I replied to her:-
"Assessing the Impact of Urban Sprawl on Soil Resources in the United States Using Nighttime "City Lights" Satellite Images and Digital Soils Maps" USGS and NASA
Abstract." Nighttime satellite images of the Earth showing city lights were merged with census data and a digital soils map in an effort to estimate the extent of developed land in the United States and the impact of development on soil resources.
The urban areas defined by "city lights" had mean population densities of 1,033 persons/km2 and 427 housing units/km2 (4.13 persons and 1.7 households/acre). Urban areas accounted for 2.7% of the surface area in the United States, an area approximately equal to the state of Minnesota or one-half the size of California."
Mankind has urbanized less than 3% of the total US You believe that not only have we changed the climate in the other 97% but also in the arctic,antarctic,Africa and the oceans.My what busy bees we have been to achieve all this with ac units, hair spray cans and car exhausts in one climatic period of 30 yrs!
Further reading at:-
http://biology.usgs.gov/luhna/chap3.html
It really started in the 1700s with expanded agriculture, and accelerated in the mid-1800s with industrialization, and has further accelerated in the last 30 years as population has increased.