Climate change: enhanced tree cover can save B'luru

Nadaprabhu Kempegowda’s Bengaluru sits at 949 metres above sea level atop the Mysore plateau. Once famed as a naturally air conditioned city of lakes and gardens, Bengaluru has experienced a hot, parched and thirsty March-April this year.

Highest ever temperature of 39.2 degree Celsius was recorded on April 24. And after much heat and dryness, the city received 46.6 mm rain within a few hours on May 6 that left several areas water-logged, uprooted at least 15 trees, damaged several electricity poles, and badly disrupted the city-traffic during peak hours. Are these impacts of climate change or natural climatic variability or is it attributable to both?

A report on climate change projections and impacts for Karnataka prepared by the BCCI-K (Bengaluru Climate Change Initiative – Karnataka) in collaboration with GGGI (Global Green Growth Institute) projects warming by less than 2 degree Celsius by 2030s and more than 4 degree Celsius by 2080s under high greenhouse gas (GHG) emission scenario for Bengaluru.

However, according to a study conducted by the Indian Institute of Science (IISc), Bengaluru, the urban heat island effect for the city has already resulted in a 2-degree rise in land surface temperature during last 2 decades. This raises a question, whether the impact of climate change is experienced sooner than projected for Bengaluru? This question needs to be considered taking into account the biophysical changes that have modified the local ecology of Bengaluru in the recent past.

Starting 1990s, Bengaluru has experienced an exponential growth, particularly in the IT and education sectors, which has earned it the distinction of and prestige as the Silicon Valley and the startup capital of India. This has gravitated professionals, students and others towards Bengaluru leading to growth in the city’s grey infrastructure in terms of roads, flyovers and underpasses, residential and commercial areas, hospitals and educational institutions.

The rapidly enlarging concretised built-up area in the Greater Bengaluru (741 km2) has indiscriminately usurped the lands previously under natural and semi-natural ecosystems like forests, wetlands, pastures, orchards and agriculture. The heat island study of IISc reports that in 1973, the area under vegetation cover and water bodies in Greater Bengaluru was 68% and 3.4%, respectively. Such spread of vegetation cover and water bodies got reduced to 46.22% and 2.6% by 1992; 45.77% and 2.26% by 2000; 42.39% and 1.8% by 2002; 28.83% and 1.57% by 2006; and 16.32% and 0.72% by 2009. Concurrently, the built-up area in Greater Bengaluru witnessed a growth of 632%. 

The soothing effect of forest cover and water bodies on local climate is well known. The traditional wisdom about the benefits of forest cover for rainfall is now supported by the latest scientific research.  Empirical research reveals that 60% of water received as rainfall in the hinterlands away from the oceans is contributed by forests and inland water bodies through evapotranspiration process. Per unit area, the quantity of transpiration moisture released by trees is 2 times or more as compared to that released from a water body by evaporation. Comparatively, built-up areas contribute virtually no moisture to the atmosphere.

Relative humidity

The moisture released by trees and inland water bodies adds to the relative humidity of air enhancing the chances of a rainfall event. Trees also release volatile organic compounds (VOC) and pollens, which act as cloud seeds. Thus, cloud formation and rainfall events are facilitated over well-forested areas due to cloud seeding and higher relative humidity. Also increased cloud cover limits the maximum temperature during summers.

Higher cloud cover and episodes of rainfall have moderating effect on summer temperatures. A tropical landscape having substantial forest cover and water bodies is able to moderate extremes of summer temperature through increased cloud formation and localised rains. Arguably, the implication of reduced forest cover and water bodies would be higher temperatures during summers.

The drastic reduction in natural environment provided by the tree cover and water bodies and the observed trends in local weather parameters prompts us to think that the capacity of Greater Bengaluru landscape to moderate summer temperature and dryness has been compromised over the past 2 decades. As per the Indian Meteorological Department (IMD) data, the number of rainy days (rainfall >2.5mm/day) during the month of April, which is hottest for Bengaluru, shows a decreasing trend; for Bengaluru City Station and Hebbal areas, the number of rainy days was reduced from 6 to 0 and 6 to 2 respectively, between 2010 and 2014 indicating increased desiccation and reduced water availability during the peak summer month.

Extensive growth of built-environment at the cost of natural environment has adversely impacted the local ecology of Greater Bengaluru and appears to be the proximate reason for weather related extremes and uncertainties. The implications of degraded ecological balance are: higher ambient temperature, reduction in afternoon showers, disruption in safe natural drainage of storm water, and reduced pollution recycling. Impacts of climate change would be farther with an intact and functional ecology at local level as also the climate models projecting the present level of warming for Bengaluru only during the next decade.

Restoration and enhancement of tree cover and water bodies in Greater Bengaluru can potentially moderate the impacts of climate change and help build necessary adaptation measures at lower cost. The public agencies for Bengaluru including the Karnataka Forest Department and Lake Development Authority are making efforts to securing the remaining green spaces, encouraging planting of tree saplings, providing green quadrants of different sizes along newly developed roads and footpaths, and restoration of lakes. These efforts are being supplemented by non-governmental agencies, private persons and others. All efforts are with the faith that Bengaluru will be able to successfully adapt and sustain itself under climate change.

(The writer is Additional Principal Chief Conservator of Forests, Water Resource Department, Karnataka)

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