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I've tried searching for this myself, but Google is saturated with the Greenhouse Gases angle of Rising Global Temperatures.

What I'm interested in is the following:

Greenhouse gases aside, to what extent do automobiles directly contribute to a rising global temperature? Has a study ever been done? Is it even a tiny portion? It seems like there are hundreds of miles of roads covered in 200-degree heaters. That heat has to go somewhere!

I saw the term "Thermal Pollution" come up, and that sounds like it could be what I'm thinking of.

I often see folks hanging out in their vehicle idling with the AC Running, and think about how inefficient it is. Of course, A/C systems simply "move" the heat from inside the vehicle to outside of it- But an idling engine in the mix is just a huge waste.

enter image description here

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    $\begingroup$ Are you referring to the "urban heat island effect"? $\endgroup$
    – f.thorpe
    Aug 2 at 21:18
  • $\begingroup$ @f.thorpe That's along the right line of thinking, but I'm curious about vehicles specifically creating the heat & the effect they can have on a broader area. Clicking through it led me to what is probably the correct term, though! "Waste Heat" $\endgroup$
    – Aww_Geez
    Aug 2 at 21:22
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    $\begingroup$ Not quite a duplicate... but related: earthscience.stackexchange.com/questions/3041/… $\endgroup$
    – f.thorpe
    Aug 2 at 23:11
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    $\begingroup$ Traffic is a problem, but this particular image has been digitally manipulated. $\endgroup$
    – gerrit
    Aug 3 at 7:43
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    $\begingroup$ The direct heat from the engines is much less significant than the Albedo change of all those miles and miles and miles of road lying in the sun, the loss of plantife over the same roads, and the disruption of water runoff from the same. $\endgroup$
    – PcMan
    Aug 4 at 8:20
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Waste heat from vehicles pales in comparison to the energy from the sun.

In 2018, total global energy consumption for transport (including road, rail, air, and sea) was 2,890,900 ktoe (kilotonnes oil equivalent), or 330 ktoe per hour (dividing through by 8,760). This converts to 3,837,900,000,000 watts consumed on the earth's surface in an average hour.

The global surface area of Earth is 510,067,420 km2. Dividing through, this works out to 7,525 W per km2, or 0.0075 W/m2. This assumes that all energy used by vehicles ends up as heat, which is a rough assumption, as only 12 to 30% of the energy used by a car is converted to kinetic energy.

In contrast, the sun radiates an average of 340 W/m2 to the Earth's surface.

This is why, despite being only 20% efficient, solar PV could power the world if each country had 100 km2 array, or about 500,000 km2 globally. That's a lot, but still less than 1/10 of a percent of the earth's total surface area.

As comments have pointed out, directly comparing waste heat from vehicles to solar irradiance isn't valid, as these energy sources are coming from and going to different places. But understanding how much greater the sun's energy is (by a factor of nearly 50,000) helps explain why there's more concern about the heat-trapping CO2 from vehicle exhaust, then the heat they directly put into the atmosphere.

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    $\begingroup$ Although it's true that waste heat is not (currently) a driver of global warming, it does not follow directly from a comparison to solar heat input. The energy imbalance due to global warming is in the order of 0.1 W/m², your waste heat estimate (which I think is an overestimate, because not all human energy consumption ends up as heat) amounts to around 7.5% of that; little, but not entirely paling in comparison. One needs to consider what happens to the (waste) heat and where the heat is coming from. $\endgroup$
    – gerrit
    Aug 3 at 7:47
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    $\begingroup$ As @gerrit points out, comparisons with total solar heat input can be a little misleading. Perhaps better to compare to radiative forcing. Net anthropogenic radiative forcing is estimated to be 1-3 W/m². Waste heat is still small in comparison though. $\endgroup$
    – jkej
    Aug 3 at 8:42
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    $\begingroup$ @gerrit fair point -- I edited to clarify. However, unless we're firing laser beams into space, doesn't all our energy consumption eventually degrade to heat in the atmosphere? $\endgroup$
    – LShaver
    Aug 3 at 13:00
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    $\begingroup$ @LShaver I like the simplified math here. Taking the energy input of the transportation industry and assuming a conversion rate is a smooth way for it to make more sense to me. So I guess if the Sun went out, all of the waste heat from traffic will only keep us 1/50000th as warm? $\endgroup$
    – Aww_Geez
    Aug 3 at 13:51
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    $\begingroup$ Not to nitpick, but @gerrit's figure of 0.1W/m² is incorrect. According to NASA, the Earth Energy Imbalance (EEI) is currently 0.3%, or about 1 W/m², making LShaver's calculated worst case example of vehicular heat a much smaller contribution to the EEI at 0.75%. $\endgroup$ Aug 3 at 23:50
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Going off the conversation in comments on another answer, I'm converting that info into an Answer.

From a 2017 article on Nature.com, compared to the heat being captured by greenhouse gasses, anthropogenic waste heat only accounts for about 1% of the temperature increase globally.

Nearly 70% of energy is consumed within cities occupying a mere 2% of the Earth’s surface area, and future scenarios indicate that global primary energy consumption will rise 1.6 times (864.7 quadrillion kJ) from 2010 to 2040 (http://www.worldenergyoutlook.org/). Although anthropogenic heat accounts for only 1% of the greenhouse gas forcing, it causes the majority of regional warming, such as urban heat islands1,2, urban boundary heights, and hourly intensity of precipitation at the city level2–5, especially at night.

https://www.nature.com/articles/sdata2017116

This article says that most of the problems caused by this issue are local to the regions creating it. Cities, industries, etc. are all creating this waste heat, but it dissipates as the distance to it increases. This means it's not a significant global issue, but it is an issue that needs to be addressed. We just need to realize that we can't ignore the 99% problem of greenhouse gasses to fix the 1% problem instead.

Some of these problems have been studied since 2006 and earlier. According to the 2020 article (on the right half of the screen), this was considered a significant enough problem to be looked into that the Osaka Heat Island Countermeasure Technology Consortium (HITEC) was created. There's also articles from 2013 and 1993 talking about this issue, so it's been looked at considerably, but the only place I can find a number assigned to the amount of this effect is from the Nature article I first referenced.

Osaka Heat Island Countermeasure Technology Consortium (HITEC) was established in January 2006, for the purpose of the development and spread of heat island countermeasure technologies, implementation of measures and verification of their effects, and the collaboration between industry, academia, government, and the private sectors [15].

https://www.sciencedirect.com/topics/engineering/anthropogenic-heat

This next article doesn't say how much this effect causes heating compared to greenhouse gasses alone, but it confirms that it's a fairly local problem in section 2. It also says that to help reduce this effect, we need to reduce the creation of greenhouse gasses. From how I read it, this means that anthropogenic waste heat and greenhouse gasses aren't two different problems, but intertwined. Not to mention that most of this anthropogenic waste heat is created by industries and homes releasing greenhouse gasses to create the heat they use and release.

Cool roofs reduce building heat-gain, create saving air conditioning expenditures, enhance the life expectancy of both the roof membrane and the building’s cooling equipment, improve thermal efficiency of the roof insulation, reduce the demand for electric power, reduce resulting air pollution and greenhouse gas emissions, provide energy savings, and mitigate UHI effects.

https://www.hindawi.com/journals/usr/2011/497524/

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  • $\begingroup$ I looked at the first article you cite -- they don't show how they derive that 1% figure, and it isn't readily available in the sources they cite either. I don't doubt that it's true -- but it would be nice to see how they derive it. $\endgroup$
    – LShaver
    Aug 3 at 22:24

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