Fri, 17 Sep 2021

Climate change is rapidly shortening Central Asia's growing season, says a new analysis of vegetation data from the past two decades. Though the study looked at native grasses and trees rather than agricultural crops, the trend could dash hopes to expand farming and add to mounting concerns about food security.

The growing season - the time between the first shoots of fresh green in the spring and the withering of vegetation in the fall - has become longer in most of the middle latitudes of the northern hemisphere, as one might expect on our warming planet.

But the reverse is happening in most of Central Asia. Growth is starting later and ending earlier, even though Central Asia is warming faster than the global average, says a peer-reviewed study published this month in Science of the Total Environment.

Changing precipitation patterns may explain this seemingly counterintuitive trend. Vegetation is "more sensitive to precipitation" than to temperature, says the study. While most of Central Asia is becoming drier, which can shorten the season, instances of increased precipitation seem to have also ended the season earlier. While precipitation patterns are a more complicated variable than temperature, in both cases - wetter or drier - the implication is that the land will yield less food.

Lizhou Wu of the Nanjing University of Information Science and Technology in China and his coauthors start with temperature and precipitation data collected between the years 2000 and 2019. This they compare to the growth of native vegetation, measured both by satellite sensors and sampling points on the ground, in the same years. Then they break this giant region - the five Central Asian states plus large parts of western China - into six ecological zones, which they dissect by altitude in increments of 1,000 meters.

(Wu et al.)

The result is a mammoth cache of data offering statistically significant evidence for shorter growing seasons. Some of the findings:

73.4 percent of the study region saw a shorter growing season in 2019 compared to 2000. The most significantly impacted areas were central and western Kazakhstan and the Syr Darya basin shared by southern Kazakhstan and eastern Uzbekistan. In the Tien Shan mountains (Kyrgyzstan, Tajikistan and stretching into Xinjiang), warmer temperatures moved the start of the growing season earlier, as much as a day per decade. However, lower precipitation tended to bring the end of the growing season forward as much as 3.7 days per decade, shortening the season overall. In westernmost Kazakhstan and large parts of Turkmenistan and Uzbekistan, rain patterns appear stable. But in wetter years, Wu and his colleagues found the season ended sooner. The same pattern affected the Tarim Basin of China's Xinjiang province, where precipitation increased more than anywhere else studied and the growing season decreased the most. And here's an interesting observation seemingly unrelated to climate change: For every 1,000-meter increase in elevation, the growing season decreases, on average, by 11.7 days. But in northern Kazakhstan, the decrease per 1,000 meters is 50.7 days.

Fortunately, the growing season is not the same as production, which in agriculture is measured in terms of net primary productivity, or NPP: what's available for herbivores to eat. Many crops can be harvested long before the end of the season. Also, as noted above, the authors base their findings on analysis of native flora - steppe grasses, shrubs like the saxaul, and trees like larch and poplar - not crops.

In fact, Wu and his colleagues found NPP rose slightly across most of the study area - except in western Kazakhstan, which saw a significant decline. Herders here rely on steppe grasslands to feed their animals. That relationship is being tragically highlighted by the extreme drought that is currently starving thousands of cows and horses in the area. Even before this year's crisis, there have been mounting signs of food insecurity across Central Asia, prompting governments to introduce export restrictions. Kazakhstan unveiled another round on July 26, this time on fodder.

Science funding often fulfills the needs of the government that pays for it. Wu and his colleagues' inquiry was supported by the Chinese Academy of Sciences, which sponsors much of today's climate and agriculture research in Central Asia. And indeed, this research dovetails with some of Beijing's strategic priorities: Numerous recent agreements with Central Asian governments tout the region's potential to feed China.

To that end, the paper is discouraging - yet more data pointing to the grim toll climate change will likely take on Central Asia. Unless, that is, it spurs local governments to address well-documented inefficiencies in their agricultural sectors. As another Chinese study that Eurasianet recently profiled has shown, there is ample room for improvement.

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