Tree Rings Reveal Historical Rainfall Patterns

Old conifers from the Hindu Kush mountains offer insights into floods and droughts in Pakistan and Afghanistan.

AsianScientist (Feb. 23, 2023) – In 2022, Pakistan witnessed extreme weather events. Severe droughts, which forced millions into food insecurity were followed by unprecedented floods that submerged one-third of the country. Climate scientists warn that such events will continue to hit Pakistan, as well as neighboring Afghanistan, across the Kabul River Basin.

However, it is not clear how frequent or severe the droughts or floods will be. Karakoram range glaciers that feed the Kabul River Basin are not losing glacial mass due to global warming. Dubbed the Karakoram Anomaly, this further complicates any projections into the future of water in the region. But looking into the past often offers clues into the future.

A study led by researchers from the Singapore University of Technology and Design, University of Malakand, Columbia University, and the Swiss Federal Institute for Forest, Snow and Landscape Research, investigated how climate influenced water availability in the region in the past. The study was published in Geophysical Research Letters.

Documented climate data for the region exists only since 1965. To see further back in time, the researchers turned to centuries-old conifer trees to study their rings. While tree rings are better known as indicators of a tree’s age (generally forming one ring per year), they also keep a visual record of the past climate.

In an interview with Asian Scientist Magazine, Hung TT Nguyen, one of the co-authors and a researcher at Columbia University, explained how they identified rainfall patterns from tree rings.

“The rings are not the same size. If there’s more rain, the tree can grow faster and that year will have a bigger [thicker] ring, and if there’s less rain, a smaller [narrower] ring. If you measure the width of each ring, you can decipher the amount of rainfall that happened in each year.”

Using these tree rings, the team was able to reconstruct the rainfall patterns in the basin for nearly the last 400 years — they couldn’t find enough healthy trees that were older. They noticed that since the turn of the twentieth century, the droughts in the Kabul River Basin have been becoming more severe and shorter in duration.

What’s happening in the Kabul River Basin isn’t an isolated phenomenon. Globally, rising temperatures are sending the water cycle into an overdrive. As the Earth warms, the atmosphere is able to hold more water vapor as air molecules have greater kinetic energy and are, thus, further apart. When the extra moisture eventually falls down, it causes heavier rains. On the other extreme, warmer air leads to extra evaporation which, in turn, causes droughts.

Consequently, dry areas are becoming drier whereas wet areas are becoming wetter across the globe. However, the severity of droughts or floods at a particular location depends on factors such as the region’s terrain and how air moisture circulates there. The researchers noted that while the dry years are getting drier in the Kabul River Basin, the wet years aren’t getting wetter.

Further complicating this phenomenon, episodes of wet periods within droughts are increasing. This finding, combined with increasing human activity in the region, emphasizes both the challenge and need for mitigating climate-induced water scarcity.

Thankfully, the research also presents a potential way to manage water resources in the region– bilateral dialogues between countries should account for historical climate data. Currently, they mainly rely on surveys of annual records of water levels in the rivers.

Nguyen cautions that the relatively short time frame of the surveys could misguide water-sharing pacts. “If you measure over a short period and happen to be measuring over a wet period, you could be over allocating the water. You should always look at the change in the longer term. Tree rings provide you with that sort of data,” Nguyen added.

Source: Columbia University ; Image: Shutterstock

The paper can be found at: Increasing Drought Risks Over the Past Four Centuries Amidst Projected Flood Intensification in the Kabul River Basin (Afghanistan and Pakistan)—Evidence From Tree Rings

Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.



Sachin Rawat is a freelance science writer & journalist based in Bangalore, India.

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