April 28, 2003

Time Scales

Yesterday I wrote a short introduction to spatial scales. Today I will write briefly about time scales.

Some timescales of interest:

  • Decision making - minutes to months
  • Attention span - about 15 minutes
  • Seasons - 3-6 months depending on latitute and other geographic factors
  • Career span - 40 years
  • Lifetime of Carbon in the atmosphere - hard to say, but a century or two is a useful approximation
  • Age of the United States and the time since oxygen was discovered - 250 years
  • Time since end of the last glacial period and the invention of agriculture - 10,000 years
  • Age of humans - 1million (1e6) years
  • Age of Earth - 4.5e9 years

    15 minutes is about 3e-5 years; thus if we just take the times outlined above and restrict our attention to human time scales, the time scales of interest span somewhere between 9 and 11 orders of magnitude. This is a huge range. Even if we decide not to worry about times longer than the age of the United States, we still have management time scales that range over 6-7 orders of magnitude.

    Let me give a concrete example. There is much concern about about the rapidly increasing concentration of CO2 in the atmosphere. It is pretty clear that most of this increase is due to the burning of fossil fuels due to rapid industrialization over the last couple of centuries. Carbon that was put into the atmosphere at the beginning of the industrial revolution has only recently cycled out of the system. Carbon that we are emitting now from our energy production and transporation systems will continue to influence the heat budget of our climate for about 2 centuries. Thus actions that we take to mitigate the impact of CO2 emissions will not have their strongest impacts for at least several decades.

    One more point on time. Think about the amont of learning and technological advance we have seen since oxygen was discovered sometime in the late 18th century. Think about what we will learn and how things will change over a comperable time into the future. Now try to imagine what kinds of policies and practices we will need in order to accomplish long-term human well being.