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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

Aww_Geez
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    Are you referring to the "urban heat island effect"? – f.thorpe Aug 02 '21 at 21:18
  • @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" – Aww_Geez Aug 02 '21 at 21:22
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    Not quite a duplicate... but related: https://earthscience.stackexchange.com/questions/3041/how-does-anthropogenic-heating-affect-global-warming?rq=1 – f.thorpe Aug 02 '21 at 23:11
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    Traffic is a problem, but this particular image has been digitally manipulated. – gerrit Aug 03 '21 at 07:43
  • I'm not sure whether you truly mean "global"! The direct effect is surely tiny, like probably all of mankind's activities because the energy that's coming from the Sun and flowing through the Earth -- which is what we are tapping into by adding greenhouse gases into the atmosphere --, together with energy from natural radioactive decay, is surely magnitudes larger. Effects are at most regional (and well understood, I believe.) – Peter - Reinstate Monica Aug 03 '21 at 08:06
  • @Peter-ReinstateMonica I do mean global - And if the effect is negligible, that'd be the answer. If the effects are purely local, wouldn't that mean that a small city in the desert would radiate all of their heat directly up and off of the planet? Surely it all gets spread around and "washed out" to a near-0 number globally; But a near-0 number is not 0. – Aww_Geez Aug 03 '21 at 14:03
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    @gerrit pretty lazily too - the white van and pickup in the centre of the image are doubled 5 lanes to the right – llama Aug 03 '21 at 17:30
  • This question is fundamentally flawed: Global warming is a shift in equilibrium temperature of the surface. Any temporary forcing will not irreversibly change this equilibrium temperature. As soon as the cars are off, the temperature returns to its initial value. – AtmosphericPrisonEscape Aug 03 '21 at 21:24
  • @AtmosphericPrisonEscape I suppose that's a matter of semantics, I'll edit out the "Global Warming" and keep "Rising Global Temperatures" – Aww_Geez Aug 03 '21 at 21:33
  • No it's not. Forcing that is only temporary cannot permanently contribute to global warming. Same reason you can't warm Mars by just exploding some nukes. You have to change the heat balance, not just the heat intake. – AtmosphericPrisonEscape Aug 03 '21 at 21:50
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    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. – PcMan Aug 04 '21 at 08:20
  • @AtmosphericPrisonEscape the "nuke Mars" idea is not to directly heat the place. It is to raise polar temperature a bit, melting a few gigatonnes of frozen co2, and using greenhouse effect from that to heat Mars. And even then the heating of the poles is not direct, but by upheaval of particulate matter, covering the reflective ices of the poles with darker grit, raising its albedo, and encouraging the sunlight to sublimate off the co2 layers. Not relevant to this question, but to point you in the right direction on your misconception. – PcMan Aug 04 '21 at 08:24
  • @PcMan I was just talking about nuking Mars for the immediate heat release, not anything you just came up with. That version of nuking Mars is akin to other nonsensical sci-fi ideas like putting windmill heaters (Green Mars trilogy) on the surface. The forcing is gone as soon as the nuke dissipates/the windmill is gone. Just to point you in the direction of thermodynamics and radiative balance. – AtmosphericPrisonEscape Aug 04 '21 at 10:05

2 Answers2

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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.

LShaver
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    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. – gerrit Aug 03 '21 at 07:47
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    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. – jkej Aug 03 '21 at 08:42
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    @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? – LShaver Aug 03 '21 at 13:00
  • @jkej I think expanding on that would make for a valuable answer here. – LShaver Aug 03 '21 at 13:00
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    @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? – Aww_Geez Aug 03 '21 at 13:51
  • @gerrit what do you think kinetic energy will end up being when the vehicle stops? ;) – Sobvan Aug 03 '21 at 21:12
  • @gerrit I think I need to think of energy input/loss of the planet like a sinking ship that is getting bailed out. As long as we bail (lose heat) as fast as we sink (gain heat), we're fine - But if the energy "imbalance" is greater than 0, we will trend in the wrong direction inevitably. Greenhouse gases are giving us a smaller bucket, and waste heat is just an extra leak in the boat. If the imbalance is indeed .1 W/m², and waste heat accounts for .0075 W/m², then 7.5% of the excess water coming in is from waste heat via transportation? – Aww_Geez Aug 03 '21 at 21:42
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    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%. – Knob Scratcher Aug 03 '21 at 23:50
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    @KnobScratcher Hmm, you're right, I remember wrongly or am out of date. My memory/source was Trenberth's energy diagram, but that one actually says 0.9 W/m², not 0.1. Thank you for correcting me. – gerrit Aug 04 '21 at 07:28
  • 'road, rail, air, and sea' (traffic) is not counting industry heat + domestic heat. And all these 3 are going up very quickly. – miguelfg Aug 29 '22 at 11:46
  • @gerrit From NASA's website: 'NASA's Clouds and the Earth's Radiant Energy System (CERES) suite of satellite sensors measure how much energy enters and leaves Earth's system' <- EEI seems to be counting our waste heat, leaving EEI due to GH gases driven to be smaller, no? – miguelfg Aug 29 '22 at 11:49
  • @miguelfg Direct human waste heat? I thought that'd be much smaller, but I don't know for sure. Good question. – gerrit Aug 29 '22 at 13:26
  • @miguelfg the question is limited to transport, but if we include total energy, we get a value of 0.04 W/m2, about 5 times higher than just transport. Still about 1/10,000 of the energy of the sun. Consider the fact that you can stand anywhere on earth when the sun is shining and feel it's heat -- but you have to stand right next to a car/building/factory to feel it's heat. – LShaver Aug 29 '22 at 14:04
  • @LShaver I think comparing it to the total energy of the sun is a big mistake to decide about 'is it our waste heat dispensable or not?'. Because how much of the total sun energy is actually kept within the atmosphere, and needed for natural processes, for reaching the 22ºC expected average temp, for wind... It'd be better to compare it to an estimated EEI.

    I'm curious about this because I'm astonished that few weak IR wavelenght bands of heat absorption by CO2 is taken as responsible for 70%(?) of GW effect. And this statement drives politics and economy right now.

     – miguelfg Aug 29 '22 at 16:07
  • @miguelfg whether the heat is coming from our waste heat, or the sun hitting the earth's surface, global warming will be a result of whether that heat is absorbed in the atmosphere or escapes back out to space. So we need to focus on greenhouse gases which trap the heat, especially since waste heat is 1/10000th of the sun's heat. But in some ways it's a moot point, since the primary energy source for our waste heat is fossil fuels, combustion of which releases GHGs... – LShaver Aug 29 '22 at 17:26
  • ... which means that reducing our waste heat will necessarily reduce our GHG emissions as well. – LShaver Aug 29 '22 at 17:26
  • @LShaver Agree on lowering combustions helps in 2 directions. My point is how much CO2 is really responsible for, because it'd still matter on actions, policies, etc. I think this is not the place of keeping the discussion, so thank you for the information. I'll keep reading about it. – miguelfg Aug 30 '22 at 13:57
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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/

computercarguy
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  • 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. – LShaver Aug 03 '21 at 22:24