Global warming



When light strikes "stuff"

Three kinds of things can happen when light strikes something, depending on frequency and the material:

  • Transmission,
  • Reflection (scattering) or
  • Absorption, followed later by thermal radiation.
  • Earth's atmosphere also transmits, reflects, and absorbs some of the light from the sun.

    Global warming

    Carbon dioxide, water vapor, methane, CFC's and others are 'greenhouse' gases. They

    • Transmit light at optical frequencies: they're transparent to visible light. They allow optical energy in to the atmosphere.
    • But they absorb / block thermal E-M waves. They don't allow thermal energy out.

    This is why both the atmosphere, and your car on a sunny day are hotter than their surroundings.

    Without some greenhouse effect the surface of earth would be much colder, perhaps inhospitable to life.


    Energy flow in the atmosphere

    Temperature Record

    • This is average temperature. There are still lots of variations.
    • Do you know the difference between weather and climate?

    The IPCC

    The Intergovernmental Panel on Climate Change (

    • has been commissioned by the UN and the World Meteorological Association to assess the status of climate change.
    • It consists of 100s of scientists, diplomats, and politicians, and attempts to reach consensus among these folks in regular reports.
    • Received the Nobel Peace Prize (2007) along with Al Gore.

    Their measured consensus is that

    • There has been an increase in world temperatures of approximately 1${}^o$F (0.5 C) since 1957,
    • which, with a likelihood of 90% is due to human activity rather than being a natural effect.

    Compare to Al Gore:

    "There is overwhelming and incontrovertible evidence of global warming".

    What about natural variation?

    The concern is not so much because of the 0.5 C ($1{}^o$F) temperature change *so far*, but rather the possibility of a further 4-6 C rise in the next 100 years.

    GreenHouse Gases - GHGs

    Greenhouse warming is due to a handful of trace gases - water vapor, carbon dioxide, methane--not nitrogen nor oxygen.

    [The gray bars indicate natural ranges in last 20,000 years.]

    [Units of radiative 'forcing': Compare to average solar power in N Indiana of about 150 W / m^2.]

    Water vapor is a GHG--GreenHouse Gas. But there's so much of it, and it comes and goes from the atmosphere very quickly.

    Carbon dioxide does not "rain" out of the atmosphere, once added it stays and stays. See: the carbon bathtub.

    Methane (natural gas) has a much higher greenhouse effect per molecule (24 X $CO_2$), but there is much less of it in the atmosphere than carbon dioxide.

    Carbon dioxide has the greatest effect on the climate.

    Other GHGs include nitrous-oxide, & CFCs.

    These data come from the IPCC - Intergovernmental Panel on Climate Change.


    Carbon-dioxide levels

    What happened around 1800?

    Not just warming...

    One way that carbon dioxide leaves the atmosphere (phew!) is it dissolves in water.

    But, when carbon-dioxide dissolves in water, the water becomes more acidic.

    Coral reefs are largely made of calcium carbonate skeletons of corals. But acidity increases the amount of calcium carbonate that can dissolve in water.



    How do we know atmosphere composition and temperature in the distant past?

    Tree rings (thickness and density) depend on temperature and go back ~11,000 yr. (Temperatures)

    Ice cores preserve air samples back about 650,000 yr. (CO2 levels)

    Temperature affects the ratio of O16 to O18. (Temperatures)

    Comparing different methods

    There seems to be a correlation between higher temps and higher carbon-dioxide levels. Usually temperature rises came first and later carbon-dioxide levels rose.


    Models of increasing sophistication attempt to project into the future

    It's not always obvious which way global warming effects should go.

    Are reports of massive glaciers breaking off of Antarctica (60 miles long in photo) evidence of global warming?

    The models of a decade ago would have predicted that increased temperatures $\Rightarrow$ greater precipitation, and more snow in Antarctica $\Rightarrow$ growth of glaciers.

    Disappearing sea ice in the Arctic (animation) [NASA images 1979-2003] is one of the more visually convincing images.

    Feedback mechanisms [writing]

    Adding carbon-dioxide to the earth's atmosphere causes many different kinds of changes to the Earth. Some of the changes might make the situation worse--this is positive feedback. Other changes might act against the rise of carbon dioxide--this is negative feedback. Modellers need to consider many of these mechanisms and try to calculate or better yet, measure whether these are large or small effects.

    Increased plant growth

    • Rising carbon dioxide levels cause a rise in temperature,
    • Plants thrive with higher levels of carbon dioxide,
    • They grow faster, and in the process take more carbon out of the air and "fix" it in their cell walls.
    • $\Rightarrow$ This reduces carbon-dioxide levels: negative feedback.

    Arctic ice

    • Rising carbon dioxide levels cause a rise in temperature,
    • This causes a general melting of Arctic sea ice,
    • White ice mostly reflects light back into space. But darker water absorbs light,
    • More energy than before stays close to the earth,
    • $\Rightarrow$ The temperature is now even warmer than it would have been without this change: positive feedback.

    For these mechanisms, decide what the final effect is going to be on temperatures or GHG levels, and say whether it is positive feedback or negative feedback:

    Release of methane

    • Rising carbon dioxide levels cause a rise in temperature,
    • The temperature of the oceans rises.
    • At the bottom of the ocean, there are large frozen deposits of methane clathrates, which might be partially released with higher temperatures,
    • The levels of methane in the atmosphere go up.
    • ...

    Cloud formation scenario I

    • Rising carbon dioxide levels cause a rise in temperature.
    • This causes increased evaporation of water into the atmosphere.
    • Perhaps the average area of Earth's surface covered with clouds increases.
    • The white tops of clouds reflect more sunlight energy back into space than the darker surface of Earth.
    • ....

    Cloud formation scenario II

    • Rising carbon dioxide levels cause a rise in temperature.
    • This causes increased evaporation of water into the atmosphere.
    • Perhaps the average area of Earth's surface covered with clouds increases.
    • Clouds act as a nice insulating "blanket".
    • Some of the IR radiation from Earth's surface that would have escaped into space is now kept "under the blanket" of clouds.
    • ....

    Uncertainty about the effect of clouds is one of the big reasons behind the IPCCs hedging.

    Who can make the biggest difference?

    Reductions totalling > 50%...

    Required reading: Muller (Climate Change) Solutions (7-page pdf)

    Image credits

    Jason Swaby, IPCC, Andreas Christen, Manuel Scheikl, David MacKay