The types of record of climate change are myriad. One type of evidence that is often neglected, but has some advantages, is analysis of the shallow thermal gradient. Imagine a tracker finding a campfire pit, touching the ground, and saying, "they left only two-three hours ago." In courses on remote sensing infrared photos are sometimes shown of a thermal shadow left on the tarmack after the planes have left. The ground remembers its past thermal history.

A bit of heat-flow science:

• steady state heat flow through a slab given by:
• q = k ((T2-T1)/h)
  • q is heat flow W m-2, energy per surface area unit
  • k = thermal conductivity W m-1°C-1
  • T2 and T1 bounding temperatures with T2>T1
  • h = thickness of a plate

Thermal diffusivity:

• k = K/(p*cp)
  • where K is thermal conductivity, p is density, cp is specific heat (pressure dependent).
• For rocks k = 15-60 km2/Ma. This means heat moves through rocks slowly.
• Relationship between depth of anomaly and age of occurrence:
  • above circa 15 m can see seasonal signal.
  • down to about 150 m a century long signal would be seen.
  • down to about 500 m a millenial signal would penetrate.
  • the ice age signal is 1 km or deeper.
• Deepening frost during spring thaw? Observation by farmers.

Discussion questions:

• What might be ideal situation to uncover this thermal record, or flip side, what are potential complications? Key to understanding this is the relationship between atmospheric T and ground T, i.e. the nature of "thermal coupling" between atmosphere and lithosphere.
• Where would be good sites?
• What will information about one locality tell you?
• What are the general results?
• What can the results be compared with to look for consistency?

Summary: This crude but robust technique provides results consistent with global warming in the last century. However, it doesn't address the causes.

Convergent validity and explanatory power in the context of multiple records.

Readings:

• Alley & Bender, 1998 (Feb. ), Greenland Ice Cores: Frozen in Time; Sci. Am., 80-85.
• Browse for a bit via some of the links below:
  • Data and summary from Russian Antarctic ice core from Univ. of Wyoming site, by .
  • Entry into National Ice Core Laboratory USGS site.
  • Joint Nordic-Japanese investigation of ice cores on Svalbard (Nordic Arctic Research Programme site).
  • Greenland Ice Sheet Project 2.
  • Ice Core Contributions to Global Change Researh: Past Successes and Future Directions By: Ice Core Working Group, May, 1998. This is a fairly in depth summary.