The Ice Chronicles: The Quest to Understand Global Climate Change

4 | Climate Change and the Rise and Fall of Civilizations

ç 4 Climate Change and the Rise and Fall of Civilizations H umans have occupied the planet for millions of years, and we know, from examination of marine sediments, that the 100,000-year sequence of ice sheet build-up and decay has been in action for at least one million years, if not longer. That means that,if natural climate alone is taken into account,we will very likely once again (in a few tens of thousands of years) have vast ice sheets return. There is also every reason to believe that the rapid climate change events that punctuate these 100,000-year cycles will continue. But what happens to the RCCEs when the ice sheets disappear and the Earth enters a milder period,like the current interglacial (the Holocene)? More important , perhaps, if rapid climate change events have occurred during the Holocene, have they had an effect on the development of civilization? ✵ climbing glaciers barefoot (with a beagle) I remember once, on an expedition to the Himalayas, waking up to the sound of our porters returning to the camp after an evening down the mountain with their families in the village below. I rolled over and looked at my wristwatch—5 a.m.These guys were amazing! They had left us late last night at 18,000 feet (5,486 meters), walked home 7,000 feet (2,132 meters) down the mountain,slept a few hours, and now they were back, ready to climb again. In the four and a half hours while they were at home, they had to eat as well, because this was during the Muslim holiday period of Ramadan, when devout Muslims eat only after sundown. 112 The Ice Chronicles ç As usual, they were barefoot, and they still carried their little beagle dog with them. Maybe it had been his barking that woke me up. Miracles of human adaptability to climatic variability, most of our porters steadfastly refused to use any footwear, even though I offered free boots to them. In fact, the boots we gave them were worth more as sale items than as shoes, and they put one porter through high school for a year. The porters could climb up 500-foot near-vertical ice falls barefoot once we had set ropes for them, following us as we would laboriously settheropesoutwhileweclimbedwithourcramponson.Ihadtoreset these ropes each day because the ice screws that held them would melt out in the noonday sun. Since the porters also wouldn’t use any of the high-altitude climbing gear, they couldn’t stay up on the mountain with us, and that’s why they had to leave each evening. But they had tremendous endurance,and they needed very little sleep. We were the first to do ice coring in the Ladakh region in the Indian Himalayas, but we couldn’t have done it without our porters, who lived so easily in an environment that challenged us to the limit every day. figure 4.1. Ladahki porters and scientists climbing within one of the icefalls on the way up to the Nun Kun plateau (20,000 feet) in 1980. No photography or maps of this site were available when we went to Nun Kun, so the three icefalls we encountered offered a surprising and formidable obstacle. Photo by Paul Andrew Mayewski (1980). Rapid Climate Change Events (RCCEs) during the Holocene To answer these questions about RCCEs and the Holocene, we turn to the Ice Chronicles and examine once again the last 110,000 years of climate record developed from the Greenland Ice Sheet Project Two (GISP2) ice core. Figure 4.2 offers plots of the change in concentration of continental source dust and sea salt in the GISP2 record. As noted in chapter 3, both dust and sea salt levels rise and fall together throughout much of the record. Increased levels of dust signal intensification of atmospheric transport from continental sources. During the coldest portions of the glacial age, the land surface was not as well stabilized by plant cover as during warmer periods, thus intensifying the availability of dusts. Because the dusts are derived from continents such as North The Rise and Fall of Civilizations 113 ç 0 20000 40000 60000 80000 100000 120000 10 1 10 2 10 3 GISP2 Seasalt, Dust (ppb) Years BP Seasalt Dust figure 4.2. Change in the concentration of continental source dust and sea salt in the 110,000year -long...