Solar News: Sunspots Return
The Sun (4): This is the fourth of a series on the Sun in the Earth-Sun climate system. The first three entries are linked at the end. This one starts with some news sent to me by Judith Lean. There’s a sunspot! There’s a news story at Spaceweather.com. And here is a picture from that website credited to the SOHO/MDI instrument. (I’ve also put a couple of news and information links at the end.)
Figure 1: This figure shows in the circle a sunspot. The minimum of sunspots at the end of the last solar cycle (#23) has extended for a long time. While long, the minimum is still not outside of the natural variability of the observed record (1 standard deviation). The polarity of this new sunspot shows that it is the beginning of Solar Cycle 24. Picture is credited to Michelson Doppler Imager on the Solar and Heliospheric Observatory.
As noted in the figure caption and the article by Irene Klotz, Sunspot Record Reveals Little to Space Weather Watchers from the American Geophysical Union’s Space Weather Website, the length of this sunspot minimum is still within the statistical norm of previous observations. Like the Earth, we have far more observations of the Sun than in the past. The climate problem brings extensive scrutiny to these observations, and hence increases our sensitivity to information being collected. This new sunspot is, seemingly, the start of the next solar cycle, # 24.
Let me summarize the points from the last three blogs. There is a measurable solar signal that we can measure in the Earth’s climate. There is about a 0.1 degree centigrade signal that can be measured over the, approximately, 11 year sunspot cycle. This variability is included in models, but models underestimate the variability. That is, the observed variability is greater than the modeled variability. This model shortcoming is an important aspect of modeling to address. Currently models do not seem able to communicate changes high in the atmosphere to the Earth’s surface. There needs to be some mechanism in the atmosphere that modulates or amplifies this signal. This is likely to be related to the state of the atmosphere, meaning persistent weather patterns. This potential relation to the atmosphere is postulated because if the solar cycle is investigated taking into account, for example, the phase of the Northern Atlantic Oscillation, the clarity of the solar signal is enhanced.
While the weakness of the modeled versus observed solar cycle is a challenge that climate modelers still need to address, in general, it is possible to evaluate what this model weakness might mean to observed and predicted climate change. For example, we have the observational information of the sun and the climate and the observed contribution of solar variability to the Earth’s warming is more than 10 times smaller than that due to greenhouse gases. And, this relative size of solar-related warming to the greenhouse gas warming is getting smaller in recent years – explicitly, the observed warming is dominated by the greenhouse gas effect, and this domination is getting larger.
In the last blog, The Sun (3) , I tried to set the foundation for thinking about the warming due to the greenhouse effect by following a ray of sunlight through the atmosphere. Ultimately, an enormous component of the radiative heating that keeps the Earth’s surface warm are due to the greenhouse gases in the atmosphere --- more that twice the heating than comes directly from the Sun. Therefore, it is relatively simple to both attribute and understand the warming at the Earth’s surface to the systematic increase of greenhouse gases.
The data show the greenhouse gas warming to be much larger than the warming associated with changes in the Sun. Since the models underestimate the solar signal, the likelihood of the model obscuring some fundamental, underlying mechanism that is fooling us about greenhouse gas warming is very low. There are many ways to add strength to this conclusion; I remind you of the series of blogs on attribution.
The plan is that the next blog will be the last in the series on the Sun-Earth climate connection. I want to point out an interesting and well written on line document The Sun and the Earth’s Climate by Joanna Haigh. The next blog will talk about solar variability in the projections of the future climate.
r
Postscript some news items:
Jeff Masters recently did an update of the 2008 sea ice. It was very low again. Here is a link.
Here is a website with some pictures important to global warming. It’s a wiki and has community participation. Global Warming Art. Here are thumbnails of all the figures in their archive.
Here is a website that documents the societal debate about global warming. Climate Debate Daily. Thanks to reader Aaron for pointing this one out.
Blogs on the Sun.
The Sun (1)
The Sun (2)
The Sun (3)
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Updated: 17:20 GMT le 05 octobre 2010
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On the Solar Trail
The Sun (3): This is the third of a series on the Sun in the Earth-Sun climate system. The first two entries are linked at the end. I am, really, heading towards the original question of the solar cycle and how solar variability is included in climate predictions. It is, however, a difficult subject and there is work to be done to get us there.
The first two blogs talked about the need for there to be mechanisms to “amplify” the change in heating associated with solar variability to understand the impact of solar variability on the Earth’s climate. One of the easiest ways to visualize how a small signal might be amplified is to think of ice. At the foundation of this reasoning is the assumption, based on observations, that the climate and weather are in some near “equilibrium” balance. This is a balance where energy coming in from the Sun is absorbed or reflected and what comes in, ultimately, goes out. In this ice example, if it got a little colder because of a decrease of solar energy, it could make more ice, which would reflect more solar energy and lead to more cooling. This interaction between cooling and ice leading to more cooling, a positive feedback, is easy to conceive.
The importance of greenhouse gases to the climate and the habitability of the Earth cannot be overstated. Without greenhouse gases the surface of the Earth would be about -18 degrees C (~zero degrees F). That is, without an atmosphere with greenhouse gases, the Sun’s radiation would just be re-emitted to space.
Careful consideration of Figure 1 demonstrates the importance of greenhouse gases. 161 watts/m^2 of solar energy is absorbed at the surface. In the absence of an atmosphere, we would expect 161 watts/m^2 to be emitted back to space. But let’s count how much energy is emitted from the surface. 17 watts/m^2 leave the surface due to thermals; this is essentially heating at the surface making hot air that rises. 80 watts/m^2 leaves the surface through evaporation and condensation of water. And 396 watts/m^2 are emitted from the Earth’s surface as infrared radiation. We have the situation where 161 watts/m^2 come to the surface and 396 + 80 + 17 = 493 watts/m^2 leave the Earth’s surface! How is this possible?
The answer is greenhouse gases. Let’s try to follow the energy. The Sun provides 161 watts/m^2, and after this visible radiation is absorbed at the surface, the surface has 161 watts/m^2 that it can give up. So this 161 watts/m^2 starts to return to space as infrared radiation. The greenhouse gases absorb and store much of this infrared radiation in the atmosphere and returns a large part of it to the surface. Ultimately, more energy, 333 watts/m^2, returns to the surface from this atmospheric store than is provided directly by the Sun. The greenhouse gases are like a blanket; they hold the heat close to the surface for a while. This exchange of heat between the atmosphere and the surface evolves to equilibrium. This 333 watts/m^2 is essentially the amount of energy that is cycled back and forth between the atmosphere and the surface. Subtract 333 from that 493 in the previous paragraph, you get 160, and that just about balances the 161 that comes from the Sun.
The point of this tour through the radiation balance is that greenhouse gases greatly alter the flow of energy from the Sun and its return to space. The temperature at the surface of the Earth is directly determined by both the Sun and the greenhouse gases. Given that, ultimately, there is more flow of energy to the Earth’s surface from the atmosphere than comes directly from the Sun, the greenhouse gases can be viewed in the spirit of an amplifier. With this idea, how the energy at the surface changes is expected to be as, or perhaps more, sensitive to greenhouse gas changes as to changes in the incoming solar energy … assuming, really, that both are incremental changes from the established equilibrium.
What are the greenhouse gases? Water vapor is the most important greenhouse gas. A lot of you ask, what is the contribution of water vapor to the greenhouse effect? It’s not a completely trivial question to answer because water changes phases and makes clouds, and clouds then contribute to the greenhouse effect even more powerfully. I think it is reasonable to say that water and clouds together account for about two thirds, say 65%, of the greenhouse warming of the surface. (I link a dense and classic paper by Ramanathan and Coakley on the subject.) Carbon dioxide is a little less than 10%.
I want to return to the figure again. On the figure there is a 40 watts/m^2 that is returned from the surface of the Earth directly to space. If you consider only water vapor, there are some wavelengths of radiation that are, essentially, completely absorbed. But there are windows where radiation goes directly to space. The greenhouse gases like carbon dioxide and methane have a big impact partly because they start to fill up those windows. And as we all know having a crack in a window can have a big impact on the temperature in the bedroom. Little things can have big effects.
r
Blogs on the Sun.
The Sun (1)
The Sun (2)
Blogs on radiative balance
Absorbing
Reflections
Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm
Figure 1: This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.
Pumping Up the Sun
The Sun (2): This is the second of a series on the Sun in the Earth-Sun climate system. The first entry, which focused on some of the basics, is linked here and at the end.
A couple of years ago I was on a review panel for NASA’s Living with a Star Program. What struck me about the proposals was what I will call the “challenges” of understanding the solar variability on the Earth’s climate. There are a number of well known ways the Sun varies: solar flares, a 27 day oscillation associated with the rotation of the Sun, and the most well known solar sunspot cycle. There are a number of ways that the Sun impacts the Earth. Some of these are through changes in the energy that is supplied to the Earth. Other changes are associated with the chemical composition of the atmosphere, for example, ozone production and destruction. A basic fact is that higher in the atmosphere the more direct is the impact of solar variability. This is a good and essential characteristic for surface dwellers; the atmosphere protects the surface from all sorts of unpleasant types of radiative energy.
In the lower atmosphere the density, the mass per unit volume, of the air is much higher than in the upper atmosphere. Because of this high density it takes more force to move air in the lower atmosphere than in the upper atmosphere. Therefore, there is not a direct way for a “storm” in the upper atmosphere to propagate down to the surface. This challenge of identifying mechanisms that can connect the upper and lower atmosphere is formidable.
One of the papers that I use in my climate change class is by Judith Lean, and entitled Living with a Variable Sun, which appeared in Physics Today in 2005. (Here’s a first on this blog. A link to a talk by Judith on YouTube.) This is a concise and clear discussion of the Sun-Earth system; it’s a good paper.
Like the Earth’s atmosphere the Sun is a dynamic place, with all sorts of fluid motions and electrical discharges. Some of this variability is made visible by sunspots on the surface of the Sun. Sunspots have, in fact, been observed for 2000 years (See Yau and Zhentao). (This has not been a systematic and quantitative record of observations.) At the time of the maximum number of sunspots there is an increase of solar energy of about 0.1%. This change is the result of two compensating solar processes. There is a reduction of solar energy of about 1 watt/meter^2 because of the darkening of the surface. There is an increase of solar energy of about 2 watts/meter^2 because of an increase of faculae above the surface of the Sun. (Lean’s paper, Figure 3, is a nice summary of it all.)
Looking a both modern and past records, there are a set of climate signals on Earth that are definitively correlated with solar variability. These include signals of surface temperature and precipitation. One of the systematic errors of climate models is that the signals that are predicted at the Earth’s surface in the climate models are smaller than the signals that are observed in the observations. The observations are that the temperature at the Earth’s surface increase about 0.1 degree Celsius with a 1 watt/meter^2 of the solar energy. This is small. Still, the fact that the climate models underestimate this change, often stated to be by a factor of 5, is an uncertainty that demands attention.
It was stated above that it has been difficult to define heuristic mechanisms that link solar variability in the upper atmosphere to signals at the surface. The bias between observations and models suggest the need for a some mechanism that amplifies the solar signal. On long time scales changes in greenhouse gases and ice are likely sources of amplification. On shorter times scales, the search for mechanisms is generally associated with modes of atmospheric variability like the Northern Atlantic Oscillation. So the question is posed does the state of the atmosphere impact the size of the response of variability associated with the Sun?
r
Blogs on the Sun.
The Sun (1)
Blogs on radiative balance
Absorbing
Reflections
Ice Water
Clouds Cool and Warm
Aerosols Cool and Warm
Here is an update of one of the iconic figures of climate change.
This figure shows how solar (visible) and terrestrial (infrared) radiation flows through the atmosphere. This is an updated figure provided by Kevin Trenberth and will appear in the Bulletin of the American Meteorological Society in the article “Earth’s global energy budget,” by Kevin E. Trenberth, John T. Fasullo and Jeffrey Kiehl.
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Updated: 11:27 GMT le 08 septembre 2008
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