Some Himalayan glaciers are advancing despite an overall retreat, according to a study on Sunday that is a step toward understanding how climate change affects vital river flows from China to India.

A blanket of dust and rock debris was apparently shielding some glaciers in the world’s highest mountain range from a thaw, a factor omitted from past global warming reports. And varying wind patterns might explain why some were defying a melt.

“Our study shows there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover,” scientists at universities in Germany and the United States wrote in the study of 286 glaciers.