No, Solar Variations Can’t Account for the Current Global Warming Trend. Here’s Why:

In part I of this series on the sun and Earth’s climate, I covered the characteristics of the sun’s 11 and 22 year cycles, the observed laws which describe the behavior of the sunspot cycle, how proxy data is used to reconstruct a record of solar cycles of the past, Grand Solar Maxima and Minima, the relationship between Total Solar Irradiance (TSI) and the sunspot cycle, and the relevance of these factors to earth’s climate system. In part II, I went over the structure of the sun, and some of the characteristics of each layer, which laid the groundwork for part III, in which I explained the solar dynamo: the physical mechanism underlying solar cycles, which I expanded upon in part IV, in which I talked about some common approaches to solar dynamo modeling, including Mean Field Theory. This installment covers how all of that relates to climate change and the current warming trend.

Solar Cycles and Earth’s Climate

The sun is responsible for nearly all of the energy entering our climate system, so it should come as no surprise that variations in Total Solar Irradiance throughout solar cycles do indeed affect Earth’s climate (Eddy 1977, Bond 2001, Solanki 2002, de Jager 2008). Knowing this, it’s natural to wonder whether solar variations are to blame for the current global warming trend. There’s nothing irrational about wondering “hey, you know that gigantic fusion reactor fireball thing in the sky? What if that thing has something to do with global warming and climate change?” I want to emphasize that this is by no means a crazy or unreasonable question to ask. It’s just that with the current warming we’re not just looking at cyclical oscillations or subtle fluctuations; we’re looking at a clear trend (NOAA 2016, Anderson et al 2013, Hansen et al 2010). And changes in solar activity are simply not sufficient to explain that rate and magnitude of the current trend (Frohlich 1998, Meehl et al 2004, Wild 2007, Lean and Rind 2008, Duffy 2009, Gray et al 2010, Kopp 2011).

It has been estimated that climate forcings attributable to solar variability have not contributed more than 30% of the global warming from 1970 – 1999 (Solanki 2003). To add insult to injury, 15 of the 16 hottest years on the instrumental record have occurred since the turn of the millennium, and more recent analyses have found that the solar activity and global temperature trends have been moving in opposite directions in recent cycles (Lockwood and Frohlich 2007 and 2008, Lockwood 2009). Moreover, researchers have found that the warming trend becomes even clearer after correcting for El Niños and volcanic and solar forcings (Foster 2011).

That’s right! Solar activity has actually declined in the last decade, and this last cycle (solar cycle 24) has been well below average amplitude (Jiang 2015, Pesnell 2016). So if changes in solar activity were the principle determinant of the recent changes, we should expect to be experiencing cooling: not warming. So this is also ruled out as a principle cause of late 20th century and early 21st century global warming and resultant climate change. Look at how solar forcings stack up against the observed temperature curve from Meehl et al 2004.

Image 3: c/o Meehl 2004

Firstly, if greenhouse gases are primarily responsible, we should expect to see little change in the amount of solar energy entering the earth’s atmosphere, but a decrease in the amount leaving. Contrastingly, if the sun is primarily responsible, we should expect to see an increase both in the energy entering earth’s atmosphere and the amount leaving. Since the mid-late 1970s, we’ve been able to measure this with satellites.

Lo and behold! It turns out that the rate of energy coming in from the sun has changed very little, while the rate at which energy leaves the earth has decreased significantly (Harries 2001, Griggs and Harries 2007, Philipona 2004, Leroy 2008, Worden 2008, Huang et al 2010). This is the proverbial smoking gun evidence that recent climate change is not due to changes in solar forcing, but rather to the greenhouse effect.

Secondly, if the warming effects were attributable primarily to the sun, then we should be seeing a very different distribution of temperatures than what we are actually observing. Specifically, warming due to solar forcing should be most prominent during the daytime and during the summer months, because these are the times during which the sun is most intensely bombarding the earth.

However, what we observe instead is that night time and winter temperatures are increasing faster than would be the case if the sun was chiefly responsible for the trend (Alexander et al 2006, Caesar et al 2006). This distribution cannot be explained by natural variability, but is consistent with the predictions of the greenhouse effect explanation (Brown 2008). The energy is entering the climate system during the day when the sun is shining, and is getting trapped by greenhouse gases, which slows down the rate at which that energy can escape the earth’s atmosphere. Alexander et al in particular found that over 70% of the land area sampled showed a significant increase in the occurrence of warm nights annual from 1951 – 2003, and a corresponding decrease in the occurrence of cold nights (Alexander et al 2006). So, here we have multiple lines of smoking gun evidence unanimously converging on the conclusion that current climate change cannot be blamed on changes in solar activity.

Could a Grand Minimum Mitigate 21st Century Global Warming?

Okay, so we know that variations in Total Solar Irradiance can’t account for the current warming trend, but what if we just lucked out and entered a new Grand Solar Minimum? How likely is it that it would stop or reverse the trend, and make the last few decades of climate science research and undesirable predictions seem like much ado about nothing? This possibility has been investigated in several papers as well. Although the predictions vary slightly insofar as the precise amounts by which TSI and temperatures would be reduced, they all arrive at reductions in TSI of no greater than a few watts per square meter, a slowing of ascending temperatures by no more than 0.1 – 0.3 °C, and therefore imply that a 21st Century Grand Minimum would (at most) slightly slow global warming down temporarily without actually stopping it (Wigley et al 1990, Feulner and Rahmstorf 2010, Jones et al 2012, Meehl et al 2013, Anet et al 2013, Maycock et al 2015). One might reason that any delay in the warming trend would be better than nothing, because it might buy some time for the innovation and implementation of mitigation and/or coping strategies, and I would not be compelled to argue against that, but the current weight of the evidence suggests that it would be of only marginal help at best.

Conclusion

In summary, solar cycles can affect earth’s weather and climate, both on decadal scales in correspondence with the 11 year sunspot cycle, as well as longer term amplitude changes associated with grand solar maxima and minima.

The prevailing scientific theory for the mechanism underlying these cycles is the solar dynamo, which explains the associated magnetic field oscillations in terms of a branch of physics called magnetohydronamics. It accounts for the observed sunspot butterfly diagrams, Sporer’s Law, Joy’s Law, and Hale’s Polarity Law, and explains the 11 and 22 year cycle periods. Mean Field theory is one of the ways in which stellar astrophysicists simplify solar dynamo model calculations, but it has its limitations.

Multiple lines of evidence suggest the current warming trend on earth is not caused by an increase in solar activity. We know from satellite data that there has been no substantial increase in the amount of solar energy (TSI) entering earth’s climate system, but less of it has been making it back out into space. Moreover, winter and night time warming has increased rapidly, which is consistent with the greenhouse effect explanation, but not with the solar forcing explanation.

Additionally, if a 21st Century Grand Solar Minimum were to occur, it would most-likely have a noticeable but small slowing effect on Global Warming and the resultant Climate Change.

What we humans should do about this is not a strictly scientific question, because it depends not only on model predictions but also on normative issues, personal values, and cost-benefit analyses of different potential solution strategies (both technological and political). However, what we do know with VERY high confidence is that global warming and climate change are happening, and that the sun is not to blame for it.

BOOM!!

Related Articles:

References:

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Anderson, D. M., Mauk, E. M., Wahl, E. R., Morrill, C., Wagner, A. J., Easterling, D., & Rutishauser, T. (2013). Global warming in an independent record of the past 130 years. Geophysical Research Letters40(1), 189-193.

Anet, J. G., Rozanov, E. V., Muthers, S., Peter, T., Brönnimann, S., Arfeuille, F., … & Schmutz, W. K. (2013). Impact of a potential 21st century “grand solar minimum” on surface temperatures and stratospheric ozone. Geophysical Research Letters40(16), 4420-4425.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M. N., Showers, W., … & Bonani, G. (2001). Persistent solar influence on North Atlantic climate during the Holocene. Science294(5549), 2130-2136.

Brown, S. J., Caesar, J., & Ferro, C. A. (2008). Global changes in extreme daily temperature since 1950. Journal of Geophysical Research: Atmospheres113(D5).

Caesar, J., Alexander, L., & Vose, R. (2006). Large‐scale changes in observed daily maximum and minimum temperatures: Creation and analysis of a new gridded data set. Journal of Geophysical Research: Atmospheres111(D5).

Cox, P. M., Betts, R. A., Jones, C. D., Spall, S. A., & Totterdell, I. J. (2000). Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature408(6809), 184-187.

De Jager, C. (2008). Solar activity and its influence on climate. Neth. J. Geosci. Geologie En Mijnbouw87, 207-213.

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Frohlich, C., & Lean, J. (1998). The Sun’s total irradiance: Cycles, trends and related climate change uncertainties since 1976. Geophys. Res. Lett25(23), 4377-4380.

Gray, L. J., Beer, J., Geller, M., Haigh, J. D., Lockwood, M., Matthes, K., … & Luterbacher, J. (2010). Solar influences on climate. Reviews of Geophysics48(4).

Griggs, J. A., & Harries, J. E. (2007). Comparison of spectrally resolved outgoing longwave radiation over the tropical Pacific between 1970 and 2003 using IRIS, IMG, and AIRS. Journal of climate20(15), 3982-4001.

Hansen, J., Ruedy, R., Sato, M., & Lo, K. (2010). Global surface temperature change. Reviews of Geophysics48(4).

Harries, J. E., Brindley, H. E., Sagoo, P. J., & Bantges, R. J. (2001). Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997. Nature410(6826), 355-357.

Huang, Y., Leroy, S., Gero, P. J., Dykema, J., & Anderson, J. (2010). Separation of longwave climate feedbacks from spectral observations. Journal of Geophysical Research: Atmospheres115(D7).

Jiang, J., Cameron, R. H., & Schuessler, M. (2015). The cause of the weak solar cycle 24. The Astrophysical Journal Letters808(1), L28.

Jones, G. S., Lockwood, M., & Stott, P. A. (2012). What influence will future solar activity changes over the 21st century have on projected global near‐surface temperature changes?. Journal of Geophysical Research: Atmospheres117(D5).

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Image Credits:

Image 3:

Meehl, G. A., Washington, W. M., Ammann, C. M., Arblaster, J. M., Wigley, T. M. L., & Tebaldi, C. (2004). Combinations of natural and anthropogenic forcings in twentieth-century climate. Journal of Climate17(19), 3721-3727.

Images 1 and 2:

Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 659–740, doi:10.1017/ CBO9781107415324.018.

Image 4:

Thoughtscapism and Making Sense of Climate Science Denial

coronal mass ejection c/o NASA

Milankovitch Cycles and Climate: Part III – Putting it All Together

In parts I and II, we looked at axial obliquity, axial precession, apsidal precession, orbital eccentricity and orbital inclination, and how their cycles can affect the climate. In this installment of the series, we’ll look briefly at how these cycles look when combined, and then discuss one of the most prominent unsolved problems raised by the theory: The 100,000 Year problem.

Putting it all together

Putting it all together

Notice that the peaks and valleys in temperature are roughly periodic, and that with the possible exception of apsidal precession, there are slight fluctuations in the periods and amplitudes of these orbital cycles. One reason for this has to do with fluctuations in solar output, but another likely reason has to do with the very causes of these orbital cycles themselves: namely, these cycles are driven by mutual gravitational perturbations between the Earth, Sun, Moon, and to a lesser extent, Jupiter and the other planets in the solar system (Borisenkov 1985, Spiegel 2010) .

These are what are referred to in physics as “n-body problems.” Isaac Newton recognized early on that some such orbital fluctuations would have to occur, but it turns out that n-body problems cannot be solved analytically for systems of n ≥ 3 (Heggie 2005). That means the resultant systems of differential equations has to be solved using what are called numerical methods, rather than being solved for a set of analytic functions describing the precise trajectories of each body in the system over time. As a result, the actual precise paths that celestial bodies take and the variations in their tilt and precession can be more complex than just a fixed elliptical path. To add insult to injury, these problems become even more complicated in General Relativity than in classical Newtonian Celestial Mechanics.

Milankovitch Cycle/Insolation animation c/o Cal Tech

Milankovitch Cycle/Insolation animation c/o Caltech

I want to also emphasize the fact that even though Milankovitch first proposed his ideas nearly a century ago, this is still an area of active research. There are still unsolved questions about how these cycles combine to help produce the glaciation data we see in the paleo-climatological records. For instance, there’s what is known as The 100,000 Year Problem. This refers to the fact that during the past million years, glacial-interglacial periods have occurred on roughly a 100,000 year cycle, which has been difficult to reconcile with the fact that the insolation changes due to the 100,000 year orbital eccentricity cycle are very small. The effect appears to exceed the magnitude of the cause.

There is no shortage of plausible hypotheses, each with their respective strengths and weaknesses, but as of yet there’s still no unified theory that explains how precisely the cycles all work together to produce the observed pattern of glacial and interglacial periods.

For instance, researchers such as Muller et al have hypothesized that orbital inclination may be more important in producing the observed 100kyr glaciation cycle than the other cycles (Muller 1997). The only physical mechanism thus far proposed for this is the possibility that different inclinations of the orbital plane may correspond to different densities of meteoroid and dust accretion. Such a change could alter stratospheric concentrations of dust and aerosols, which would change the amount of sunlight reflected back into space. So far there’s no evidence for sufficiently different amounts of accretion at different orbital inclinations for this to be the case, but it’s a testable hypothesis whose strengths and weaknesses are outlined by the lead author here (Muller 1995).

Other researchers have been able to reproduce the 100,000 year cycles in models involving the non-linear phase locking of interactions between the known orbital forcings and internal oscillations in the climate system (Tziperman et al 1997). The basic idea behind the non-linear phase locking models is that axial obliquity and/or precession may act as pacemakers for the glaciation cycle in a manner that is distinct from models based on mere amplifications of the effects of the eccentricity cycle.

The minutia of the actual physical mechanisms involved in the non-linear phase locking models is left quite vague. No definite conclusion is attempted regarding whether the dominant cycle is obliquity, precession or both; the models work just as well with CO2 changes driving glaciation in synchrony with the orbital cycles as they do with the orbital cycles driving them with CO2 changes merely amplifying the signal. But that’s because the goal with that paper was merely to figure out whether such models could reproduce the cycle. The same lead author (Tziperman) has also co-authored work that explored a sea ice triggering mechanism for glaciation (Gildor 2000).

Others have even argued that the last 800,000 or so of climate records extends insufficiently far back to establish that the apparent 100,000 year glaciation cycle and its relationship to the eccentricity cycle are even statistically significant (Wunsch 2004).

This is not the only unresolved problem related to Milankovitch cycles. In addition to the 100k year problem, there’s also a similar 400k year problem, which exists because a strong variation in the eccentricity cycle doesn’t appear to correspond to an extra strong 400k period climatological cycle.

Then there’s also what’s called the “Stage 5” Problem: aka the Causality Problem (Oppo et al 2001). This refers to the fact that the penultimate interglacial period (corresponding to Marine Oxygen-Isotopic Stage 5) appears to have occurred about 10k years prior to the forcing hypothesized to have caused it. Another issue is what’s called the Split Peak Problem, which refers to the fact that eccentricity cycles have cleanly resolved variations at both 95k and 125k years which don’t appear to translate into two cleanly resolved peaks in insolation (Zachos et al 2001).  Instead, what’s observed is a single peak on a roughly 100k frequency.

So, as you can see, determining the precise manner in which the combinations of these cycles affect global climate is no easy task, and we still don’t know everything.

That said, what we DO know is that these cycles are not sufficient to explain the rate of the current warming. For one, they occur over much longer periods of time than the current trend (on the order of tens or hundreds of thousands of years versus a couple hundred years).

Moreover, Earth’s Orbital Eccentricity is nearly circular, and both Axial Obliquity and Axial Precession are currently changing in opposition to the warming trend; Axial Obliquity is getting smaller: not larger, which means if anything that we in the Northern Hemisphere should be cooling (or at least not warming). Similarly, precession is changing such that it should be moderating the warming, but it’s not. If anything, other variables (i.e. human activities) may be delaying the next glacial period (Berger 2002). So, even though we don’t know everything about how Milankovitch cycles affect the climate, we do know that they can’t explain the current warming trend.

References:

Berger, A., & Loutre, M. F. (2002). An exceptionally long interglacial ahead?.Science297(5585), 1287-1288.

Borisenkov, Y. P., Tsvetkov, A. V., & Eddy, J. A. (1985). Combined effects of earth orbit perturbations and solar activity on terrestrial insolation. Part I: Sample days and annual mean values. Journal of the atmospheric sciences,42(9), 933-940.

Ellis, R., & Palmer, M. (2016). Modulation of ice ages via precession and dust-albedo feedbacks. Geoscience Frontiers.

Gildor, H., & Tziperman, E. (2000). Sea ice as the glacial cycles’ climate switch: Role of seasonal and orbital forcing. Paleoceanography15(6), 605-615.

Heggie, D. C. (2005). The classical gravitational N-body problem. arXiv preprint astro-ph/0503600.

Muller, R. A., & MacDonald, G. J. (1995). Glacial cycles and orbital inclination. Nature377(6545), 107-108.

Muller, R. A., & MacDonald, G. J. (1997). Spectrum of 100-kyr glacial cycle: Orbital inclination, not eccentricity. Proceedings of the National Academy of Sciences94(16), 8329-8334.

Oppo, D. W., Keigwin, L. D., McManus, J. F., & Cullen, J. L. (2001). Persistent suborbital climate variability in marine isotope stage 5 and Termination II. Paleoceanography, 16(3), 280-292.

Rial, J. A. (2004). Earth’s orbital eccentricity and the rhythm of the Pleistocene ice ages: the concealed pacemaker. Global and Planetary Change41(2), 81-93.

Rial, J. A. (1999). Pacemaking the ice ages by frequency modulation of Earth’s orbital eccentricity. Science285(5427), 564-568.

Spiegel, D. S., Raymond, S. N., Dressing, C. D., Scharf, C. A., & Mitchell, J. L. (2010). Generalized Milankovitch cycles and long-term climatic habitability.The Astrophysical Journal, 721(2), 1308.

Tziperman, E., Raymo, M. E., Huybers, P., & Wunsch, C. (2006). Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing. Paleoceanography21(4).

Wunsch, C. (2004). Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change. Quaternary Science Reviews23(9), 1001-1012.

Zachos, J. C., Shackleton, N. J., Revenaugh, J. S., Pälike, H., & Flower, B. P. (2001). Climate response to orbital forcing across the Oligocene-Miocene boundary. Science, 292(5515), 274-278.

Photo Credits:

Incredio – https://commons.wikimedia.org/wiki/File:MilankovitchCyclesOrbitandCores.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=45036243

 

Milankovitch Cycles and Climate: Part I – Axial Tilt and Precession

The theory of Milankovitch cycles is named after Serbian astronomer and geophysicist, Milutin Milanković, who in the 1920s postulated three cyclical movement patterns related to Earth’s orbit and rotation and their resultant effects on the Earth’s climate. These cycles include axial tilt (obliquity), elliptical eccentricity, and axial precession. In aggregate, these cycles contribute to profound long term changes in earth’s climate via orbital forcing.

Axial Obliquity: The Earth’s rotational axis is always tilted slightly; currently, its axis is about 23.4 degrees from the vertical. Alternatively, you could say that its equatorial plane is tilted about 23.4 degrees relative to its orbital plane. This tilt is responsible for Earth’s seasons. During the Northern Hemisphere (NH) summer, Earth is further away from the Sun than it is during the NH winter due to its slightly elliptical orbit, yet it receives more sunlight because it’s tilted towards the Sun. During this same time period, the Southern Hemisphere (SH) is tilted away from the Sun, which is why NH summer coincides with SH Winter and vice versa. Contrastingly, during the NH winter, the Earth is closer to the Sun, yet receives less sunlight because it’s tilted away from it. During that same period, the SH is tilted towards the Sun, and is thus experiencing summer.

However, that axial tilt slowly varies between about 22.1 degrees and 24.5 degrees over long quasi-periodic cycles of roughly 41,000 years. The last maximum is estimated to have occurred around 8,700 BCE, and the next minimum should occur roughly around the year 11,800 CE. A more exaggerated tilt corresponds to more severe seasons: warmer summers and colder winters. As you may have guessed, less exaggerated tilt corresponds to milder seasons: cooler summers and warmer winters. The latter phases can lead to increased glaciation. This is because cooler summers mean less ice loss per year, and warmer winters mean more precipitation (rain or snow) to build up ice sheets. Now, you might be wondering why exaggerated tilt wouldn’t build ice sheets with its extra cold winters, but remember that the freezing point of water at 1 ATM of pressure is still going to be 0 degrees Celsius. Reaching negative 50 degrees C in the winter isn’t likely to facilitate much greater glaciation than reaching negative 10 degrees C, and those extra cold winters would involve less precipitation. To add insult to injury, the extra hot summers would melt greater portions of the existing ice each year. That’s why smaller axial tilt values are thought to correspond to increased glaciation and larger tilt values to deglaciation. Moreover, greater surface areas of ice cover can function to resist warming via the ice-albedo feedback (or snow-albedo feedback), which I mentioned briefly in my article on how continental drift affects climate (here and here).

Axial Tilt (photo credit).

Axial Tilt (photo credit).

Axial Precession: At any given obliquity, the direction of the earth’s rotational axis can “wobble” around the vertical in its own cycles (called precession) even while maintaining a more or less constant angle between the rotational axis and the vertical. This is caused by gravitational influences on the earth from the sun and moon. It takes roughly just under 26,000 years for the earth to complete an entire cycle of precession. Estimates differ from different sources, in part due to the fact that the rate of precession is not constant. This is also the reason earth’s axis points either towards Polaris or Vega as the “North Star” roughly every 13,000 years.

Axial Precession.

Axial Precession.

In the contrasting case (i.e. precession in the opposite phase of its current configuration), NH Winters would occur when Earth was furthest away from the sun and summers would occur when it was closest. That would mean extra hot summers, and thus more glacial melting. It would also mean extra cold winters, but those colder winters also correspond to less precipitation. That’s why our current precession should be more conducive to building the NH ice sheets, but the opposite is occurring due to reasons we’ll delve into soon enough. Again, loss of ice also means less help from the ice-albedo feedback effect, which could otherwise help resist further warming. Although axial precession does not affect total annual insolation, it can have a profound effect on where and when that solar energy is distributed, and consequently on the formation or disintegration of ice sheets. Right now, the northern hemisphere is closer to the sun in the NH Winter and further away in the NH Summer. This makes NH Summers less hot and NH Winters less cold than would be the case if Earth were in the opposite phase in its precession cycle. Presumably, the warmer NH Winters should be conducive to more precipitation (snow fall), which would contribute to glaciation, whereas the moderate summers would be conducive to less glacial melting than if the precession were in the opposite configuration from its current phase.

Keep in mind that the magnitude of these seasonal effects also depends on how eccentric our orbit is around the sun, and neither obliquity nor precession affects the total amount of energy coming in from the sun. Their immediate warming or cooling effects are only regional, but regional warming can lead to global warming by altering ocean circulation patterns, redistributing heat throughout the oceans, and consequently causing the oceans to release stored CO2, by decreasing its solubility, which can drive additional warming via the greenhouse effect.

In part II, we’ll look at three other orbital cycles: orbital eccentricity, apsidal precession and orbital inclination. After that, we’ll look at their combined effects on climate in part III, and then discuss the limitations of our current knowledge by examining some unsolved problems regarding the relationship between these cycles and Earth’s glaciation cycles.

References:

Capitaine, N., Wallace, P. T., & Chapront, J. (2003). Expressions for IAU 2000 precession quantities. Astronomy & Astrophysics412(2), 567-586.

Hays, J. D., Imbrie, J., & Shackleton, N. J. (1976, December). Variations in the Earth’s orbit: pacemaker of the ice ages. American Association for the Advancement of Science.

Martin, P., Archer, D., & Lea, D. W. (2005). Role of deep sea temperature in the carbon cycle during the last glacial. Paleoceanography20(2).

Schmittner, A., & Galbraith, E. D. (2008). Glacial greenhouse-gas fluctuations controlled by ocean circulation changes. Nature456(7220), 373-376.

Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E., & Barker, S. (2010). Ventilation of the deep Southern Ocean and deglacial CO2 rise. Science,328(5982), 1147-1151.

Toggweiler, J. R., Russell, J. L., & Carson, S. R. (2006). Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages.Paleoceanography21(2).

How Continental Drift affects Climate: Part I – Plate Tectonics, Albedo, and the SnowBall Earth Hypothesis

About 700 – 800 MYA, during the Neoproterozoic era (in the late Precambrian), it has been proposed that the Supercontinent, Rodinia occupied an equatorial position on the Earth. Perhaps counter-intuitively, this tropical arrangement of the continents may have set the stage for a massive glaciation known as “Snowball Earth” during the Cryogenian period, despite equatorial regions receiving the most sunlight due to the orientation of the Earth with respect to the sun. Indeed, the fact that equatorial regions receive the most sunlight is part of the reason why tropical rainforest biomes exist where they do. So how could this have happened? Why would an equatorial continental arrangement render such a glaciation more likely? More on the Snowball Earth hypothesis in a moment: More generally however, what factors are capable of inducing changes in the climate?

There are several of them. First of all though, what do we mean by climate, and how does it differ from weather?

Weather refers to short term local fluctuations in temperature, precipitation, humidity, sunlight, atmospheric pressure, cloud cover and wind patterns, whereas climate refers to the average distribution of weather patterns over longer periods of time (usually decades or longer).

It’s no secret that average global temperatures have increased appreciably since the beginning of the industrial revolution. It’s a subject of endless and vehement political controversy, particularly surrounding the subject of anthropogenic causation via increased atmospheric concentrations greenhouse gases – primarily through the burning of fossil fuels – and what it might mean for policy makers.

Weather station temperature records, paleoclimatological data from ice cores and dendroclimatology (tree ring data), loss of Arctic glaciers and sea ice, and rising sea levels all converge on the conclusion that both land and sea surface temperatures have risen significantly in the last 150 years or so, and are breaking new records even as I type this.

I can delve into the current global warming phenomenon, its causes, why it’s considered a problem, and the public debate associated with it in a later post. Today however, I want to talk about something else. So for now, suffice it to say that global warming is expected to lead to broader climate change more generally. But we know that climate has changed naturally in the past, so what are the factors which can cause it?

There are several variables capable of changing earth’s climate, including the following: Plate tectonics (continental drift), changes in earth’s orbit (Milankovitch cycles), changes in solar output (sun spots and solar flares), volcanism, asteroid, comet and meteoroid impacts, as well as changes in the composition of the atmosphere (i.e. the greenhouse effect).

Plate Tectonics:

Plate tectonics is the scientific theory that the Earth’s outer shell consists of a network of rigid plates which glide slowly over the Earth’s mantle, interact with each other a various ways, and which are responsible for continental drift. The lithosphere includes the Earth’s rigid outer crust as well as the top layer of the mantle. The plates glide over the layer beneath that, a viscoelastic layer known as the asthenosphere, which is elastic and ductile under short term stress, and capable of convection and flow under long term stress. The motion of lithospheric plates results from a combination of influences such as gravity, convection currents and various forces associated with the rotation of the earth.

Tectonic plates can interact by pushing together along convergent boundaries, whereby one plate can slide beneath another other in a process called subduction. They can also pull apart at divergent boundaries; this is what occurs in seafloor spreading along oceanic ridges. They can also undergo lateral motion at what are called transform boundaries, which is when two adjacent plates slide past each other horizontally. Transform boundaries are usually associated with ocean floors where they offset the divergent sea floor spreading at ocean ridges. However, they can also occur on land, such as the San Andreas Fault in California, for example. In actuality, plates can also deform and interact in more complicated ways, but these are the Classical Plate Tectonics interactions covered in your typical 101 treatment.

plate_tectonics

U.S. Geological Survey/ map by Jose F. Vigil

Plate tectonics have been shaping the earth’s geography for billions of years via continental drift, and are responsible for the current layout of the continents (see these animated reconstructions here and here). Continental drift via the movement of tectonic plates can affect earth’s climate by changing the sizes and locations of both land masses and ice caps, and by altering ocean circulation patterns, which are responsible for transporting heat around the earth, which in turn affect atmospheric circulation processes. For instance, changes in continental area at higher latitude can lead to corresponding changes in the area of permanent ice cover, which can lead to what’s called ice-albedo feedback (or snow-albedo feedback). Earth’s albedo is simply the proportion of light from the sun that gets reflected back into the atmosphere. An albedo of 0 corresponds to a perfect black body (total light absorption), whereas an albedo of 1 corresponds to a perfect white body (total reflection).

So how does this relate to the Rodinia Supercontinent, and the Snowball Earth hypothesis? The idea here is that tropical continents reflect more light than Open Ocean, thus absorbing less heat. Today, by contrast, most of the heat is absorbed by the tropical oceans. Since the equator receives more sunlight than other latitudes, that means that a significant portion of incoming sunlight would have been reflected rather than absorbed.

Snow and ice have even higher albedos than rock and soil, so all that was needed was some sort of triggering mechanism to set a runaway ice age effect in motion: cooling leads to more snow and ice, which reflects more light, which leads to more cooling, which leads to more snow and ice, and so on and so forth.

Credit: MIT Artist concept of a planet-wide Ice Age on Earth. Credit: iStockphoto

Credit: MIT Artist concept of a planet-wide Ice Age on Earth. Credit: iStockphoto

I should mention that although the presence of glaciers during the Cryogenian is not disputed, the snowball earth hypothesis is far from settled science. There’s even a less extreme variant that has been dubbed the Slush Ball Earth hypothesis. Regardless of its details, or whether it’s even correct, the point of this section is that a tropical distribution of continents makes such a self-reinforcing feedback loop possible, and it serves as an example of what a dramatic effect continental drift can have on climate change over the long term.

In part 2, I’ll finish up the Rodinia Supercontinent and Snowball Earth example by discussing candidates for a possible triggering mechanism that could have initiated such a reinforcing albedo feedback loop, and discuss how climate can be affected by perhaps a more familiar result of plate tectonics: mountain ranges.

References:

CNRS (Délégation Paris Michel-Ange). (2011, October 12). ‘Snowball Earth’ hypothesis challenged. ScienceDaily. Retrieved October 2, 2016 from

Fairchild, I. J., & Kennedy, M. J. (2007). Neoproterozoic glaciation in the Earth System. Journal of the Geological Society164(5), 895-921.

Hoffman, P. F. (2005). 28th DeBeers Alex. Du Toit Memorial Lecture, 2004. On Cryogenian (Neoproterozoic) ice-sheet dynamics and the limitations of the glacial sedimentary record. South African Journal of Geology108(4), 557-577.

Jacobsen, S. B. (2001). Earth science: Gas hydrates and deglaciations.Nature412(6848), 691-693.

Li, Z. and Bogdanova, S. and Collins, A. and Davidson, A. and De Waele, B. and Ernst, R. and Fitzsimons, I. et al. 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research. 160: pp. 179-210.

Riguzzi, F., et al., Can Earth’s rotation and tidal despinning drive plate tectonics? Tectonophysics (2009).

Tsvetsinskaya, E. A., C. B. Schaaf, F. Gao, A. H. Strahler, R. E. Dickinson, X. Zeng, and W. Lucht, Relating MODIS derived surface albedo to soils and landforms over Northern Africa and the Arabian peninsula, Geophys. Res. Lett.,29(9), doi:10.1029/2001GL014096, 2002.

Watson C.S., King M.A., White N.J., Church J.A., Legresy B., & Burgette R.J. (2015). Unabated global mean sea-level rise over the satellite altimeter era. Nature Climate Change,5(6), 565-568. doi:10.1038/nclimate2635