By Sahana Ghosh
Thiruvananthapuram, Dec 24 (IANS/Mongabay): Is the devastating 2018 flood in Kerala linked to climate change? Based on a study that does not find any long-term increase in the mean or heavy monsoon rainfall in the state, researchers argue that attributing the event to climate change can be "difficult".
The analysis is by researchers at the Water and Climate Lab at the Indian Institute of Technology-Gandhinagar, who looked at the event from the lens of hydroclimatology -- the effect of large water bodies on climate.
Factoring in the long-term (1951-2017) changes in mean and extreme precipitation, air temperature and total run-off in Kerala, researchers said that during this 66 year period, the state witnessed a significant decline in monsoon rainfall while the temperature rose.
"Therefore, long-term trends indicate drying and warming in the monsoon season in Kerala. Kerala had two remarkable extreme rain events in 1924 and 1961, which is prior to the global warming era," Vimal Mishra, the lead author of the study, told Mongabay-India.
"Therefore, we hypothesise that the 2018 event may be similar to earlier extreme events and also driven by the natural variability of climate. However, the role of climate warming on 2018 flood in Kerala needs to be diagnosed," Mishra said.
The 2018 event threw up a "surprise" from the trends perspective.
"Both extreme rainfall and run-off during 1951-2017 show a decline. Moreover, there has not been a considerable increase in the frequency of extremes associated with rainfall in the state. Therefore, from the trends perspective the 2018 event was surprising," Mishra explained.
"Climate change has posed profound implications on extreme precipitation and flood events globally. Therefore, there is no doubt that we should take climate change seriously in terms of the rising frequency of extreme events," said Mishra.
Until we can scientifically attribute the 2018 flood to climate warming we can't say this was caused by climate change, he reasoned.
"Since there is no increase in mean and extreme precipitation in Kerala over the last six decades, the extreme event during August 2018 is likely to be driven by anomalous atmospheric conditions due to climate variability rather anthropogenic climate warming," the study said.
The researchers also questioned if the flooding in Kerala was caused by the human activities other than climate change.
They believe the severity of the flood and the damage caused might be affected by several factors, including land-use and/or land cover change, antecedent hydrologic conditions, reservoir storage and operations, encroachment of flood plains, and other natural factors.
Climate scientist Arpita Mondal, who works on detection and attribution of hydroclimatic change, agrees with one aspect of the study in that attributing hydrological events are particularly difficult.
"We must also understand that floods are hydrological events, and not necessarily an atmospheric one. Hydrology is what happens to rain when it falls on the ground," IIT-Bombay's Mondal told Mongabay-India.
"Therefore, the translation of heavy rainfall to floods is also dependent on the catchment characteristics and local factors, such as antecedent moisture conditions, land-use characteristics such as perviousness of the surface, floodplain encroachment, reservoir releases and so on," she said.
Since these are complicated, interlinked processes, she said, attributing hydrological events are particularly difficult. "This is one aspect highlighted in this study, and I completely agree with that," Mondal said.
Ecologist Rajiv Chaturvedi said that generally for Kerala, published literature pointed to increasing droughts, so it's no surprise that the trends associated with extreme and total rainfall in the state for 1951-2017 do not show an increase, but decrease.
"It is agreed by most of the researchers that in Kerala the key problem today is drought, and drought is going to be the future problem as well (based on high resolution climate model projections) and not flood," Chaturvedi said.
Mishra emphasised that research is needed to understand what impact the historic land cover change in Kerala caused on the severity of flood in August 2018.
Reservoirs might have played a key role in at least worsening the flood situation in the state during August 2018, the researchers stressed.
For instance, the group had earlier reported that excess rainfall had caused above-normal reservoir storage even before the onset of extreme rainfall (August 15-16).
"The extreme rainfall in the catchments of major reservoirs forced reservoir operators to open the gates and release a considerable amount of stored water, which might have added to the severity of floods in the state," they observed in the study.
But India's Central Water Commission (CWC), in its report on the Kerala floods, has held that "dams in Kerala neither added to the flood nor helped in reduction of the flood" while squarely holding the heavy rainfall responsible for the disaster.
According to historical records, the region witnessed one of its most severe floods in 1924 and the CWC report noted that the rainfall of August 15-17, 2018 in Kerala was "almost of the same order as that of rainfall which occurred during July 16-18, 1924".
Chaturvedi believes the sudden flood was so devastating (from the dam management perspective) as water resources in the state were managed for drought mitigation (keeping dams in full capacity).
"And prolonged drought in the state could certainly be linked to climate change going by previous studies. Kerala is certainly experiencing the impacts of climate change in the form of declining rains, and its subsequent effect on vegetation productivity and carbon stock growth in vegetation," the ecologist said.