Blog Archive - March 2008
Air Capture
[This is one of an occasional series on the science of mitigation/adaptation/geo-engineering that we hope to continue. Since this isn't our core expertise, we'd especially appreciate balanced contributions from other scientists.]
One of the central challenges of controlling anthropogenic climate change is developing technologies that deal with emissions from small, dispersed sources such as automobiles and residential houses. Capturing these emissions is more difficult as they are too small to support infrastructure, such as pipelines, and may be mobile, as with cars. For these reasons, proposed solutions, such as switching to using hydrogen or electricity as a fuel, rely on the carbon-free generation of electricity or hydrogen. That implies that the fuel must be made either by renewable generation (wind, solar, geothermal etc.), nuclear or by facilities that capture the carbon dioxide and store it (CCS).
There is however an alternative that gets some occasional attention: Air Capture (for instance, here or here). The idea would be to let people emit the carbon dioxide at the source but then capture it directly from the atmosphere at a separate facility.
The removal of carbon dioxide directly from the atmosphere is a natural phenomenon that occurs in the surface ocean or during photosynthesis. Ocean absorption is a result of both the higher concentration of CO2 in the atmosphere and the alkaline nature of seawater (Note that this absorption that is leading to the “other” CO2 problem, ocean acidification - which may prove detrimental to coral reefs and other organisms that use carbonate). Land-based air capture is an effort to enhance this mechanism at an industrial scale so that CO2 can be removed from the atmosphere under controlled conditions. Given that it is performed under controlled conditions, we can use more alkaline solutions to improve the rate of capture without adversely affecting the biosphere.
Industrial air capture is based on the absorption of CO2 using alkali earth metals such as sodium or potassium. The process is a variant of the Kraft Process used in most pulp and paper mills and as such, benefits from a long industrial history. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which an electrical voltage is applied across the carbonate solution to release the CO2. While simpler, the electrical process consumes more energy as it splits water at the same time. It also depends on electricity and so unless the electricity is renewable or nuclear, will result in the storage of more CO2 than the chemical process.
If the technology is well established and, aside from the oxygen combustion of lime, dates back over 50 years, what stops it from being used and what might change in the future?
The main barrier is the efficiency of the energy requirements during the reducing process. Air capture requires energy to move the air, manufacture the absorbing solutions and solids as well as to produce the oxygen, fuel and make up chemicals. All of these items will result in additional CO2 emissions, which reduce the efficiency and therefore the benefits. The second important consideration, and maybe the dominant one, is cost. Air capture has to be more economical than the proposed alternatives (hydrogen, electricity, renewables, greater efficiency etc.). It should be stated clearly that air capture is not a viable alternative to capture at large, point source emitters such as power plants since it will always be more efficient to capture and store carbon dioxide from more concentrated streams. So while there are any non-CCS fossil fuel plants, Air Capture is a non-starter.
But recent suugestions have re-thought air capture as a thermal process. The early incarnations of air capture used electricity as the energy source and therefore depended on carbon-free sources. A thermal Air Capture system uses heat that can be generated on-site, reducing the inefficiencies associated with producing electricity, but of course it still needs a source of (carbon-free) heat. Notably, this implies that air capture could reduce greenhouse gas emissions independently of developments in the power generation or transportation sector. Preliminary experimentation has shown that causticization can occur at ambient temperatures and that conventional vacuum filtration is sufficient to avoid large evaporation penalties. These developments reduce the total energy required for the process by about half compared to the conventional method and thereby reduce the amount of CO2 that would need to be sent to storage.
However, the cost of air capture is still basically unknown. Estimates have varied wildly and real numbers will only come from pilot projects over the next few years. In some sense, that puts this technology on par with the hydrogen economy with expansion potentially starting sometime after 2015. For now there are far easier (efficiency) and cheaper (power plants) ways of reducing emissions of CO2 and so air capture is not a replacement for other efforts to reduce emissions. But in the long run, all carbon sources will require mitigation - including the transportation sector - and at that time air capture could be the most cost effective option for some sources. It is not any kind of panacea though.
Venus Unveiled
Something over a week ago I had the pleasure of making my way up to the little ski resort of La Thuile in the Val D'Aosta to learn about the latest results from the Venus Express mission. (You can imagine it was a tough decision to go to La Thuile and hear real scientists talking about Venus when I could have instead been listening to luminaries such as Mark Morano drone on at the Heartland Institute pseudoscience bash. ) My own connection with the Venus Express meeting came about through some work I've been doing on habitability of the newly discovered "Super Earth" extrasolar planets like Gliese 581c. Many of us think these may be "super-Venuses" rather than "Super-Earths," so it seemed like time to touch base with the people working on our own Venus. The fact that we can put together the same bits of physics we use to understand global warming on Earth in order to understand the interplay of the carbon dioxide greenhouse with sulfuric acid clouds on Venus is a testament to the fundamental power of climate science, and gives the lie to Claude Allègre's oft stated claim that there is no such thing as a science of climate. Altogether, it was a thrilling meeting.
The Venus Express mission was described in this earlier RealClimate article, and you can read more about the mission at the VEX home page. Venus Express was done on the cheap, mostly using instruments cobbled together from leftover hardware from Mars Express and the Rosetta comet mission. The results have been nonetheless spectacular, and La Thuile provided a suitably spectacular venue in which to discuss them. This meeting was one in the series of Rencontres de Moriond in which scientists get together for a week of intensive discussion of leading-edge topics in physics — plus equally intensive skiing, climbing, hiking and enjoyment of good Northern Italian cooking. If you've ever read any of Jeremy Bernstein's accounts of how he got involved in mountaineering through his attendence at particle physics meetings conducted in similar circumstances, you'll know the general idea about how such things work.
It's a great way to shake loose creative thinking. And it's one of those things that makes real science so much fun. Perfectly aside from the setting, it was a thrill to see the vigor of this field, and the influx of talented new young postdocs and graduates students, with all their fresh ideas and enthusiasm. I hope to give just a bit of the flavor of what went on during that eventful week.
A Field Guide to Venus
Venus has an Earthlike mass and surface gravity, the latter being 8.9 meters per second per second, only slightly less than Earth's.
Venus is in a very nearly circular orbit about the Sun with orbital period (year) equal to 224.65 Earth days. Venus rotates much more slowly than Earth, however, and this has many consequences for the atmospheric dynamics, since it greatly reduces the Coriolis accelerations that do so much to organize Earth's large scale atmospheric circulations. In fact, the rotation of the planet is retrograde — i.e. opposite in direction to the rotation of the orbit. The siderial day on Venus — the period with which the star patterns would repeat, if you could see the stars from the surface — is 243 Earth days, but since this is in the retrograde direction, it adds to the angular velocity of the planet relative to the Sun. Thus, the rate of rotation relative to the Sun is 1/224.65 + 1/243 rotations per Earth day, leading to a solar day of 1/(1/224.65 + 1/243), or 116.7 Earth days. This is the time between sunrises, as would be seen from the planet's surface.
You might think that the long day would result in the dayside heating to extreme temperatures while the darkness-plunged nightside plummeted to relatively frigid values. In fact, because of the dense 92 bar atmosphere, it takes a very long time for most of the atmosphere to heat up or cool down, and there is little day/night variation over most of the depth of the atmosphere. Higher up, however, there is a diurnal and seasonal cycle, as illustrated by the black vs. green lines in the accompanying sketch — Venus Express in fact found indications that the diurnal cycle extended deeper into the atmosphere than this traditional sketch suggests, with significant temperature variations penetrating to 45 km. altitude. The atmosphere of Venus is nearly pure carbon dioxide, with a few percent of nitrogen thrown in. It also contains traces of water vapor, which though tiny, contribute significantly to the greenhouse effect of the atmosphere.Most of the greenhouse effect comes from the carbon dioxide, however, which by itself is sufficient to raise the surface temperature most of the way toward its observed value of around 470C. A key feature of the atmosphere of Venus is the sulfuric acid cloud deck. These clouds account for the high reflectivity of Venus, but because they also reflect infrared back to the surface (unlike water clouds, which absorb and emit), they have a warming effect as well, and constitute the second most important factor in the greenhouse effect of Venus after carbon dioxide. Radiation model calculations demonstrate that the clouds have a pronounced net cooling effect on the planet, when both factors are taken into account. The cloud deck comes from combinations of sulfur dioxide with water, but the nature of the sulfur cycle allowing the cloud deck to be maintained is currently a matter of considerable uncertainty.
The VEX instruments
For a full list of the complement of instruments on Venus Express you can take a look at the VEX Instrument Summary .For the most part, I'll focus on data from VIRTIS and SPICAV-SOIR. VIRTIS is a spectral imager which observes patches of Venus in a set of wavelengths ranging from the ultraviolet (0.25 microns) to the near infrared (5 microns). On the night-side VIRTIS infrared yields thermal emission, which can provide information about cloud structure and temperature, as well as information about atmospheric constituents
On the day-side VIRTIS infrared images are dominated by reflection of the near-infrared component of sunlight; the absorption of solar near-infrared also provides valuable information about atmospheric constituents, as well as information about cloud structure that is complementary to the night-side thermal emission. SPICAV/SOIR is a spectrometer with somewhat different characteristics; it returns high-resolution ultraviolet images , which reveal interesting aspects of atmospheric dynamics. The reflection of short wavelengths like ultraviolet gives a good indication of the occurrence of cloud particles, but the utility of ultraviolet observations is enhanced by the presence of an as-yet unidentified ultraviolet absorber in the atmosphere of Venus, which shows up in the form of dark streaks on ultraviolet images. Besides being useful as an ultraviolet imager, SPICAV/SOIR is used with a technique called occultation, in which the attenuation of starlight or sunlight passing through the atmosphere provides information about the vertical profile of various atmospheric constitutents, including sulfur dioxide, water vapor (and its various isotopes), carbon monoxide, carbonyl sulfide and oxygen. SPICAV is derived from spare parts from a similar instrument (SPICAM) that flew on Mars Express, but SOIR, which adds infrared channels useful for solar occultuation measurements, was newly developed for Venus Express.
Venus Express also carried a thermal infrared spectrometer, PFS, which was intended to study wavelengths longer than 5 microns. Thermal emission in these wavelengths is important to the understanding of the radiation budget of Venus. Unfortunately, this instrument was the one disappointment in an otherwise spectacularly successful mission, as the PFS was rendered inoperative by the failure of a critical shutter to open. But no worries — the instruments that did work provide a great wealth of new material to think about.
Venus Express sports a lightweight radio-science package, VeRa. Radio occultation is a low tech but highly valuable workhorse of planetary observation. By observing the refraction of radio waves passing through the atmosphere, one can obtain density profiles, since the index of refraction is proportional to density. From density and the hydrostatic relation (i.e. pressure is the weight of all the fluid above you) it is possible to reconstruct temperature profiles if you know what the atmosphere is made of. On Venus, radio occultation can observe the atmosphere down to about 45 km. altitude from the surface.
Venus: A dynamic atmosphere
Venus is not the featureless cue-ball you'd think it is from low-resolution observations in the visible spectrum. Observations of Venus in the infrared and ultraviolet spectrum show a variety of intriguing wave and vortex patterns. The highly dynamic nature of the upper atmosphere of Venus is not a new discovery, but the increasingly sophisticated observations have continued to enrich our understanding of Venus atmospheric dynamics. Indeed a new era of Venus meteorology is dawning. For multiple reasons, the deep atmosphere of Venus is a fairly quiescent place: the low rotation rate of Venus makes it easy for the dense atmosphere to transport heat and keep temperatures horizontally uniform, while the mass of the atmosphere and the limititation of infrared cooling by the dense carbon dioxide atmosphere even out the diurnal and seasonal cycle. Moreover, through a combination of reflection, scattering and absorption, only a trickle of sunlight reaches the surface to drive convection and other atmospheric circulations. Things can happen more rapidly in the upper part of the atmosphere, which can also support stronger temperature gradients. Keep in mind that the top 1% of the mass of the atmosphere of Venus is about like the whole atmosphere of Earth, and that there is plenty of dynamics and temperature variations at least down to the 2 bar level (about 45 km above the surface). There is plenty of active atmosphere to keep dynamicists happy. If you will, you can think of Venus as consisting of a dynamically active 2 bar "atmosphere" atop a sluggish deep "ocean."
Now let's take a look at some of the circulation patterns revealed by Venus Express. The patterns are made visible through the modulation of the cloud distributions, and for the most part reflect motions taking place at altitudes of 45-60 km. In the night-side infrared, clouds show up as dark patches or streaks, since they block upwelling infrared; relatively cloud-free areas are bright. The image at the right shows the night-side VIRTIS infrared image, taken looking toward the South Pole, on the right half of the image. The left half is a day-side observation in the visible spectrum, from reflected sunlight. You can see a concentrated vortex structure near the pole, and an intriguing spiral cloud pattern, probably due to the differential rotation of the atmosphere — i.e. the fact that it is swirling like a bathtub vortex and not in rigid-body rotation. It is generally believed that there is subsiding motion near the pole, but the extent of vertical motions associated with these cloud patterns is not well known. Some of the patterns are interpreted as variations in cloud top height (hence temperature) induced by the vertical motion field, rather than as variations in cloud thickness. Many of the eddies give an appearance very much like two-dimensional vortices having little vertical motion, while other cloud and wave patterns look more like Earth's boundary layer convective cloud streets (modulated by wind shear), or like gravity waves.
The South Polar vortex shows a dipole pattern very much like has been seen earlier at the North Pole. Here's an example of the evolution, viewed by VIRTIS in an infrared channel thought to be mostly responding to cloud top height (see 'South-polar features on Venus similar to those near the north pole' by G. Piccioni et al. Nature,29 November 2007.) The vortex you are seeing is about 2000 km. across.
And this color image of the vortex, taken in the 5 micron band, shows the dipole structure more clearly. The bright yellow region is the dayside.
Although the surface of Venus rotates only slowly, the upper atmosphere has taken on a rotation rate of its own, and air around the 50km level circles the planet with a period of roughly 5 days. This is called super-rotation, because the rotation is in the same sense as the rotation of the planet, but stronger. It allows the upper atmosphere to support substantial temperature gradients, by providing a Coriolis acceleration which can offset pressure gradients associated with temperature variations. The super-rotation is not actually a rigid-body rotation, but has a very distinctive profile in latitude. Venus Express has provided new observations of the zonal wind pattern using cloud-tracking methods, an example of which is shown to the right. Note particularly the uniform velocity in the nightside winds, extending from 70S to the Equator; the polar regions are in something closer to rigid body rotation. What accounts for this pattern? In all theories, the transport of momentum by transient eddies is critical to redistributing the angular momentum and creating low-latitude super-rotation. Thus, the improved understanding of eddy dynamics from Venus Express will help us to determine the character of the eddies and their transports. An essential question remains to be answered: What are these eddies and where to they come from? Much prevailing thinking ascribes the eddies to instabilities of horizontally sheared jets — the barotropic instability — but the images themselves do not give a very clear impression of jet instability.
In a related notable result, the modelling team from the Laboratoire de Meteorologie Dynamique, Paris has achieved a very convincing simulation of super-rotation in a new Venus general circulation model. Though the deep atmosphere of Venus is sluggish, its dynamics is nonetheless crucial since it is this circulation which brings angular momentum from the surface to the upper atmosphere; this circulation is also critical for atmospheric chemistry, since it transports gases to the surface where they can react to form minerals, and (more speculatively) transports volcanic outgassing to the cloud deck.
Higher in the atmosphere, the extreme temperature difference between the dayside and the nightside, due to solar absorption in the atmosphere on the dayside, drives a circulation flowing from the hot dayside to the cold nightside. This circulation also gives rise to another fascinating phenomenon, the oxygen airglow. On the dayside, extreme ultraviolet from the Sun can actually break up carbon dioxide molecules into their component parts, which liberates free oxygen atoms. These are carried to the cold nightside, where they recombine into O2, releasing energy in the 1.27 micron infrared range. Venus express carried out many new observations of the airglow phenomenon and the circulation in these extreme upper reaches of the atmosphere — which also bear on the processes allowing planetary atmospheres to escape to space.
Venus atmospheric chemistry: The cloud deck
Using occultation methods, Venus Express has shed far more light on atmospheric chemistry than I can begin to go into here, but I will at least mention a few of the results. Quite remarkably, the remote-sensing techniques can detect not only the profiles of water vapor in the atmosphere, but also the various isotopes, notably the heavier version — deuterated water, HDO, in which deuterium substitutes for one of the hydrogens. In the Earth's upper atmosphere, the HDO to H2O ratio is depleted relative to the water vapor in the lower atmosphere, because the heavier HDO condenses out more readily when it rains. On Venus, the situation is the opposite, and HDO is greatly enriched relative to lower level water vapor. This occurs because water vapor is broken apart by sunlight, and the lighter hydrogen escapes more readily than the heavier deuterium. Venus Express also found, though, that oxygen is escaping from the atmosphere in the ratio expected from breakup of water, suggesting that non-thermal escape processes in which chemical reactions give an extra kick to atoms, are important. Putting together a consistent picture which simultaneously accounts for the oxygen escape and the deuterium enrichment will tell us much about the mechanisms by which planets lose atmospheres, and perhaps shed light on issues affecting habitability of extrasolar planets.
Because the sulfuric acid cloud deck has such a profound impact on the climate of Venus, the chemistry of the Venusian sulfur cycle is of great interest. What is the lifetime of the clouds, absent resupply? Venus Express measurements, and associated laboratory experiments, are helping to clarify these issues as well. Carbonyl Sulfide has been observed in the atmosphere, and there are good indications that the formation of polysulfur (S2, S4, etc.) and possible subsequent precipitation plays a role in the sulfur cycle. The nature of sulfur dioxide resupply connects up with the contentious issue of catastrophic resurfacing of Venus, which I'll take up shortly.
Peeking at the surface
With such a thick CO2 atmosphere, you'd think it would be utterly impossible to see the surface of Venus in the infrared. The issue of "saturation" of the absorption of infrared by CO2 has been discussed previously on RealClimate, but it turns out that even with 92 bars of CO2 in the atmosphere, Venus is not saturated throughout the infrared spectrum. There is a narrow window region in the vicinity of 1 micron wavelength, which allows the surface to be observed in the infrared. Venus Express has exploited this window to make maps of infrared emission from the surface, which, combined with topography data from the Magellan radar altimiter, allow an estimate of surface emissivity. Hopefully, this will shed some light on the minerology of the surface, which is largely mysterious. Just as a sample of the new data, here is an image showing the surface brightness anomaly overlain with radar topography. This image comes courtesy of Joern Helbert and his student Nils Müller, and of course as for all results derived from VIRTIS, the VIRTIS P.I's Giuseppe Piccioni and Pierre Drossart deserve a round of applause as well.
One of the most exciting questions concerning the nature of Venus is whether the planet has undergone catastrophic resurfacing in the relatively recent past.. Venus has no plate tectonics to gradually engulf part of the surface. Unlike Earth or Mars, impact craters are uniformly distributed over the surface. By some reckonings, there appear to be no old craters, as witnessed by the apparent lack of craters in a state of partial degradation. This would argue for the entire surface having been engulfed or flooded over with magma some time in the past half billion years or so. However, the morphology of the surface lends itself to varying interpretations, and these were the subject of a genteel debate between David Grinspoon (for resurfacing) and Sue Smrekar (in the opposing camp). I can't say that anybody struck a knockout blow, but it was certainly informative, and serves as another example of the way real scientists try to hash out uncertainties and conflicting theories. There is no herd mentality here, any more than there is in studies of Earth's climate. It's in the DNA of scientists to poke at theories all the time, and never cease their questioning. "Consensus" does not consist in agreeing on everything, but rather in agreeing on a common set of tools and methodologies, as well as on a set of results that can be considered settled to a sufficient degree that further results can be confidently built upon them. Consensus of this sort exists for Venus as well as for Earth, and nobody makes a fuss about it.
The question of resurfacing has major implications for the climate history of Venus. No active volcano has yet been observed on Venus, and it has been conjectured by David Grinspoon that perhaps Venus goes through cycles of extreme volcanism and sulfur resupply to the cloud-deck, followed by long quiescent periods in which the clouds dissipate and leave the planet's surface much hotter than its already torrid temperature. This is indeed a frontier area of planetary science, and one which engages phenomena extending from the interior to mineral reactions at the planet's surface, and onward to photochemistry in the outer reaches of the amosphere.
The Earth seen from Venus: A Pale Blue Dot
Modern planetary probes are so versatile they can be re-programmed to do things their original designers never anticipated. With colleagues at the University of Chicago, I'm doing that now, using a minerology probe on the Spirit and Opportunity rovers on Mars to examine atmospheric argon. Taking a leaf from the Pale Blue Dot observations of Earth from Galileo, popularized by Carl Sagan (look. here. for a detailed report of those observations), Venus Express mission scientists have gotten the bright idea of using observations of Earth from Venus to test methods for searching for habitable extrasolar planets. The next generation of extrasolar planet-finders will return spectra of the planets averaged over the entire planetary disk, so learning how to make the most of "single pixel astronomy" is of the utmost importance.
Already, observation of the Earth spectrum by VIRTIS has detected Earth's CO2, oxygen and water vapor. Observations by SPICAV show the absorption feature characteristic of ozone. Work is underway, spearheaded by Darren Williams, to see if Earth's oceans can be detected through the characteristic sun glint they produce.
At the meeting I also had the pleasure of meeting Enric Pallé, a young astronomer who is probably best known to regular RealClimate readers for his attempts to estimate trends in Earth's albedo through ground-based measurements of lunar Earthshine. . Enric's connection with the Earth from Venus session comes about because he has been looking at ways to measure a planet's length of day by examining fluctuations in reflected sunlight; he has also been trying to detect Earth's vegetative "red edge" through Earthshine observations. Enric tells me that he has refined his somewhat controversial Earth albedo estimates, and these no longer show the mysterious and striking long-term albedo trends reported in the original paper. To tell the whole story, though, I should note that the satellite based CERES albedo estimates (which showed an opposite trend to Earthshine) have evidently also been revised, owing to discovery of a drift due to deterioration in a filter. We'll have to wait for Enric's paper to see the details, but estimates of albedo fluctuations appear to be converging, according to preliminary results he showed me. This is science at its best, and a reminder both that detecting trends from satellites is difficult, and that one should avoid building elaborate and earthshaking interpretations on data from novel observing techniques like Earthshine until they have time to mature. We all could have been spared a lot of grief if the Alabama boys who pushed their erroneous microwave satellite temperature trends over surface records for so long had been as diligent as Enric in re-examining their methods.
What a week!
All in all, quite a week! And despite the lack of good snow on the cross-country trails, I did manage to get in a few nice excursions up the valley, with the help of a prodigious supply of red klister. I am looking forward to seeing what another year of Venus Express observations brings.
A Galactic glitch
After having watched a new documentary called the 'Cloud Mystery' - and especially the bit about the galaxy (approximately 2 - 4 minutes into the linked video clip) - we realised that a very interesting point has been missed in earlier discussions about 'climate, galactic cosmic rays and the evolution of the Milky Way galaxy.
It is claimed in 'The Cloud Mystery', the book 'The Chilling Stars', and related articles that our solar system takes about 250 million years to circle the Milky Way galaxy and that our solar system crosses one of the spiral arms about every ~150 million years (Shaviv 2003).
But is this true? Most likely not. As we will discuss below, this claim is seriously at odds with astrophysical data.
Here is a little background on the Milky Way: The arms of spiral galaxies are not constant entities in time. They are results of gravitational instabilities in the disk or are induced by external companions. These instabilities are moving mass 'overdensities' containing old stars and gas, but also newly formed stars recently created from local collapse of the overdense gas.
Arms move around a spiral galaxy with a pattern speed that is defined by the mass distribution. This pattern speed differs from the motion of individual stars, just like the speed of an ocean wave differs from the movement of water particles. Estimating the pattern speed is difficult, as it is not coupled to the motion of individual stars but can only be inferred indirectly. For this reason it has not yet been reliably measured for our Milky Way – unlike for some other spiral galaxies, for which our clear and unobstructed view from the outside allows an estimate.
So how did Shaviv come up with this number?
Measuring the rotational velocity of stars in the Milky Way disk or other spiral galaxies is straightforward. The rotation is not rigid, but depends on the encircled mass inside the orbit of a star, including the Dark Matter, a yet unknown but solidly established source of gravitational attraction. It is easy and a standard technique to measure rotation curves of galaxies as a function of radius, and this is also possible for the Milky Way.
The two different rotating velocities of arms and stars have a different radial dependence - to first order the arms get preserved as entities while the stars further out have much smaller angular velocities than stars further inside - so the relative velocity of a star with respect to the nearest spiral arm will depend on its distance from the centre of the galaxy. At a certain radius, the radius of co-rotation, the two velocities are identical and a star at this radius has zero relative velocity with respect to the spiral arm pattern. It stays “forever” in the same spiral arm – or outside of it.
What are the best estimates for the relative velocity of the Sun with respect to the spiral arm pattern of the Milky Way? As mentioned, the pattern speed of the spiral arm in the Milky Way has not been firmly established.
When investigating other spiral galaxies, however, it was found that almost independently of the wide range of possible assumptions on which the pattern speed estimate was based, the radius of co-rotation follows a simple law: rcorot=r0 * (3.0 +/- 0.5), where r0 is the scale length of the exponential disk of the galaxy (the surface brightness of spiral galaxies drops very close to exponentially from the center to the outside, setting a characteristic size scale). This was measured by Kranz et al. 2003.
Since the Milky Way is a completely normal spiral galaxy, we can apply this result to it. The scale length of the Milky Way disk has recent estimates ranging from 2.6 kilo-parsec (kpc, 1pc=3.3 light years) from the SDSS survey (Juric et al. 2008), through 2.8 kpc (Ohja 2001) to 3.5 kpc (Larsen & Humphreys 2003).
We also know the Sun's distance to the galactic center well, 7.9 +/- 0.4 kpc (Eisenhauer et al. 2003), which means that the range of values for rcorot=9.1 +/- 1.9kpc. In other words, from this calculation the co-rotation radius of the Milky Way is between 7 and 11 kpc, and at 8 kpc our Sun is close to or at the radius of co-rotation. It almost certainly is not 6 kpc further inside, as Shaviv (2003) claims.
Shaviv (2003) lists in his Table 3 a number of values for the pattern speed of the spiral arms, taking from publications ranging from 1969 to 2001, two years before his article. In these papers the derived relative motion of the Sun relative to the arms ranges from Omegarel=+13.5 km/s/kpc to -4km/s/kpc, and includes estimates that are close to zero (-4km/s/kpc < than Omegarel < +7), i.e. a location near the radius of co-rotation in the majority of the publications, and most of the more recent ones. However, he selectively disregards most of these results.
If we add the above evidence that the radius of co-rotation lies at 9kpc distance and not further out, and convert this to relative velocities, e.g. by using the Milky Way rotation curve by Merrifield 1992, we obtain Omegarel =+3.2 km/s/kpc with an error range from -2.5 to +7.1km/s/kpc, and including zero. Shaviv's derived "period for spiral arm crossing" of p=134 +/- 25Myr for four spiral arms is well outside the range derived from these values.
So it seems that Shaviv's "periodicity" estimate for crossing of spiral arms by the sun does not hold up under scrutiny when using current astronomical results as the work by Kranz et al. This comes in addition to the previously shown fact that the correlation of cosmic ray flux with paleoclimatic data proposed by Shaviv and Veizer (2003) only arises “by making several arbitrary adjustments to the cosmic ray data” (Rahmstorf et al. 2004).
Note also that the question of current climate change is quite another matter from that over time scales of many millions of years – despite Shaviv’s remarkable press-release claims that “The operative significance of our research is that a significant reduction of the release of greenhouse gases will not significantly lower the global temperature”. As we repeatedly pointed out over the years: that global warming over the past decades is not linked to cosmic rays is clear from the fact that the cosmic ray measurements over the past 50 years do not show any trend (Schiermeier 2007).
Remarkably, the poor scientific basis of the galactic cosmic ray hypothesis seems to be inversely related to the amount of media backing it is getting. At least 3 documentaries ('The Climate Conflict', the 'Global Warming Swindle', and now 'The Cloud Mystery') have been shown on television – all with a strong thrust of wanting to cast doubt on the human causes of global warming.
The global cooling mole
To veterans of the Climate Wars, the old 1970s global cooling canard - "How can we believe climate scientists about global warming today when back in the 1970s they told us an ice age was imminent?" - must seem like a never-ending game of Whack-a-mole. One of us (WMC) has devoted years to whacking down the mole, while the other of us (JF) sees the mole pop up anew in his in box every time he quotes contemporary scientific views regarding climate change in his newspaper stories.
The problem is that the argument has played out in competing anecdotes, without any comprehensive and rigorous picture of what was really going on in the scientific literature at the time. But if the argument is to have any relevance beyond talking points aimed at winning a debate, such a comprehensive understanding is needed. If, indeed, climate scientists predicted a coming ice age, it is worthwhile to take the next step and understand why they thought this, and what relevance it might have to today's science-politics-policy discussions about climate change. If, on the other hand, scientists were not really predicting a coming ice age, then the argument needs to be retired.
The two of us, along with Tom Peterson of the National Climatic Data Center, undertook a literature review to try to move beyond the anecdotes and understand what scientists were really saying at the time regarding the various forces shaping climate on time human time scales. The results are currently in press at the Bulletin of the American Meteorological Society, and Doyle Rice has written a nice summary in USA Today, and an extended version based on a presentation made by Tom at the AMS meeting in January is on line.
During the period we analyzed, climate science was very different from what you see today. There was far less integration among the various sub-disciplines that make up the enterprise. Remote sensing, integrated global data collection and modeling were all in their infancy. But our analysis nevertheless showed clear trends in the focus and conclusions the researchers were making. Between 1965 and 1979 we found (see table 1 for details):
- 7 articles predicting cooling
In other words, during the 1970s, when some would have you believe scientists were predicting a coming ice age, they were doing no such thing. The dominant view, even then, was that increasing levels of greenhouse gases were likely to dominate any changes we might see in climate on human time scales.
We do not expect that this work will stop the mole from popping its head back up in the future. But we do hope that when it does, this analysis will provide a foundation for a more thoughtful discussion about what climate scientists were and were not saying back in the 1970s.
536 AD and all that
"during this year a most dread portent took place. For the sun gave forth its light without brightness… and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear."
This quote from Procopius of Caesarea is matched by other sources from around the world pointing to something - often described as a 'dry fog' - and accompanied by a cold summer, crop failures and a host of other problems. There's been a TV special, books and much newsprint speculating on its cause - volcanoes, comets and other catastrophes have been suggested. But this week there comes a new paper in GRL (Larsen et al, 2008) which may provide a definitive answer….
It's long been known that tree-rings (such as the one pictured from Arizona) often show an extremely small growth ring for AD 536 (you can count back from the marked AD 550 ring). In fact, if you look at the mean anomaly in a whole range of tree ring constructions, this event stands out along with 1601 and 1815 (known volcanic events) as being exceptional over the last 2000 years.
These data match the written sources quite well. However, tying it to a cause has always been plagued with problems of chronology.
An initial attempt to tie this event to a volcanic pulse in the Dye3 ice core in Greenland foundered when the chronology was revised to put it 20 years earlier. However, there has recently been a concerted effort to place all the Greenland ice cores on a common timescale based on annual layer counts (Vintner et al, 2006). Because all the cores are being counted together, ambiguities in one can be corrected by reference to the others. Once the dates have been better established, the sulphate records (which generally show the impact of volcanic aerosols) can be examined to see if they line up. And low and behold, they do:
The second peak is dated at 534 AD which is close enough to 536 AD given the one or two year uncertainty in counting. Note that the 534 AD peak is actually smaller than the one a few years earlier. In assessing the importance of an eruption though, it isn't enough to have just a peak in Greenland. That could simply signify an eruption that was close by. Instead, people look for a matching peak in Antarctica. This signifies that the eruption was likely tropical and the aerosols were carried into both hemispheres by the stratospheric circulation. Here is where previous attempts often faltered. The dating of ice cores in Antarctica is less exact than in Greenland because the accumulation is slower (it doesn't snow as much). However, the relatively new Dronning Maud Land (DML) core has comparable resolution to the Greenland ones, and this one does have a clear sulphate peak at about 542 +/- 17 years. That is good enough to be a match to the 536 AD peak in Greenland. The correction you'd need to make to align them exactly would also fix some other apparent offsets for smaller events in the subsequent 100 years.
So it probably was a volcano, somewhere in the tropics, and it was likely the size of Tambora in 1815. There has been some speculation that it was an earlier eruption of Krakatoa (which went off again in 1883), but that is uncertain, as are the numerous consequences such as the fall of the Rome or the rise of Islam which have been attributed to this event. While not exploring that too deeply, this quote from Michael the Syrian indicates dramatically the potential for climate events like this one to really spoil your day:
"The sun was dark and its darkness lasted for eighteen months; each day it shone for about four hours; and still this light was only a feeble shadow … the fruits did not ripen and the wine tasted like sour grapes."
|
