CO2, Hothouse and Snowball

Gareth E. Roberts

Department of Mathematics and Computer Science College of the Holy Cross Worcester, MA, USA

Mathematical Models MATH 303 Fall 2018 November 12 and 14, 2018

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 1 / 42 Lecture Outline

The and the Earth’s history

Consequences of Global Warming

The long- and short-term carbon cycles and

The Snowball Earth hypothesis

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 2 / 42 Chapter 1 Historical Overview of Science

Frequently Asked Question 1.3 What is the Greenhouse Effect?

The powers Earth’s climate, radiating energy at very short Earth’s natural greenhouse effect makes as we know it pos- wavelengths, predominately in the visible or near-visible (e.g., ul- sible. However, activities, primarily the burning of fossil traviolet) part of the spectrum. Roughly one-third of the solar fuels and clearing of forests, have greatly intensiﬁ ed the natural energy that reaches the top of Earth’s is reﬂ ected di- greenhouse effect, causing global warming. rectly back to space. The remaining two-thirds is absorbed by the The two most abundant gases in the atmosphere, nitrogen surface and, to a lesser extent, by the atmosphere. To balance the (comprising 78% of the dry atmosphere) and (comprising absorbed incoming energy, the Earth must, on average, radiate the 21%), exert almost no greenhouse effect. Instead, the greenhouse same amount of energy back to space. Because the Earth is much effect comes from molecules that are more complex and much less colder than the Sun, it radiates at much longer wavelengths, pri- common. Water vapour is the most important , and

marily in the infrared part of the spectrum (see Figure 1). Much (CO2) is the second-most important one. , of this thermal radiation emitted by the land and ocean is ab- nitrous oxide, ozone and several other gases present in the atmo- sorbed by the atmosphere, including clouds, and reradiated back sphere in small amounts also contribute to the greenhouse effect. to Earth. This is called the greenhouse effect. The glass walls in In the humid equatorial regions, where there is so much water a greenhouse reduce airﬂ ow and increase the temperature of the vapour in the air that the greenhouse effect is very large, add-

air inside. Analogously, but through a different physical process, ing a small additional amount of CO2 or water vapour has only a the Earth’s greenhouse effect warms the surface of the planet. small direct impact on downward infrared radiation. However, in

Without the natural greenhouse effect, the average temperature at the cold, dry polar regions, the effect of a small increase in CO2 or Earth’s surface would be below the freezing point of water. Thus, (continued)

FAQ 1.3, Figure 1. An idealised model of the natural greenhouse effect. See text for explanation. Figure: The Greenhouse Effect. Source: “Historical Overview of Climate Change

Science,” IPCC AR4, (2007) p. 115. 115

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 3 / 42 The Greenhouse Effect

Energy from the Sun reaches the Earth in short wavelengths, mostly in the visible spectrum (e.g., ultraviolet). About half of this is absorbed by the Earth’s surface (land and oceans).

In order to achieve energy balance, the Earth radiates this energy back. But since the Earth is much colder than the Sun, it radiates energy in much longer wavelengths (infrared spectrum).

Due to their chemical properties, greenhouse gases (water, CO2, methane) increase the ability of the atmosphere to absorb radiation in the infrared spectrum. Parts of this radiation are then re-emitted back toward the Earth, warming the planet.

Without the greenhouse effect, the Earth’s average surface temperature would be well below the freezing point of water ( #2 from Lab #3 predicts −18.24◦C).

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 4 / 42 Figure: The electromagnetic spectrum indicating the frequency differences between the Earth and Sun. Source: Yochanan Kushnir https://www.learner.org/courses/envsci/visual/visual.php? shortname=electromagnetic_spectrum

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 5 / 42 Figure: The Observatory located on Mauna Loa on Island. The observatory consists of 10 buildings from which up to 250 different atmospheric parameters are measured by a group of 12 NOAA/ESRL and other agency scientists and engineers.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 6 / 42 Figure: The Keeling Curve. CO2 concentrations (red) measured from the in Hawaii from March 1958 to August 2018. Seasonally corrected data shown in black.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 7 / 42 The Keeling Curve

Measurements of carbon dioxide (CO2) concentrations in the atmosphere recorded at the Mauna Loa Observatory

Study began by (1928–2005) of the Scripps Institution of in 1958; now run by his son . Methane measurements began in 1983.

Remote location ideal because measurements are not impacted by local vegetation or human activity, and wind patterns bring well-mixed air samples. Even though it is near a volcano (which would tend to increase CO2 concentrations), the prevailing winds suppress this effect.

Air samples taken hourly, every day, with the same measuring method, for 60 years. “The most important data set in modern climate research.”

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 8 / 42 Quadratic Fit to Mauna Loa Data 420

400

380 2 360 CO

340

320

300 1950 1960 1970 1980 1990 2000 2010 2020 Year

Figure: Regression analysis with a quadratic ﬁt (blue) indicates a positive second derivative (recall Lab #1). R2 = 0.9935.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 9 / 42 Figure:CO 2 concentrations over the course of 2017 indicate a maximum in May and a minimum at the end of September. Why? Red bars are weekly means; blue bars are monthly averages.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 10 / 42 Earth’s Past Climate: Cores

Figure: Some ice cores contain ice 800,000 years old (carbon dating). The proportions of different oxygen and hydrogen help reconstruct ancient temperatures; air trapped in bubbles can be analyzed to determine past levels of greenhouse gases.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 11 / 42 articles

the discussion of our new data sets to the upper 3,310 m of the To calculate DTI from dD, we need to adopt a curve for the change , that is, down to the corresponding to marine in the isotopic composition of sea water versus time and correlate it 18 26 stage 11.3. with Vostok. We use the stacked d Osw record of Bassinot et al. , Lorius et al.4 established a glaciological timescale for the ﬁrst scaled with respect to the V19-30 marine sediment record over their climate cycle of Vostok by combining an ice-ﬂow model and an ice- common part that covers the past 340 kyr (ref. 27) (Fig. 2). To avoid accumulation model. This model was extended and modiﬁed in distortions in the calculation of DTI linked with dating uncertain- several studies12,13. The glaciological timescale provides a chronol- ties, we correlate the records by performing a peak to peak adjust- 18 ogy based on physics, which makes no assumption about climate ment between the ice and ocean isotopic records. The d Osw forcings or climate correlation except for one or two adopted correction corresponds to a maximum DTI correction of ,1 8C control ages. Here, we further extend the Extended Glaciological and associated uncertainties are therefore small. We do not attempt 12 Timescale (EGT) of Jouzel et al. to derive GT4, which we adopt as to correct DTI either for the change of the altitude of the or our primary chronology (see Box 1). GT4 provides an age of 423 kyr for the origin of the ice upstream of Vostok13; these terms are very at a depth of 3,310 m. poorly known and, in any case, are also small (,1 8C). The overall amplitude of the glacial–interglacial temperature Climate and atmospheric trends change is ,8 8C for DTI (inversion level) and ,12 8C for DTS, the Temperature. As a result of fractionation processes, the isotopic temperature at the surface (Fig. 3). Broad features of this record are content of snow in East (dD or d18O) is linearly related thought to be of large geographical signiﬁcance (Antarctica and part to the temperature above the inversion level, TI, where precipitation of the Southern Hemisphere), at least qualitatively. When examined forms, and also to the surface temperature of the precipitation site, in detail, however, the Vostok record may differ from coastal28 sites

TS (with DTI ¼ 0:67DTS, see ref. 6). We calculate temperature in East Antarctica and perhaps from West Antarctica as well. changes from the present temperature at the atmospheric level as Jouzel et al.13 noted that temperature variations estimated from 18 18 DTI ¼ðDdDice 2 8Dd OswÞ=9, where Dd Osw is the globally aver- deuterium were similar for the last two glacial periods. The third aged change from today’s value of d18O, and 9‰ per 8C is and fourth climate cycles are of shorter duration than the ﬁrst two the spatial /temperature gradient derived from deuterium cycles in the Vostok record. The same is true in the deep-sea record, data in this sector of East Antarctica21. We applied the above where the third and fourth cycles span four precessional cycles relationship to calculate DTS. This approach underestimates DTS rather than ﬁve as for the last two cycles (Fig. 3). Despite this by a factor of ,2 in Greenland22 and, possibly, by up to 50% in difference, one observes, for all four climate cycles, the same Antarctica23. However, recent model results suggest that any under- ‘sawtooth’ sequence of a warm interglacial (stages 11.3, 9.3, 7.5 estimation of temperature changes from this equation is small for and 5.5), followed by increasingly colder interstadial events, and Antarctica24,25. ending with a rapid return towards the following interglacial. The

Depth (m) 0 500 1,000 1,500 2,000 2,500 2,750 3,000 3,200 3,300

280 260 240 (p.p.m.v.) 2 220 a 2 CO C)

200 0 ° –2 –4 b 700 –6 –8 ( Temperature 600 (p.p.b.v.)

4 500 c CH 400 –0.5

0.0 (‰)

d atm

0.5 O 18 δ

N 100 ° 1.0 50 e 0

Insolation J 65 –50 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000

Age (yr BP) Figure 3 Vostok time series and insolation. Series with respect to time (GT4 thermal conductivity chromatographic detector has been replaced by a ﬂame

timescale for ice on the lower axis, with indication of corresponding depths on the ionization detector which measures CO2 after its transformation into CH4. The

Figure:top axis)of: Timea, CO2; b, isotopic series temperature data of theatmosphere from ice (see text); corec, CH4; samplesmean resolution of in the LakeCO2 (CH4) proﬁle Vostok, is about 1,500 Antarctica. (950) years. It goes up to 18 −2 a =d, CO2,d Oatm; and be, mid-June= Antarctic insolation at 658 N temp. (in W m ) (ref. changes 3). CO2 and CH4 , cabout= 6,000 Methane. years for CO2 in theSource: fractured zones “Climate and in the bottom and part of the measurements have been performed using the methods and analytical pro- record, whereas the CH4 time resolution ranges between a few tens of years to 5,9 atmosphericcedures previously describedhistory. However, of the the CO past2 measuring 420,000 system has been years4,500 from years. The the overall Vostok accuracy for CH ice4 andcore, CO2 measurements Antarctica,” are 620 p.p.b.v. Petit,slightly et. modiﬁed al., inNature order to increase399 the, sensitivity June of 3, the CO 1999,2 detection. pp. The 429–436.and 2–3 p.p.m.v., respectively. No gravitational correction has been applied. © 1999 Macmillan Magazines Ltd | VOL 399 | 3 JUNE 1999 | www.nature.com 431

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 12 / 42 Figure: Very strong correlation between temperature and CO2 concentrations from the Vostok ice core data. Concentrations over the last 420,000 years range from 185 to 300 ppmV (parts per million by volume).

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 13 / 42 Math 5490 9/8/2014

Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Atmospheric CO2 (Vostok data) Atmospheric CO2 (Vostok data)

400 400

350 350

300 300 (ppm) (ppm) 2 250 2 250 CO CO

atmos 200 atmos 200

150 150

100 100 ‐450 ‐400 ‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0 ‐450 ‐400 ‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0 time (Kyr) time (Kyr)

Petit, et al, Nature 399 (June 3 1999), pp.429-436 Petit, et al, Nature 399 (June 3 1999), pp.429-436

Math 5490 9/8/2014 Math 5490 9/8/2014

Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Atmospheric CO2 & Temperature (Vostok data) Vostok Data 12 10 Current 8 conditions 6 are well 4

C) 2

outside the ° (

T 0 range δ recorded in ‐2 the ice core ‐4 data. ‐6 ‐8 ‐10 150 200 250 300 350 400 450 CO2 (ppm) Pam Martin, University of Chicago, 2010

Math 5490 9/8/2014 Figure: A scatter plot of the temperature changeMath versus 5490 9/8/2014 CO2 concentration for the data, showing a clear correlation between temperature and CO2 (Petit, et. al., Nature 399, June 3, 1999, pp. 429–436). The green dot represents current conditions. Figure Source: Dick McGehee, Univ. of Minnesota and MCRN, lecture slides.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 14 / 42 Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Vostok Data 12 10 8 Extrapolate 6 linear 4

C) 2 ° ( regression to T 0 δ 400 ppm ‐2 CO2. ‐4 ‐6 ‐8 ‐10 150 200 250 300 350 400 450 CO2 (ppm) http://www.carleton.edu/departments/geol/DaveSTELLA/Carbon/long_term_carbon.htm

Math 5490 9/8/2014 Math 5490 9/8/2014

Richard McGehee, University of Minnesota 3 Math 5490 9/8/2014

Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Atmospheric CO2 (Vostok data) Atmospheric CO2 (Vostok data)

400 400

350 350

300 300 (ppm) (ppm) 2 250 2 250 CO CO

atmos 200 atmos 200

150 150

100 100 ‐450 ‐400 ‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0 ‐450 ‐400 ‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 0 time (Kyr) time (Kyr)

Petit, et al, Nature 399 (June 3 1999), pp.429-436 Petit, et al, Nature 399 (June 3 1999), pp.429-436

Math 5490 9/8/2014 Math 5490 9/8/2014

Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Atmospheric CO2 & Temperature (Vostok data) Vostok Data 12 10 Current 8 conditions 6 are well 4

C) 2

outside the ° (

T 0 range δ recorded in ‐2 the ice core ‐4 data. ‐6 ‐8 ‐10 150 200 250 300 350 400 450 CO2 (ppm) Pam Martin, University of Chicago, 2010

Math 5490 9/8/2014 Math 5490 9/8/2014

Anthropogenic Warming Anthropogenic Warming

What Determines the Earth’s Temperature? What Determines the Earth’s Temperature?

Vostok Data 12 10 8 Extrapolate 6 linear 4

C) 2 ° ( regression to T 0 δ 400 ppm ‐2 CO2. ‐4 ‐6 ‐8 ‐10 150 200 250 300 350 400 450 CO2 (ppm) http://www.carleton.edu/departments/geol/DaveSTELLA/Carbon/long_term_carbon.htm

Figure: A naive linear extrapolation indicates aMath temperature 5490 9/8/2014 rise of over 10◦C! Math 5490 9/8/2014 Figure Source: Dick McGehee, Univ. of Minnesota and MCRN, lecture slides.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 15 / 42 Richard McGehee, University of Minnesota 3 CO2 concentrations over the last 800,000 years Source: NOAA Earth System Research Library, Global Monitoring Division https://www.esrl.noaa.gov/gmd/ccgg/data-products.html

Excellent resource for data and graphs on CO2 and Methane averages, a Greenhouse Gas Index, CO2 tracker, etc.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 16 / 42 Impacts of Global Warming

1 Rising sea level and increased coastal ﬂooding.

2 Longer and more damaging wildﬁre seasons.

3 More powerful and damaging hurricanes largely caused by rising ocean temperatures.

4 Increase in extreme weather events (heat waves, coastal ﬂooding, ). This has obvious impacts on our food supply.

5 Ocean acidiﬁcation (higher acidity near surface due to excess carbon dioxide) and long-term damage to coral reefs.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 17 / 42 The Destructive 2017 Hurricane Season

Figure: Three simultaneous hurricanes on Sept. 8, 2017: Katia (left), Irma (center) and Jose (right). The year 2017 saw 17 storms and 10 hurricanes, 6 of which were Category 3 or higher. Image Source: NOAA Suomi NPP satellite

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 18 / 42 The Great Thaw in July 2012

Figure: Around 97% of the experienced melting during the middle of July in 2012 (pink and red), up from a usual average of 50%. Most of the melt water refreezes; however, if the entire sheet were to melt, sea level could rise 24 feet!

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 19 / 42 Disappearing in Greenland

R: NAT. HIST. MUS. DENMARK MUS. HIST. NAT. R: L: DANISH GEODATA AGENCY; HANS HENRIK THOLSTRUP/ HENRIK HANS AGENCY; GEODATA DANISH L:

Photographs of Ujaraannaq Valley in southwest Greenland in summer in 1936 (left) and in 2013 show that several glaciers have disappeared.

28 JULY 2016 | VOL 535 | NATURE | 483 ǟ ƐƎƏƖ !,(++ - 4 +(2'#12 (,(3#"Ʀ / 13 .\$ /1(-%#1  341#ƥ ++ 1(%'32 1#2#15#"ƥ ǟ ƐƎƏƖ !,(++ - 4 +(2'#12 (,(3#"Ʀ / 13 .\$ /1(-%#1  341#ƥ ++ 1(%'32 1#2#15#"ƥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ ɥ Decline of the Greenland Ice Sheet

Figure: “Nobody expected the ice sheet to lose so much mass so quickly," Isabella Velicogna, geophysicist at the University of California, Irvine. Source: “The great Greenland meltdown,” E. Kintisch, Science, Feb. 23, 2017, doi:10.1126/science.aal0810

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 21 / 42 Figure: 2017 Global Signiﬁcant Weather and Climate Events. Source: NOAA Global Climate Report https://www.ncdc.noaa.gov/sotc/global/201713

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 22 / 42 Figure: Figure source: http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/ Carbon/long_term_carbon.htm

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 23 / 42 Long-Term

Cycle occurs over time scales on the order of 100,000 to a million years.

Carbon dioxide is added to atmosphere through volcanic activity on land and at mid-ocean ridges (e.g., Paciﬁc rim, hot springs at Yellowstone National Park).

Rain brings CO2 down into the oceans forming sediments ().

Shifting of () causes sediments to heat up, releasing CO2 back into the atmosphere.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 24 / 42 Figure: The long-term Carbon cycle — a million-year feedback loop.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 25 / 42 Silicate Weathering A loop or thermostat to help control the Earth’s temperature

Rainwater containing dissolved CO2 falling on silicate rocks (granite, feldspar) replaces a silicon atom with a carbon atom, eventually producing carbonate (limestone, marble) and silicon dioxide (quartz). This is known as silicate weathering.

CO2 + CaSiO3 −→ CaCO3 + SiO2

CO2 (gas) replaced by SiO2 () Due to volcanic activity and intense heat, the carbon atom in gets replaced by the silicon atom and the CO2 gets released, completing the long-term carbon cycle.

CaCO3 + SiO2 −→ CO2 + CaSiO3 The entire process is temperature dependent: speeds up for warmer temp.; slows down for cooler temp.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 26 / 42 222 The Open Atmospheric Science Earth’sJournal, 2008, Volume Climate 2 in the Cenozoic Era Hansen et al.

(a) Global Deep Ocean δ18O Antarctic Ice Sheet

C) 12 0 °

N. Hemisphere Ice Sheets δ 8 ↑ 1 18 Paleocene-Eocene O ( Thermal Maximum 4 2 0 / 00 ) Deep Ocean 0 3 Temperature ( ↑ Ol-1 4 Glaciation 5 60 50 40 30 20 10 0 (b) Global Deep Ocean Temperature Figure: Oxygen16 isotope records for foraminifera shells found in deep ocean -4 δ18O + 12 sediment cores12 provide a temperature record-2 ( forδ18O the- 4.25) past 65 million years C) (Cenozoic era).° Source: Hansen, et. al., “Target Atmospheric CO2: Where Should Humanity Aim?” The 8 Open Atmospheric Science Journal 2 (2008), pp. 217–231.

4 Temperature ( 0 60 50 40 30 20 10 0 Time (My BP) Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 27 / 42 Fig. (3). Global deep ocean (a) 18O [26] and (b) temperature. Black curve is 5-point running mean of 18O original temporal resolution, while red and blue curves have 500 ky resolution.

3.1. Cenozoic Carbon Cycle Between 60 and 50 My ago India moved north rapidly, 18-20 cm/year [50], through a r egion that long had b een a Solid Earth sources and sinks of CO are not, in general, 2 depocenter for carbonate and organic sediments. Subduction balanced at any given time [30, 47]. CO is removed from 2 of car bon-rich cr ust w as s urely a lar ge s ource o f CO out - surface reservoirs by: (1) chemical weathering of rocks with 2 gassing and a prime cause of global warming, which peaked deposition of on the ocean floor, and (2) burial of 50 My ago (Fig. 3b) with the Indo-Asian collision. CO must organic matter; weathering is the dominant process [30]. CO 2 2 have then decreased due to a reduced subduction source and returns primarily via metamorphism and volcanic outgassing enhanced w eathering wit h upl ift of t he Him alayas/Tibetan at locations where carbonate-rich oceanic is being sub- Plateau [51]. Since then, the Indian and Atlantic Oceans have ducted beneath moving continental plates. been m ajor de pocenters for c arbon, but s ubduction of c ar- 12 13 Outgassing and burial of CO2 are each typically 10 -10 bon-rich crust has been limited mainly to small regions near mol C/year [30, 47-48]. At times of unus ual p late tectonic Indonesia and Central America [47]. activity, such as rapid subduction of c arbon-rich ocean crust Thus atmospheric CO declined following the Indo-Asian or s trong oroge ny, t he i mbalance be tween out gassing a nd 2 collision [44] and climate cooled (Fig. 3b) leading to Antarc- burial can b e a significant fr action of t he one -way carbon tic glaciation by ~ 34 My. Antarctica has b een more or less flux. Although negative feedbacks in the geochemical carbon glaciated ever since. The rate of CO drawdown declines as cycle reduce the rate of surface reservoir perturbation [49], a 2 atmospheric CO de creases due to ne gative fe edbacks, i n- net i mbalance ~1012 m ol C/year c an be maintained ove r 2 cluding the effect of de clining atmospheric temperature and thousands of ye ars. S uch an imbalance, if c onfined to t he growth rates on we athering [30]. These negative feed- atmosphere, woul d be ~ 0.005 ppm/year, but a s C O is d is- 2 backs te nd t o c reate a ba lance be tween c rustal out gassing tributed a mong s urface re servoirs, t his i s onl y ~ 0.0001 and dra wdown of C O2, which ha ve be en equal w ithin 1-2 ppm/year. T his r ate is n egligible co mpared to the p resent percent over the past 700 ky [52]. Large fluctuations in the human-made atmospheric CO 2 increase of ~ 2 ppm/year, yet size o f the A ntarctic ice sheet h ave o ccurred in th e p ast 3 4 over a m illion y ears s uch a cr ustal im balance al ters at mos- My, possibly related to temporal variations of plate tectonics pheric CO by 100 ppm. 2 [53] a nd out gassing ra tes. The re latively c onstant a tmos- Earth’s Climate in the Cenozoic Era

We have good geological records of the Earth’s climate (100 mya) from measuring oxygen isotopes (δ18O) in the deep ocean sediment cores. CO2 records only date back 800,000 years.

From 65–35 mya, the Earth was a hothouse with no polar ice caps. Small and rodents exist, (e.g., mice, rabbits). Warm period conﬁrmed by the spread of warm-adapted and mammals into high , as well as magnesium/calcium ratios in marine fossil shells.

About 35 mya, for reasons not well understood, Earth shifted dramatically into a glacial cooling period. One theory is that Asia and India collided, forming the Himalayas and Tibetan plateau. The newly formed mountains absorbed huge amounts of CO2 through silicate weathering, thus reducing the greenhouse effect dramatically and cooling the planet enough for ice sheets to begin to form.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 28 / 42 Target CO2 Level?

“Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 fell to 450 ± 100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.” — Hansen, et. al., “Target Atmospheric CO2: Where Should Humanity Aim?” The Open Atmospheric Science Journal 2 (2008), pp. 217–231.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 29 / 42 Chapter 7 Couplings Between Changes in the and Biogeochemistry

Figure 7.3. The global carbon cycle for the 1990s, showing the main annual ﬂ uxes in GtC yr–1: pre-industrial ‘natural’ ﬂ uxes in black and ‘anthropogenic’ ﬂ uxes in red (modi- ﬁ ed from Sarmiento and Gruber, 2006, with changes in pool sizes from Sabine et al., 2004a). The net terrestrial loss of –39 GtC is inferred from cumulative emissions minusFigure: atmospheric increaseThe minus short-term ocean storage. The carbonloss of –140 GtC cycle from the ‘vegetation, indicating soil and detritus’ anthropogenic compartment represents the (human)cumulative emissions from land use changeimpacts (Houghton, 2003), (red) and requires versus a terrestrial the biosphere natural sink of 101 GtC biological (in Sabine et al., given and only as chemical ranges of –140 to –80 weathering GtC and 61 to 141 GtC, respectively; other uncertainties given in their Table 1). Net anthropogenic exchanges with the atmosphere are from Column 5 ‘AR4’ in Table 7.1. Gross ﬂ uxes generally have uncertainties of more thanprocesses ±20% but fractional amounts (black). have beenRates retained to achieve are overall given balance inwhengigatons including estimates ofin fractions carbon of GtC yr–1 per for riverine year. transport,Source: weathering, deep ocean burial,Couplings etc. ‘GPP’ is annual between gross (terrestrial) changes primary production. in Atmospheric the climate carbon content system and all cumulative and ﬂ biogeochemistry,uxes since 1750 are as of end 1994. IPCC AR4 (2007), p. 515. change (primarily deforestation) (Table 7.1). Almost 45% of uptake associated with regrowth. The ‘airborne fraction of total combined anthropogenic CO2 emissions (fossil fuel plus land emissions’ is thus deﬁ ned as the atmospheric CO2 increase as a use) have remained in the atmosphere. Oceans are estimated to fraction of total anthropogenic CO2 emissions, including the net have taken up approximately 30% (about 118 ± 19 GtC: Sabine land use ﬂ uxes. The airborne fraction varies from year to year et al., 2004a; Figure 7.3), an amount that can be accounted for mainly due to the effect of interannual variability in land uptake Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 30 / 42 by increased atmospheric concentration of CO2 without any (see Section 7.3.2). change in ocean circulation or biology. Terrestrial have taken up the rest through growth of replacement vegetation 7.3.1.3 New Developments in Knowledge of the Carbon on cleared land, land management practices and the fertilizing Cycle Since the Third Assessment Report effects of elevated CO2 and N deposition (see Section 7.3.3). Because CO2 does not limit signiﬁ cantly Sections 7.3.2 to 7.3.5 describe where knowledge and in the ocean, the biological pump does not take up and store understanding have advanced signiﬁ cantly since the Third anthropogenic carbon directly. Rather, marine biological cycling Assessment Report (TAR). In particular, the budget of of carbon may undergo changes due to high CO2 concentrations, anthropogenic CO2 (shown by the red ﬂ uxes in Figure 7.3) via feedbacks in response to a changing climate. The speed with can be calculated with improved accuracy. In the ocean, newly which anthropogenic CO2 is taken up effectively by the ocean, available high-quality data on the ocean carbon system have however, depends on how quickly surface waters are transported been used to construct robust estimates of the cumulative and mixed into the intermediate and deep layers of the ocean. A ocean burden of anthropogenic carbon (Sabine et al., 2004a) considerable amount of anthropogenic CO2 can be buffered or and associated changes in the carbonate system (Feely et al., neutralized by dissolution of CaCO3 from surface sediments in 2004). The pH in the surface ocean is decreasing, indicating the the deep sea, but this process requires many thousands of years. need to understand both its interaction with a changing climate The increase in the atmospheric CO2 concentration relative and the potential impact on organisms in the ocean (e.g., Orr to the emissions from fossil fuels and cement production only et al., 2005; Royal Society, 2005). On land, there is a better 2 is deﬁ ned here as the ‘airborne fraction’. Land emissions, understanding of the contribution to the buildup of CO2 in the although signiﬁ cant, are not included in this deﬁ nition due atmosphere since 1750 associated with land use and of how to the difﬁ culty of quantifying their contribution, and to the the land surface and the terrestrial biosphere interact with a complication that much land emission from logging and changing climate. Globally, inverse techniques used to infer the clearing of forests may be compensated a few years later by magnitude and location of major ﬂ uxes in the global carbon

2 This deﬁ nition follows the usage of C. Keeling, distinct from that of Oeschger et al. (1980).

515 Changes in Atmospheric Constituents and in Chapter 2

6 CDIAC, http://cdiac.esd.ornl.gov/; WDCGG, http://gaw.kishou.go.jp/wdcgg.html.

138 Understanding and Attributing Climate Change Chapter 9

also occurred over several decades during the ﬁ rst half of the The fact that climate models are only able to reproduce 20th century, followed by a period of more than three decades observed global mean temperature changes over the 20th century when temperatures showed no pronounced trend (Figure 3.6). when they include anthropogenic forcings, and that they fail to Since the mid-1970s, land regions have warmed at a faster rate do so when they exclude anthropogenic forcings, is evidence than oceans in both hemispheres (Figure 3.8) and warming over for the inﬂ uence of on global climate. Further evidence the SH was smaller than that over the NH during this period is provided by spatial patterns of temperature change. Figure (Figure 3.6), while warming rates during the early 20th century 9.6 compares observed near-surface temperature trends over the were similar over land and ocean.

9.4.1.2 Simulations of the 20th Century

There are now a greater number of climate simulations from

684 Recap

The greenhouse effect is required to keep the Earth at a habitable temperature.

We have excellent records of CO2 concentrations over the past 800,000 years, recently from observatories such as Mauna Loa, and historically from ice core data. The ice core data reveal a strong correlation between the amount of greenhouse gases in the atmosphere and the temperature. A long-term carbon cycle exists from natural processes (volcanoes, silicate weathering) that serves as a thermostat for global temperature. It acts on very long time scales.

Over the past 420,000 years, CO2 concentrations have been in the range 185–310 ppmV. Now they are over 400 ppmV and climbing, resulting in a 1◦C rise in temperature over the 20th century mean. There is very strong evidence that human activity (e.g., burning of fossil fuels) is responsible for this.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 33 / 42 Snowball Earth

by Paul F. Hoffman and Daniel P. Schrag

Ice entombed our planet hundreds of millions of years ago, and complex animals evolved in the greenhouse heat wave that followed

ur human ancestors had it rough. Saber-toothed Aside from grinding glaciers and groaning sea ice, the only stir Figure: Paul F. Hoffmancats and woolly mammoths and may Daniel have been day- P.comes Schrag, from a smattering Scientiﬁcof volcanoes forcing their hot American heads , January O to-day concerns, but harsh climate was a consum- above the frigid surface. Although it seems the planet might ing long-term challenge. During the past million years, they never wake from its cryogenic slumber, the volcanoes slowly 2000, pp. 68–75.faced one ice age after another. At the height of the last icy manufacture an escape from the chill: carbon dioxide. episode, 20,000 years ago, glaciers more than two kilometers With the chemical cycles that normally consume carbon thick gripped much of North America and Europe. The chill dioxide halted by the frost, the gas accumulates to record lev- delivered ice as far south as New York City. els. The heat-trapping capacity of carbon dioxide—a green- Dramatic as it may seem, this extreme climate change pales house gas—warms the planet and begins to melt the ice. The in comparison to the catastrophic events that some of our ear- thaw takes only a few hundred years, but a new problem Roberts (Holyliest Cross) microscopic ancestors endured aroundCO 6002, million Hothouse years arises and in Snowballthe meantime: a Earth brutal greenhouse effect. Any crea-Mathematical Models 34 / 42 ago. Just before the appearance of recognizable animal life, in tures that survived the icehouse must now endure a hothouse. a time period known as the , an ice age pre- As improbable as it may sound, we see clear evidence that vailed with such intensity that even the tropics froze over. this striking climate reversal—the most extreme imaginable Imagine the earth hurtling through space like a cosmic snow- on this planet—happened as many as four times between 750 ball for 10 million years or more. Heat escaping from the million and 580 million years ago. Scientists long presumed molten core prevents the oceans from freezing to the bottom, that the earth’s climate was never so severe; such intense cli- but ice grows a kilometer thick in the –50 degree Celsius cold. mate change has been more widely accepted for other planets All but a tiny fraction of the planet’s primitive organisms die. such as Venus [see “Global Climate Change on Venus,” by

68 Scientiﬁc American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. Fire and Ice

genetic change in a short time, be- cess stiﬂed, the carbon dioxide in cause organisms that can most the atmosphere stabilizes at a level quickly alter their genes will have high enough to fend off the ad- the most opportunities to acquire vancing ice sheets. If all the conti- traits that will help them adapt nents cluster in the tropics, on the and proliferate. other hand, they would remain Hot-spring communities widely ice-free even as the earth grew separated geographically on the icy colder and approached the criti- surface of the globe would accumu- cal threshold for a runaway freeze. late genetic diversity over millions Some say the world will end in ﬁre, The carbon dioxide “safety of years. When two groups that switch” would fail because car- start off the same are isolated from Some say in ice. bon burial continues unchecked. each other long enough under dif- From what I’ve tasted of desire We may never know the true ferent conditions, chances are that trigger for a snowball earth, as at some point the extent of genetic I hold with those who favor ﬁre. we have but simple theories for mutation will produce a new spe- But if it had to perish twice, the ultimate forcing of climate cies. Repopulations occurring after change, even in recent times. But each glaciation would come about I think I know enough of hate we should be wary of the planet’s under unusual and rapidly chang- To say that for destruction ice capacity for extreme change. For ing selective pressures quite differ- the past million years, the earth ent from those preceding the gla- Is also great has been in its coldest state since ciation; such conditions would also animals ﬁrst appeared, but even favor the emergence of new life- And would sufﬁce. the greatest advance of glaciers forms. 20,000 years ago was far from the Martin Rudwick may not have —Robert Frost, critical threshold needed to plunge gone far enough with his inference Fire and Ice (1923) the earth into a snowball state. that climatic amelioration follow- Certainly during the next several ing the great Neoproterozoic ice hundred years, we will be more age paved the way for early animal evo- event has occurred since that time. But concerned with humanity’s effects on cli- lution. The extreme climatic events them- convincing geologic evidence suggests mate as the earth heats up in response to selves may have played an active role in that no such glaciations occurred in the carbon dioxide emissions [see “The Hu- spawning multicellular animal life. billion or so years before the Neopro- man Impact on Climate Change,” by We have shown how the worldwide terozoic, when the sun was even cooler. Thomas R. Karl and Kevin E. Trenberth; glacial deposits and carbonate rocks in The unusual conﬁguration of conti- Scientiﬁc American, December 1999]. the Neoproterozoic record point to an nents near the during Neopro- But could a frozen world be in our more extraordinary type of climatic event,Roberts a ter (Holyozoic times Cross) may better explain howCOdistant, Hothouse future? and Snowball Earth Mathematical Models 35 / 42 snowball earth followed by a briefer snowball events get rolling [see illustra- 2We are still some 80,000 years from but equally noxious greenhouse world. tion on page 70]. When the continents the peak of the next ice age, so our ﬁrst But what caused these calamities in the are nearer the poles, as they are today, chance for an answer is far in the fu- ﬁrst place, and why has the world been carbon dioxide in the atmosphere re- ture. It is difﬁcult to say where the spared such events in more recent histo- mains in high enough concentrations to earth’s climate will drift over millions of ry? The ﬁrst possibility to consider is keep the planet warm. When global tem- years. If the trend of the past million that the Neoproterozoic sun was weak- peratures drop enough that glaciers cover years continues and if the polar conti- er by approximately 6 percent, making the high- continents, as they do in nental safety switch were to fail, we the earth more susceptible to a global Antarctica and Greenland, the ice sheets may once again experience a global ice freeze. The slow warming of our sun as prevent chemical erosion of the rocks be- catastrophe that would inevitably jolt it ages might explain why no snowball neath the ice. With the carbon burial pro- life in some new direction. SA

The Authors Further Information

PAUL F. HOFFMAN and DANIEL P. SCHRAG, both at Harvard Origin and Early Evolution of the Metazoa. Edited by University, bring complementary expertise to bear on the snowball J. H. Lipps and P. W. Signor. Plenum Publishing, 1992. earth hypothesis. Hoffman is a ﬁeld geologist who has long studied an- The Origin of Animal Body Plans. D. Erwin, J. Valentine cient rocks to unravel the earth’s early history. He led the series of ex- and D. Jablonski in American Scientist, Vol. 85, No. 2, pages peditions to northwestern Namibia that turned up evidence for Neo- 126–137; March–April 1997. snowball earth events. Schrag is a geochemical oceanogra- A Neoproterozoic Snowball Earth. P. F. Hoffman, A. J. pher who uses the chemical and isotopic variations of coral reefs, Kaufman, G. P. Halverson and D. P. Schrag in Science, Vol. deep-sea sediments and carbonate rocks to study climate on timescales 281, pages 1342–1346; August 28, 1998. ranging from months to millions of years. Together they were able to The First Ice Age. Kristin Leutwyler. Available only at www. interpret the geologic and geochemical evidence from Namibia and to sciam.com/2000/0100issue/0100hoffman.html on the Scientiﬁc explore the implications of a snowball earth and its aftermath. American Web site.

followed. After hundreds of millions of G years of burial, these now exposed A SCHR rocks tell the story that scientists ﬁrst . began to piece together 35 years ago. In 1964 W. Brian Harland of the Uni-

versity of Cambridge pointed out that P DANIEL OF TESY OUR

glacial deposits dot Neoproterozoic rock C

70 Scientiﬁc American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. Figure: Hoffman (right) and Schrag (left) at their ﬁeld site in Namibia studying rocks from the Neoproterozic era (1000–540 Mya). The light-colored boulder likely traveled within a glacier during a runaway snowball event and fell to the muddy seaﬂoor when the ice melted. Both at Harvard, Hoffman is a ﬁeld geologist with a long and storied career of studying ancient rocks while Schrag is a geochemical oceanographer who studies the chemical and isotopic variations of coral reefs, deep-ocean sediments, and carbonate rocks. (Hoffman and Schrag, “Snowball Earth,” Scientiﬁc American, Jan. 2000, p. 70)

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 36 / 42 Snowball Earth Theory

Around 740 and 635 Mya, glaciers covered nearly all of the Earth’s surface. Evidence comes from ancient rocks and glacial debris found near sea level in the tropics. Glaciers near the equator can only exist at 5,000 meters above sea level (or higher).

Ice and snow have a much higher rate of reﬂectivity (albedo) than do water and land. The more ice on the planet, the higher the albedo and thus less energy is received by the Earth (less Ein). This creates an ice-albedo feedback that can overwhelm the greenhouse effect and drive the planet toward a snowball state.

During this time period, the continents were clustered around the equator. Thus, silicate weathering was allowed to continue unabated as the glaciers slowly moved towards the equator. This meant CO2 was continually being pulled from the atmosphere as the temperatures dropped.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 37 / 42 Realizing that the glaciers must have covered the tropics, Harland became the ﬁrst geologist to suggest that the earth SOUTH CHINA AUSTRALIA had experienced a great Neoproterozoic SIBERIA ice age [see “The Great Infra-Cambrian KAZAKHSTAN NORTH AMERICA Glaciation,” by W. B. Harland and M.J.S. Rudwick; Scientiﬁc American, AFRICA August 1964]. Although some of Har- INDIA land’s contemporaries were skeptical WEST AFRICA about the reliability of the magnetic data, other scientists have since shown EASTERN that Harland’s hunch was correct. But AND SOUTH AMERICA NORTHERN SOUTH AMERICA EUROPE ANTARCTICA no one was able to ﬁnd an explanation for how glaciers could have survived the HEIDI NOL tropical heat. At the time Harland was announcing EARTH’S LANDMASSES were most likely clustered near the equator during the global glaciations that took place around 600 million years ago. Although the continents have his ideas about Neoproterozoic glaciers, since shifted position, relics of the debris left behind when the ice melted are exposed at physicists were developing the ﬁrst dozens of points on the present land surface, including what is now Namibia (red dot). mathematical models of the earth’s cli- mate. Mikhail Budyko of the Leningrad Geophysical Observatory found a way Figure:idea During in the thejournal runaway Science snowballa year and events,a outcrops Earth’s across continents virtually every were continent. to explain tropical glaciers using equa- clusteredhalf aroundago. If we the turn equator. out to be(Hoffman right, the andBy Schrag,the early “Snowball1960s scientists Earth,” hadScientiﬁc begun tions that describe the way solar radia- Americantale, does Jan. mor 2000,e than p. 70)explain the myster- to accept the idea of plate tectonics, tion interacts with the earth’s surface ies of Neoproterozoic climate and chal- which describes how the planet’s thin, and atmosphere to control climate. lenge long-held assumptions about the rocky skin is broken into giant pieces Some geographic surfaces reﬂect more Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 38 / 42 limits of global change. These extreme that move atop a churning mass of hotter of the sun’s incoming energy than oth- glaciations occurred just before a rapid rock below. Harland suspected that the ers, a quantiﬁable characteristic known diversiﬁcation of multicellular life, cul- continents had clustered together near as albedo. White snow reﬂects the most minating in the so-called Cambrian ex- the equator in the Neoproterozoic, based solar energy and has a high albedo, plosion between 575 and 525 million on the magnetic orientation of tiny min- darker-colored seawater has a low albe- years ago. Ironically, the long periods of eral grains in the glacial rocks. Before do, and land surfaces have intermediate isolation and extreme environments on the rocks hardened, these grains aligned values that depend on the types and dis- a snowball earth would most likely have themselves with the magnetic ﬁeld and tribution of vegetation. spurred on genetic change and could dipped only slightly relative to horizon- The more radiation the planet reﬂects, help account for this evolutionary burst. tal because of their position near the the cooler the temperature. With their The search for the surprisingly strong equator. (If they had formed near the high albedo, snow and ice cool the at- evidence for these climatic events has poles, their magnetic orientation would mosphere and thus stabilize their own taken us around the world. Although be nearly vertical.) existence. Budyko knew that this phe- we are now examining Neoproterozoic rocks in Australia, China, the western U.S. and the Arctic islands of Svalbard, we began our investigations in 1992 along the rocky cliffs of Namibia’s Skeleton Coast. In Neoproterozoic times, this region of southwestern Africa was part of a vast, gently subsiding continental shelf located in low south- ern latitudes. There we see evidence of glaciers in rocks formed from deposits of dirt and debris left behind when the ice melted. Rocks dominated by calcium- and mag- nesium-carbonate minerals lie just above the glacial debris and harbor the chemical evidence of the hothouse that

followed. After hundreds of millions of G years of burial, these now exposed A SCHR rocks tell the story that scientists ﬁrst . began to piece together 35 years ago. In 1964 W. Brian Harland of the Uni-

versity of Cambridge pointed out that P DANIEL OF TESY OUR

glacial deposits dot Neoproterozoic rock C

70 Scientiﬁc American January 2000 Snowball Earth Copyright 1999 Scientific American, Inc. Escaping Snowball

Figure: Koh Xuan Yang, Beyond Earthly Skies (Blog), http://beyondearthlyskies.blogspot.com/2013/09/ snowball-earth-thawing-snowball.html

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 39 / 42 nomenon, called the ice-albedo feed- tinuity of life. Also, once the earth had The key to the second problem—re- back, helps modern polar ice sheets to entered a deep freeze, the high albedo versing the runaway freeze—is carbon grow. But his climate simulations also of its icy veneer would have driven sur- dioxide. In a span as short as a human revealed that this feedback can run out face temperatures so low that it seemed lifetime, the amount of carbon dioxide of control. When ice formed at latitudes there would have been no means of es- in the atmosphere can change as plants lower than around 30 degrees north or cape. Had such a glaciation occurred, consume the gas for photosynthesis and south of the equator, the planet’s albedo Budyko and others reasoned, it would as animals breathe it out during respi- began to rise at a faster rate because di- have been permanent. ration. Moreover, human activities such rect sunlight was striking a larger surface The ﬁrst of these objections began to as burning fossil fuels have rapidly area of ice per degree of latitude. The fade in the late 1970s with the discovery loaded the air with carbon dioxide feedback became so strong in his simula- of remarkable communities of organ- since the beginning of the Industrial tion that surface temperatures plummet- isms living in places once thought too Revolution in the late 1700s. In the ed and the entire planet froze over. harsh to harbor life. Seaﬂoor hot springs earth’s lifetime, however, these carbon support microbes that thrive on chemi- sources and sinks become irrelevant Frozen and Fried cals rather than sunlight. The kind of compared with geologic processes. volcanic activity that feeds the hot Carbon dioxide is one of several gas- udyko’s simulation ignited interest springs would have continued unabated es emitted from volcanoes. Normally Bin the ﬂedgling science of climate in a snowball earth. Survival prospects this endless supply of carbon is offset modeling, but even he did not believe seem even rosier for psychrophilic, or by the erosion of silicate rocks: The the earth could have actually experi- cold-loving, organisms of the kind living chemical breakdown of the rocks con- enced a runaway freeze. Almost every- today in the intensely cold and dry verts carbon dioxide to , one assumed that such a catastrophe mountain valleys of East Antarctica. which is washed to the oceans. There would have extinguished all life, and and certain kinds of bicarbonate combines with calcium yet signs of microscopic algae in rocks occupy habitats such as snow, porous and magnesium to produce car- up to one billion years old closely re- rock and the surfaces of dust particles en- bonate sediments, which store a great semble modern forms and imply a con- cased in ﬂoating ice. deal of carbon [see “Modeling the Geo- G A SCHR . TES ANS AL F AL ARBONA TESY OF DANIEL P DANIEL OF TESY ST Y OUR AP C AP C CR C

ROCKY CLIFFS along Namibia’s Skele- ton Coast (left) have provided some of the best evidence for the snowball earth hypothesis. Authors Schrag (far left) and Hoffman point to a rock layer that repre- S sents the abrupt end of a 700-million-year- old snowball event. The light-colored boulder in the rock between them proba- bly once traveled within an iceberg and

CIAL DEPOSIT CIAL fell to the muddy seaﬂoor when the ice A VERSON

GL melted. Pure carbonate layers stacked

A HAL A above the glacial deposits precipitated in IPP the warm, shallow seas of the hothouse aftermath. These “cap” carbonates are the only Neoproterozoic rocks that exhibit

TESY OF GALEN P GALEN OF TESY large crystal fans, which accompany rapid OUR

C carbonate accumulation (above).

Snowball Earth Scientiﬁc American January 2000 71 Copyright 1999 Scientific American, Inc. Figure: Pure carbonate layers, typically found in shallow, warm waters, situated directly above the glacier rocks are evidence of an ensuing hothouse. The crystal fans suggest a rapid carbonate accumulation. Sudden variation in the carbon isotopes also supports the relatively rapid transition from snowball to hothouse.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 40 / 42 Escaping Snowball

The snowball state would last around 10 million years! Huge amounts of CO2 would build up from volcanoes enabling the greenhouse effect to warm the planet enough for the ice to melt near the equator.

Open water forming in the tropics absorbs more solar energy, helping to accelerate the increase in global temperatures. Now the ice-albedo effect works in the opposite direction: less ice and more water means a lower albedo and more solar radiation being absorbed by the planet.

Global temperatures rise so quickly (“in a matter of centuries?!”) that the effect of silicate weathering is not enough to halt a runaway path to a hothouse planet.

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 41 / 42 Life Finds a Way

One of the major objections to Snowball Earth comes from biologists: how could life have survived?

Microbes have been discovered on seaﬂoor hot springs that thrive on chemicals rather than sunlight. Cold-loving organisms have been found in cold and dry mountain valleys in East Antarctica. Cyanobacteria and some types of algae can survive in snow, porous rock, and in dust particles.

The 11 animal phyla (our genetic ancestors) all emerged from the time period of the last snowball event, known as the .

“Ironically, the long periods of isolation and extreme environments on a snowball earth would most likely have spurred on genetic change and could help account for this evolutionary burst.” — Hoffman and Schrag

Roberts (Holy Cross) CO2, Hothouse and Snowball Earth Mathematical Models 42 / 42