Rsing sea level increases the damage from storms, says S.Ananthanarayanan.

Goa, Maharashtra and Gujarat are limping back after cyclone Taukte. Estimates of the cost of the damage and restoration are yet to come. And then, there is the cyclone, Yaas, striking the eastern states of Bengal and Odisha. In the same month of May, last year, cyclone Amphan, had struck Odisha and Bengal (and Bangla Desh) and caused damage priced at $ 14 billion. There is no telling what it would be this time, but a study published in the journal, Nature Communications, says the damage resulting from extreme climate events increases with the rise in sea level.

There is ample documentation to show that there has been an 18 cm (over 7 inches) rise in the average sea level since 1900. With the globe warming much faster in the last few decades, we can expect an even greater rise within a much shorter time. Warming itself has been linked to more frequent and intense weather events. That sea level rise would multiply the damage caused by the events is ominous.

Benjamin H. Strauss, Philip M. Orton, Klaus Bittermann, Maya K. Buchanan, Daniel M. Gilford, Robert E. Kopp, Scott Kulp, Chris Massey, Hans de Moel and Sergey Vinogradov, from Climate Central, a group of scientists in Princeton, Stevens Institute of Technology and Rutgers University in New Jersey, Tufts University, Boston, Vrije Universiteit Amsterdam, Potsdam Institute for Climate Impact Research, Germany, US Army Corps of Engineers, Washington, DC and Binera, Inc. in Maryland look into the role of human activity in Hurricane SANDY, which struck the East Coast of the US in 2012. The cost of the damage was $ 60 billion. The group writing in Nature Communications use simulation with different sea levels and find that 13.5% of the damage was a result of sea level rise due to human activity.

Hurricane Sandy, which wreaked havoc all along the east coast of USA, caused 300-year record high flood levels in the New York Metropolitan Area, “flooding streets, tunnels and subway lines and cutting power in and around the city.” The Nature Communications paper concedes that the hurricane was worsened by having struck right when it was high tide and again by having come in at almost right angles with respect to the New Jersey coastline. Despite these factors, the paper says, the mean sea level had a direct connection with the flood levels experienced. And modeling establishes the extent of damage due to sea level rise arising from human activity.

The factual basis of the simulation is the observation that the sea level is rising, and rising faster, with the ice sheet and glacier melt being the major contributors. Data from tide gauges and satellite ranging shows that the sea level rose by 1.4 mm each year, between 1900 and 1990, but by 2.1 mm a year between 1970 and 2015, by 3.2 mm a year between 1993 and 2015 and by 3.6 mm a year between 2006 and 2016. We can see that the rate of rise has increased by 150% since 1970.

And then, in the context of 95% of the global temperature rise, which leads to ice sheet melting, being due to human activity, the team assesses the contribution of human activity to sea level rise, globally and in the New York region. The method of assessment was to construct a table where the sea level rise, over a period, was corelated with the change in the mean temperature during the period. There is a fraction of sea level rise that is due to other reasons. This is factored out and the contribution of human activity is calculated.

Accordingly, the sea level rise was computed, based on (i) the elements that contribute to the rise and (ii) empirical data relating sea level rise with temperature rise. For the first computation, referred to as a budget-based estimate, the main contributors were seen to be the melting of land-bound ice sheets and glaciers, the rise due to expansion of sea-water with rising temperature, and changes in water stored in inland reservoirs (like dams). And these are affected by the changes in the shape of the earth itself, mainly due to changes in the ice-loading at the poles. And from this analysis, the team works it out that 9.8 cm out of the 18 cm rise in sea level from 1900 to 2012 was on account of the warming that arose from human activity.

The second approach was to estimate the sea level, over the centuries, based on different geological sources, and records, like tide-gauge readings kept for mercantile shipping, the world over. The record shows that the sea level rose a tenth of a millimeter every year for first eight centuries CE and fell by two tenths of a millimeter during the 2°C cooling of the mini ice age in the 11th to the 15th centuries. But it picked during the 19th and 20th centuries, faster than anything during the previous 27 centuries. Modeling sea level rise shows that but for human activity, the sea level rise in the 20th century would have been less than 51% of what is seen.

Damage due to sea level rise

Having thus found that at least 49% of the rise in the sea level is on account of human activity, the paper looks at what part of the damage caused by Hurricane Sandy should be pinned on human activity. With only small damage due to wind, 98% of the damage and loss came from coastal flooding, the paper says.

This exercise was carried out using a procedure for modeling the near shore hydrodynamics of events like storms, developed by the U.S. Army Corps of Engineers. The topography and water depth data, and the wind and atmospheric pressure during Hurricane Sandy were used to develop the model, and the flood levels in the simulation were found to match the actual flooding. The model was then applied to discover flooding with lower levels of the mean sea surface. With the help of a damage model based on the actual flooding, it was then possible to work out what the damage would have been with lower mean sea levels, to start with.

Flood maps were created using data of land elevation data and water elevations from the model, after applying due corrections. Census data was used to assess the land, housing and population that was exposed, and damage was calculated using a system developed by the U.S. Federal Emergency Management Agency. The method is based on records of payments by insurance companies and even if the method is not accurate determination of losses, it provides reliable comparative costs of floods simulated from different baseline sea levels. The method, the paper says, yields the fraction of the damage that can be attributed to sea level rise due to human action.

The result of the computation is that $ 8.1 billion, or 13.5% of the losses caused by Hurricane Sandy was driven by sea level rise associated with anthropogenic, or human caused climate change. “The same general approach demonstrated here may be applied to other past and future coastal storms to estimate sea-level-linked anthropogenic climate damages from those events,” the paper says. But the message is that with sea levels set to rise in the coming decades, the losses due to extreme climate events are also set to rise.

< ------------------------------------------------------------------------------------------ Do respond to : response@simplescience.in -------------------------------------------