On 12 December, Cyclone Vardah made landfall just north of Chennai at around 2 pm. The accompanying winds, with gusts up to 120 kilometres per hour, caused plenty of damage to property and ecosystems and resulted in at least two dozen deaths. For its residents, still fresh with the memory of the devastating floods last year and the tsunami 12 years ago, Vardah was a grim reminder of how vulnerable the coastal city is to natural disasters.
The Bay of Bengal is a well-known hotspot for tropical cyclones, also known as hurricanes or typhoons in other parts of the world. In 1999, the Supercyclone Paradip in coastal Odisha with winds of up to 260 kilometres per hour, left tens of thousands missing or dead and destroyed nearly 300,000 homes. The world record for human destruction by any disaster, sadly, is much higher and goes to Cyclone Bhola, which made landfall in Bangladesh (then, East Pakistan) in 1970 and killed over half a million people.
In New Orleans, another coastal city that has had its own share of troubles, Hurricane Katrina in 2005 turned out to be its worst tropical cyclone. Over 1,800 lives were lost and some 400 square kilometres of islands, levees and coastal wetlands, constituting the first line of defence, were destroyed. Economic losses were to the order of $100 billion with residential property losses alone appearing as the destruction of 300,000 homes.
Cyclones and the science of climate change
After Katrina, in a well-known blog called realclimate.org, several notable atmospheric scientists asked the question filling the minds of the informed public: Was this a result of global warming?
Their answer was perhaps disappointing to those who had hoped for something more clear-cut: “There is no way to prove if or not Katrina was affected by global warming. For a single event, regardless of how extreme, such attribution is fundamentally impossible,” Stefan Rahmstorf and his colleagues argued. They said that higher levels of greenhouse gas concentrations in the atmosphere caused by anthropogenic emissions are responsible for increased radiative forcing, that is to say, more energy that is transmitted to the atmosphere and oceans. That ends up “loading the dice” in favour of more severe weather, but any single event is the outcome of both deterministic and stochastic or random factors.
Climate models and weather prediction have improved significantly in the past decade, but attribution faces the same fundamental challenge when it comes to matching individual events with global climate change. The Earth’s atmosphere and oceans may be subject to well-known laws of physics, but there are many non-linear processes that literally relate the flapping of butterflies (in this case, microphysical evaporation and condensation phenomena within individual clouds) to the unravelling of large-scale weather phenomena elsewhere. The best one can say is something that sounds like this: the conditions of possibility of severe weather are increased x-fold when radiative forcing levels in the atmosphere due to anthropogenic activities are increased by y percent.
Notwithstanding this caveat, and basing their conclusions on new evidence in recent years, several scientists have been somewhat more intrepid about making the connection between clusters of weather phenomena and climate change. In 2016, the US National Academy of Sciences invited a group of experts to assess the state of the field of attribution studies associated with climate change.
Their conclusion was that while it is still an ill-posed question to ask whether or not a single event is the consequence of climate change, one might find reasonable levels of confidence to attribute certain weather anomalies to climate change. Based on observational studies and models, they concluded that ascribing weather disasters with human-induced climate change seem to work best when trying to connect the former with extreme heat or cold conditions over large regions or unprecedented drought in an area.
Well-suited to explain extreme hot and cold conditions
The heat wave in Western Russia in 2010, for instance, could be reasonably well associated with increased greenhouse gas concentrations during the past century. Similarly, conditions generating ongoing droughts in California and Western Australia are considered to be more likely because of global warming than in its absence.
Over much of the coast surrounding the Bay of Bengal, tropical cyclones remain an ongoing threat. To what extent do attribution studies tell us anything about how they might be linked to climate change? The NAS report is cautious about attributing the observed increased intensity of tropical cyclones to climate change, because of the relatively small scale of the events and the lack of “ensemble” data.
Tropical cyclones are relatively short-lived phenomena of limited extent. They usually last no more than a few days and occupy an intense zone of around 100-200 square kilometres. It is difficult, therefore, to expect their detailed features to be predicted well by global atmosphere-land-ocean models that try to simulate broad measures of climate such as temperature, pressure and precipitation, over long periods and numerous large cubes of the earth’s atmosphere about 200 kilometres on each side. Without having better ways to understand the details, only broad indications are available.
Climate change appears a plausible cause
One feature, which has been well known for some time is that tropical cyclone intensity — implying its drop in pressure, wind speeds, and rainfall — is fuelled by sea surface temperatures (SSTs). In the Bay of Bengal, following a global trend, these have increased by at least half a degree over the past half century . The relationship between tropical cyclone activity and SST is complex and often operates non-linearly upon crossing thresholds. It is also fairly well established that the intensity of tropical cyclones goes up dramatically beyond certain temperature ranges.
Altogether, these pieces of information indicate that climate change vastly improves the conditions for the creation and sustenance of intense tropical cyclones. In doing so, it significantly raises the probability for many of these severe systems to make landfall in populated areas along the coast. Thus, even if a court of law might not be able to admit a direct link between greenhouse emissions increases and the damage incurred by a farmer in the Kaveri delta because a destructive cyclone of very high winds and energy caused extensive ruin, it could still conclude that climate change made the damage more likely.
Protecting coastal areas: a three-way approach
Coastal areas vulnerable to tropical cyclones might face even further problems indirectly because of climate change. Sea level rise, expected to be at least a metre, probably more, this century, will raise storm surge heights, as will higher SST. This will result in more coastal erosion and flooding during intense storms and generate considerable damage to otherwise protective ecosystems, infrastructure and people along the coast.
Perhaps the best way to think about responding to future disasters then would be to take a three-pronged approach. One, is to build a strong line of defence for immediate disaster relief and repair work to get people to safety, bring infrastructure back online and ensure that basic services are restored quickly. Two, adequate planning for long-term intensification of these disasters must be undertaken. This would involve a variety of engineering and participatory processes to ensure a just transition to a climate-proofed coastal existence. Three, a frank discussion must be had on the sheer foolishness of “creating value” along the shoreline from private businesses as well as via government actions.
In the long run, it may just be a better idea to retreat from the most vulnerable coastal regions, even if they currently support vast population and to try to restore at least some of the ecosystems that have provided natural flood control and other protection. The question is no longer whether climate change is something we should worry about in the distant future, but how human aggravation of a vast economic enterprise is already moving the world towards metaphorical icebergs and whether we have the wherewithal to turn the Titanic around in time.
 Jadhav, SK, and AA Munot. “Increase in SST of Bay of Bengal and its consequences on the formation of low pressure systems over the Indian region during summer monsoon season.” Mausam 58, no. 3 (2007): 391.
First Published On : Dec 16, 2016 17:08 IST