Whether urban planners and architects are aware of the meaning of "externality" is a good question, however this word belongs to economics, it is enormously important. I should correct myself and say, not only for architects and urban planners.
What is "externality"? Externality is something that is not directly involved in the cost of a good, but affects the living standards. A good example of externality is: the beautiful landscape surrounding an office building, which allows the employees to constantly appreciate it. Therefore, the enjoyable view can slightly enhance the productivity of the company.
On the other hand, a negative example of this spillover is: when we refuel our cars, the price of gas includes costs for extraction of oil, manufacturing (investments on R&D), taxes and the presumible profit. However, is this the real price of gas? Using gas implies in CO2 dispersion, and consequently, it also pollutes the local environment, decreasing the air quality and enhancing the probability of respiratory illness outbrakes. Obviously, it will also have impacts on the people, generating stress, more likelihood for illness and so on.
And how can we relate the negative externality with the climate change? I conducted an experiment incorporating data of anthropogenic energy (sensible heat and latent heat) of Kanto Region in August of 2006. It is said that air temperature peaks in August, therefore the air conditioning demand increases, and it generates a vicious circle that enhance air temperature.
However, our daily activities also affect the local wind velocity. A default run in a numerical simulation weather prediction model showed that the average air velocity measured at 10 meters in Tokyo is 2.53753 m/s, whilst the inclusion of anthropogenic energy enhances the velocity to 2.92886 m/s, a difference of 0.391336 m/s.
In addition to that, the water molecules amount in a volume of dry air (mixing ratio) is altered, and it is related to the extra energy input. For the levels close to the ground, the extra heat will provide more energy, and the water molecules will tend to be uplifted. For example, in August for Tokyo, the amount was 16.894 g/kg in the default run, and the inclusion of anthropogenic energy meant that the mixing ratio amount was 16.7437 g/kg measured at 2 meters height. A gap of 0.1502853 g/kg.
Therefore, urbanization not only enhances air temperature, but also enhances air velocity, and decreases air humidity at the bottom levels, which means a drier and hotter environment, and we could think about the outbreak of respiratory illness outbrakes.
Although my results are merely based on investigation, I am very sure that tackling the contemporary problems requires an enormous combined effort of urban planners, architects, medical doctors and economists.
Perhaps, the lie of the prices do not input the environmental burden costs, however, the effects are crystal-clear.