I've been wondering this for a while but I have not yet encountered an explanation.

This is from my understanding of physics, which is by no means expert, so sorry for my crude explanation:

Energy within earth can be considered a closed system; it transforms but cannot be created or destroyed -- and from what I understand, heat seems to be its most natural form, so it will always end up like that in some way.

Two things affect the total sum of energy on earth: radiation into space will drain energy (and is limited because of the presence of atmosphere). Radiation from the sun adds energy to the system.

Basically the sun is our only real source of energy (and we can consider it limitless, since when the sun is exhausted, we're over anyway).

The way I understand it, solar panels increase the efficiency of how we 'harvest' this solar energy, reflecting less of it back into space, and turning more of it into en energy (in this case, electrical). So we take more energy from the sun by putting solar panels in place. But the amount of energy that is removed from the system stays the same.

Hence the total sum of energy on earth increases (more) when we use solar panels. So how come we consider them to be a way to counter global warming, instead of a contributing factor?

edit: since all answers are about comparing solar cells to fossil fuels, let me clarify a bit more.

I understand that fossil fuels will contribute more to climate change than solar cells -- but I just wanted to clarify that is seems to me that both are a net negative (not if you replace one by the other). In other words, that the idea that solar cells are 100% clean (apart from production cost), is not really true, then. Wind of hydro however, would be, since they use energy that is already present in the earth system. (and of course what we really need to do is require less energy)

  • Comments are not for extended discussion; this conversation has been moved to chat. – David Z Oct 14 at 6:04

11 Answers 11

up vote 19 down vote accepted

Simple back-of-the-envelope calculation:

This figure shows our best estimates of the radiative forcing from different anthropogenic (human-caused) phenomena:

Radiative forcing figure

It shows that the radiative forcing from anthropogenic $\mathrm{CO_2}$ is the largest component at approximately $1.5 \,\mathrm{W/m^2}$.

The total world electricity generation is approximately $25000$ TWh/year. This is approximately $2.9 \,\mathrm{TW} = 2.9\cdot 10^{12} \,\mathrm{W}$ . The toal surface area of Earth is approximately $510 \,\mathrm{million \,km^2}$, which is $510 \,\mathrm{trillion \,m^2} = 510\cdot 10^{12} \,\mathrm{m^2}$. So if the ratio between heat produced and electricity generated was $1$ for all electricity production, the radiative forcing from this would be:

$$ \frac{2.9\cdot 10^{12} \,\mathrm{W}}{510\cdot 10^{12} \,\mathrm{m^2}} = \frac{2.9}{510} \,\mathrm{W/m^2} \approx 0.0057 \,\mathrm{W/m^2} $$

Now, efficiencies for commercial photovoltaic panels currently does not go much higher than $20\%$, so if we want to do a worst case calculation for a scenario with only solar panels at current total electricity generation, we might want to use a heat-electricity ratio of $5$. This gives $\sim 0.03 \,\mathrm{W/m^2}$. This is still a factor of $50$ smaller than the radiative forcing from anthropogenic $\mathrm{CO_2}$ at current levels.

Conclusion:

This was just a simple calculation to show that heat from electricity generation is a negligible contribution to global climate change, even if we change all electricity generation to photovoltaics. And this calculation doesn't even bother to take into account that photovoltaics don't absorb all light, that the surface they are placed on also absorbed some light, or that many alternative sources for electricity (most notably fossil fuel combustion and nuclear power) also generate heat. If we did that, the net radiative forcing from switching all electricity generation to photovoltaics would be even more negligible.

  • If we compare 20% efficient PV cells against a baseline of 100% waste heat, we should multiply that figure by 0.8, not by 5. – gerrit Oct 9 at 16:38
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    @gerrit If the solar panels have 20% efficiency, that means the incident energy on them must be 5 times the electricity produced. If we assume that all that energy is absorbed and converted into heat eventually (even the electricity), we should multiply the electricity generated by 5 to get the amount of heat generated. – jkej Oct 10 at 9:34
  • But you're comparing against 100% waste heat, which would imply 0% efficiency. If all electricity generation produced 100% waste heat, we would not produce any electricity at all. The heat produced by 20% efficient electricity production is 0.8 times the heat produced by 0% efficient electricity production, for the same production capacity. – gerrit Oct 10 at 11:23
  • @jkej thanks for an excellent answer -- I'm a bit wary towards calling anything 'negligible', considering that's what got us in this situation in the first place :) But I guess having a zero-net effect is a pipe dream ... – Joeri Hendrickx Oct 10 at 12:22
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    @gerrit Maybe the term "waste heat" is incorrect here. Maybe I should have just written heat. When I wrote "100% waste heat", I just meant that the ratio between heat generated and electricity generated was 1. This ratio is of course larger than 1 for solar panels, as well as for fossil fuel combustion and nuclear power. In fact it is most often basically the inverse of the efficiency. I see now that I perhaps used a confusing terminology, but I still think this is the right way to do the calculations. By multiplying with 0.8, you would be saying that less heat than electricity is generated. – jkej Oct 10 at 13:25

The purpose of solar cells is to generate electricity. This can replace the electricity generated by burning fossil fuels for electricity. The fact that it's becoming practical to run vehicles on electricity means we can also replace the fossil fuels burned to power vehicles, which makes things even better.

But let's concentrated on generating electricity. Your analysis ignores two things, one minor and one absolutely crucial:

  1. The minor point: Generating electricity by burning fossil fuels also adds heat to the planet. For example, only about 1/3 of the energy liberated by burning coal in a coal power plant is turned into electricity; the rest is waste heat.
  2. The major point: Fossil-fuel power plants continually produce CO2.

This post on RealClimate does an excellent job of going through the details. To take an unrealistic extreme case, they assume that solar cells are perfectly black (albedo = 0), and they ignore the fact that real solar cells are sometimes installed on already dark surfaces (such as roofs). In order to generate the current world electricity supply of 2 trillion watts, perfectly black solar cells would add about 6.7 trillion watts due to waste heat. As they point out, the efficiency of fossil fuel plants means 2 trillion watts of electrical power would be accompanied by about 6 trillion watts of waste heat.

So if you replace fossil fuel power plants with solar-cell power plants, you don't really change the waste heat production.

But you do change the CO2 production, and that's crucial, because the heat added to the atmosphere by adding CO2 is orders of magnitude larger than the waste heat from the power-generation process itself. (This is a continuing process: every second you run the fossil-fuel power plants, you add more CO2 to the atmosphere.)

... by the time a hundred years have passed, the heat trapped each year from the CO2 emitted by using coal instead of solar energy to produce electricity is 125 times the effect of the fossil fuel waste heat. And remember that the incremental waste heat from switching to solar cells is even smaller than the fossil fuel waste heat. What’s more, because each passing year sees more CO2 accumulate in the atmosphere, the heat trapping by CO2 continues to go up, while the effect of the waste heat from the fossil fuels or solar cells needed to produce a given amount of electricity stays fixed.

(You can, if you like, argue that getting rid of electricity generation entirely -- closing all power plants, solar or fossil-fuel-powered -- would be marginally better than converting electricity generation to solar. But that's a very small difference, and not really an option if you want to continue to have some kind of human civilization on the planet.)

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    To be fair, we've had some kind of human civilization on the planet for considerably longer than we've had electricity generation. – OrangeDog Oct 9 at 14:17
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    OK, yes -- "modern" human civilization, then. – Peter Erwin Oct 9 at 14:20
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    practical to run vehicles on electricity — as long as they're on land. We're quite far from practical application of electrical ocean freighters (unless nuclear, but that has other problems) or electrical aeroplanes. – gerrit Oct 9 at 16:34
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    @Anoplexian Really it's more about "we generate electricity using the currently-incoming heat instead of releasing both heat and CO2 that were captured long ago in fossil fuels." – SirTechSpec Oct 9 at 22:16
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    Something to make extra clear is that global warming isn't caused by internal combustion engines and power plants directly heating the air. That's basically none of it. Global warming comes from the sun warming the Earth, and it's the greenhouse effect from CO2 that just causes us to hold on to more of the Sun's energy. – Vectorjohn Oct 10 at 22:16

Solar panels are good for global warming because they cause less (much less) surface heating than harvesting equivalent energy due to burning fossil fuels does.

An oversimplified model

To see why this is consider a (very much oversimplified!) model of what the temperature of the surface should be. In this model we'll just consider incoming sunlight and I will assume that all of this makes it to the surface (this is wrong, but good enough).

Two things happen at the surface:

  • some proportion of the sunlight is reflected and (by the same assumption as above) goes straight back out to space;
  • some proportion is absorbed, and causes the surface to get hot.

Because the surface is hot, it now radiates as a (or as an approximate) black body, and most of this radiation is in the infrared (if it's not, then you probably are not interested in living on this planet, as its surface is visibly glowing).

Unfortunately the atmosphere is not transparent to infrared, so some of the outgoing radiation from the surface gets absorbed in the atmosphere and then reradiated, and some of this reradiation comes back down to the surface. This process is fairly complicated because you need to know what wavelengths the atmosphere is not transparent to, and then solve a bunch of hairy radiative-transfer equations, not to mention dealing with clouds, convection, wind &c &c &c.

But there's a simple, and obvious, physicist's answer: the end result of this process around infrared (what a climate scientist would call 'longwave') in the atmosphere is that the surface ends up a little warmer than you would expect if there were no atmosphere.

(Note I have completely ignored the IR component of the incoming radiation from the Sun, which a proper model should not do.)

So, OK, now we'll consider two adjustments to this model: solar panels, and burning fossil fuels.

Solar panels

Solar panels capture some of the visible / UV light from the Sun, and turn it into infrared (via running machines &c and ultimately heating). It's not completely obvious whether solar panels lower or raise the proportion of incoming sunlight which is reflected directly (do they have higher or lower albedo than the surface they covered in other words) but lets assume they lower it, so their net result is to lower the albedo of the surface and to increase the amount of infrared being radiated. This then increases the surface temperature slightly.

Fossil fuels

These do nothing to the albedo, (actually, they do: they lower it due to soot, but they also dump soot into the atmosphere which makes it less transparent to visible light and this is all a complicated process which we will ignore but which matters a lot in fact). They create two things:

  • approximately the same amount of infrared as solar panels for the same amount of energy, which slightly heats the surface (this heating comes from energy previously trapped in the fossil fuels, and captured there long ago from the Sun);
  • $\mathrm{CO_2}$, in large quantities.

Unfortunately $\mathrm{CO_2}$ is one of the components of the atmosphere which absorbs and reradiates infrared, and so this emission of $\mathrm{CO_2}$ increases the surface temperature by the process roughly described above.

Which causes more surface heating?

To know which of these causes more heating you have to actually model the system in some reasonable detail (and it's just because these models end up as rather complicated that allows denialists a way in). But one way in is to compare the amount of energy coming from the Sun (and being radiated back) and the energy humans generate.

The solar constant, which is the flux of power from the Sun crossing the Earth's orbit is about $1360\,\mathrm{W/m^2}$, and this means that the amount of power the Sun delivers to the Earth at the top of the atmosphere is about $1.7\times 10^{17}\,\mathrm{W}$. Human power generation in 2013 was about $1.8\times 10^{13}\,\mathrm{W}$.

This means that the energy flux from the Sun is about $10^4$ times bigger than human power generation: even a relatively tiny change in how much of this contributes to surface heating will completely dwarf any heating due to human power generation. Another way of thinking about this is that all human power generation is about $0.04\,\mathrm{W/m^2}$. The imbalances in solar flux due to changes in greenhouse gasses are of the order of $1\,\mathrm{W/m^2}$: far more.

The simple-minded blackbody model

Another way of seeing this is to consider a mindless blackbody model: assume there is no atmosphere and that the Earth is a perfect blackbody being illuminated by the Sun: what would its temperature be. Well a little thought shows you that it would sit at a temperature of

$$T_S = \left(\frac{F}{4\sigma}\right)^\frac{1}{4}$$

Where $F$ is the incoming solar flux, and $\sigma$ is the Stefan-Boltzmann constant. And this turns out to be $278\,\mathrm{K}$, or about $5\,\mathrm{C}$. This is colder than it really is on average but it's a decent first estimate.

So now, let's do it for human power generation. The formula here is

$$T_H = \left(\frac{H}{4\pi R^2\sigma}\right)^\frac{1}{4}$$

Where $H$ is human power generation and $R$ is the radius of the Earth.

And this is about $28\,\mathrm{K}$. And remember energy flux goes as the fourth power of temperature ($\sigma T^4$): human power generation is not anywhere near warming the planet significantly. You can easily see this by considering the difference between a planet warmed entirely by the Sun and one where human power generation is added:

$$ \begin{align} \Delta T &= \left(\frac{F}{4\sigma} + \frac{H}{4\pi R^2\sigma}\right)^\frac{1}{4} - \left(\frac{F}{4\sigma}\right)^\frac{1}{4}\\ &\approx 0.007\,\mathrm{K} \end{align} $$

This is completely negligible.

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    One small quibble on an excellent answer. You say "Unfortunately the atmosphere is not transparent to infrared." Actually, it's very lucky the atmosphere is not transparent to infrared, because it it were, we would be living on a frozen, uninhabitable planet. – Peter Shor Oct 9 at 13:54
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    @PeterShor: good point. I meant 'unfortunately, since it makes the sums harder' really! – tfb Oct 9 at 16:27
  • I think the most important part of this is the sentence "energy flux from the Sun is about 10^4 times bigger than human power generation". The power we use doesn't actually make a squat of difference, compared to changing the balance of radiation input/output. +1 for that, and consider giving it more emphasis – craq Oct 10 at 21:27

solar panels increase the efficiency of how we 'harvest' this solar energy, reflecting less of it back into space, and turning more of it into en energy (in this case, electrical

I’m no expert, but I don’t think that’s true.

Sunlight falling on a solar panel produces some electricity and some heat, and some energy gets reflected back. Same for sunlight falling NOT on a solar panel, except for the electricity bit. I don’t think there’s any inherent reason why a solar panel would reflect less. The total heat+electricity produced by a panel might be the same or even less than the heat produced by dirt, rock, or asphalt. And obviously the product will be actually useful, unlike waste heat.

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    Well, solar panels are deliberately optimized to absorb as much sunlight as possible and reflect as little as possible (since any sunlight that gets reflected off of them certainly can't be converted into electricity). That's why they're usually black. This of course isn't a proof that they reflect less sunlight than dirt and rock, but it's a circumstantial argument. – tparker Oct 10 at 0:56
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    Something to consider is that actually letting electricity do work (e.g. in an electric motor) produces heat, so in some sense the electricity, wires, etc. could be considered to simply move the location at which heat is produced from the solar panel to somewhere else. Another thing to consider is that solar panels are typically covered by a sheet of glass or plastic, both of which are materials which can often directly reflect a significant amount of light back into space, as opposed to asphalt, which would absorb it and then reradiate it as mostly-infrared blackbody radiation. – Some Guy Oct 10 at 1:24
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    @SomeGuy: Letting electricity do work doesn't produce heat; it converts electricity to heat. That electricity would have been heat anyway if the sun had hit some asphalt rather than a solar panel. – cHao Oct 10 at 15:57
  • @cHao: Yes, that was the point I was trying to make. The solar cell produces less heat locally because it generates electricity instead, but the electricity reproduces that "saved" heat elsewhere when it eventually is used to do work. So the electric wires essentially just act to transport the heat from the location of the solar cell to somewhere else, via an intermediate transformation to, and then from, electricity. – Some Guy yesterday

The benefit of a solar panel power plant as a replacement for coal(or other fossil fuel) power plant is manifold:

1) less fossil fuel is burned, so it can be used differently, for example as a source of various organic chemicals;

2) much less pollution of the environment near the plant;

3) mere operation of a solar panel releases little to no greenhouse gases

...

That said, manufacturing, installing and maintaining solar panels does produce greenhouse gases and pollution. Solar panel has a lifetime, after which a new one has to be manufactured and the old one disposed off. The pollution may be much better than burning the fossils, but it depends on how the life cycle of the panels is managed.

(I understand it's still better than burning fossil fuels, but I still think it's a net negative)

I think this is correct. Most electrical energy from the power grid, when consumed, contributes to heating the planet (synthesis of energy rich chemicals may be an exception but I think the energy thus consumed is negligible when compared to the rest). When 1kWh of useful energy is generated:

  • by burning fossil fuels, much more kWhs of waste heat are released, and also greenhouse gases are released;

  • by harvesting energy from solar radiation, much less waste heat is released and almost no greenhouse gases are released. But, still some waste heat is released.

Energy within earth can be considered a closed system; it transforms but cannot be created or destroyed -- and from what I understand, heat seems to be its most natural form, so it will always end up like that in some way.

Earth is not a closed system. It does exchange energy with the surrounding space, via EM radiation and gravity (tides) and there is also interaction with cosmic particles (solar wind...).

Basically the sun is our only real source of energy (and we can consider it limitless, since when the sun is exhausted, we're over anyway).

There are also fossil fuels, which provide energy without help from the Sun. And there are also radioactive elements like uranium, which are a substantial source of available energy, again without any help from the Sun.

The way I understand it, solar panels increase the efficiency of how we 'harvest' this solar energy, reflecting less of it back into space, and turning more of it into en energy (in this case, electrical).

If you are comparing having a solar panel as opposed to having none, then yes, less is reflected and more is stored on the surface of the Earth.

But the amount of energy that is removed from the system stays the same.

Not sure what you mean here; the amount of energy radiated to space is decreased by introducing large areas of solar panels, because what would have been reflected towards space, will now be absorbed on the surface of the solar panels and partially transformed into electric energy.

Hence the total sum of energy on earth increases (more) when we use solar panels. So how come we consider them to be a way to counter global warming, instead of a contributing factor?

Because it is assumed that the energy they provide will cause people to decrease the burning of the fossil fuels. This will decrease production of greenhouse gases, but not their amount in the atmosphere. So the immediate benefit is in stopping the human-caused increase of amount of the greenhouse gases. In time, the amount of greenhouse gases may even decrease by natural processes (plant/algae growth) and the atmosphere may cool down.

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    Thanks -- what I meant with 'our only source of energy' is that fossil is, when you look back at it, also solar energy that has been absorbed by flora and fauna (indirectly) and then fossilized and stored for a very long time, right? And which would take an extremely long time to resupply, should we want it to. – Joeri Hendrickx Oct 9 at 13:55

This crude approximation is essentially correct:

Two things affect the total sum of energy on earth: radiation into space will drain energy (and is limited because of the presence of atmosphere). Radiation from the sun adds energy to the system.

TL;DR - solar panels don't contribute to global warming because they neither increase radiation from the sun nor decrease radiation into space.

I'll try to elaborate on why not while maintaining a similarly crude approximation.

Earth has energy coming in from the sun, and going out into space as it glows. Temperature is (roughly) energy per matter, so the average temperature of Earth is essentially determined by how much energy it has.

The temperature will be stable when the energy is coming in at the same rate it's going out. The temperature will be changing when the energy is coming in and going out at different rates.

The rate of energy going out (how much Earth glows) depends on its temperature and on what its outer layers are made of. In essence, different materials glow at different rates, so under the same incoming light the stable temperature will be different for different materials.

The "greenhouse effect" refers to increasing the stable temperature by changing the materials that the outer layers of are made of - by increasing the proportion of dimmer "greenhouse gases" in the atmosphere.

(It's worth noting that there can be a big gap between the current temperature and the stable temperature, and on the scale of human lifespans it can take a long time to reach the stable temperature.)

Putting solar panels at the bottom of the atmosphere doesn't significantly change the radiation rate. On average, solar panels don't absorb more light than rocks, or water, or leaves (which are basically biological solar panels), and even if they did it wouldn't make much difference - light reflected up from the ground doesn't go directly out into space, it gets scattered through the atmosphere first, so the composition of the atmosphere still determines the outgoing radiation rate.

What solar panels do is make the energy they collect available for our use. Sunlight that lands on almost anything else is only useful to us indirectly. Solar panels cannot change the average temperature of Earth, they only change where some of the incoming energy goes, directing it to electrical power systems.

I can see where you're coming from - and why answers comparing fossil fuels don't answer your question - but it is a misconception about energy.

I always find water analogies help give a more intuitive understanding, so let's imagine some system with a tank of water with a pipe letting the water flow down (this is our equivalent of the sun, the pipe being our energy getting to earth).

Now, at the bottom you can let this water splash down into a bowl or you could put a water wheel in the way. Now think about the size of the ripples you'd expect in the two cases. Without a water wheel the water has had nothing to impede it, so the ripples are large. With the water wheel you've slowed your water down (taking some energy to spin the wheel), and we get smaller ripples.

Our solar panels take the energy from the sun that would have been hitting the earth anyway so the same amount of energy is imparted either way.

A point to note:

Solar panels, being black, are much less reflective. The icecaps are good reflectors, but if we were to cover the icecaps in black solar panels then we're looking at much less of the sun's energy leaving the system. In that case solar panels would be increasing global warming.

I like your model and you phrased it the way I think, so maybe this will help:

In general your heat model is correct, but consider that the solar panels that have been on the planet for ages (plants) have actually removed heat by turning it into chemical energy instead of heat energy.

This chemical energy has been stored underground instead of becoming heat and has been subtracting from the atmospheric heat rather than adding to it. The chemical energy that isn't stored underground is used to power ALL animal life on the planet (Which eventually becomes heat or more buried chemical energy).

If you took all the electricity from many efficient solar cells and put the energy into a form of energy that wasn't heat you'd actually be subtracting net heat.

Since much of the energy reflected by a mirror is caught by the atmosphere and clouds it is kept as heat anyway, a solar panel could actually reduce heat since you could store the solar energy as chemical or potential energy instead of turning it to heat (I picture using all this solar energy to lift a mountain or something, not terribly practical but I'm pretty sure it would reduce heat until you dropped the mountain).

Of course this chemical/potential/??? energy will change to heat eventually anyway as is currently happening with fossil fuels.

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Your analysis would be correct to the extent that solar panels are more efficient energy absorbers than dirt, concrete, etc. and being considered isolated by themselves. However, when considered in the context of replacing generators that use fossil fuels (which is the intent of solar panel generators), then there would be a net negative contribution to the global warming. To make this clear, lets assume a solar panel generator contributes 1 unit to the global warming and a fossil fuel generator contributes 10 units, then, by replacing the fossil fuel generator with the solar panel generator, one would save 9 units (1 - 10 = - 9), However, if we do not replace the fossil fuel generator, then the solar panel generator would add to the global warming (1 + 10 = 11).

There are two parts to this:

  1. Replacing energy from other sources i.e. replacing fossil fuel sourced energy with solar energy
  2. Replacing some original surface with a solar array

When you deploy a solar array, you change the albedo at that location. Some of the incident energy is reflected back, but what is not will ultimately end up as heat. What isn't converted to heat in the array itself will eventually become heat somewhere. If the array was deployed on an arctic snow field, that will increase the net energy absorbed by the planet. If it was deployed on a dark rooftop, it might decrease the energy absorbed. Given the total surface of the planet exposed to sunlight, it would take a lot of solar arrays to amount to any practical difference. Also, the arctic is a lousy place to set up a solar array because the Sun never gets very high in the sky - low angle means more atmosphere and therefore less energy makes it to the surface; also, there are long periods of darkness. The best places to put solar arrays might have low albedo to begin with. Rooftops are great candidates because they can supply the buildings directly, reducing transmission losses.

When you deploy a solar array and put its electrical energy to use, you replace energy generated by other sources, such as fossil fuel. Less fossil fuel consumed means less greenhouse gas emissions which affects the burden of greenhouse gas in the atmosphere which affects the ability of the planet to radiate heat into space. It doesn't take much of difference in greenhouse gas levels to have a measurable effect on global temperatures.

A key point in all this is that it isn't the heat generated by burning fossil fuels that is the global warming problem, it's the effect all that greenhouse gas has on the planet's ability to cool itself. If we stopped all burning of fossil fuels tomorrow, it wouldn't have a measurable effect on the energy balance between what Earth receives from the Sun and what it radiates to space - there are many orders of magnitude difference between the energy arriving from the Sun and the energy we produce. If we suddenly stopped burning all that fossil fuel, what would happen is that atmospheric carbon dioxide levels would gradually decrease and the Earth would become a slightly better radiator of heat energy, so would cool down a bit.

To sum up, the benefit of solar energy as it relates to global warming is the displacement of fossil fuels and the associated reduction in greenhouse gas emissions.

  • And the third part of fossil energy required to make the solar collectors. – KalleMP Oct 15 at 9:01

To explain it crudely, the earth is warmer because the surface is warm, not because the air is warm. Solar panels don't contribute to global warming because, while they do absorb energy from light, they also reflect a lot of light back out into the atmosphere. That energy (presumably) eventually makes it back out into space.

On the other hand, burning fossil fuels produces by-products comprised of heavy elements that stay in the air and cover the planet like a massive blanket. Radiation from the sun is therefore trapped within the atmosphere by the greenhouse effect, continuing to heat the surface.

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