Global warming, a frequently discussed topic, has evoked considerable social concerns. Globally, the average surface air temperature increased by 0.5 °C in the 20th century. It is estimated that the global average air temperature will further increase by 1.5–4.5 °C by the year 2100. The increase in air temperature was mainly due to the increasing emissions of greenhouse gases, among which carbon dioxide (CO2) is the most dominant one, accounting for three-quarters of the total emissions. Urban areas are generally warmer than their surrounding rural environments due to the high proportion of imperious surface and high levels of fossil fuel combustion. Therefore, the impacts of climate change may be exacerbated in urban areas.

Urban forests can play an important role in mitigating the impacts of climate change by reducing CO2 in the area. They can reduce the levels of atmospheric CO2 through sequestration and reduce CO2 emissions by conserving energy used for heating and cooling. Trees and shrubs can transform CO2 into above- and below-ground biomass through photosynthesis, a process called carbon sequestration, and store carbon in the form of stems, branches, or roots. In addition, urban forests decrease building cooling demand by shading and evapotranspiration in the summer and reduce heating demand through decreasing wind speed in the winter. The decrease in both cooling and heating demand lowers carbon emissions from fossil fuel combustion.

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Urban  Vegetation  Affects  CO2 Concentrations in cities

The variation in, and intensity of, CO2 sources depend on the habits of citizens, local economies, and local climates. Therefore, designing a clear seasonal dynamic of CO2 is difficult and remains site-specific. A common condition for many urban areas is that a shallower boundary layer (i.e. the lower part of the atmosphere in which gas exchanges take place) in colder seasons, due to a decrease of convective movement of the air, generally leads to a lower dilution of CO2 emitted from anthropogenic and biogenic sources into the atmosphere leading to an immediate increase in atmospheric CO2 concentration. The presence of vegetation in urban areas can affect the concentration and use of CO2. In the city of Rome, Gratani and Varone (2005) showed an increase in mean annual CO2 concentration from 1995 (367 ppm) to 2004 (477 ppm). The annual dynamics showed a peak in winter, which was highly correlated with traffic density and atmospheric stability. The researchers found that during weekends when traffic density was reduced by 72%, CO2 concentrations were also lower. The daily trend in Rome’s city center showed a peak in the early morning, corresponding with rush hour as well as the hours when the atmosphere was more stable. Park et al. (2013) showed that CO2 concentrations over two urban sites (one in a residential area and one inside an urban forest) followed a similar diurnal variation, with maximum values occurring during the night and minimum values occurring during the daytime. Also, a similar seasonal variation was observed, with a maximum value during the non-growing season (early spring) and a minimum value during the growing season (summer). Interestingly, due to photosynthesis, the rate of CO2 sequestration over the urban forest was high during daytime in the growing period, while this was not observed in the residential site. In central London, Helfter etal. (2011) studied the emission/sequestration of CO2 with micrometeorological techniques and found that CO2 exchange was mainly controlled by fossil fuel combustion (e.g. traffic, commercial and domestic heating). The researchers also found that exchanges were affected by changes in natural gas consumption for heating, but also that to a lesser extent, photosynthetic activity controlled the seasonal variability.

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The efficiency in carbon storage varies with species. A thorough study was conducted in documenting the carbon sequestration potential of various Indian vegetative species. The city of Kochi, India, is situated at 9° 58′ N 76° 13′ E and has a tropical monsoon climate. Due to its seaside location and near the equator, Kochi experiences little seasonal temperature change and moderate-to-high levels of humidity. The annual temperature ranges from 23 to 31 °C. Conifers have the highest carbon storage, followed by deciduous, evergreen, and bamboo trees. Many studies from India show that native Indian trees such as Azadirachta indica (Neem), Tamarindus indica (Tamarind), Ficus religiosa (Peepal), and Madhuca longifolia (Mahua) are more efficient in carbon sequestration. It has also been reported that native and evergreen species showed better performance in biomass and carbon sequestration than the introduced and deciduous species.

A study conducted in Vadodara city found that the trees planted on the roadsides remove 73.59 tonnes of CO2 representing a 22% estimate of the total CO2 production of a city which is around 159.47 tonnes. Street vegetation in this populated city of Gujarat turns out to be an effective way of offsetting urban carbon emissions.

While inventorying all trees having sizes larger than 5 cm diameter at breast height (dbh) from a one-hectare area of the Cooum river bank at Chennai city, found that about 2 tonnes of carbon were sequestrated by the urban forest in a year. The study indicates that the existence of a greater number of small trees with lower mean stem growth contributes less to biomass as well as carbon sequestration, compared to bigger and high-growth rate trees. Another study found that a 20-year-old silver oak shade tree may sequester up to 41.8 Mg/ha of carbon, indicating that the carbon stock in young urban trees is insignificant. Carbon sequestration, on the other hand, is dependent on tree growth rates because it reflects a possible increase in biomass.

Pune city in India has been reported to have very less area of urban forest available for sequestration in comparison with the amount of carbon emission from the city. The standing biomass of the above-ground woody sections of the trees in Pune City was determined. The city of Pune and its many sectors have a carbon emission and sequestration potential of 15,000 tonnes per year. The city of Pune releases 780,000 tonnes of carbon each year, but its trees only absorb 2% of that, resulting in a 98-percent atmospheric overload (Waran & Patwardhan, 2001).