Alcove

Science, culture, complexity

Tag: climate models

  • Onward and spillward

    ‘The Lunacy Of Rebuilding In Disaster-Prone Areas’, Noema, April 25, 2024:

    In the months after Hurricane Katrina, New Orleans proposed a flood control program unlike any other in U.S. history. Developed by the Bring New Orleans Back Commission, a diverse group of stakeholders appointed by the mayor, the resulting plan called for large parts of the city to be converted from longstanding residential zones to floodable parks. Released to the public in the form of a map, large green circles were positioned over neighborhoods where owners would be forced into buyouts. These were some of the most historic districts in a very historic city … and almost exclusively in majority Black and marginalized neighborhoods.

    Christened in the press as the “Green Dot” map, the proposal ranks among the most profoundly unsuccessful plans ever issued by a municipal body and would never be put to a vote in the city council. … The Green Dot map’s remarkably brief tenure can be attributed in part to its proponents’ failure to adhere to the most basic rule of community planning: Never designate the where before building support for the what.

    “Building support”. What a quaint idea. Everyone should be doing it the way India’s doing it: don’t ask anyone. That way “building support” is redundant and “where” starts to really mean “anywhere”.

    ‘Expert committee clears plan to rebuild washed-out Teesta dam in Sikkim’, The Hindu, January 28, 2025:

    Fourteen months after a devastating glacier lake outburst flood in Sikkim washed away the Teesta-3 dam – the state’s biggest hydropower project – and killing at least 100, an expert committee of the environment ministry has recommended that the dam be reconstructed.

    Instead of the older structure that was part rock and part concrete, the new dam will be entirely concrete – reportedly to increase its strength – and its spillway will be capable of managing a peak flow of 19,946 cubic metres a second (cumecs), thrice the capacity of the former dam, which was 7000 cumecs.

    Sounds reasonable, right?

    The new design incorporates a “worst-case scenario” – meaning the maximum possible rain in the upstream glacier lake, modelled by the India Meteorological Department, in the South Lhonak region over the next 100 years influencing further downstream modifications.

    Now all we have to do is wait for the flood that will show up the IMD’s model — a fate models have often had to contend with this century, especially when dealing with rainfall.

    ‘The value of attributing extreme events to climate change’, The Hindu, May 24, 2024:

    It is worth understanding how these ‘rapid extreme event attributions’ are performed. The most important concept is the change in probability: in this case, climate scientists contrasted the conditions in which the heatwaves occurred against a counterfactual world in which climate change did not happen. The conditions that prevail in the counterfactual world depend on the availability of data from our world. When there isn’t enough data, the researchers run models for the planet’s climate without increasing greenhouse gas emissions and other anthropogenic forcing. Where there is sufficient data, they use trends in the data to compare conditions today with a period from the past in which human effects on the planet were relatively minimal.

    [But] the data are hardly ever sufficient, especially for rainfall, and almost never for extreme rainfall events. Climate models are also notoriously bad at properly capturing normal rainfall and worse at extreme ones.

    Thus, the environment ministry keeps the gates open to a new dam with a 59,838-cumec spillway in future.

  • A muffling of the monsoons

    New research conducted at the Potsdam Institute for Climate Impact research suggests that global warming could cause frequent and severe failures of the Indian summer monsoon in the next two centuries.

    The study joins a growing body of work conducted by different research groups across the last five years that demonstrate a negative relationship between the two phenomena.

    The researchers, Jacob Schewe and Anders Levermann, defined failure as a decrease in rainfall by 40 to 70 per cent below normal levels. Their findings, published on November 6 in the Environmental Research Letters, show that as we move into the 22nd century, increasing temperatures contribute to a strengthening Pacific Walker circulation that brings higher pressures over eastern India, which weaken the monsoon.

    The Walker circulation was first proposed by Sir Gilbert Walker over 70 years go. It dictates that over regions such as the Indian peninsula, changes in temperature and changes in pressure and rainfall feedback into each other to bring a cyclic variation in rainfall levels.  The result of this is a seasonal high pressure over the western Indian Ocean.

    Now, almost once every five years, the eastern Pacific Ocean undergoes a warm phase that leads to a high air pressure over it. This is called the El Nino Southern Oscillation.

    In years when El Nino occurs, the high pressure over the western Indian Ocean shifts eastward and brings high pressure over land, suppressing the monsoon.

    The researchers’ simulation showed that as temperatures increased in the future, the Walker circulation brings more high pressure over India on average, even though the strength of El Nino isn’t shown to increase.

    The researchers described the changes they observed as unprecedented in the Indian Meteorological Department’s data, which dates back to the early-1900s. As Schewe, lead author of the study, commented to Phys Org, “Our study points to the possibility of even more severe changes to monsoon rainfall caused by climatic shifts that may take place later this century and beyond.”

    A study published in 2007 by researchers at the Lawrence Livermore National Laboratory, California, and the International Pacific Research Centre, Hawaii, showed an increase in rainfall levels throughout much of the 21st century followed by a rapid decrease. This is consistent with the findings of Schewe and Levermann.

    Similarly, a study published in April 2012 by the Chinese Academy of Sciences demonstrated the steadily weakening nature of the Indian summer monsoon since 1860 owing to rising global temperatures.

    The Indian economy, being predominantly agrarian, depends greatly on the summer monsoon which lasts from June to September. The country last faced a widespread drought due to insufficient rainfall in the summer of 2009, when it had to import sugar and pushed world prices for the commodity to a 30-year high.