Humans have been influencing complex weather conditions for the last century, resulting in some areas getting increasingly drier while others see wetter conditions. According to new research published in Nature, researchers say these trends are only likely to continue with worsening consequences for humans.

To come to their conclusions, scientists detected a “fingerprint” of human-driven global warming, affecting patterns of drought, moisture, and temperatures across the world dating back to 1900. In this particular study, the team went beyond precipitation levels and looked at soil moisture – the measure of precipitation against evaporation – to determine systemic changes in the hydroclimate already underway. Computer models paired with long-term observations of tree rings dating back as far as 900 years allowed the team to estimate soil moisture before and after the Industrial Revolution to determine if the two synced up.

For the first time, they have identified the long-term global effects on vital water supplies that provide for crops and cities globally.

“We asked, does the real world look like what the models tell us to expect?” said study co-author Benjamin Cook in a statement. “The answer is yes. The big thing we learned is that climate change started affecting global patterns of drought in the early 20th century. We expect this pattern to keep emerging as climate change continues.”

Altogether, the team broke the study down into three periods. First, 1900 to 1949, where the global-warming fingerprint was the most obvious. During this time, drying of soils was observed in Australia, most of Central and North America, Europe, the Mediterranean, western Russia, and southeast Asia. However, it got wetter in western China, central Asia, the Indian subcontinent, Indonesia, and central Canada – predictions indicated by both the computer models and tree ring analysis.

Regions projected to become drier or wetter as the world warms. More intense browns mean more aridity; greens, more moisture. (Gray areas lack sufficient data so far.) Adapted from Marvel et al., Nature, 2019

Flash forward to between 1950 and 1975, when the weather seemed to be a bit more erratic. Here, researchers say enormous amounts of industrial aerosols were poured into the air before modern pollution controls became the norm, impacting cloud formation, rainfall, and temperatures, which ultimately masked the effects of greenhouse gas.

Beginning in the 1970s, many countries implemented strict clean-air laws that allowed atmospheric aerosol levels to level off or decline. However, greenhouse gas emissions continue to increase into the present day, resulting in increased temperatures and the global-warming signature on the hydroclimate that has become increasingly obvious in recent years.

“If we don’t see it coming in stronger in, say, the next 10 years, we might have to wonder whether we are right,” said lead author Kate Marvel. “But all the models are projecting that you should see unprecedented drying soon, in a lot of places.”

Compounding factors such as increasing population and more blaring demand for water will only cause agricultural production to continue drying the climate, according to the scientists. They say it could result in much of the world becoming permanently arid, while others may see more rainfall, rising temperatures, and more evaporation of moisture in soils. 

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