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Loss of green cover create 'heat island' over Nairobi

By The Conversation | December 17th 2020 at 11:39:49 GMT +0300

Part of Nairobi City (PHOTO: FILE)

Nairobi is one of the fastest growing cities in Africa. The city’s population, which was about two million people a decade ago, now stands at over four million.

This puts a lot of pressure on the natural environment.

Areas of vegetation around rivers and forested land have declined in the city due to encroachment and infrastructure development, namely roads and buildings. It has been reported that Nairobi has lost 22 per cent of its green spaces cover in the period between 1988 and 2016.

Changes in land cover in Nairobi are mainly due to policy changes – such as zoning – that increase the plot area covered by a building relative to the total plot size.

For instance, areas close to the central business district that were residential in land use have seen recent conversions to commercial or office space land use. This changes the landscape.

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Areas covered by impervious surfaces such as tarmac, metal or concrete have higher surface temperatures compared to vegetated land cover. This is because vegetation provides shade and, through evapotranspiration, cools cities.

Impervious surfaces such as concrete absorb more solar radiation than they reflect. This leads to a phenomenon known as urban heat island.

Urban heat island refers to when temperatures in the city are higher than those of surrounding areas that have more vegetation cover. The effect is more pronounced at night when impervious surfaces re-radiate heat that is absorbed during the day into the atmosphere.

Quality of life

Through our work, we have found that an urban heat island is already manifesting over Nairobi. Data over Nairobi indicates that average air temperatures increased from 18.8 degrees Centigrade in the 1950s to 19.5 degrees in 2000s.

This situation is likely to become more pronounced, given the ongoing environmental modification by construction, such as the Nairobi Expressway.

This urban micro-climate will have an influence on the quality of life of city residents. Although the intensity of Nairobi’s heat island has not reached alarming levels of directly causing death, with ongoing developments, this is bound to eventually happen.

The most direct effect on health from the urban heat island is heat health risk. Heat can worsen pre-existing conditions such as heart and lung disease, kidney problems, diabetes, and asthma. It can also affect occupational performance or lead directly to death.

Extremely high temperatures of more than 27°C for a period of time, such as during heat waves, puts people with health issues particularly at risk.

Nairobi Expressway Climate Change
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