The Plausibility of Global Warming.

Art Hobson

Professor Emeritus of Physics

University of Arkansas, Fayetteville, ahobson@uark.edu

 

This article is based on a less technical article published on 28 February 2009 in my regular bi-weekly column in the Northwest Arkansas Times of Fayetteville, Arkansas.  I recommend that you write a regular column, or at least an occasional letter to the editor, in your local newspaper as an excellent and needed way for scientists to communicate with the public about scientific topics as well as about science-related societal topics.

A 2008 Pew Poll found that less than half of all Americans agree with the scientific consensus that global warming is real and human-caused.  This contrasts with the scientific consensus, as expressed in the 2007 Intergovernmental Panel on Climate Change report by over 2000 climate scientists who assessed tens of thousands of peer-reviewed scientific papers published during 2000-2005.  That report stated that “warming of the climate system is now unequivocal” and that there is “very high confidence” that this is at least partly due to humans.  As further testimony to the strength of the scientific consensus about global warming, see Ref. 1.

I witnessed this consensus in February 2009 at the national meeting of the American Association for the Advancement of Science (AAAS), probably the world’s  most prestigious large scientific organization.  Al Gore was invited to speak.  The large lecture hall was overflowing.  Gore received a standing ovation both at the beginning and end of his talk.  The enthusiasm for this man who has so raised public awareness of global warming was palpable.

Public skepticism about global warming stems partly from a gut feeling that human actions are too puny to warm our huge planet.  Allow me to explain why this gut feeling is wrong.  For this, we must understand the greenhouse effect of Earth’s atmosphere.

It’s true that the anthropogenic energy input to the planet is relatively small.  Earth absorbs 1.1 x 1017 W from the sun.2  In 2006, humans consumed an average 14.5 terrawatts (TW) of power from fossil and nuclear fuels,3 at an energy efficiency of roughly 0.4, for a total of 36.2 TW of thermal energy produced from fossil and nuclear resources combined.  Thus the anthropogenic input is only 0.033% of the solar input.  The energy from the sun (plus the small amount produced by humans and other processes such as volcanism) must, on the long-term average, be radiated by Earth into space.  If, for example, Earth radiated 1% less energy than the energy that comes in from the sun, our planet would quickly warm and the increased temperature would cause Earth to radiate more strongly until, eventually, the radiated energy balanced the input energy.

The amount of energy radiated by Earth is determined by Earth’s overall temperature, just as a hot plate’s radiative properties (white hot, red hot, dim-red, no visible glow) are determined by its temperature.  When this balance is put into the form of a couple of simple equations, the overall Earth temperature required to balance the incoming energy from the sun can be calculated to be -19oC.

But Earth’s surface isn’t this cold.  -19oC would freeze the oceans, for one thing.  The answer to this dilemma is that -19oC is the temperature of the top of the atmosphere; this extreme outer surface must be at -19oC for the planet to “glow” sufficiently to balance the sun’s energy.  Like a warm but not visibly glowing hot plate, this low-temperature glow is invisible infrared radiation, not visible light.

Earth’s surface is much warmer than -19oC, because the surface lies under a thick atmospheric blanket.  Like the blanket on your bed on a cold night, the bottom of this blanket is much warmer than the top.  Just like a blanket, the atmosphere warms us by trapping some of the infrared radiation that the surface emits toward space.  This trapping is called the “greenhouse effect” but the greenhouse analogy is rather inaccurate and “blanket effect” would be more accurate.  Like all analogies, the blanket analogy is not perfect: It doesn’t capture the minor fraction of Earth’s energy that is moved around in the atmosphere by convection (the movement of warm or cold air) and by latent heat effects (water vapor condensing and water evaporating), but it does capture the major effect, namely radiation.4

The warming “insulation” in Earth’s greenhouse blanket involves only a few trace chemicals known as greenhouse gases, and not the nitrogen, oxygen, and argon that form far more than 99 percent of the atmosphere.  This insulation is almost entirely water vapor and CO2, forming far less than one percent of our atmosphere.  These few water and CO2 particles are such efficient absorbers of infrared radiation that they cause the bottom of the greenhouse blanket to be nearly about 33 degrees warmer than the top, resulting in a pleasant +14oC average surface temperature.

Three-quarters of the greenhouse effect is caused by water vapor, one-fifth by CO2, and the remainder by other trace chemicals which I’ll neglect.  Humans and ongoing natural processes can’t alter the amount of water vapor by much, because the atmosphere can hold only so much water before it condenses into droplets and “rains out.”  But humans can and have altered the amount of CO2.  Before the industrial age, for at least the past 800,000 years (as determined from polar ice core observations) there were never more than about 280 CO2 particles in the atmosphere for every million air particles.  Now there are 386.  We know that most of the excess CO2 comes from fossil fuels (the rest comes from de-forestation) because this long-buried fossil carbon  contains no radioactive 14C while atmospheric carbon does, and the gradually decreasing 14C/C fraction can be detected in tree rings since the beginning of the industrial age around 1750.

So humans have increased the greenhouse blanket’s CO2 insulation (which forms about 20% of the blanket’s overall insulation) by about 38% (280 ppm to 386 ppm).  In view of the 33-degree natural greenhouse effect, it’s not surprising that this has raised Earth’s temperature by 0.8 degrees, with 3.5 degrees expected by 2100 if present emissions continue.  What we’ve done is analogous to replacing a cotton blanket with a wool blanket.  On a cold night, the top of both blankets are at the same temperature but the bottom of the wool blanket is much warmer because wool retains more of your body’s heat.

So global warming is unsurprising, in view of the significant human alteration of the greenhouse blanket.  It’s not the total energy input by humans that’s causing global warming, it’s the greenhouse gas input.

What is surprising is humankind’s lackadaisical and skeptical response.  Judging by the scientific consensus, and by the audience response to Al Gore’s AAAS talk, we’d better get smart fast.

 

References

  1. Naomi Oreskes, “The scientific consensus on climate change,” Science 306, 1686 (2004).
  2. Robert Romer, Energy: An Introduction to Physics (W. H. Freeman and Company, San Francisco, 1976), p. 383.
  3. U.S. Energy Information Administration, International Energy Outlook 2008.
  4. See e.g. Richard Wolfson, Energy, Environment, and Climate (W. W. Norton & Company, New York, 2008), p. 368.