'The cities tried to keep an 18-month supply of water in their reservoirs,' says Sever. Not only did drought make it difficult to grow enough food, it also would have been harder for the Maya to store enough water to survive the dry season. In that time, trees and vegetation can grow back there while you slash and burn another area to plant in.'Д«ut what if you don't let the land lay fallow long enough to replenish itself? And what if you clear more and more fields to meet growing demands for food? 'We know that for every 1 to 3 years you farm a piece of land, you need to let it lay fallow for 15 years to recover. The Maya deforested through the use of slash-and-burn agriculture вЂ“ a method still used in their old stomping grounds today, so the researchers understand how it works. 'We propose that increases in temperature and decreases in rainfall brought on by localized deforestation caused serious enough problems to push some but not all city-states over the edge.'

'We believe that drought was realized differently in different areas,' explains Griffin. Sever and Griffin found the stele and other ruins hidden for more than 1,000 years during an expedition that relied on NASA remote-sensing technologies to pinpoint sites of ancient settlements. Archeological records reveal that while some Maya city-states did fall during drought periods, some survived and even thrived.ДЄbove: Deep in the Guatemalan jungle, Sever and Griffin study a crumbled 'stele,' a stone pyramid used by the Maya to record information or display ornately carved art. The results are telling, but more research is needed to completely explain the mechanisms of Mayan decline. Loss of all the trees caused a 3-5 degree rise in temperature and a 20-30 percent decrease in rainfall.' 'We modeled the worst and best case scenarios: 100 percent deforestation in the Maya area and no deforestation,' says Sever. They isolated the effects of deforestation using a pair of proven computer climate models: the PSU/NCAR mesoscale atmospheric circulation model, known as MM5, and the Community Climate System Model, or CCSM. He and his team used computer simulations to reconstruct how the deforestation could have played a role in worsening the drought. 'They had to burn 20 trees to heat the limestone for making just 1 square meter of the lime plaster they used to build their tremendous temples, reservoirs, and monuments,' explains Sever. Sign up for EXPRESS SCIENCE NEWS delivery

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