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)

The Tropical Cooling Debate (21 k yrs ago) (Chapter 12, p. 224-228) 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

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

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

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

Land-Based Evidence for a Large Tropical Cooling Drop of the ice line Descent of the lower limit of mountain glaciers by 600-1000 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

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)

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

Thank You!