Monday, March 23, 2015

Anthropogenic Aerosols, Climate Change and Geoengineering

By Oscar A. Escobar

Last Update January 14, 2016:

Newest -A23K-04: Evidence of Clear-Sky Daylight Whitening: Are we already conducting geoengineering?
Newest -[press review] Media responses to Charles Long on "Unintentional Geoengineering"
-SA article at bottom
-March 25 Event: Dimming the Sun: How Clouds and Air Pollution Affect Global Climate
-IPCC Fourth Assessment Report (AR4) - Surface Radiation
-Further comments below the update.
-NASA - GLORY Mission video

Aerosols are minute particles suspended in the atmosphere.

Oxford definition: A colloidal suspension of particles dispersed in air or gas.

Sulfur aerosols in the stratosphere are the most controversial and popularly talked about in solar radiation management discussions. 

A short article from NASA about what aerosols are. My comments below the article.
(My emphasis)
 “Atmospheric Aerosols:

What Are They, and Why Are They So Important? Aerosols are minute particles suspended in the atmosphere. When these particles are sufficiently large, we notice their presence as they scatter and absorb sunlight. Their scattering of sunlight can reduce visibility (haze) and redden sunrises and sunsets. Aerosols interact both directly and indirectly with the Earth's radiation budget and climate. As a direct effect, the aerosols scatter sunlight directly back into space. As an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth's energy budget. Aerosols also can act as sites for chemical reactions to take place (heterogeneous chemistry). The most significant of these reactions are those that lead to the destruction of stratospheric ozone. During winter in the Polar Regions, aerosols grow to form polar stratospheric clouds. The large surface areas of these cloud particles provide sites for chemical reactions to take place. These reactions lead to the formation of large amounts of reactive chlorine and, ultimately, to the destruction of ozone in the stratosphere. Evidence now exists that shows similar changes in stratospheric ozone concentrations occur after major volcanic eruptions, like Mt. Pinatubo in 1991, where tons of volcanic aerosols are blown into the atmosphere (Fig. 1).

Fig. 1 The dispersal of volcanic aerosols has a drastic effect on the Earth's atmosphere. Following an eruption, large amounts of sulfurr dioxide (SO2), hydrochloric acid (HCL) and ash are spewed into the Earth's stratosphere. Hydrochloric acid, in most cases, condenses with water vapor and is rained out of the volcanic cloud formation. Sulfur dioxide from the cloud is transformed into sulfuric acid (H2SO4). The sulfuric acid quickly condenses, producing aerosol particles which linger in the atmosphere for long periods of time. The interaction of chemicals on the surface of aerosols, known as heterogeneous chemistry, and the tendency of aerosols to increase levels of chlorine which can react with nitrogen in the stratosphere, is a prime contributor to stratospheric ozone destruction.”
NASA FS-1996-08-11-LaRC - August 1996

So about... 

Anthropogenic Aerosols, Climate Change and Geoengineering

Scientists generally shy away from comparing sulfur dioxide (SO2) emission between volcanos and anthropogenic sources.  After asking a question prompted by one of his articles in Yale Climate Connections, science journalist David Appell, pointed out to me that in comparing current SO2 emissions between anthropogenic and volcanic eruptions sources… “the question isn't so much about comparing that to volcanoes, but of how much of that gets to the stratosphere”.

I think David Appell’s point is very important, particularly in respect to how long the aerosols stay in the atmosphere (they stay longer in the stratosphere than in the troposphere), how far away they travel and various other reasons such as chemical interactions.  But, for me as a layperson, making that type of comparisons and thinking in more concrete terms helps in understanding the scientific jargon and science concepts it describes.

Take the following figures of yearly global anthropogenic sulfur dioxide for example:

Over 100 Tg/yr (100,000 Gg/yr) of anthropogenic SO2 (Sulfur Dioxide) [1] [2]

Another important and potent aerosol but with differnt properties is Black Carbon (BC) with over 9.1 Tg/yr [3] [4]

Those are big quantities, but for some of us, lay people, a frame of reference is needed to comprehend that magnitude. So a careful comparison between anthropogenic and volcanic emissions is helpful:

The Mount Pinatubo Eruption of 1991 injected about 20Tg of SO2 into the atmosphere [5] with some thinking half of that (10Tg) went  into the stratosphere over the course of a few days. [6]

The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines was the second-largest volcanic eruption of the 20th century. [6a]


Since 2000 there have been a number of modest volcanic eruptions, the three most significant in 2008, 2009, 2011, each of which expulsed between 1Tg to 1.5 Tg of Sulfur Dioxide. [6]
Yearly volcanic emissions vary between 1.5 to 50 Tg. [7]

Ok. So now I understand better.  

Mount Pinatubo’s injection of SO2 into the atmosphere during the 1991 eruption was extraordinary… regardless of how much actually went into the stratosphere.

But even more extraordinary is the fact thatwe’ are currently emitting into the atmosphere over 5 times the amount of Mount Pinatubo’s eruption in 1991, or the equivalent of up to 100 or more ‘moderate eruptions’ worth of Sulfur Dioxide every year! With international shipping alone emitting, by 2005, over 12 Tg/yr. [1] 

Now going back to ‘where in the atmosphere’ the SO2 is going to, and how they may affect climate…

“Aerosols have very roughly the same ability to scatter sunlight back to space, wherever they are in the atmosphere.” [8] This ability is countered when they act to form nuclei for high cirrus clouds, which act to trap, and reflect, outgoing long wave radiation back to earth; [9] and when or if they act to form thin low laying clouds over Greenland’s ice sheet. [9a] 

But yes, aerosols in the stratosphere stay there longer-- a year or more [10] while those lower in the troposphere tend to stay there only for a week or two, [11], all depending on various factors. These are short ‘residence’ times when compared to carbon dioxide, which can stay in the atmosphere for centuries, and is the reason why sulfur dioxide is classified as a Short Lived Climate Forcer/Pollutant.

The scientific consensus is that stratospheric aerosols like SO2 from volcano eruptions cool the global climate. Err… except when large volcanic eruption cause “winter warmig!” [12] [13]

Although in occasions I have repeated here in this blog the mantra that “sulfur aerosols cool the climate” a more accurate concept is that they have masked global warming in some regions. 

Over time there have been various conflicting studies and reports pointing to anthropogenic sulfur aerosols as the main or a significant cause of what is being called... ‘the global warming pause or hiatus.’ [14][15] While others argue that recent small volcanic eruptions may be the main aerosol source and therefore responsible for the pause, [16] [17] with still some others theorizing it is due to various other factors like cycles of ‘natural variability’ and heat uptake by the oceans. [18] [19] 


For the most part, the environmental, social, legal, economical, ethical and political considerations of the effects of 100 Tg/yr, or over 5 yearly Pinatubo’s worth of anthropogenic SO2 emissions into the atmosphere is largely absent on Climate Geoengineering discussions. 

With some actually dismissing these concerns out of hand.

Perhaps the most recent exception that I know of from opponents to deliberate climate geoengineering is Clive Hamilton Hamilton who briefly writes about anthropogenic SO2 aerosols in his book Earth Masters – The Dawn of the Age of Climate Engineering. 2013 (Chapter 3. p70-71 paperback).

Although as I said before, this topic of tropospheric aerosols is largely absent from the geoengineering discussions, it receives attention only in passing, and perhaps tangentially, for example when studies and reports speak of a possible spike in global temperatures by 2020 or 2025, [20] which is coincidentally the time frame given for some SO2 pollution control laws.[21] Also the year 2020 has been referenced as a start date for SRM geoengineering activities. [8]  

I think not discussing tropospheric aerosols is a glaring and concerning omission by the environmental and political communities, as well as others. Especially in light of reports of accelerated ocean acidification, [22] and taking into consideration the role that international shipping may be playing in this acceleration with its 12 Tg/yr of acidifying sulfur and CO2 emissions.  The accelerated and record Arctic ice loss, [23] taking in consideration the role of black carbon and other radiation trapping factors. [9] The multiple global reports on persistent and record breaking droughts; [24] [25] [26] and other environmental factors such as the effects on biodiversity, ocean acidification and health.

All these and other global environmental events that are (rightfully in my view) attributed to carbon dioxide and other forms of pollution, have also a very strong correlation to sulfur aerosols, and while correlation does not imply causation, in my view it certainly does call for exhaustive and prompt investigation within a geoengineering framework.

So why is it important that anthropogenic aerosols be investigated within a geoengineering framework?

1. Termination shock. Magnitude of expected raise in temperatures when climate forcing from the anthropogenic tropospheric aerosols is terminated [27] [28].  

 Are we in time to avoid it or are we already locked-in to termination shock?

2. Socio-technical lock-in. Both SRM ramp up and/or a continuation of business as usual SO2 emissions, are forms of socio-technical lock-in. [29]

How to avoid it?

3. Moral hazard. “Moral hazard occurs under a type of information asymmetry where the risk-taking party to a transaction knows more about its intentions than the party paying the consequences of the risk.” 

Are the facts about anthropogenic aerosols important facts? 

Who benefits from ignorance?

Fossil fuels subsidies, [30][31][32] Arctic oil exploration, [33] new Arctic shipping routes, [34] [35] Arctic mining, [36], Arctic militarization, [37][38] etc. Are they not expressions of ongoing moral hazard? [39]

Who benefits? Who is paying the price?

To date has the debate or lack thereof been an exercise in Hegelian dialectics with a prescribed outcome?

Will we continue down this path? 


March 25, 2015 

I don't know if this event is a result of this blog, I found the event posting yesterday March 24.
(My emphasis)

Environmental and Urban Studies Program, Distinguished Scientist Lecture Series, and Bard Center for Environmental Policy present

Dimming the Sun:
How Clouds and Air Pollution Affect Global Climate [40]

Beate Liepert
Senior Research Scientist - Climate & Solar Energy
NorthWest Research Associates

Wednesday, March 25, 2015

While anthropogenic greenhouse gas emissions are well known contributors to global warming, the additional impact of short-lived air pollution (e.g. anthropogenic aerosols) is not well understood. The potential for this air pollution to modify climate or even mask global warming will be introduced and the wider implications of aerosol-induced global dimming, including geoengineering prospects, will be discussed.

Beate Liepert is scientist and artist. Climate change and specifically the cycling of water and energy in a changing world are overarching themes of her research. Highlights of her research are the discovery of the phenomenon “global dimming”, which is the increase in atmospheric transparency, and its responsibility for masking parts of the global warming signal in the 20th century. She further pioneered research on the causes of dimming and its implications for the water and carbon cycle on Earth. Beate received a PhD in Meteorology from the Ludwig Maximilians University in Munich, Germany.

In 1996 she moved to New York City to become Doherty Research Scientist at Columbia University. Beate contributed a section on global dimming to the IPCC 4th Assessment Report “Scientific Basis” (chapter that won the Nobel Prize for Peace in 2007. While living in New York City, Beate attended the Certificate Program of Fine Arts at Parsons New School of Design. She moved to Seattle in 2009, and became partner and principal investigator at NorthWest Research Associates, a cooperative for basic and applied research.

Beate’s current research focuses on shifting weather patterns with global warming, and novel solar energy innovations. As artist Beate collaborates with the Seattle based artist, educator and designer Marisa Vitiello on a series of automatism drawings. In 2015 Marisa and Beate have been selected by Spaceworks Tacoma to do an installation in the downtown storefront program and in the Evergreen Association of Fine Arts Gallery Open Abstract Show in Seattle Washington.

Here is the IPCC (AR4) chapter referred to in the event posting from Bard College:

IPCC Fourth Assessment Report (AR4)                  
Climate Change 2007: The Physical Science Basis
Chapter 3 Observations: Surface and Atmospheric Climate Change [41] Surface Radiation
(My emphasis)

The energy balance at the surface requires net radiative heating to be balanced by turbulent energy fluxes and thus determines the evolution of surface temperature and the cycling of water, which are key parameters of climate change (see Box 7.1). In recent years, several studies have focused on observational evidence of changing surface radiative heating.

Reliable SW radiative measurement networks have existed since the 1957–1958 International Geophysical Year.

A reduction in downward solar radiation (‘dimming’) of about 1.3% per decade or about 7 W m–2 was observed from 1961 to 1990 at land stations around the world (Gilgen et al., 1998; Liepert, 2002).

Additional studies also found declines in surface solar radiation in the Arctic and Antarctic (Stanhill and Cohen, 2001) as well as at sites in the former Soviet Union (Russak, 1990; Abakumova et al., 1996), round the Mediterranean Sea (Aksoy, 1997; Omran, 2000), China (Ren et al., 2005), the USA (Liepert, 2002) and southern Africa (Power and Mills, 2005). Stanhill and Cohen (2001) claim an overall globally averaged reduction of 2.7% per decade but used only 30 records.

However, the stations where these analyses took place are quite limited in domain and dominated by large urban areas, and the dimming is much less at rural sites (Alpert et al., 2005) or even missing altogether over remote areas, except for identifiable effects of volcanic eruptions, such as Mt. Pinatubo in 1991 (Schwartz, 2005).

At the majority of 421 analysed sites, the decline in surface solar radiation ended around 1990 and a recovery of about 6 W m–2 occurred afterwards (Wild et al., 2004; 2005).

The increase in surface solar radiation (‘brightening’) agrees with satellite and surface observations of reduced cloud cover (Wang et al., 2002b; Wielicki et al., 2002a; Rossow and Dueñas, 2004; Norris, 2005b; Pinker et al., 2005), although there is evidence that some of these changes are spurious (see Section 3.4.3).

In addition, the satellite-observed increase in surface radiation noted by Pinker et al. (2005) occured primarily over ocean, whereas the increase observed by Wild et al. (2005) was restricted to land stations.

From 1981 to 2003 over central Europe, Philipona and Dürr (2004) showed that decreases in surface solar radiation from increases in clouds were cancelled by opposite changes in LW radiation and that increases in net radiative flux were dominated by the clear-sky LW radiation component relating to an enhanced water vapour greenhouse effect.
Alpert et al. (2005) provided evidence that a significant component of the reductions may relate to increased urbanisation and anthropogenic aerosol concentrations over the period (see also Section 7.5).

This has been detected in solar radiation reductions for polluted regions (e.g., China; Luo et al., 2001), but cloudiness changes must also play a major role, as shown for European sites and the USA (Liepert, 2002; Dai et al., 2006).

In the USA increasing cloud optical thickness and a shift from cloud-free to more cloudy skies are the dominating factors compared to the aerosol direct effects. Possible causes of the 1990s reversal are reduced cloudiness and increased cloud-free atmospheric transparency due to the reduction of anthropogenic aerosol concentrations and recovery from the effects of the 1991 eruption of Mt. Pinatubo. See Box 3.2 for more discussion and a likely explanation of these aspects.

From Box 3.2: The Dimming of the Planet and Apparent Conflicts in Trends of Evaporation and Pan Evaporation

increased clouds impose a greenhouse effect and reduce outgoing LW radiation (Philipona and Dürr, 2004), so that changes in net radiation can be quite small or even of reversed sign. Recent re-assessments suggest increasing trends of evapotranspiration over southern Russia during the last 40 years (Golubev et al., 2001) and over the USA during the past 40 or 50 years (Golubev et al., 2001; Walter et al., 2004) in spite of decreases in pan evaporation. Hence, in most, but not all, places the net result has been an increase in actual evaporation but a decrease in pan evaporation. Both are related to observed changes in atmospheric circulation and associated weather.”

According to David Appell, citing a more recent 2014 paper by Santer et al. (Volcanic contribution to decadal changes in tropospheric temperature doi:10.1038/ngeo2098 paywalled) stratospheric optical depth* has increased 4% - 7% per year from 2000 to 2009. If that is correct, that would make it... 

A total increase of 62% in the stratospheric optical depth in the 2000-2009 decade alone.

I will e mail Dr. Liepert to find out if there will be video recording of her discussion, and if this would be available for public viewing. Also I will include a copy of this post.

*"Optical depth measures the attenuation of the transmitted radiant power in a material. Attenuation can be caused by absorption, but also reflection, scattering, and other physical processes." [42] 
In other words how much light goes through. Higher optical depth means less light goes through.

NASA - GLORY - The Particle Puzzle

A public domain video by NASA Goddard

On 4 March 2011, the Aerosol Polarimetry Sensor (APS) was lost as a consequence of the failed launch of the Glory Mission. [43]


Newest: [press review] Media responses to Charles Long on "Unintentional Geoengineering"

Newest: A23K-04: Evidence of Clear-Sky Daylight Whitening: Are we already conducting geoengineering?

July 22, 2015
Natural aerosols explain seasonal and spatial patterns of Southern Ocean cloud albedo
Science Advances 17 Jul 2015: Vol. 1, no. 6, e1500157 DOI: 10.1126/sciadv.1500157

July 2, 2015
How to Misinterpret Climate Change Research
By Gayathri Vaidyanathan and ClimateWire | April 23, 2015

 [1] Anthropogenic sulfur dioxide emissions: 1850–2005

[2] The last decade of global anthropogenic sulfur dioxide: 2000–2011 emissions

[3] Trend in Global Black Carbon Emissions from 1960 to 2007
Rong Wang et al
Environ. Sci. Technol., 2014, 48 (12), pp 6780–6787
DOI: 10.1021/es5021422
Publication Date (Web): May 13, 2014

[4] Study estimates global black carbon emissions up 72% from 1960-2007; BC emissions intensity down 52%
Green Car Congress - 31 May 2014

[5] Global tracking of the SO2 clouds from the June, 1991 Mount Pinatubo eruptions
Gregg J. S. Bluth
Article first published online: 7 DEC 2012
DOI: 10.1029/91GL02792

[6] An overview of geoengineering of climate using stratospheric sulfate aerosols
Philip J Rasch et al
The Royal Society – Philosophical Transactions A
DOI: 10.1098/rsta.2008.0131 Published 13 November 2008
“For example, the eruption of Mount Pinatubo is believed to have injected approximately 10 Tg S (in the form of SO2) over a few days”

[6a] The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines
USGS - U.S. Geological Survey Fact Sheet 113-97

[7] Emissions from volcanoes
Christiane Textor, Hans-F. Graf, Claudia Timmreck, Alan Robock
Emissions of Atmospheric Trace Compounds
Advances in Global Change Research Volume 18, 2004, pp 269-303

[8] A Case for Climate Engineering
David Keith (2014)

[9] Possible influence of anthropogenic aerosols on cirrus clouds and anthropogenic forcing
J. E. Penner – Feb 3, 2009
Atmos. Chem. Phys., 9, 879-896, 2009

[9a] Thin, Low Arctic Clouds Played an Important Role in Widespread 2012 Greenland Ice Sheet Melt
National Science Foundation - Press Release 13-060 - April 3, 2013

[10] Does air pollution—specifically particulate matter (aerosols)—affect global warming?
Summary prepared by M. Baker (University of Washington) and reviewed by B. Ekwurzel, N. Cole, P. Frumhoff, and S. Shaw (UCS).
Union of Concerned Scientists 

[11] Estimates of residence times of sulfate aerosols in ambient air
E.A. Bondietti, C. Papastefanou∗∗
Science of The Total Environment
Volume 136, Issues 1–2, 15 August 1993, Pages 25–31

[12] Winter warming from large volcanic eruptions
Alan Robock and Jianping Mao - 7 DEC 2012
DOI: 10.1029/92GL02627

[13] Workshop “High-latitude volcanic eruptions and climate: filling the gaps”, 5–6 November 2014
Bolin Center for Climate Research

[14] New study blames 10-year lull in global warming on China coal use, air pollution
By Andrew Freedman – Capital Weather Gang – WP – 07/05/2011

[15] Global is the new local: Pollution changes clouds, climate downstream
By Carol Rasmussen, - NASA's Earth Science News Team - January 26, 2015

[16] Volcanic aerosols, not pollutants, tamped down recent Earth warming, says CU study
March 1, 2013 - Natural Sciences, Environment, Institutes, Cooperative Institute for Research in Environmental Science (CIRES)

[17] Volcanic contribution to decadal changes in tropospheric temperature
Published online 23 February 2014
Benjamin D. Santer et al - Nature Geoscience 7, 185–189 (2014) doi:10.1038/ngeo2098

[18] Cool Pacific Ocean Slowed Global Warming
By Becky Oskin – Live Science - February 26, 2015

[19] Cause of global warming hiatus found deep in the Atlantic Ocean 
Phys.Org – Aug 21, 2014

[20] Global warming likely to accelerate after “pause”, say scientists
By Sophie Yeo – RTCC - Last updated on 24 February 2015

[21] Sulphur oxides (SOx) – Regulation 14
IMO International Maritime Organization

[22] Ocean Acidification
Summary for Policymakers
Third Symposium on the Ocean in a High-CO2 World
The International Geosphere-Biosphere Programme (IGBP)

[23] Arctic sea ice winter maximum may be record low
Yereth Rosen - Alaska Dispatch News - March 9, 2015

[24] U.S. Drought Monitor

[25] Little relief in Central America's food crisis
By Claire Luke - 19 February 2015

[26] Taps Start to Run Dry in Brazil’s Largest City

[27] A new Geoengineering Model Intercomparison Project (GeoMIP) experiment designed for climate and chemistry models
S. Tilmes et al.
Geosci. Model Dev., 8, 43–49, 2015

[28] 4. Post-Implementation Stage - Termination Problem
Ethics of Geoengineering Online Resource Center
University of Montana

[29] Geoengineering: issues of path-dependence and socio-technical lock-in
Rose Cairns (SPRU – Science and Technology Policy Research)
October 2013
Wires Climate Change - Advanced Review
Published Online: Jun 27 2014
DOI: 10.1002/wcc.296

[30] Fossil Fuel Subsidies Are Twelve Times Renewables Support
by Bloomberg News - July 29, 2010

[31] Energy subsidies aren’t just for renewables, fossil fuels get the lion’s share
By The BDN Editorial Board,
Posted Feb. 19, 2015

[32] Fossil fuel clampdown could finally be on its way
By Matt TimmsThursday, World Finance - February 26th, 2015

[33] Shell oil drilling in Arctic set to get US government permission
By Terry Macalister – The Guardian – March 22, 2015

[34] The Melting Arctic: A Fragile Frontier of Riches and Risk Opens for Business
By Sabrina Shankman, InsideClimate News - Mar 17, 2015

[35] China Can Play Key Role in Arctic Shipping
By MarEx 2015-03-21

[36] Future Arctic: More Mining, More Shipping and More Tourists
By Benjamin Hulac and ClimateWire - March 13, 2015

[37] Could the Arctic be the next Crimea?
BY Duncan Depledge - Postdoctoral researcher, Royal Holloway
Quartz – March 23, 2015

[38] Russia's Arctic pivot is a massive military undertaking
JEREMY BENDER – Business Insider - MAR. 12, 2015

[39] Moral Hazard: “In economics, moral hazard occurs when one person takes more risks because someone else bears the burden of those risks. A moral hazard may occur where the actions of one party may change to the detriment of another after a financial transaction has taken place.

Moral hazard occurs under a type of information asymmetry where the risk-taking party to a transaction knows more about its intentions than the party paying the consequences of the risk. More broadly, moral hazard occurs when the party with more information about its actions or intentions has a tendency or incentive to behave inappropriately from the perspective of the party with less information.”
Moral Hazard - From Wikipedia, the free encyclopedia

[40] Dimming the Sun:
How Clouds and Air Pollution Affect Global Climate [40]

Beate Liepert
Senior Research Scientist - Climate & Solar Energy
NorthWest Research Associates
Wednesday, March 25, 2015

[41] IPCC Fourth Assessment Report (AR4)
Climate Change 2007: The Physical Science Basis
Chapter 3 Observations: Surface and Atmospheric Climate Change

[42] Optical Depth
From Wikipedia, the free encyclopedia

[43] Glory Mission Science

Other readings

Climate forcing growth rates: doubling down on our Faustian bargain
Environmental Research Letters Volume 8 Number 1
James Hansen et al 2013 Environ. Res. Lett. 8 011006 doi:10.1088/1748-9326/8/1/011006

Top Lessons to be Learned from Warming ‘Hiatus’
David Appell  —  Yale Climate Connections --- March 5, 2015

Added March 30th.

 Equilibrium climate sensitivity in light of observations over the warming hiatus 

 Natural variability, radiative forcing and climate response in the recent hiatus reconciled 

The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change

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A #Geoengineering #Climate Issues blog - Geoingeniería by Oscar and Jocelyn Escobar is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.Licencia Creative Commons
A #Geoengineering #Climate Issues blog por Oscar y Jocelyn Escobar se distribuye bajo una Licencia Creative Commons Atribución-NoComercial 4.0 Internacional.