The secret was in the trees.
Looking at tree rings from ancient trees, scientists in a new study say they've finally found the fingerprint of human-caused global warming on drought and rainfall patterns worldwide from as far back as 1900.
Amazingly, tree ring data is an accurate gauge to determine past climates: The rings are thinner in years when it's dry and may not grow at all in stressful conditions like drought.
In fact, researchers can use tree ring data to "find" droughts as far back as 1400, centuries before reliable weather data was available.
According to NASA, "we now know that greenhouse gases caused by humans have been affecting global drought since the early 20th century."
Climate change, aka global warming, occurs because fossil fuels such as oil, gas and coal are burned to power our world. This burning process releases carbon dioxide, methane and other greenhouse gases into our atmosphere.
"The big thing we learned is that climate change started affecting global patterns of drought in the early 20th century," said study co-author Benjamin Cook of the NASA Goddard Institute for Space Studies and Columbia University’s Lamont-Doherty Earth Observatory. "We expect this pattern to keep emerging as climate change continues."
This is the first study to provide historical evidence connecting human-generated emissions and drought at near-global scales between 1900 and 2005.
Lead author Kate Marvel, a climate modeler at Goddard and Columbia University, said, “It’s mind boggling. There is a really clear signal of the effects of human greenhouse gases on the hydroclimate.”
It's also the first time researchers have identified long-term global effects on the water supplies for crops and cities around the Earth.
In a warming world, some regions are expected to get drier, while others will get wetter. The study suggests this pattern will continue:
"All the models are projecting that you should see unprecedented drying soon, in a lot of places,” Marvel said.
Many of the areas expected to dry out are centers of farming, and could become permanently arid. “The human consequences of this, particularly drying over large parts of North America and Eurasia, will likely be severe,” the study said.
The study was published Wednesday in the peer-reviewed British journal Nature.
This article originally appeared on USA TODAY: The secret was in the trees. Humans are making droughts worse