快猫短视频

Up to around 750 AD, the Mayan civilisation was thriving. Dozens of new monuments were being built every year in what is now Mexico and central America. But by 900 monument building ceased altogether and some cities in the southern lowlands were abandoned. What happened?

One idea is that a prolonged drought was to blame, and now we have the best evidence yet that this was the case. Analysis of 鈥渇ossil water鈥� has shown that there was half as much rain as usual between 800 and 1000, and that at times during this period there was 70 per cent less rainfall.

During prolonged dry periods gypsum may precipitate out of lake waters and be deposited in sediments. The presence of gypsum deposits in Lake Chichancanab in Mexico provided the first evidence of prolonged droughts around the time of the Mayan decline. But just how severe were these droughts?

To find out, Nicholas Evans at the University of Cambridge, UK, and colleagues extracted ancient water trapped in the gypsum and analysed its isotopic ratios. Water molecules that contain, say, an oxygen-17 atom instead of the more common oxygen-16 are heavier and less like to evaporate. Heavier water molecules therefore accumulate in the lake during times of low rainfall and high evaporation.

Domino effect

While isotopic analysis has been around for decades, Evans鈥� team has developed a method that for the first time makes it possible to simultaneously measure the levels of all the different isotopes of oxygen and hydrogen, revealing a wealth of information: rainfall levels, water temperature and even humidity levels.

鈥淲e get the full climate picture,鈥� says Evans, who is using it to study past climate in many other places and says it could even reveal what Mars was like in the past.

While this work cannot prove cause and effect, there is growing evidence from around the world that many periods of upheaval and war coincided with climate change. The drought could have had a domino effect, Evan says, with a lack of food leading to unrest, warfare and political disintegration, and the eventual downfall of the Maya鈥檚 ruling elite.

鈥淭he changes [in rainfall] were considerable, but not overly dramatic,鈥� says Eelco Rohling of the Australian National University, who . 鈥淚n other words, they are a good illustration that no dramatic changes in climate are needed to cause enormous problems. This truly is the lesson humanity should learn for our future.鈥�