Tuesday, January 7, 2014

Short-Lived Greenhouse Gases and Global Climate Change

What is the Role for Short-Lived Climate Pollution in Mitigation Policy

By S. Tom Bond, Retired Chemistry Professor & Resident Farmer, Lewis County, WV

There was another significant article about global warming published in the December 13 Science, journal of the American Association for the Advancement of Science (AAAS).  It concerns the effects of the several short-lived pollutants which have significant influence on the progress of global warming.

As everyone is aware by this time, carbon dioxide, CO2, is the principal cause of global warming, which is observed by many signs from warming of the North Polar region, the decline of glaciers all over the world, the Northward migration of the growing season, and thawing of permafrost, to dozens of others.  This is because of the huge amount of  CO2 being released by populated and industrial societies in addition to the amount coming into the atmosphere from natural causes. Sometime last year the average monthly concentration of  CO2 all over the earth passed 400 parts per million, up from 280 in the preindustrial past.  It is known that over 20% of it will be around thousands of years from now.
This effect is because global warming pollutants are able to block longer wave radiation (principally infrared) coming from space from being reflected back into space.  With CO2 the gas stays in the atmosphere for a very long time, hundreds of years.  There is a small proportion of each of the pollutants in the atmosphere naturally, but it is removed over time.  For CO2 the time is so long that the increases added by the now 7,000,000,000 humans is constant and each new addition accelerates the warming.  The warming would continue at a constant rate and the temperature would increase even if no more CO2 was added.
The short-lived pollutants of concern are methane, black carbon, tropospheric ozone and hydrofluorocarbons.  Methane, the reader will be familiar with, is the principal constituent of natural gas. There has been some considerable discussion of it as a global warming gas, too.  It leaks from production, transmission and use of natural gas, from ruminant animals, rice paddies and various natural sources.  Like shale wells, it has a short half-life (time until half the activity is gone).  Shale wells are half as productive in about three years.  The half life of methane in the atmosphere is about seven years.  However, the heat retaining effect is given as 34 times as great as CO2 in the first hundred years.  So it is removed from the atmosphere, but is huge while it lasts.
Black carbon is formed from incomplete combustion of fuel.  Diesel exhaust is an example and so is the smoke from burning debris from cleared forest, which is extensive in the tropics, because of formation of corporate farms.  It is a solid in such small particles they are carried in the air.  The effect last days to weeks.  Black carbon is sometimes given credit for much of the melting taking place on the Greenland ice sheet. The surface appears at times to be comparatively dark, absorbing radiation, rather than the pristine white which reflects it.  It also affects melting in other snowy places.
Tropospheric ozone is found in the layer of air from seven and one half to twelve miles above the surface.  It is caused by nitrogen oxides, carbon monoxide and certain volatile organic compounds, such as xylenes and methane.  Internal combustion engines, industrial processes and chemical solvents produce these compounds which are known as “ozone precursors.”  They also cause smog in cities and increasingly in heavily drilled places, such as the Marcellus region.  The chemistry is complex, beyond us here, but we should note that ozone also occurs in the upper atmosphere in what is known as the ozone layer.  There ultra violet radiation coming from the sun strikes ordinary oxygen, O2, causing  formation of ozone, O3, and removing 97-98% of the ultraviolet.  If all UV got through, sunburn would be much more difficult for humans and animals and plant life would be inhibited.
Hydrofluorocarbons are manufactured compounds of hydrogen and carbon in which one or two hydrogen atoms have been replaced with fluorine.  None exist in nature. They are used in refrigeration and air conditioning, as propellants for aerosols and for solvent cleaners.  They have up to 3300 times the heat retaining capacity of CO2.  Fortunately, they are used in small quantities and alternatives are replacing them.   Hopefully, they will be taken care of under the Montreal and Kyoto International agreements.
Together, at this time, the short-lived global warming gases account for about one third of the present greenhouse forcing, according to the article.  What is needed, the authors say, is a program to reduce them along with reducing output of CO2, which advocates call “hybrid climate mitigation.”  It would be possible to reduce the short-lived climate pollutants while working on reducing long term pollutants, but it must be handled delicately.
The problem is to get a balance.  With emphasis on either to the neglect of the other, the problem can get worse, rather than better.  The short term pollutant effects involve decades while the long term pollutant effects centuries.  If the emphasis goes on the long term pollutant, CO2, the political apparatus and the public might neglect the effect of short term effect of the short lived climate pollutants.  If the effort is made on the short term pollutants, CO2 may be neglected, allowing it to build up over time.
The authors state, “Because it is the most dominant green house gas, near complete reduction of CO2 is the only way to limit the rise of global temperatures and to avoid the risk of  catastrophic impacts.  But partial reduction in CO2 emissions over the next few decades will produce minimal relief from climate impact until mid-century because of the long time scales of CO2 in the atmosphere and the momentum of climate change due to the CO2 already emitted.”
One way to diminish climate impact in the next few decades is to also reduce emissions of short-lived climate pollutants.  It is possible that reducing CO2 will also reduce some of the short-lived climate pollutants.  (If we look at them, methane in varying amounts is emitted by extraction of coal, oil and natural gas and black carbon by coal and oil, and ozone to some extent by all high temperature ignition.)
If political and economic emphasis is put on reducing CO2 alone, this can lead to continued emissions due to the slow nature of innovation in that area.  If emphasis is put on short-lived climate pollutants, no new technology is needed, so the technology can be implemented as soon as political and economic commitments occurs.  This includes such things as stopping leaks and reducing biomass (forest clearing debris) burning.  Switching from hydrocarbon burning as the world’s principal energy source is the only way to substantially reduce CO2 emissions.  But that requires technology changes, economic (profitability) changes and political changes, since the present legal structure is set up to facilitate hydrocarbon energy.
The authors believe “the best way to prevent slowing of CO2 mitigation efforts is to emphasize parallel strategies for reducing short-lived climate pollutants and CO2.”  The problem of preventing disastrous climate change is more complex than we generally realize, but certainly no less important and no less uncertain. 
The article is “What Role for Short-Lived Climate Pollution in Mitigation Policy”,  Science, Vol. 342, Page 1323, December 13, 2013.