Tuesday, May 27, 2014

Climate Change Part 9: Slow Carbon Fast Carbon

Note: This program first aired May 24, 2014.

It should be clear by now, if you’ve been following the show for the past couple of months, that the current global climate change is all about the carbon. Its happened before, in the PETM, and it is happening now. Green house gases are accumulating in the atmosphere, preventing the escape of more and more infrared radiation. This is causing the energy in the climate system to increase, resulting in rising average temperatures and a pattern of climate instability in this period of rapid transition.

We know on a simple level, where the carbon is coming from—out our tail piles and smoke stacks. It comes from the burning of fossil fuels. What I want to look at today is how the carbon got into the fossil fuels in the first place. Were do fossil fuels come from?

For practical purposes, the amount of carbon in our world is fixed. Yes, elements can change form in nuclear reactors, like power plants, and the sun, but for our purposes here on Earth there is a finite amount of carbon, distributed among various reservoirs, or parts of the system where it tends to accumulate. Amazingly over 99% of the carbon on Earth is held in the rock reservoir in the form of limestone and other carbon containing rocks, marine sediments, and fossil fuels. That means that all the changes we are experiencing and that are predicted to occur in the near future are the result in minute changes in surface carbon reservoirs, minute relative to the total amount of carbon on Earth. Surface reservoirs include the atmosphere, the oceans, and all living material-the biosphere. Carbon moves between various reservoirs as a result of various biogeochemical processes; up here on the surface carbon gets taken out of the atmosphere and goes into the living biomass or gets absorbed into the ocean. Carbon can be released from the ocean as well, and when living material biodegrades, it releases carbon back into the atmosphere. These various mechanisms are known as the fast or short term carbon cycle. The cycling of carbon in and out of the rock reservoirs by contrast, happens very slowly, at the bottom of the ocean and any where rock is exposed to the elements, thus, getting carbon into or out of rocks is part of the slow or long term carbon cycle. To summarize, small amounts of carbon cycle rapidly through reservoirs in the biosphere on the surface of the Earth, and larger amounts of carbon move into and out of the rock reservoir very slowly, and in general, the amount that goes in is in balance with the amount that goes out. When the amount of carbon in the system is steady, generally the climate is steady, all other things being equal.

What this means is that the carbon that is building up in the atmosphere right now is carbon that is coming out of the rock reservoir. This is carbon that generally moves very slowly—when that carbon came out of the atmosphere and was sequestered in the rock reservoir during the Carboniferous period about 325 million years ago, it took millions and millions of years to do so. What we are doing is unsequestering that carbon very rapidly, we are moving carbon out of the rock reservoir much faster than normally happens, and much faster than natural processes can move carbon back into the rock reservoir-we’re withdrawing much faster than nature can deposit. That’s where, on a large scale level, the carbon is coming from. It literally is extra carbon in the system, carbon that hasn’t been in the fast, surface carbon cycle for over 300 million years. That’s why we call them fossil fuels.

Is it possible that carbon could exit the rock reservoir this fast naturally? Possibly there is some period in the geologic record where there was extreme weathering or exposed coal burning or some other global event that initiated a sharp decrease in the carbon in the rock reservoir. But whether the carbon imbalance results from some natural event (as may have happened in the geologic past) or from our own insatiable use of this carbon from the rock reservoir, the impact is the same—rapidly changing conditions here on the only place we have to live, the only place we’ve ever had to live. Buckle your seat belts folks. Its going to be a rough ride.

You can find a transcript of this program, as well as contact information and references on our website, head to www.weru.org and look for the Show Notes link. Our music is from Stanley Watson’s Portrait of Don Potter, performed by MDI guitarist Kevin Morse. Thanks for listening, and as always, join us next week for another look at the world around us.


This site from the University of California Berkeley’s Museum of Paleontology is a wealth of information, and resources for teachers:

A good, vetted source for information on climate change, from the American Museum of Natural History: http://www.amnh.org/exhibitions/past-exhibitions/climate-change

Climate Change: The Science of Global Warming and Our Energy Future Dr. Edmond Mathez, Columbia University Press, 2009

The slow carbon cycle, from NASA’s Earth Observatory: http://earthobservatory.nasa.gov/Features/CarbonCycle/page2.php

Saturday, May 17, 2014

A Lovely Book: The Urban Bestiary

  Note: This program first aired on April 26, 2014. 

As a writer it’s a rare and unexpected delight to encounter a book you identify so completely with or admire so much that you wish you had written it. The wickedly funny How Animals Have Sex is one such book for me. Carl Safina’s  gorgeous The View from Lazy Point is another. Annie Dillard’s A Pilgrim at Tinker Creek is so good I had to stop reading it, if you can believe that. I feared I would devour it too quickly, and in my greed forget to savor each perfect line and verse. As a writer at the beginning of her career, these books fill me with giddy inspiration, and anticipation for what I may someday be able to produce, if I am lucky, if I am tenacious, if I work as hard as I can.

One of the benefits, and perhaps curses of small town living is that everyone thinks they know everyone else’s business, so when my local librarian mentioned that she had heard about a book that she thought I would like, and had ordered it for me I was a tad taken aback. I am used to making my own decisions about what I read, and value immensely the pleasure of the discovery of a book that piques my interest, so I was more than a little skeptical about my librarian’s presumption. But I figured, so what, its just a book. I don’t have to read it.

It turns out that in my heart, I owe that librarian an apology and a huge debt of gratitude. The book she took it upon herself to request for me, The Urban Bestiary by Lyanda Lynn Haupt is a gift, and is getting placed on that sparsely populated top shelf among the books I desperately wish I had written myself. Her premise is simple; she takes the medieval concept of the bestiary, a written compendium of animal lore, and applies it to modern urban living. Each chapter chronicles an organism increasingly encountered in urban or near urban settings, habitats humans have appropriated for themselves. The mammals and birds she describes, are, with an exception or two, common everywhere in North America; most of us encounter squirrels and chickadees on a daily basis. While this may sound like a mundane book about the animals one might encounter on a trip to the city park, it is so much more than that. Personal accounts of animals (and even a tree) are interwoven with natural history, human and wild life interactions and ecological philosophy in an impeccable balance that never strays into the esoteric. Haupt’s pitch is perfect when it comes to blending the details of the specific life histories of this species or that with the philosophical context for why we feel the way we do about these animals. In the introduction Haupt confesses that she made an effort to literally write most of the book outside, as readers we reap the rewards of this endeavor. This is a thoughtful, mindful book that has its feet planted firmly on the ground.

I have made it my life’s work to pursue that which makes me feel more grounded, more attuned, more connected to the natural world. This book made me feel all of that, but even more, The Urban Bestiary made me feel connected to another human being, some one who feels the same way I do, who thinks the same thoughts and holds the same values. Some one who has read all the books I want to read, and has devoted deep and careful time to all the ideas I’ve tried to think, someone living perhaps a parallel universe version of my life. What I found in this book was not just beautiful words about our relationship with nature, but in fact, an easing of my loneliness. Somewhere out there, there’s some one just like me, and I found her, she wrote this book.

So read this book, read it sitting outside in the spring sunlight, read it by the window where you watch your bird feeder, read it lying beneath your favorite tree. Read it and prepare to be cracked open just a little bit, I can assure you it feels good, it feels really good. Read it and you will remember all the things you used to know without knowing about how your body is made of the same earth as trees and chickadees and squirrels and stars. I guarantee you will feel less alone as a result.


Lyanda Haupt’s official website: http://www.lyandalynnhaupt.com/

Lyanda Haupt’s blog about an “urban earthen household” http://thetanglednest.com/

Climate Change Part 8: Paleocene Eocene Thermal Maximum (PETM)

Note: This program first aired on May 10, 2014.

I promised last week that we would look at a time in Earth’s climate history when climate changed significantly due to the same mechanism as we are experiencing now, that being, increased atmospheric carbon dioxide. Recall that carbon dioxide is a green house gas, and green house gasses absorb outgoing infrared radiation, which is the form of energy by which the Earth loses heat. By absorbing the radiation, these gasses reradiate that energy back out, some of it back to Earth. The higher the concentration of green house gasses, the more energy is absorbed in the atmosphere and radiated back to Earth, rather than simply being lost to space. That is what is happening right now, and like I said, it has happened before too.

The last time that green house gasses were the primary driver for large scale climate change on Earth was about 56 million years ago. The warming occurred at the end of a geologic epoch called the Paleocene, and in fact caused a significant enough change in the geologic and fossil records that Earth scientists recognize the beginning of a new epoch during this time, the Eocene. Epochs are simply units of geochronological time, and the boundaries between them are marked by significant changes in layers of rock, because these differences in the rocks indicate that conditions on Earth were changing too. This warming event was significant enough to change conditions on Earth, and marks the boundary between the Paleocene and the Eocene, which has earned it the name the Paleocene Eocene Thermal Maximum, or PETM for short.

It isn’t 100% clear what initiated the PETM, but the leading theory is that there was major volcanism associated with the tail end of the break up of Pangea, and that this released enough carbon dioxide to raise the global average temperature 1 to 2 degrees centigrade. The Paleocene was already a much warmer place than Earth is today; for example there were no glaciers on Antarctica, though it was cold, and there was permafrost there. Based on evidence from the stratigraphic record, scientists believe that this small initial warming kicked off several positive feedback loops which then dramatically raised atmospheric carbon dioxide and global temperatures. The number one positive feedback that is hypothesized to have had the largest impact was the melting or vaporization of methane hydrates from the continental shelves.

And what are methane hydrates you might ask? Methane is the shortest of all the hydrocarbons, it is a single carbon atom bonded to four hydrogen atoms. It forms from biological activity, the break down of organic material by bacteria, but it can also be formed abiotically, which is why we can find it in outer space. At the right temperatures and pressures, methane molecules can get trapped inside a lattice work of water molecules. This usually happens in underwater sediment and in deep permafrost. The temperature has to be cold and the pressure has to be high. The thought is that in the Paleocene, Earth was already much warmer than it is now, so perhaps the conditions required for methane hydrates were already near their limit. The initial relatively mild warming then pushed the climate system rapidly over the edge and warmed the water enough to vaporize the methane hydrates. That is the most widely accepted hypothesis, though the exact mechanisms are definitely still debated by climate scientists. What isn’t debatable is the fact that methane is a green house gas. It has some different properties than carbon dioxide to be sure, mostly having to do with its reactivity, and it infact eventually degrades into carbon dioxide, but a steady input of methane into the atmosphere will have a warming effect on the climate which is exactly what mostl likely happened in the PETM.

The thing about the PETM that is raising so many alarms is, that while it is our best and most recent analog for the current greenhouse gas driven warming, there are some important differences as well, and they have to do with timing. During the PETM the rate of increase of green house gasses (mostly methane) into the atmosphere is estimated to be on average about 5 billion tons per year. That translates into a warming trend of 0.025 degrees centigrade per 100 years. It warms, but slowly. You get big warming events if this slow warming goes on consistently (which it did in the PETM, for nearly 200,000 years). Conditions on Earth change, but again, slowly. They changed enough to show up in the rock record, and they also induced migration of various early mammals, driving the evolution of most of the mammal orders we see on Earth today. Contrast that with today’s scenario. Our average green house gas input is something more like 30 to 35 billion tons per year, remember, in the PETM it was 5 billion. That input translates into a 1 to perhaps as much as 4 degree centigrade increase in temperature in 100 years. That is what keeps climate scientists and conservation biologists up at night, the potential speed of the current warming is exactly the kind of abrupt climate change event that triggers mass extinctions, the Earth’s reset button. There is a great deal of uncertainty about what the future will hold, but most of that uncertainty has to do with which feedback loops will get triggered when, like the warming feedback that melted the methane hydrates in the PETM, turning a mild warming event to a climatic anomaly that stamped its signature on the geologic record and left a biological legacy that shaped the world we enjoy today.


Weather Underground, of all places, is a great climate change resource: http://www.wunderground.com/climate/PETM.asp?MR=1

Curious list of articles about the PETM, compiled after a scientific paper was published in 2009; the paper got a lot of press, most of it off the mark. http://tugpullpushstop.blogspot.com/2009/07/resources-on-palaeocene-eocene-thermal.html

Climate scientist Gavin Schmitt on the PETM: http://www.realclimate.org/index.php/archives/2009/08/petm-weirdness/

US Geological Survey on Methane Hydrates: http://woodshole.er.usgs.gov/project-pages/hydrates/primer.html

A scientific journal article about the PETM and attempts to model it: http://rsta.royalsocietypublishing.org/content/368/1919/2395.long

Climate Change Part 7: Green House Gases 2

Note: This program first aired May 5, 2014.

When last we talked about climate change, we left off with the idea that more energy comes into Earth’s climate system than leaves. Heat is essentially building up in the atmosphere, less heat is escaping back into space than is coming in in the form of sunlight. The reason for this is a change in the amount of greenhouse gasses in the atmosphere.

When we look up into the sky we see what we think of as nothing. But far from it, the sky is full of matter, billions upon billions upon billions of atoms are floating around up there. And these atoms, just like many others, absorb infrared radiation, which is simultaneously being released by every atom everywhere, every atom that isn’t absolute zero. The Earth is radiating infrared radiation, as a result of having been hit with light from the sun. That light (a narrow relatively high energy band of the electromagnetic spectrum) gets converted into a longer wavelength, lower energy form, infrared radiation, and infrared radiation is the form in which the Earth loses the energy that it originally gained as light from the sun. If there were no atmosphere, if when we looked into the sky what we really saw was nothing, the Earth would be much colder than it is now because all the infrared radiation it is emitting would easily escape into space. Much like a human being, running around in the winter naked. Heat readily leaves the warm human body, and with nothing there to stop it, the human rapidly cools. That naked human can slow the escape of heat from her or his body by putting on clothing, creating a warmer microclimate around the body, thus reducing their convective, evaporative, and radiative losses.

The Earth loses energy by all of those means as well. Sunlight comes in and warms the air. Warm air rises, transporting that heat higher into the atmosphere, moving it closer to space. Likewise, water absorbs that light energy and warms up, evaporating more readily. When water changes phase from liquid to gas, it takes a great deal of energy with it. When that water vapor makes its way up into the atmosphere and condenses into water droplets to form clouds, it releases that heat, again, high in the atmosphere, where it is more easily lost to space. The current imbalance in Earth’s energy budget is due to the radiative losses (or lack there of), the fraction of the sunlight that makes it to the surface of the Earth that doesn’t just go into heating air or evaporating water. That light is simply absorbed by matter here and reradiated back out as infrared radiation.

That infrared radiation doesn’t all make it back out into space right away, because of green house gasses. The green house gasses are water vapor, carbon dioxide, ozone, methane, nitrous oxides, and chloroflourocarbons. All of them except the CFC’s are naturally occurring gasses, and without them, the Earth would not be livable, so green house gasses aren’t a bad thing. Each of them absorbs specific wavelengths of infrared radiation, with water vapor absorbing the widest range of wavelengths (that is why it is the most important green house gas!). There is a gap in the absorption spectrum of water vapor though, from about 8 to 14 microns, a micron is a micro meter, a millionth of a meter. That gap is essentially a hole that infrared radiation can escape through, because any radiation with wavelengths between 8 to 14 microns won’t be absorbed! Most of the green house gasses overlap their absorption spectrums with water vapor, meaning they absorb the same kinds of wavelengths that water vapor does. But carbon dioxide is different. It absorbs wavelengths of around 12 and 13 microns, meaning, it partially plugs that 8 to 14 micron hole in the water vapor absorption spectrum. Because the hole that infrared radiation can escape through is getting smaller due to being partially plugged by carbon dioxide, less heat can escape the atmosphere. When less heat escapes, more heat stays around to warm things up here on Earth.

Remember, there has always been a lag time between heat getting radiated from Earth and heat getting lost to the atmosphere. That is what makes Earth’s average temperature greater than zero, and that is a good thing, at least for us. When climate is stable, that “lag time” is consistent, which means the back log of heat leaving through the atmosphere is consistent. What is happening now is, due to the extra carbon dioxide  in the atmosphere, the lag time is increasing, and the back log of heat is getting bigger, so Earth’s average temperature is warming. And it is just due to the slight narrowing of this atmospheric window. That is why we pay so much attention to carbon dioxide as a green house gas, it is the atmospheric factor that is effecting the biggest change in the heat balance of the Earth. Next time we will look at an example from Earth’s history, of a period that experienced the same kind of rapid warming we are beginning to, as it may give us a sense of what we can expect.


Scroll down this page from Texas A&M University to see a graph of the absorption spectra of several green house gasses: http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm

Climate Change Part 6: Green House Gases

Note: This program first aired on April 12, 2014.

So far we’ve talked about how the amount of energy reaching the Earth can change, though differences in the amount of energy the Sun emits and variations in what the Earth reflects back into space as light. The last big player in climate change affects not how much energy reaches the Earth, but how much is kept here.

Recall that the energy that drives the climate system comes from the Sun arriving as light. Light is a high energy, short wavelength form of radiation, and the Sun emits light because it is so hot. About 30% of the light that hits Earth is reflected back into space, due to the reflectivity, or albedo, of certain Earth surfaces. That leaves approximately 70% to actually reach Earth and enter the climate system. Of that 70%, about 20% of the visible light is absorbed by gasses in the atmosphere. We’re more familiar with the phenomenon of atmospheric gases absorbing certain wave lengths of solar radiation when we talk about the stratospheric ozone layer. The ozone molecule, O3, is found at low but important concentrations 15 to 30 kilometers above the surface of the Earth. It plays an important role in the history of life on Earth, because it absorbs some of the non visible radiation that comes from the sun, ultraviolet light. UV light is particularly damaging to cells (skin cancer anyone?), and without the filtering effect of stratospheric ozone, it would have been hard for life to evolve, or the life that did evolve, would look pretty different.

It turns out that ozone absorbs visible light as well, as does water vapor in the atmosphere, and even carbon dioxide to a limited extent. Any non reflective particulates (dust, soot) can also absorb light. This is where that first 20% of energy entering the climate system goes—its absorbed by gases and particles in the atmosphere.

The last 50% of the light energy from the sun makes it to the surface of the Earth and is absorbed there by everything down here-rocks and soils, plants, water, and sunbathers. When that as light energy gets absorbed it gets transformed into infrared radiation, otherwise known to us a heat. The gases in the atmosphere, the rocks and soils, plants, water and sunbathers aren’t hot enough to emit the energy as visible light, so it down cycles into heat. That 70% of light energy from the sun that hits the Earth all gets turned into heat, which is the form in which it is reradiated back into space which is eventually where it will all end up.

When the gases in the atmosphere absorb light and radiate out infrared radiation or heat they do so in all directions. Each molecule sends out its little bit of transformed heat energy, and that energy can then hit another nearby molecule, heating it up, before that second molecule radiates out the heat, and so one. Because the gases in the atmosphere are up so high, quite literally closer to space, it is much easier for the heat they are emitting to make it back out into space. When we get high enough above Earth, the atmosphere is no longer the atmosphere, there are so few air molecules there we call it the exosphere. With so few molecules, when a molecule does radiate some heat, the chances of another molecule absorbing it are much lower, and the chances of it simply going into space are much higher. The net result of this is that a high percentage of the light energy that is absorbed by the gases in the atmosphere gets reradiated back into space, without ever helping us out down here on the surface.

Down here closer to Earth all that heat radiation is a great deal further from space. Again, radiation is happening in all directions, but some of that direction is right back to the surface of the Earth. So simply by distance it is harder for heat at the Earth’s surface to get out into space. The physical process of heat radiation transferring from atom to atom, and the randomness of the direction of travel all play into this. The net effect of this is that of the 50% of the light energy of the sun that reaches the surface of the Earth, a lower proportion is radiated back out into space (at least immediately). This time lag in the functional reemmission of infrared radiation from the surface of the Earth is the reason that we have a livable climate here, why the average temperature of the Earth is something higher than absolute zero. And it is due in part to this backscattering of infrared radiation I’ve just described. But it is mostly due to gases in the atmosphere, green house gases. The specific mechanics of how this process works will be our topic next week.

Climate Change Part 5: The Parts of the system 2

Note: This program first aired on April 5, 2014.

We talked last week about one of the ways that the amount of energy affecting the Earth’s energy balance can change, that being if the amount of sunlight reaching the Earth changes. Changes over time in solar output, as well as minute changes in the distance of the Earth from the sun can influence the energy balance, but only on very long time scales, or to very small degrees, or both.

Another way that Earth’s energy balance can be disrupted is through a change in albedo. Albedo is the reflectivity of a surface, the higher the albedo, the more light is reflected. Reflection is an important concept to understand when thinking about Earth’s energy balance. The sun’s energy comes to Earth as light, but is quickly transformed when that light is absorbed by the oceans, land and atmosphere and is reradiated as heat. Albedo refers to the portion of the sun’s light that is NOT absorbed, but instead simply reflected back into space as light. It hits us as light, and bounces back to the universe as light. Its what makes us visible as the “Pale Blue Dot” that Carl Sagan and the Voyager space craft made famous. Because the light isn’t absorbed and transformed, it does not play any role in Earth’s energy budget. Currently Earth’s albedo is around 0.3, meaning 30% of the light that hits Earth is immediately reflected back to space.

How does that number change? The easiest way is to change the amount of snow and ice on the surface of the Earth, as snow and ice are white, and thus have the highest albedo of all Earth surfaces. Desert sand and grasslands can also have relatively high albedo, but nothing approaching the bright white of freshly fallen snow. When less surface is covered with highly reflective material, less sunlight is reflected, which means that more sunlight gets absorbed by the Earth’s climate system.

Here’s the thing with the albedo of snow, though, and it allows us to introduce another important concept in climate change, the concept of feed back loops, and it goes something like this: the more snow you have, the more albedo you have, the more light you reflect, the less energy stays in the climate system, the cooler it gets, the more snow you get. Likewise, the less snow you have, the less albedo you have, the less light you reflect, the more heat stays in the climate system, the warmer it gets, the less snow you have. These are feedback loops, in which the results of an interaction then influence subsequent interactions. When the interactions result in ever increasing values (and in this case the value we are looking at is temperature), for example the less snow we have, the less we reflect and the warmer it gets as a result, further influencing the amount of snow and thus reflectivity that can remain,  we call that a positive feedback. When the interactions result in decreasing values, the more snow, the more reflection, the less energy in the system, the cooler it gets, that’s a negative feedback. It adds a layer of complexity to understanding the climate system. When the solar output varies, climate can change, but when Earth’s climate changes, it doesn’t influence solar output. Solar output is a truly independent variable. Albedo varies, but often in response to a change in climate. Albedo can change climate, but can also be changed by climate. Complexities like this are why predicting climate change is so incredibly difficult and requires the world’s most powerful super computers to accurately model.

There are a couple other albedo issues to look at. Snow and ice are white and reflective, but what about clouds? They’re white. And its true, clouds can have high albedo, and result in a negative forcing or net cooling influence on climate. But…but, clouds are also very good at trapping heat, infrared radiation, which results in a positive forcing or net warming influence. The albedo of clouds depends strongly on how thick they are and how high they are, and the formation of clouds depends on how much water vapor is in the atmosphere and how much the atmosphere is cooled (water vapor condenses out of air and forms clouds when an air mass reaches its dew point). The effect of clouds is highly variable, and thought, at this point to have a small negative, or cooling impact on the climate system.

The effect of volcanic aerosols is the other place we talk about reflectivity relative to climate. When a volcano erupts, especially if it is a big eruption and it erupts straight up, it ejects lots of material up into the stratosphere. Some of this material is sulfuric acid, which then forms particles called aerosols. Because these aerosols  have been injected way up into the stratosphere, they can disperse around the planet in a matter of weeks, and stay there for a year or more.  They are light colored and hence increase the albedo very high in the atmosphere, which decreases the amount of light energy that reaches the lower atmosphere and surface. Net global cooling is often observed in the year or two after a major volcanic eruption, due in part to this temporary increase in upper atmosphere albedo.

We’ve looked at solar output and now albedo, next time we will look at another big driver of climate change, the so called green house gasses in the atmosphere.


Cloud Albedo from the Earth Observatory: http://earthobservatory.nasa.gov/Features/Clouds/clouds.php

Climate Change Part 4: The Parts of the System

Note: This program first aired on March 29, 2014.

Last time, we talked about the energy balance, or budget sheet for Earth. Earth’s climate at any given time results from the equilibrium of energy coming in and energy going out. When more energy comes in than leaves, the climate warms, when less energy comes in than leaves, the climate cools. As we are at the beginning of a projected warming period, we need to  look at the pieces of the climate system to see what might be causing the changes we are observing.

This begs the question then, what are the things that can change the energy balance of the Earth’s climate system? The first one doesn’t have anything to do with us, it originates out in space, that being of course, the sun. Anything that can change the total amount of energy from the sun that makes it to Earth can have the potential to change the climate system, if everything else stays the same. Returning to our household budget analogy, changes to the amount of solar radiation are like changes to your pay check. If you get a raise, but nothing else about your budget changes, you end up saving more. If you take a pay cut but maintain the same budget, your savings decrease.

As a star, the sun has a natural evolution that changes the amount of energy it emits. Over the past several billion years, the sun has gotten hotter. It will get hotter, and then cooler as it continues to evolve over then next several billion years. This isn’t a factor in current climate change, because this solar evolution takes place over a huge time scale, billions of years. There is a shorter, 11 year cycle of solar activity, related to sunspots that some scientists have tried to correlate to changes in solar radiation, with varying degrees of success. Other ways that solar radiation can vary are through the Milankovich cycles, periodic changes to the shape of the Earth’s orbit, called eccentricity, the timing of perihelion and aphelion (the times when the Earth is closest to and furthest from the Sun),  changes to the timing of equinoxes and solistices called precession and changes to the angle of Earth’s rotational tilt, which varies from 21.5 to 24.5 degrees. When these cycles come into phase, say the Earth is at perihelion (or as close as it gets to the sun during its orbit) during an especially eccentric (or maximally elliptically shaped) orbit, at the same time as the summer solstice and when the tilt is at its 24.5 degrees, all three factors add up to maximize the amount of sun hitting the hemisphere experiencing summer. These cycles can amplify or dampen seasonal effects and in conjunction with other climate feedback systems can induce rapid climate shifts. For example, they are thought to play a role in the glacial advances and retreats of the past two million years. On a much, much smaller time frame, we know that regionally, the amount of sun reaching the Earth varies, we call this phenomenon the seasons. In the winter less sunlight is reaching us than in the summer. When we talk about global climate we average these variations out because in winter here, its summer some where else on Earth, but seasonal variation can influence climate, particularly when in conjunction with other climate change drivers. Mostly though we think of these variation as being averaged out.

So, in conclusion, it is possible for the output of the sun to vary, but the changes are either too miniscule or operate on too long a time scale to account for the totality of the observed changes to current global average temperatures. Next time we will look at the other independent components of the climate system, albedo, or reflectivity of the Earth, which influences how much energy is reflected away from Earth and therefore not available to enter the climate system at all, and the components of the climate system that absorb light and heat, influencing how that heat either escapes or recirculates in Earth’s atmosphere and oceans.

Real Climate (website run by climate scientists) is an excellent source of timely and topical info and perspective by those in the thick of this research. Here is a post about solar forcing and how relevant it is (or isn’t): http://www.realclimate.org/index.php/archives/2005/07/the-lure-of-solar-forcing/

Cute little slide show about Milankovich cycles: http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch/