1. Lecture 25: Tropical Cooling Debate
Ch. 12
This lecture was prepared on 1/23/2005 (Sunday).
Took me 5 hours (from 8:00pm to 1:00am)

2. The Tropical Cooling Debate
(21 k yrs ago)
(Chapter 12, p )
How cold were the glacial tropics?
Evidence for a small tropical cooling
Evidence for a large tropical cooling
Was the actual tropical cooling medium-small?
Relevance of global tropical temperatures to future climate

3. Difference between LGM and Today
CLIMAP (Climate Mapping and Prediction) Project
Began in 1970s, published its first map in 1976 and then 1981
Reconstructing the Last Glacial Maximum
LGM August SST
Mainly based on ocean sediments
Difference between LGM and Today
Overall: 4°C cooler than today
N Atlantic: 8°C cooler
N Pacific: 2-4°C cooler
Tropical oceans: 1-2°C cooler

4. What Caused The Tropical Cooling?
Insolation was close to today
Ice sheets were too distant.
Greenhouse gases must have been a major factor.

5. Ocean-Based Evidence for a Small Tropical Cooling
CLIMAP
Distribution of plankton species depends on ocean water temperature.
During LSM, high-latitude cold-adapted species moved to the tropics  a large cooling in the tropics. But …
Tropical cooling: 1.5°C
Biochemical Composition
Relative abundance of alkenone molecules is sensitive to ocean water temperature.
Tropical cooling: less than 2°C
Oxygen isotope measurements
Difference in δ18O values (LGM and today) = difference by ice sheets + difference by ocean temperatures
Tropical cooling: 2-3°C

6. Land-Based Evidence for a Large Tropical Cooling
Drop of the ice line
Descent of the lower limit of mountain glaciers by meters in the tropics.
Lapse-rate cooling: 6.5°C/1000 meters
Tropical cooling: 4-6°C
Descent of the upper limit of forests
Tropical cooling: 5°C
Temperature-sensitive noble gases (xenon, krypton, argon, neon) in groundwater
SW USA and SE Brazil cooling: 5°C

7. Was the Actual Tropical Cooling Medium-Small?
Ocean-based evidence: small cooling; Land-based evidence: large cooling
Critics of small cooling
Plankton relatively insensitive to temperatures at low latitudes
Food more important than temperature for survival
The Pacific is a difficult region to apply CLIMAP
Seafloor sediments poorly preserved (altered by dissolving)
Critics of large cooling
Drier glacial tropical climate increases lapse cooling rate
from present-day 6.5°C/km toward 9.8°C/km of dry air
Mountain glaciers poorly dated
Descent of vegetation due to lower CO2
Where is the truth?
Somewhere between 1.5°C (CLIMAP) and 5°C (land evidence)

8. Relevance of Glacial Tropical Temperatures to Future Climate
Lower values of greenhouse gases caused glacial tropical cooling (1.5 to 5ºC); how large the future warming will be in response to large increases in greenhouse gases?
CO2 = 190 ppmv (LGM) 280 ppmv (preindustrial, 47% higher)  381 ppmv (in 2006) ? (by 2100)
CH4 = 350 ppbv (LGM) 700 ppbv (preindustrial 100% higher)  1751 ppbv (in 2006) ? (by 2100)
Tropical cooling between 1.5°C (CLIMAP) and 5°C (land evidence)
Greenhouse gases by 2100: doubling of the preindustrial values?
This range matches the range of uncertainty about Earth’s CO2 sensitivity simulated by GCMs

9. Thank You!