Has there been any investigation into the effects of asphalt on global warming.
Sounds like a silly question, but consider the millions of miles of blacktop road in the United States alone, add in asphalt shingles on rooftops, and you probably have several million square miles of asphalt heat traps.
Is it possible this would have an effect on the global climate?
Is that supposed to be a complement to this question? 
The answer is, again, no.
Asphalt represents a surface heat-absorbing phenomenon called albedo – the diffuse non-directional reflectivity of radiation. A high density of asphalt roads and roofs and other urban phenomena does indeed create an urban heat island effect, but the urban heat island is relatively insignificant in the overall global warming equation, and is accounted for in temperature records.
Right, local warming, not global warming.
There’s not that much road surface … not compared to ocean surface …
So according to you, asphalt is accepted as having a discrete effect on increased temperatures in urban areas.
The combined local warming as described above may not be considered as the major factor in global warming, but it must be a contributor none the less by definition.
True, but how is that observation related to the OP? Ocean surface area pretty much exceeds anything else in this world.:smack:
Unmentioned in this discussion is the tremendous amount of surface area in non-metro areas that is occupied by asphalt (roads, roofs, etc.) that exceed that of urban areas… The cumulative effect is probably more than that of the documented urban hot-spots, so the OP’s question isn’t silly when considering total effect… And certainly should not be discounted off hand.
According to a table at Wikipedia, worn asphalt has higher albedo than open ocean, and perhaps higher albedo than conifer forest in summer.
New concrete has higher albedo than anything in the table except ice and snow.
An estimate of 3% of Earth’s landmass as urbanised. Being generous, if all of that was asphalt, including the parks, backyards, nature strips etc, it still equates to about 1% of total global surface area.
As well as being low, its probably growing steadily but slowly. I would expect that some of the bigger more proximal climate modifiers have annual variabilities that dwarf that 1% figure.
The biggest effect by far of all that asphalt, concrete and roofs is on drainage. Wheres the precipitation used to soak into the ground and eventually the aquifer, it now runs off into drainage, rivers and to the ocean.
In the UK; if you resurface your front drive, it has to be water absorbent or include drainage to soakaways.
The OP is asking how the area of asphalt effects global climate … my point is that this effect is minimal compared to the effects of ocean water … perhaps even less than 1% and that’s the general limit for consideration … for a 5ºC global temperature increase, asphalt contributes 0.05ºC at best … in of itself this is safely ignored …
However there is a danger of claiming too many factors can be safely ignored … each of the various tiny contributions to global warming when added all together may well contribute more significantly than what we should ignore … unfortunately ripping out all the asphalt roads and replacing them with gravel isn’t a viable option … and concrete production is a major source of greenhouse gases …
They paved paradise / And put up a parking lot
With a pink hotel, a boutique / And a swinging hot SPOT
Don’t it always seem to go / That you don’t know what you’ve got / ‘Til it’s gone
They paved paradise / And put up a parking lot
There are about 4 million miles of road in the US. I’ll estimate the average width is about 50 feet. So that comes to 38,000 square miles of road. Which is almost exactly 1% of the area of the US. That’s actually a larger fraction than I would have guessed. Not all of the roads are asphalt of course.
That reminds me of a question that someone asked on a science call-in radio show today: if we start mining the moon for minerals as some have proposed, won’t the mass reduction affect the tides? The answer, of course, is theoretically “yes”, but it would be so tiny as to not be measureable. There’s a difference between “effect” and “effect significant enough to be worth measuring or calculating”.
Moreover, although I did say that a high density of asphalt-like surfaces would create a heat island effect, urban heat islands are not usually primarily due to albedo at all, and in fact depending on what was there before and the nature of the urban environment, there’s no reason to automatically assume that the urban albedo is necessarily lower than the natural terrain it replaces. Indeed much of the UHI is actually due to the fact that the modified surface affects the storage and transfer of heat, water and airflow-- things like changes in runoff, effects on heat retention, and lack of evapotranspiration.
So what, if the contribution is insignificant?
Why on earth would you assume that it’s being “discounted offhand”? Climate research has looked at albedo changes due to human activity – or more generally, at land use land cover changes – very carefully. One important conclusion is that although UHIs are real local phenomena, they have negligible impact on large-scale trends. A few fringe pundits have tried to raise questions about the extent to which UHI has biased the global temperature record but such claims have not been statistically rigorous and the general consensus is that any such residual biases in broad regional trends are insignificant. Global temperature records adjust for UHI effects and the upper limit to the maximum potential bias that UHI might introduce into the records is very small, which is borne out by the fact that urban and rural temperature trends are very similar.
It’s also interesting that when you look at all land use land cover changes changes due to human activity, the net effect is (a) a negative, not a positive, climate forcing, and (b) very small compared to climate forcing due to greenhouse gases. The reason for (a) is that the majority of such changes are due to deforestation, and the resulting land use is primarily croplands and pasture which is higher albedo. The reason for (b) is the enormous power of solar radiation that is leveraged by even small changes in atmospheric radiative transfer properties from increased greenhouse gases operating over the entire planet.
To put this in perspective, the radiative forcing from net post-industrial CO2 alone is currently around +1.82 W/m[sup]2[/sup], and somewhere around +2.3 W/m[sup]2[/sup] for all GHGs combined, and more than double that when water vapor feedback is included. Meanwhile all LULC changes combined produce a negative forcing of about -0.15 W/m[sup]2[/sup].
The total amount of croplands and pastures in the pre-industrial era was around 8 million km[sup]2[/sup]. Today it’s 50 million km[sup]2[/sup], or nearly 20 million square miles, much of it from deforestation, which is a hell of a lot of land area to subject to higher albedo, which makes road surface albedo pretty insignificant and renders the net anthropogenic land use effect a negative forcing.
Of the 4 million miles of road in the USA, only 2.7 million miles is paved (cite). And 50 ft average seems excessive, since most roads are 2-lane. So I think it’s closer to half a percent. Still more than I expected though.
Because (per the thread responses at the time) the question was discounted offhand. To summarize the drift of conversation that prompted my comments:* Asphalt is an inconsequential factor because I/we say so.* At least that is the way I took it.
Now that the reasoning behind the aforementioned “drift” has been revealed in subsequent posts, I am beginning to understand the point.
Thanks for the insight.