Global Climate Change and the Greenhouse Effect

 

Notes from an old guest lecture given by

Prof. Sarah Gille
Scripps Institution of Oceanography and
Dept of Mechanical and Aerospace Engineering
sgille@ucsd.edu

1.  The Earth has a greenhouse.

The sun is the major supplier of energy for the Earth's atmosphere and ocean.   The sun's energy input (at the Earth's distance from the sun) is S=1376 W/m2.    Roughly three quarters of this energy is reflected by clouds and ice, so the earth only takes in  This energy is distributed over the illuminated part of the Earth, which has an area of pi R2.   The total area of the earth is 4pi R2, so the average energy received on earth is 344 W/m2.   A portion of this energy (alpha) is reflected by clouds and ice, so the earth receives (1-alpha)344 W/m2 = 241 W/m2.

The energy that comes into the Earth's atmosphere and ocean balances the energy that the Earth sends into space.  (If they didn't roughly balance, the Earth would very rapidly heat up. )   Outgoing radiation is longwave blackbody radiation and is equal to sigma T4.   The Boltzman constant, sigma = 5.7 x 10-8 W/(m2K4).

If sigma T4 = (1-alpha)S/4, we can predict the temperature of the Earth to be:  T=((1-alpha)/sigma S/4)1/4 = 255 K or about -18C.  The average temperature of the Earth is quite a bit warmer, essentially because the Earth's atmosphere acts like a greenhouse, trapping heat.

The Earth's atmosphere actually lets solar radiation pass through to reach the Earth's surface, but it readily absorbs and reradiates outgoing longwave radiation.  We can represent this by creating a cloud layer that absorbs some fraction of longwave radiation (epsilon) and reradiates this energy.  Then we write an energy balance for the lower atmosphere below the cloud and one for the region above the cloud.   Above the cloud:  S/4 = alpha S/4 + sigma Tc4  + (1-epsilon)sigma Te4.   Below the cloud:  (1-alpha)S/4 + sigma Tc4 = sigma Te4.    This gives two equations and two unknowns.  Solving for Te4, we find that, if epsilon = 1, the temperature should be a factor of 21/4 warmer than in the previous example, or about 30C.  In reality, temperature on Earth is somewhere between -18 and 30C, but clearly we need an absorbing layer in the atmosphere to explain why this planet is warm enough for us to live here.

2.  Greenhouse gases in the atmosphere have increased significantly since 1850.

The major greenhouse gases are all 3 atom molecules:  H2O, CO2, CH4, N2O, etc.  With the exception of water vapor, most greenhouse gas concentrations have increased substantially since the start of industrialization.  CO2, for example, has increase from about 280 ppmv to 360 ppmv since 1850, and it has been monitored closely at Mauna Loa since the mid 1950's by Dave Keeling of Scripps Institution of Oceanography.  CO2 increases are essentially entirely due to the burning of fossil fuels.  Methane and N2O have also increased substantially.

Here's a summary of greenhouse gas increases and the effective increase in radiative warming:
 

gas

pre-1850

now

radiative forcing

CO2

280 ppmv

360 ppmv

1.56 Wm2

CH4

700ppbv

1714 ppbv

0.47 Wm2

N2O

275 ppbv

311 ppbv

0.14 Wm2

Greenhouse gas concentrations continue to rise in the atmosphere, and the overall impact of this increase is not fully understood, although an obvious guess is that we'd predict the Earth to warm as greenhouse gas concentrations increase.
 

3.   The climate forecast is for warmer weather.

In response to rising greenhouse gas concentrations and the overall uncertainties in the climate impact, in 1988 the Intergovernmental Panel on Climate Change was formed with a mandate to assess scientific data, assess socio-economic impacts of rising greenhouse gases, and formulate a response.  That led to full reports in 1990 and 1995.  The 2000 report is due out any day now.  These are excellent resources for finding out about greenhouse warming.

In addition, in December 1997, the Kyoto Protocol was  negotiated.  This  is a formal agreement to reduce emissions globally, while allowing nondeveloped countries to develop.  It commits countries to reduce greenhouse gas emissions to a fixed percentage of 1990 levels (92% for most of Europe and 93% for the US)---since most developed countries have continued to build highways and cars and have expanded our populations, the commitments in the Kyoto Protocol will be very challenging.

In the US, treaties are ratified by the Senate, and the Senate has not yet attempted to discuss ratifying the Kyoto Protocol.  See  Kyoto Protocol text  or  commentaries  or search the web for more information.

Sources for  further information