Urban Climate

A recent journal article by MZ Jacobson et al points out some of the climatological impacts of urbanization on local climate. The land use change that urbanization leads to is known to impact climatological conditions through changes in soil moisture evaporation, transpiration (evaporation from plants), heat absorption and advection (wind).

In this article the authors used models to study the impact of the change in urban extent in Beijing. Beijing provides a good case due to the extreme nature of changes there - the urban extent of the city quadrupled between 2000 and 2009.

Key Points from the article:

• Beijing's expansion created a ring of impact in the new portion of the city
• Without considering the impact of more vehicles and other sources of human caused polution - urbanization's impact on climatological conditions alone slowed winds and increased pollution vertical dilution and increased ground level temperature, and ozone

Interestingly the authors used crowdsourced data on road surface area in their study.

Take homes:
This study found that urbanization changes local climate in these ways - it:

• increases ground level temperature
• decreases ground level humidity
• decreases horizontal movement of air (wind, advection)
• increases vertical movement of air (convection)
• reduces reflectivity of the surface (albedo) causing the retention of more solar energy

These impacts:

• decreased surface pollution by promoting vertical mixing
• but increased surface ozone (O3) due to other changes in the chemical profile of the air

None of these impacts takes into account vehicle use or other pollution sources.

Ring of impact from the mega-urbanization of Beijing between 2000 and 2009 (pages 5740–5756) Mark Z. Jacobson, Son V. Nghiem, Alessandro Sorichetta and Natasha Whitney Article first published online: 19 JUN 2015 | DOI: 10.1002/2014JD023008

Land use change and global warming

An interesting tidbit that showed up over at Nature's "Nature Reports: Climate Change" in a story on what might be the discovery of the missing carbon sink.

Albedo effect

Other scientists have also recently come to the conclusion that northern forests, although critically important in maintaining biodiversity, might be less important in slowing climate change than tropical forests. Govindasamy Bala and Ken Caldeira found that tropical forests help cool the Earth in two ways: by storing carbon and also by reflecting the suns warming rays back to space⁵. "Unlike tropical forests, high latitude forests darken the Earth's surface, causing the earth to absorb more sunlight, an effect that is most pronounced in snowy regions. This darkening of the surface has a warming influence that can be stronger than the cooling influence of carbon storage in these forests," says Caldeira. This suggests that removing high-latitude forests would have a net cooling effect on the planet, whereas removal of tropical forests would result in warming.

What is interesting here is what has been happening for the past 30 to 50 years. Northern forests have been growing and tropical forests have been shrinking. Thus, according to the above, this should lead to warming. How much of this land use change impact is included in the models of climate change? Not sure.