Time%20Scales%20 - PowerPoint PPT Presentation

View by Category
About This Presentation
Title:

Time%20Scales%20

Description:

Time Scales Human vs Physical Processes A big source of trouble Most policy makers and voters just don t get it Time scales of physical processes roughly ... – PowerPoint PPT presentation

Number of Views:23
Avg rating:3.0/5.0
Slides: 12
Provided by: Preferr959
Learn more at: http://www.cabrillo.edu
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Time%20Scales%20


1
Time Scales Human vs Physical Processes
  • A big source of trouble Most policy makers and
    voters just dont get it

2
Time scales of physical processes roughly scale
with size bigger size longer time scale
  • Size can be. Length, mass, thermal
    capacitance.
  • For significant changes in position to happen, we
    look at Newtons 2nd Law
  • --- F ma Forcemass x acceleration

3
For movement under influence of force
  • Solving for dt time scale, gives
  • Time scale (m/F) dv
  • So, larger mass means longer time scale, for a
    given forcing.
  • Time scale for what? For significant change in
    position

4
  • Now, mass goes up very rapidly with physical
    dimension
  • For a block of any material of a given density,
  • Therefore, time scale goes up rapidly with
    physical size.
  • --Microbes dance like crazy
  • --People move like normal
  • --Big things like oceans and atmospheres take
    years or decades to change visibly.
  • --Stars take millions or billions of years to
    change significantly.

5
For temperature change, its THERMAL MASS thats
important as well
  • This is the heat equation. The rate of change
    of the internal energy u of an object is
    proportional to the conductivity k divided by the
    thermal capacitance times density, times the
    gradient of the gradient of the internal energy
    over space

6
Without Getting Lost in Differential Calculus
  • lets just note that solving for dt shows it is
    proportional to both mass density and thermal
    capacitance (remember, water has a very high
    thermal capacitance)
  • times a physical size over which the gradient of
    internal energy changes

7
The Essence of the Physics is this
  • that the time scale for significant change in
    the thermal energy of the system is proportional
    to the thermal mass which is to be forced (add
    heat to something twice as big, itll change
    temperature only half as fast, in everyday
    language)
  • and inversely proportional to the amount of
    forcing (force it twice as hard, itll change in
    half the time, in everyday language)
  • Ts Mt/Ft

8
Implications?
  • The atmosphere is a tiny fraction of the oceans
    mass, and thermal mass. Its time scale for change
    is therefore much faster
  • .but not fast enough as well see
  • The time scale for the ocean to completely turn
    over and come to equilibrium with a constant
    atmosphere (if we had one) is roughly 1000 years
  • The time scale for significant change in the
    atmospheric forcing due to human-caused CO2 is a
    few decades. CO2 levels in the atmosphere have
    increased by 50 in 100 years. Thats a change
    approaching of order 1.
  • Unfortunately, a time scale of 50-100 years falls
    between the cracks as far as us humans

9
Its an UNFORTUNATE time scale
  • 50-100 years for significant climate change
    unfortunate
  • If it were many centuries, we could forget about
    it perhaps and let smarter, wealthier people of
    the future deal with it
  • If it were just a few years, like e.g. impending
    WW II, wed motivate, wed do what was necessary
    spending 10, 20 even 30 of world GDP to
    solve the problem before it became serious.
  • If it were less than 4 years, even our
    politicians might be motivated to focus on the
    truth, instead of short term pay-offs from Big
    Oil or pressure from right wing ideologs
  • Its long compared to human attention spans, and
    its long enough that ignorant people can feel
    if this was serious, itd be changing obviously
    right now. Slow non-existent, in this kind of
    mind
  • Its too long, and its too short.
  • Its. Unfortunate for our kids, and future
    generations. We just dont deal well with this
    kind of time scale. The risk is that well do
    little things in order to salve our conscience
    but avoid the big changes necessary to actually
    head off disaster

10
(No Transcript)
11
Key Points Time Scales
  • Time scales for physical climate processes are
    longer for systems of higher thermal mass Mt
  • Time scales for physical processes are faster for
    stronger forcings Ft
  • Time Scale Mt/Ft
  • Simple calcs show 50-100 yrs for significant
    climate change very unfortunate! Too long to
    keep our personal attention, too short for the
    safety of our children and future generations
About PowerShow.com