Radiated Energy from the Sun: Is It Diminishing?

[Jimmy B]

Is the radiated energy that reaches the Earth from the Sun diminishing?
Some meteorologists think it is due to the reduction of Sun spots. Is this the normal 11 year cycle, or is there more to it?

 

[Drakkith]

It's not due to the sun spots directly, but to the underlying reduction in magnetic activity.
Still, the reduction is only about half a watt per square meter less than average. See below.

 

[Jimmy B]

Could this reduction in the Suns energy have any effect on the Earths climate?
On the TV the other day, a weather man for the BBC said that we might be heading for another medieval ice age.

 

[Drakkith]

I'm sure it has a small effect on Earth's climate, but unless it stays below average for a prolonged period of time we will be fine.

 

[Vanadium 50]

The absolute magnitude of that scale is small. It would correspond to a direct change in the Earth's temperature of 0.2C. Furthermore, an 11 year cycle is too fast and the Earth has too much thermal inertia for this to be a big effect.

It is true that the Little Ice Age coincided with the solar Maunder Minimum. That was a ~500 year period, so the 11 year argument doesn't apply. However, it's not known if this is cause and effect: it's not known how global the Little Ice Age was, there are other candidate explanations, and even if this was due to solar activity, it might have been indirect: temperature changes due to cloud cover changes rather than direct solar forcing.

 

[russ_watters]

I've also heard it said that we are overdue for a real ice age, but the same logic applies: a few years near the solar minimum is far too short to trigger one.

 

[D H]

Could this reduction in the Suns energy have any effect on the Earths climate?
On the TV the other day, a weather man for the BBC said that we might be heading for another medieval ice age.

The Sun undergoes a solar cycle that is about 11 years long. It also apparently undergoes longer oscillations where the sunspot is drastically reduced or even nonexistent. The Sun appears to be heading toward a period where sunspot count is at least somewhat reduced. The last solar minimum was rather long, and the current solar maximum is rather weak.

Nobody knows whether the Sun is heading for a Maunder Minimum type of event, or even a lesser Dalton Minimum type of event. While something does appear to have changed in the Sun, saying that this means we're entering a grand minimum is bad science. Solar scientists don't have enough of an understanding of the Sun to be able to say that.

Even if the Sun is heading for grand minimum, the connection with climate is highly suspect. The "little ice age" was a span of about 500 years that started in the mid 13th century and ended in 19th where Europe had colder than normal winters. The Maunder Minimum was a span of 50 years in the latter half of this interval where sunspots were pretty much nonexistent. The Maunder Minimum could not have caused the little ice age for the simple reason that the little ice age started 300 years before the Maunder Minimum.

 

[Jimmy B]

How low would the Suns intensity have to go before it started global cooling?
Using Boltsman's radiation law, and putting in 5855 Kelvin for the Suns temperature, I get an average intensity at
the top of the Earths atmosphere of 1369.39 Watts/m^2.
This is obviously incorrect according to the graph, can someone please tell me why?

 

[Drakkith]

How low would the Suns intensity have to go before it started global cooling?
Using Boltsman's radiation law, and putting in 5855 Kelvin for the Suns temperature, I get an average intensity at
the top of the Earths atmosphere of 1369.39 Watts/m^2.
This is obviously incorrect according to the graph, can someone please tell me why?

The Sun is not a perfect black body, so the emitted radiation spectrum will be slightly off. But you're only 2 watts/m² off, still pretty close.

 

[goldust]

Still, the reduction is only about half a watt per square meter less than average.

I would wager that, even though the reduction is small, over time, the effect accumulates and the balance of Earth's climate system tips over. Prior the Maunder minimum, the Sun's output decreased by a tiny bit. Over many years, the reduction in temperature from Earth's input / output equilibrium accumulated to the point that a prolonged cold period resulted. It's not really about how small the effect is, it's really about how long an effect lasts. Just my 2 cents.

Suppose, the Sun's output is constant at a value over time, then the Earth's temperature stays constant after getting into equilibrium from heat coming to Earth and heating escaping Earth. Suppose now the Sun's output suddenly decreases by a tiny bit. Then, each year, more heat escapes Earth than received from the Sun as compared to previously. The effect from each year would accumulate and therefore eventually plunge Earth into an ice age, no matter how small the reduction in the Sun's output should it be a prolonged episode in reduction in output.