Glaciers in the High Mountains of Asia (HMA), like most mountain glaciers around the world, are retreating due to climate change. However, in the Karakoram mountains of northwest HMA, glaciers have remained stable, and in some cases, have actually advanced. A recently published study in Geophysical Research Letters delved into one of the potential drivers behind this climatic irregularity, irrigation.
The idea that irrigation, a human-induced change to the environment for agricultural production, could regionally counteract climate change, another human-driven change, might seem a bit far-fetched. And to Remco J. de Kok, Obbe A. Tuinenburg, and Walter W. Immerzeel, three of the authors of the study who spoke with GlacierHub, it did at first start out as a wild idea. Nevertheless, previous studies, including Tuineburg’s own Ph.D. thesis, found that evaporation from irrigation in India was being transported by atmospheric winds to Himalayan areas where it fell as snow or rain, likely contributing to the increased glacial mass observed.
The most famous of the advancing glacier areas is the Karakoram anomaly, first coined in 2005 by Ken Hewitt. According to Immerzeel, the term gained traction in subsequent remote sensing studies that partly confirmed the anomaly. As it turned out, the Karakoram range was not the only region with stable or advancing glaciers. Studies published in 2013 and 2017 also found positive mass balances in the Pamirs and the Kunlun Shan mountains northeast of the Karakorams.
Prior research pointed to atmospheric circulation patterns and a particularly seasonal pattern (precipitation was concentrated in winter). These studies were conducted across a large spatial scale, and therefore their proposed mechanisms should support stable and advancing glaciers uniformly across the region. Yet, the glaciers just south of Karakoram show some of the highest glacial melt rates in the region, notes de Kok, while at the same time glaciers in the Kunlun Shan region northeast of Karakoram exhibit positive mass balances. These differences in a relatively small geographical area led the authors to consider two hypotheses: either the glaciers are responding differently to similar climatic changes or that the glaciers are experiencing different climatic changes.
To assess these hypotheses, the authors selected China’s Tarim basin. According to de Kok, “It is adjacent to the areas of largest glacier growth and has some of the biggest increases in irrigation.” Next, the authors needed to assess whether recent irrigation increases in the basin were impacting neighboring mountain climates in a way that would be conducive to glacial growth.
The authors utilized a regional climate model known as the Weather Research and Forecasting model, or WRF for short. The model was run under two scenarios: a historical or stagnant scenario and a recent change scenario. The historical scenario represented the difference between model runs with modern irrigation and no irrigation, along with atmospheric greenhouse gas concentrations (GHG) held at 1900 levels. On the other hand, the recent change scenario represented the intensification of irrigation over the period 2000 to 2010 and a concurrent increase in GHG concentrations.
By applying these two scenarios, the authors were able to more accurately depict climatic changes and evaluate whether the impact of irrigation is significant in comparison to climate change. The authors were then able to compare the two to see if either had a dominant influence on the regional climate.
After running the models, the first apparent impacts of irrigation were an increase in evapotranspiration, a decrease in summer daytime temperatures, and an increase in atmospheric moisture directly over the basin.
Then things got interesting.
The model runs showed increased summer snowfall in Kunlun Shan, Pamir, and northeast Tibet. These increases were largest for the historical scenario with 1900 GHG levels and modern irrigation, showing that the increase was primarily caused by irrigation, not GHG. The authors also analyzed changes in net radiation, the difference between incoming solar and outgoing terrestrial radiation. Across most of HMA, net radiation increased; conversely, in Kunlun Shan, net radiation decreased. This decrease is a result of a decrease in incoming solar radiation due to increased cloud cover and the increase in snow cover, which has a high albedo.
The decrease in net radiation was found to counteract climate change’s enhanced greenhouse effect. Strikingly, when GHG were raised to current levels and irrigation was held at zero, the model results revealed an increase in net solar radiation across the entire region, signaling that irrigation is the principal reason for negative net radiation in Kunlun Shan.
While it was clear that increases in irrigation are leading to favorable conditions for glacier growth, where was this increased moisture coming from?
Model results pointed to the hypothesized Tarim basin as the main source of the increased moisture in the Kunlun Shan. The authors were able to corroborate this by conducting two model runs for the Tarim basin, one with no irrigation and one modern irrigation, while the rest of the region was held at modern levels. These runs revealed an increase in snowfall and a decrease in net radiation only when the Tarim basin had modern irrigation, confirming its influence.
The irrigation of the Tarim basin is creating an advantageous environment for glacial growth, but the study is unable to attribute just how much this mechanism is contributing to the positive mass balance of the Kunlun Shan, a topic that will be the focus of future research, according to de Kok.
There’s also the question of sustainability for the recent increase in irrigation in the region as groundwater becomes more and more stressed and adverse ecological impacts in the otherwise arid Tarim basin take hold. A future reduction in irrigation could be bad news for glaciers, as the authors note it might “mean that the anomalous glacier mass balance in Kunlun Shan is of temporary nature.” Nonetheless, in a world where melting glaciers have become the new norm, stable glaciers, even if fleeting, are a welcomed respite.