New Research Shows How Different Greenhouse Gases Adversely Affect Crops

One of the tropes circulated by climate deniers has been that "Excess carbon dioxide is good for plants and especially crops". In other words, CO2 even at exceptionally high levels, is not really a pollutant, but something to help us attain better crop yields. While climate scientists and environmentalists have tried to correct this, i.e showing that excess CO2 actually propels "super weeds" which kill crops and that necessitates ever stronger weedicides like agents as: 2, 4- D, dicamba and paraquat, the effects have been nominal. (See also:The Wall Street Journal  June 4, 2010, p A16, 'Superweeds Trigger New Arms Race')

The deniers, for their argument, probably invoked the (summary) chemical equation most of us learned in high school biology for photosynthesis:

6 CO2 +   6 H20 + light energy  ->  C6 H12 O6  + 6 O2

Thus, the carbon dioxide (6 molecules) combined with six of oxygen and adding in radiant energy (sunlight) gives glucose plus oxygen. What could be wrong? Well, a lot it there is too much carbon dioxide.

But this example, illustrates the recurring problem of exactly how any greenhouse gas affects crop yields. Now thanks to new work by Drew T. Shindell, e.g.

http://onlinelibrary.wiley.com/doi/10.1002/2016EF000377/full

We are finally getting some useful answers. While it's been known that carbon dioxide is the largest driver of climate change, Shindell found that other anthropogenic emissions cause more damage to actual crop yields.  Among the culprits named in the past: methane, halocarbons (used in refrigeration and air conditioning),  and black carbon (from biomass and fossil fuel burning)

In his paper published in Earth's Future, Shindell sought to create a more complete picture of human-caused emissions.  He did this by assembling data from his previous studies as well as the IPCC's fourth and fifth assessment reports and crop yield data from previous work.  Shindell then created a model to determine the effect that individual greenhouse gases have on global temperatures, precipitation, carbon dioxide and ozone - all of which affect crop success.

Shindell's model revealed that in the short term (i.e. 1st decade after release of emissions), the greatest damage to crops per ton comes from black carbon and halocarbons. Methane emissions come next, given that the gas (CH4) increases surface ozone which causes harmful chlorosis, i.e. a yellowing of leaves.

Shindell found the effects of CO2 are more complicated, For example, as the amount in the atmosphere increases, crop yields initially do as well. But as the CO2 emissions continue to contribute to climate warming the overall impact becomes negative with any benefits originally accruing basically being outweighed by deleterious effects after only 10 years.

Overall, Shindell's model indicates that up to 93 percent of crop losses through the rest of the century will be caused from non-carbon dioxide emissions with methane the most damaging. (Let us also bear in mind the most massive methane outgassing, as from melting permafrost in the Arctic, is triggered by much higher warming from carbon dioxide)

The most intriguing aspect of Shindell's paper is in where it shows how mitigation procedures can increase crop yields. For reduced CO2 alone, for example, crop yields would go up 3 percent. If methane is suitably reduced (which also means CO2 and warming is reduced) we can expect a 16 percent increase in crop yields.

On the other hand, under a high emissions scenario for all greenhouse gases (likely under Trump) there will be nearly 25 percent crop losses by the end of the century. Given the world's population is expected to balloon to 11 billion or more, we are looking at extreme hunger, starvation. This will be in the most densely populated regions such as Africa.

Without a doubt, Shindell's work provides a solid basis for decisions by policy makers. The question now is whether they will even read his work, and then act on it.

More Rapid Ice Sheet Melt Discovered In Greenland

View of rapidly darkening region in SW Greenland, settled on ice sheet surface.

According to the results of a new study reported in the journal, The Cryosphere on March 3rd, see e.g. http://bit.ly/Cryo-Dark

the ongoing warming of Greenland is speeding changes to the crystalline structure of fallen snow in such a way that snow pack more readily absorbs solar energy. This has resulted in a change in albedo (surface reflectivity) which in turn has further propelled the transformation to more heat absorbent snow and more melting. This has led to a vicious cycle currently playing a role in the growing losses of snow and ice from the region.

This according to Sarah Doherty a co-author of the journal article and atmospheric scientist at the University of Washington. 

The process basically entails the subtle alteration of the shape of snowflakes, which then tend to become more rounded, enabling them to more easily merge with others to form larger grains once they've landed. Doherty explains that as Greenland warms the merging occurs more rapidly. She adds that because larger grains don't have as many crystal boundaries or as rough textures, they tend to reflect sunlight back into space less effectively than finer-grained snow. This results in a lower of albedo. The process is especially pronounced at near infrared wavelengths (700- 2500 nm) that make up half the solar energy the Earth receives.

According to Doherty, quoted in Eos - Earth & Space Science News (March, p. 5):

"You have a self-reinforcing mechanism. When you have larger snow grains you end up with a darker snow pack. It is actually absorbing more sunlight which accelerates melting."

This new phenomenon pointed out by Doherty and her team is interesting because atmospheric scientists previously thought the more rapid rates of melting of Greenland's ice sheet were due to black carbon settling on the snow.  This carbon -mainly as  aerosolized pollution and soot from wildfires - would limit snow's ability to reflect sunlight causing melting.

In fact, the latest research shows the amount of black carbon in the air has been slowly declining after peaking around 1920, according to an article published in Science, e..g.

http://bit.ly/Blk-Carbon

Note, however, that diminished particulates (as black carbon) reaching Greenland doesn't mean they don't play a role, as Doherty et al noted. They point out, in fact, that as the top layer of snow melts it leaves behind a deposit of tiny particles trapped inside it. These dark particulates further reduce the reflectivity of the ice, increasing warming and causing more melting.

Doherty explains (ibid.) that the melting snow exposes more dirt which can then be picked up by the wind and deposited on the ice - in other areas- which also makes it melt faster

This study and others related to it are of immediate import to the welfare of coastal cities - like Miami Beach - currently coping with different degrees of flooding due to rising sea levels. In fact, any major melting of Greenland's ice sheet could threaten many coastal cities because of the sea level rise caused globally. In addition, the influx of fresh melt-water entering the sea could also disrupt ocean circulation patterns and harm marine organisms, the Doherty team concluded.

It bears noting that we need to carefully monitor albedo changes occurring.  Already, researchers based at the National Oceanic and Atmospheric Administration (NOAA) using the NASA Moderate Resolution imaging Spectro-radiometer (MODRIS) and an advanced, high resolution NOAA radiometer, have found that between 1996 and 2012 Greenland's summertime reflectivity if the ice sheet has decreased by about 2 percent per decade.

In contrast to the nonsense about claims of "global warming" on Mars - for which there is no hard evidence of any albedo change- we do have hard evidence of such changes here on Earth. If we know what's good for us we will pay attention to them and cease making excuses to ignore them. Which we do at our own peril, and this is no "global warming alarmist conspiracy" as computer techie Kort Patterson seems to believe.

See also:

http://brane-space.blogspot.com/2011/05/greenland-arctic-ice-acceleratingso-why.html