Weather and climate are perennial and relevant issues; with weather often being the first topic of social conversations and weather forecasts being some of the most watched media around the world, while climate and climate change are similarly prevalent and pressing. However, weather and climate are often mixed in the public consciousness, and even researchers not expert in atmospheric sciences can confound them.

Weather and climate are actually part of two specific fields: meteorology and climatology, respectively. While both refer to atmospheric conditions, the difference lies in how they consider time. The weather is the atmospheric state over a short period of time, from a few minutes to months – and includes the chemistry and physics inside a cloud and the movement and development of large high and low pressure areas. In contrast, climate characterises the average of this state, over much longer periods of time, from a few years to millennia – and react, for example, to human activities (gas emission and land use) or to the amount of sunlight received by the Earth.

Indus river
The Indus river in the Himalayas. Before human influence, land cover was mainly affected by climate variability

These phenomena might be affected by external factors such as changes in the Earth’s orbital parameters, Sun activity, an asteroid impact, or volcanic eruptions. But some factors can also originate from the climate system itself, which includes the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere.

In the Indus River catchment these phenomena have specific variability and impacts. Monsoon lows in summer, and western disturbances in winter bring the water needed for agriculture. More intense instances of either can cause widespread floods, whereas a “break” or stop in the succession of disturbances, will lead to a drought event that can endanger crops. Changing climate would see drought or flooding events repeat themselves year after year and eventually lead to durable change affecting human activity, as might have happened 4000 years ago.

A hurricane striking Pakistan in May 1999. Such phenomenon is typical of the meteorological scale

For TwoRains I will be looking at seasonal, decadal and millennial time scales to assess the impact of a changing climate on Indus populations. The millennium is the time scale across which orbital forcing evolves, and it has a significant impact on monsoon activity, natural land cover, hydrological networks and the way that humans moved through and occupy regions during the mid-Holocene. At lower time scale, we have inferred from palaeoclimate samples that sharp drought events in the Indus region has occurred. In order to understand their origin, I will be using long simulations of the Earth’s climate and long series of observation. Finally, I plan to look at the seasonal scale – which brings us to the “two rains” of the project title. The Indus area is subjected to two wet seasons, and we know that Indus populations adapted their farming practices to these two seasons. No one has ever look at those specific seasons, mainly because paleoclimate samples can’t distinguish the two. With the better understanding of the past climate dynamics offered by models, I might be able to reckon the long term evolution of the two rainy seasons.

To conclude, while we are interested in the past climate of the Indus region, we must understand it at very different time scales, which are affected by different factors, and require different methods of analysis.