‘Amid unprecedented Ganga-basin drying, river-specific studies vital’

Earth is warmer now than it has been in the past 100,000 years. This warming seems to be accelerating, with 2023-2025 becoming the warmest three-year period on record and also breaching the 1.5°C threshold for the first time, according to the European Union’s Copernicus Climate Change Service (C3S).

The only way to know this is the warmest the planet has been in the last 100,000 years is by reconstructing temperature and precipitation datasets through paleo-climatological records of tree rings, ice cores, sea sediments, coral reefs and others.

If we zoom into the River Ganga basin, what does this data say about the recent drying? To understand this, Down To Earth spoke with Kaustubh Thirumalai, expert in paleo-climatology and associate professor at the department of geosciences, University of Arizona. 

Edited excerpts:

Akshit Sangomla (AS): According to your research, the droughts in the Ganga Basin over the last few decades were the worst in 1,300 years. How sure can we be about this?

Kaustubh Thirumalai (KT): Our research uses paleo-climate data on wetness vs dryness from a tree-ring-based dataset called the Monsoon Asia Drought Atlas (MADA), and we specifically focus on the Ganga river basin, applying hydrological modelling to convert MADA values over the last millennium into basin streamflow.

First, let me talk about MADA and how that factors into our story: It is a reconstruction of past drought and wetness across South and Southeast Asia that extends back more than a thousand years. It is based primarily on tree-ring records, which preserve year-by-year information about moisture availability.

In our study, we use MADA data alongside a hydrological model (to convert MADA dryness / wetness into standardised streamflow anomalies), which allows us to place recent observed droughts in the Ganga basin in a long-term historical context, showing how unusual recent decades have been compared to natural monsoon variability over the last 1,300 years. Importantly, MADA does not rely on modern instruments or models, but on natural climate archives themselves. 

When we compare the 1991-2020 drying (using 30-year backward-moving means of standardised streamflow anomalies) against all other 30-year windows back to 700 CE, the recent period is the most negative in the record.

In that sense, the “worst in 1,300 years” refers to the relative position of 1991–2020 within the reconstructed distribution, not just a single-year extreme.

Regarding uncertainty: The reconstruction is built as an ensemble (multiple plausible reconstructions), and the paper reports a 95 per cent confidence interval envelope from that ensemble. The key point is that the recent multi-decadal drying sits outside the range of comparable events across the pre-instrumental portion of the record, even accounting for uncertainty.

That said, there are important caveats: The underlying proxy network (tree-ring data used to reconstruct drought metrics) is less dense earlier in time, especially prior to the 14th century, which can reduce fidelity for very early centuries. 

AS: You have also highlighted that most climate models fail to capture this drying trend in the Ganga? Why is that and how can it be improved?

KT: A major result is that while state-of-the-art CMIP6 models generally reproduce the warming trend, most do not reproduce the observed drying / streamflow decline in that specific spatial domain over recent decades. In our analysis, only a small subset of models captures the drying tendency, and even those tend to underestimate the magnitude. Why might this happen? Here are some possible reasons:

• South Asian monsoon rainfall is sensitive to anthropogenic aerosols (and their spatial distribution), which are still challenging to represent accurately in models.

• Land–atmosphere processes and irrigation: Irrigation and land-use changes can alter surface fluxes and regional temperature gradients in ways that can affect monsoon circulation; these processes are often simplified or inconsistently represented.

• Resolution and regional dynamics: The monsoon involves sharp gradients, orography, mesoscale convection, complex cloud physics, and air–sea coupling that coarse-resolution global models struggle with.

• Improving these aspects in the model might help improve accurate spatial simulation. 

AS: What does paleo-climatological research inform us about the major river systems of India, such as the Brahmaputra, Narmada, Godavari, Krishna and others? Is similar drying / droughts being observed there as well? 

KT: Paleo-hydroclimate information exists for parts of India, but coverage is uneven by basin and by proxy type. More studies like ours are needed to look at how coarse-scale proxy field reconstructions can be coupled to local hydrological forcing.

Moreover, even though most subcontinental rainfall is controlled by the summer monsoon rains, the Indian subcontinent does not behave as one coherent hydroclimate unit (especially concerning rain outside northern hemisphere summer).

Teleconnections (ENSO / IOD), aerosol forcing, irrigation and regional circulation shifts can create strong spatial contrasts (such as drying in some subregions alongside increases elsewhere).

So, I would be cautious about implying that the same magnitude of unprecedented drying is already established for every basin in the way it is for the Ganga reconstruction in this study. Basin-specific attribution is important, but there are efforts from my colleagues towards constraining pan-Indian-river drought under global warming. One example is this 2025 report Increased Drought Synchronicity in Indian Rivers Under Anthropogenic Warming.

AS: What does a possible future look like for the Ganga and other river systems? 

KT: Models often project that a warmer atmosphere can hold more moisture, and many projections show increasing precipitation on average in parts of South Asia. However, our results emphasise complexities.

For one, the observed decadal-scale recent drying is not well reproduced by most models, so confidence in near-term regional projections is limited. Moreover, risk is not only about mean rainfall: Changes in seasonality, dry spell length, extremes and groundwater withdrawals matter for streamflow and water availability.

Even if mean precipitation increases later this century, multi-year drought risk and water management stress can remain high due to demand, groundwater depletion and variability.

All of these factors compound alongside socioeconomic growth and change of emissions and landscapes in India and around the globe as well. I think we can conclude that variability and extremes will matter more for impacts, especially under continued warming and human water use. 

AS: With warming accelerating in recent years and the world set to breach the Paris Agreement threshold of 1.5°C by the end of this decade, what can paleo-climatology tell us that would be useful for us in the future, much warmer world? 

KT: Paleo-climatology is the only way that we will continue to obtain “out of sample” information to constrain future changes under exponentially increasing anthropogenic CO2 forcing. Given that instrumental observations are too short (around 100-200 years), and perhaps already compromised by early anthropogenic CO2 forcing, paleo-climate estimates of climate change under Earth system conditions that are radically different from the past two centuries will help us make progress in deciphering the machinery of the climate system — our best path towards navigating and anticipating climate dynamics under a much warmer world.