[amalia (OP)]

Angela sent us a very good question:
As humans continue to overpopulate the planet, does the heat that they generate contribute to global warming? Imagine a compost bin getting hot... it’s just like we’re the bacteria and the earth is the compost bin! If humans do contribute, what %age of blame would we be awarded to us in comparison to, say, cars?
Do you know the answer?

[Bored chemist]

We certainly don't contribute much.
If we didn't eat the food, it would decay and as you know from compost heaps, decay produces heat.
The total energy released by the degradation of, for example, an apple is the same whether it is eaten by someone, or crushed and turned to cider (where the yeast take some energy) and then drunk, or just left to rot on the ground.

[evan_au]

The heat from human activity is indirect.

We pump CO₂ and other greenhouse gases into the atmosphere.
This retains a tiny bit more of the Sun's enormous incoming radiation.
That has produced a measurable warming in the past century.

It's a bit like being in bed, and putting on a doona.
Your power output hasn't changed much, but the temperature under the blankets has increased significantly.

[Petrochemicals]

Quite alot, if we heat the air as is seen in the chemical reactions of fossil fuels, this allows more water to evapourate as the humidity maximum is raised. Water on the surface can therefore  harness the power of the sun to evapourate rather than being forced inno emmisivity of the energy it recieves from the sun, adding more energy to the atmosphere rather than it being re-emitted to space.

[jeffreyH]

One person alone on earth would produce a negligible effect. Seven billion or so produce a larger but still insignificant effect. The point is it is a matter of a scale.

The scale becomes important when the composition of the atmosphere is being changed. The rate of that change is also of critical importance.

The other concern is what elements of this changing composition are having the largest negative effects on our environment.

The answer can elude us since we have nothing within our direct experience to compare it to. The effects can also be subtle and accumulative. We may not even realise the worst culprits until it is too late to mitigate the damage.

[Bored chemist]

Quite alot, ...

Petrochemicals is on record as saying he doesn't understand how this works.
https://www.thenakedscientists.com/forum/index.php?topic=78055.msg586482#msg586482

[MarkPawelek]

It depends on how it contributes, and on how we measure that contribution.

Consider this. We can split global land surface temperatures into 2 main groups. Ocean air sheltered, OAS, regions and Ocean air affected, OAA, regions (see ref 1). When we look at each separately we see there's been no global warming for OAS regions since 1950s.  Warming is all in OAA regions. That is, of course, totally incompatible with a greenhouse gas effect. Greenhouse gas warming should not discriminate between OAA and OAS regions. It begs the question: How can we explain the OAA warming, without OAS warming?  Easily. Consider than 95% of people live on just 5% of the land surface. It follows that only 5% of people live in the other 95% of land. People generally prefer to live closer to the coast; which means the vast majority of people live in OAA regions. We also measure temperatures close to where we live.  Many temperatures are biased by an urban heat island effect, UHI. This UHI is predominantly experienced as a warming at night; just like the greenhouse gas is supposed to act!

Because many surface climate stations are located close to populations and people build more building, parking lots, etc., overtime the great majority, 92.1%, of stations in, for example, the USA are subject to UHI corruption. These stations should not be used for monitoring land surface temperature. I see no reason to believe the situation elsewhere in the world is any better.



(1) "Temperature trends with reduced impact of ocean air temperature", 2018, Frank Lansner, Jens Olaf Pepke Pedersen; https://doi.org/10.1177/0958305X18756670"

[Bored chemist]

Greenhouse gas warming should not discriminate between OAA and OAS regions.

The effect of greenhouse gas  driven warming depends on surface reflectivity which differs between land and sea.
Did you post the other stuff because you...

Like fairy stories.

[Kryptid]

An average adult male produces about 100-120 watts of body heat: http://large.stanford.edu/courses/2016/ph240/stevens1/
The current world population is about 7.7 billion. If you multiply the two, you'd get 770 - 924 gigawatts (although this is an over-estimate, since much of the population is made up of children). The total solar energy received by the Earth is around 173,000,000 gigawatts: http://news.mit.edu/2011/energy-scale-part3-1026 That would make the total body heat contribution from human bodies less than 0.00045% of the contribution from the Sun. So it's practically non-existent.

[chris]

An average adult male produces about 100-120 watts of body heat: http://large.stanford.edu/courses/2016/ph240/stevens1/
The current world population is about 7.7 billion. If you multiply the two, you'd get 770 - 924 gigawatts (although this is an over-estimate, since much of the population is made up of children). The total solar energy received by the Earth is around 173,000,000 gigawatts: http://news.mit.edu/2011/energy-scale-part3-1026 That would make the total body heat contribution from human bodies less than 0.00045% of the contribution from the Sun. So it's practically non-existent.

Excellent answer; thank you for injecting that clarity.

To add a further component to this, the rate of energy release from within the planet (radiogenic heating etc) is close to 50 terawatts; this alone is 50 times greater than the human contribution.

[evan_au]

the great majority, 92.1%, of stations in, for example, the USA are subject to UHI corruption.

That's why space-based instruments play an important role in measuring global average temperatures.
- It's also why Donald Trump doesn't want NASA or NOAA looking at the Earth from space - because he can't handle the truth.

See: https://en.wikipedia.org/wiki/Satellite_temperature_measurements