Data analysis suggests that this wind-driven annual weather pattern — which typically starts in June and lasts through September — is changing. However, it is too early to blame it on climate change.
Even minor variations can have far reaching impacts on South Asia’s nearly 1.5 billion people. The monsoon brings four-fifths of the Indian subcontinent’s annual rainfall, which sustains much of its agriculture, power generation and basic human needs.
Climate scientists at Stanford University, US, published a paper in April 2014 saying they have found, through advanced statistical tests, some changing patterns in South Asian monsoons since 1980.
They noticed that while the average total rainfall during the monsoon season has declined, variability during peak months has increased. This means there are now more extreme wet spells and more frequent dry spells. These, in turn, raise the risk of droughts and floods. 
"There are many predictions that global warming should cause heavier downpours and more frequent dry spells,” the study’s senior author and associate professor of environmental earth system science, Noah Diffenbaugh says. “That's what we've found here, but India is a complex region, so we want to be sure before we point the finger at global warming or any other cause."
Whatever the exact cause, both scientific analysis and anecdotal evidence indicate more uncertainties in monsoon behaviour. This calls for greater regional cooperation in monitoring, sharing data and multi-disciplinary research. It also challenges governments to adjust development plans to be more resilient, and come up with strategies for coping with disasters occurring on a more regular basis.
Monsoons happen in other tropical regions too, but they are especially strong in South Asia. For six months of the year, they blow in one direction, from southwest to northeast. They reverse direction during the other half of the year.
The summer monsoon, also known as the southwest monsoon, comes from East Africa and is pulled eastwards by the rotation of the earth.
It typically arrives in the Maldives and Sri Lanka in late May, shortly after which it enters the Indian subcontinent from the southernmost state of
Kerala. Over the next few weeks it heads northwards, finally reaching the Himalayan states of Nepal and Bhutan. 
An ample and timely monsoon boosts harvests, drives production and generates wealth. A delayed or inadequate monsoon, on the other hand, causes many problems for communities and governments.
In India, where over 60 per cent of farming is rain-fed, monsoon rains are vital for major crops such as rice and sugarcane. Former finance minister Pranab Mukherjee, now president, once described the monsoon as his country’s “real finance minister”.
Rulers, farmers and traders across South Asia have been anxiously watching the monsoon for centuries. Scientific record keeping and analysis started in the 19th century, and meteorologists continue their quest to better understand and forecast the phenomenon.
As one researcher put it, the single biggest scientific challenge is to understand how the winds and rains behave during each monsoon — the so-called active and break events. 
A full assessment of this year’s monsoon is yet to be made. In April, meteorologists of eight member states of SAARC (South Asian Association for Regional Cooperation), after meeting in Pune, India, said the region was likely to get “below normal to normal rainfall” this year.
Experts assessed several factors known to influence the monsoon, including El Niño-Southern Oscillation (ENSO) conditions over the equatorial Pacific, Indian Ocean Dipole (IOD) conditions over the Indian Ocean, winter and spring northern hemisphere (NH) snow cover, and land surface temperature anomalies. 
This multiplicity of forces makes precise predictions harder. To cope, economic planners and disaster managers must use the latest — albeit imperfect — evidence for decision making.
Their task is compounded by the fact that the monsoons eventually interact with the Himalayas, the youngest mountain range on the planet. Its fragile southern slopes — when hit by heavy rain — can trigger rapid onset of disasters.
A powerful example was in mid-June 2013, when landslides and flash floods in the northern Indian state of Uttarakhand killed more than 5,700 people and caused large-scale devastation. The tragedy started with a cloudburst, which breached a natural dam. It highlighted gaps in India's disaster preparedness, as well as neglect of environmental concerns in hilly areas. 
On 2 August, a landslide in the Sun Kosi, a major tributary of Nepal’s Kosi river, suddenly blocked the river’s course. It created an artificial lake which inundated houses, destroyed or submerged roads, and damaged a hydropower dam and transmission lines. Over 150 were killed, and many hundreds were displaced. At one point, it threatened downstream floods in the eastern Indian state of Bihar. 
The Nepali Times, in an editorial, said climate change has made Himalayan water and infrastructure management more unpredictable. It pointed out how dozens of swollen glacial lakes are on the verge of bursting, while monsoons have become erratic with cloudbursts and unseasonal blizzards.
The editors urged: “It is best not to underestimate the dangers of Himalayan geology, and the fury of its rivers. Future urban planning, design of infrastructure and hydropower projects have to factor in the risk of floods of biblical proportions.” 
That caution applies to all countries that share the monsoon and the Himalayas. Taking chances with the seasonal wind is not an option when so many lives and economic prosperity are at stake.
This does not mean fragile mountain ecosystems and other areas should remain underdeveloped. Guided by best available scientific evidence and advice, planners can balance infrastructure development, land use change, urbanisation and water resource management.
At the same time, they have to be prepared to deal with the monsoonal surprises that are now more frequent.
Nalaka Gunawardene is a Colombo-based science writer, blogger and development communication consultant. He is also a trustee of SciDev.Net. The views in this column are his own.
This article has been produced by SciDev.Net's South Asia desk.
References Extremes in wet, dry spells increasing for South Asian monsoons, Stanford scholars say. Stanford News, 28 April 2014.
 Monsoon of South Asia. Wikipedia.
 South Asian monsoon variations hard to fathom. SciDev.Net, 4 July 2012.
 Consensus statement of the South Asian Climate Outlook Forum (SASCOF-5), Pune, India, 14-23 April 2014.
 Indian floods highlight dangers of rushed development. SciDev.Net, 11 July 2013.
 A flood of floods. Nepali Times, 2 August 2014.
 Calculated Risk. Editorial in Nepali Times, 8-14 August 2014.