Re: Roughly how much of the Earth's surface is covered in asphalt or tarmac?

This is a suggestion for 'climate control' that has been considered 
seriously. As we cut down forrests and build roads and cities we are 
changing the reflectivity of the surface of the Earth and this causes 
changes in the radiative balance between the incoming and reflected 
energy. Reflected energy does not alter the climate while energy that is 
absorbed (in dark surfaces for instances) become heat in the environment 
before it escapes to space once more from the top of the atmosphere.

It is possible to estimate the effects of this "albedo" changes. Albedo 
is a fancy word for 'reflectivity'. In a climate model you can set the 
model Earth reflectivity and then calculate the equilibrium conditions. 
Then you can alter the reflectivity and recalculate the equilibrium 
temperature and look for any changes.

I have done this and the result is that a 1% change in reflectivity will 
change the global mean temperature by  about 0.4 degrees Centigrade. So, 
if you changed the reflectivity of every surface area on Earth by 1% 
temperatures would on average change by 0.4 C. (darker surfaces = warmer 
world)

Now, we are not yet able to change the reflectivity of every surface of 
the Earth so we must take into account the proportion of the area we can 
change before answering your question.

I am not aware what the area of the cities and roads on Earth are but 
would estimate that it is something like the area of Ireland or Denmark 
or Belgium or something like that.

Belgium's area is about 0.006% of the Earth's surface, but 70% of the 
Earth's surface is water and if we assume we will not be able to change 
that we end up with Belgium being something like 0.01% of the Earth's 
land surface. If we changed the reflectivity of that area by 1% we could 
except a temeprature change of about 0.0044 degrees C. If we changed the 
reflectivity by more we could get proportionaly larger effects. Let us 
say we could change the surface reflectivity by 50%, then the expected 
temperature change would be 0.2 degrees C, which is not unmeasurable. 
Global average temperature has changed by perhaps 0.6 or 0.8 degrees C 
since the mid 1800's, in comparison.

Does land surface change its reflectivity by that much when we build on 
it?

Well, if we think of cutting down a forrest and then surfacing the area 
with black asphalt we are in fact not canging the reflectivity by much 
because a forest is very dark seen from above. In fact, by planting 
ecological forest on light-coloured land, such as deserts, we are helping 
the Earth warm up!

Roads built on light sand may easily halve the reflectivity of the 
surfacea, however. Most cities tend to be light-coloured when seen from 
above. If you use Google Earth to look for cities from above you can 
almost always spot them because they seem brighter than the landscape 
nearby. That is due to the use of concrete in so many surfaces.

I hope the above calculations help to answer your question. I may be 
wrong by the area estimates, but you can scale the answers accordingly if 
you feel that you know the area of built-up regions on Earth.