And there have been claims that the drying of the region happened at about the right time, c. 1900 BCE. But the science on this is very iffy, and there have been lots of other claims. The latest, coming from a major study across a wide swath of India, suggests that the greatest drying occurred around 2200 to 2000 BCE. When they announced this dating, the archaeologists involved said that they had found the cause of Indus civilization's decline.
Of course 2200 BCE is 300 years before Indus Civilization began its precipitous decline, and 300 years is a long time to hang on without enough water. So the same group of archaeologists set about figuring out how people adapted to that change. Did they grow different crops, put more emphasis on herding, extend irrigation, move to someplace wetter? No. So far as this study showed, people changed nothing at all.
We argue that rather than being forced to intensify or diversify subsistence practices in response to climatic change, we have evidence for the use of millet, rice, and tropical pulses in the pre-urban and urban phases of the Indus Civilization. This evidence suggests that local Indus populations were already well adapted to living in varied and variable environmental conditions before the development of urban centers. It is also possible that these adaptations were beneficial when these populations were faced with changes to the local environment that were probably beyond the range of variation that they typically encounteredTranslated into English, that seems to mean that people did not have to change because their existing style of farming was already flexible enough to cope with a great deal of variation in rainfall.
I personally love this result, because I have gotten tired of climate change being used lazily an an explanation for every change in human society. People are adaptable, and thrive in all but the driest and coldest climates. I can imagine a scenario in which the climate continued to worsen and the early success people had adapting to declining rainfall without major changes only made them more vulnerable when things got too bad for their system to continue. But that is not what this study shows; it shows the Indus civilization sailing through what the authors identify as the period of most profound change in the climate over the past several thousand years.
I would say that leaves the relationship between the drying climate and the end of Indus Civilization as much up in the air as ever.