/ m2.   If the CO2 concentration changes by 2 ppm/ yr. that means after a decade the cumulative radiative heating will be 10 x  2 W/ m2.  = 20 W/ m2.  The additional question then becomes: can the cumulative added sulfate aerosols after a decade be enough to impact the radiative heating from CO2 increase?   This is especially critical given the rate of greenhouse warming is likely more rapid than has hitherto been reported. 

The WSJ authors, meanwhile, acknowledge another problem with the sulfate aerosols is that they likely will increase erosion of the ozone layer - exposing humans to more ultraviolet radiation, see e.g.

The WSJ authors suggest instead that calcium carbonate (Ca CO3) particles be used instead, but this idea is no better and perhaps worse. We definitely do not need to add to the carbonate stock whether via "reflective" particulate matter or other.    For reference one can cite the Eos Transactions paper, Progress Made in Study of Ocean’s Calcium Carbonate Budget’, which noted that sedimentary carbonates represent the largest reservoir of carbon on Earth. The author also noted that “a third of the anthropogenic CO2 that has been added to the atmosphere since the middle of the 18th century has been absorbed by the oceans."

As the CO2 concentrations exceed 500 ppm, that threshold will be reached. In other words, like by the year 2060. Increases of atmospheric CO2 also increase concentrations of inorganic carbon, mainly in the forms pCO2 and HCO3. A side effect is to also diminish the pH of sea water. The author notes p. 374:

Future decreases in sea water pH (and CO3(-2)) concentration will decrease the saturation state of the waters with respect to Ca CO3”.

This means that spillover becomes much more likely as the saturation threshold is lowered, with masses of CO2 released additionally into the atmosphere. Melting ice from glaciers, etc. – far from being an assisting agent to a new ice age, will reduce further the sea water pH and accelerate the release of CO2 from the oceans. Leading to much much warmer conditions and even more acidic oceans than exist currently (likely pH of 6 or less by 2075). Perhaps  rendering the seas so acidic as to kill off the  phytoplankton, the main producers of oxygen for our planet.

The takeaway here is that even using aerosol reflective particulates as a "supplement" will not help us unless  the total mass of carbon can also be reduced  significantly from its current 45 - 50 gigatons per year. What the Harvard researchers plan will do is provide a brief "feel good" solution that can boost optimism for a bit, and perhaps - if we're really lucky - postpone the advent of the runaway greenhouse effect for a few years.  

One thing the Harvard profs who wrote the WSJ Review piece are right about is (ibid.):

"The greatest concern about solar geoengineering is political: the fear that giving it serious attention  will crowd out long overdue steps to cut carbon pollution."

As I indeed showed, unless carbon pollution is also  cut - and significantly  this aerosol plan using either sulfates or calcium carbonates is a non-starter.

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