Dr R Krishnan: ‘Tropical cyclones, especially in the Arabian Sea, are becoming more and more frequent’

Even then we can offer conclusions only in terms of probabilities. Sometimes these probabilities are very clear. There could be a very high probability that a certain event could not have occurred in the absence of climate change. But, at other times, the data is not very conclusive. So, we then say we are not sure.

https://youtube.com/watch?v=A64il87rj3k%3Fsi%3DjX44jNc7fYqjWJZl

On a particular region being more vulnerable to rainfall events

I think we need to do a region-by-region study. An earlier study showed that extreme event trends are not uniform across India. In some parts, the rate of increase is different and in some parts, the trends are different. They are all very much co-dependent. For example, in western India, the empty sea has a link to the Indian Ocean Dipole (a climate pattern affecting water temperatures) events as well. So the regional heavy precipitation has a link to the circulation pattern, the modes of variability, the way the water vapour is channelled to that region from source and so on. We can expect that in many regions but heavy precipitation is projected to increase in a global sense.

Weather systems also play a major role. When  monsoon systems are organising themselves, that is the time you can expect these events to produce a lot of rainfall. They are not going to be the same throughout the year.

On uncertainty about extreme events

The early warning systems developed by the Indian Meteorological Department (IMD) have made tremendous progress. We had Cyclone Biparjoy this year but the model forecasts helped in disaster preparedness. The models are performing well, especially for tropical cyclones and the active phases of monsoon synoptic systems. The Numerical Weather Prediction (NWP) centres are helping as well in weather prediction.

On climate change causing extreme rainfall and flooding, like in Chennai in 2015

If you want to do a model-based attribution, then it depends on what we are referring to as a particular event. For example in Uttarakhand, in 2023, there was a low-pressure system going from the Bay of Bengal and a flow of moisture from the Arabian Sea.  To assess the role of climate change, we have to do two sets of experiments – with and without greenhouse gases. We may have to do hundreds of experiments for each of them with a fairly high-resolution model. That is going to take some time though it’s not impossible. We require a lot of computing resources. Greenhouse gases are assessed in a climate context. It is very different when we are doing a weather prediction. So it has to be cleverly designed. In principle, it is possible, but even if you have these resources, experiments are going to take time. And by the time you do that, you will have another event occurring and we will end up chasing events. We have to probably take a very pragmatic approach. Also, model to model, there can be differences, so we need to coordinate this big effort.

On other kinds of climate change impact over the Indian subcontinent

Tropical cyclones, especially in the Arabian Sea, are becoming more and more frequent. We have earlier mostly seen them in the Bay of Bengal, so a part of this phenomena at least has to do with climate change. Some work is already going on already but if the Arabian sea cyclones are going to increase, they’re going to affect storm surges and already, the sea level has gone up. Sea levels are rising in many coastal areas. This is mostly because of the increase in the mean sea level in the region itself. Once the mean sea level increases, storms can lead to extreme events in coastal regions.

Melting glaciers in the Himalayan region are going to impact precipitation, like what happened in  Pakistan last year. The meltwater run-off and the increased precipitation could lead to large-scale flooding. With so much infrastructure development taking place in urban areas and constricting traditional drainage channels, flooding is a given. It’s not just meteorology but mitigation involves hydrological management and governance issues.

On the Indian subcontinent being not as warm as the global average

Generally, temperature changes are most pronounced in the polar regions. One possible reason could be the Ice-Albedo feedback, which basically means a higher absorption of the solar radiation  with the melting ice exposing a greater land surface. This leads to amplified warming in the high-latitude region. This is also affected by circulation changes. In tropical areas, the warming is less compared to the high latitudes. One reason is the formation of clouds, which reduce the radiation reaching the surface. The water vapour feedback increases warming. Also, there was some effect of the aerosols that cooled large areas in the northern hemisphere, according to the IPCC (Intergovernmental Panel on Climate Change). Emissions from North America, Europe (especially during the 1970s and 80s) China and even India have offset the rise in temperature because they cut down incoming solar radiation. Without that, the temperature increase could have been much higher.

In the tropics, water vapour feedback causes warming. Then when precipitation happens, the soil is going to get wet, there is going to be more evapotranspiration and latent heat fluxes. So it is going to be complicated. Therefore, some areas in the extreme northwest of India could see a temperature rise. As the land surface is not wet, especially during the pre-monsoon months, the temperatures can go up. The land surface processes are going to be a very key factor in controlling temperature changes over the subcontinent. With increased monsoon precipitation, the temperature increase may not be that much.

Audience Questions

On floods in Himachal Pradesh being a man-made disaster

Other than rainfall events, there have been many developmental changes in the region that have altered the land use and land cover. We need to calibrate models for a particular region. Say you have stripped an area of its forest cover, it would make the land barren. We are basically changing the Albedo (proportion of light reflected from a surface) of the region. When we have more forest cover, we are absorbing more radiation, whereas when we are taking out the forest cover, we’re reflecting back incoming radiation. When the land is heated during the pre-monsoon months and triggers a very heavy deep convection and localised heavy precipitation, there is a sudden influx of moisture.

On forest fires and heat waves

A forest fire depends on many complex factors. When the land surface dries out, there is less soil moisture and a continuous high-pressure system. There is a kind of sinking motion over the area, which is very conducive to the development of a forest fire. That’s why forest fires happen in the Indonesian region or Australia during an El Nino event (meaning below-average levels of rainfall). It also depends on the vegetation, the kind of trees you have and the oil that comes out of these trees. In the Australian region, they’re highly flammable.