Energy Balance and the Snowball Earth Models

Author: Tina Gale

Advisor: prof. dr. Lučka Kajfež Bogataj

Ljubljana, 14th April 2004

Picture available from the University of Bristol Website

Picture 1: Stages of snowball Earth.

2

Abstract Seminar presents some basic aspects of energy balance in Earth system. In order to show, how extreme climate condition are possible, there are two different climate models described. The first model includes energy transport between latitudinal zones. Second simulation of climate behaviour is more advanced and only results are presented. At the end, the stages of so called snowball Earth episode are described.

Table of Contents

1 Introduction ........................................................................................................................ 3

2 Earth’s Energy Balance...................................................................................................... 4

2.1 Radiation .................................................................................................................... 4 2.2 Albedo and Distribution of Insolation........................................................................ 4 2.3 Energy Flow ............................................................................................................... 5 2.4 Greenhouse Model ..................................................................................................... 7

3 Zonal Models...................................................................................................................... 9

3.1 Theoretical Background ............................................................................................. 9 3.2 Calculations.............................................................................................................. 12 3.3 Stable and Unstable Ranges ..................................................................................... 13 3.4 Climate Forcing with GISS GCM............................................................................ 13

4 Frozen and Fried............................................................................................................... 15

4.1 Snowball Earth Prologue.......................................................................................... 15 4.2 Snowball Earth at Its Coldest ................................................................................... 16 4.3 Hothouse Aftermath ................................................................................................. 18

5 Conclusion........................................................................................................................ 19

6 References ........................................................................................................................ 19

3

1 Introduction In 1964, Brian Harland at Cambridge University postulated that the Earth had experienced a great Neoproterozoic ice age. He pointed out that Neoproterozoic glacial deposits are widely distributed on virtually every continent. The first paradox was the occurrence of glacial deposits within types of marine sedimentary strata characteristic of low latitudes. Glaciers near the equator today survive only at 5.000 meters above sea level or higher, and at the worst of the last ice age they reached no lower than 4.000 meters. What could cause glaciers to reach sea level near the Equator? Which were the contributors that unfroze world, frozen completely for 10 million years? And how the living organisms survived? To find the answers we have to comprehend theoretical background first.

Picture 2: Geological eras

At the time Harland was examining Neoproterozoic glacial deposits, physicists were developing the first mathematical models of the Earth climate. Mikhail Budyko of the Leningrad Geophysical Observatory found a way to explain tropical glaciers using equations that describe the way solar radiation interacts with the earth's surface and atmosphere to control climate.

4

2 Earth’s Energy Balance 2.1 Radiation The Earth's climate is fundamentally controlled by the way that solar radiation interacts with the Earth's surface and atmosphere. At the present time in its evolution the Sun emits energy at a rate of Q = 3,87 1026 W. Earth receives

H0 = Q/4πr2 = 1369 W/m2

of radiation from the Sun at average Earth – Sun distance r = 1,5 1011 m. Wien’s law says, that wavelength of maximum radiation flux is inversely proportional to temperature of emitter. Since surface temperature of the Sun is Tsun = 6000 K, most energy emitted is in the visible spectrum and 95 % of total energy lies between 0,25 and 2,5 µm. Earth’s surface is cooler than the Sun, on average has about 5-10 ºC. Therefore, Earth emits longer (infrared) waves. The boundary between short-wave and long-wave is selected at wavelength of 4 µm.

Picture 3: Curves of radiation flux for the surface of the Sun and Earth, respectively.

2.2 Albedo and Distribution of Insolation The surface albedo is a measure of how much radiation is reflected; snow has a high albedo, seawater has a low albedo and land surfaces have intermediate values that vary widely depending on the types and distribution of vegetation. When snow falls on land or ice forms at sea, the increase in the albedo causes greater cooling, stabilizing the snow and ice. This is called ice-albedo feedback, and is an important factor in the waxing (and waning) of ice sheets. Albedo (α) Water < 0,1 Vegetation 0,03 to 0,2 Bare land 0,2 to 0,3 Clouds ~0,5 Ice and snow 0,5 to 0,9 Planetary average* ~0,3 *Planetary average albedo is mostly due to clouds (2/3) and snow/ice (1/3). Table 1: Albedo for different terrestrial surfaces.

5

Let's take all the insolation coming to the top of atmosphere as 100 units. Then: • 3 % of insolation is scattered into space, • 19 % is reflected into space by clouds, • 9 % is reflected into space by ground surface.

The sum of these is 31% and this is the global albedo of the earth-atmosphere system. That is, the Earth returns directly back to space slightly less than 1/3 of the insolation it receives. The global albedo varies depending on the atmospheric condition between 29 and 34 %. The remained insolation is absorbed:

• 17 % of insolation is absorbed by atmospheric gases and dust, • 3 % is absorbed by clouds, • finally, only 49 % of insolation reaches the ground.

Picture 4: Distribution of insolation.

2.3 Energy Flow By learning about how energy enters and leaves the Earth and achieves equilibrium we can figure out the effective temperature of the Earth as well as the temperature at the surface. One of the oldest and best known types of atmospheric science models are energy balance models. The term "balance" suggests that the system is at equilibrium and no energy is accumulated. The model attempts to account for all energy coming in and all energy going out of some system, in this case the Earth. In a simple global energy balance, the only variable is the temperature of the Earth, usually signified as Te, known as the ‘effective planetary temperature’ or ‘emission temperature’ of the Earth. Cross-sectional area of the Earth intercepting the solar flux = π Re

2. Solar radiation incident on the Earth = H0 π Re

2 = 1,74 × 1017 W. Solar radiation absorbed by the Earth = π Re

2 (1 - α) H0 = 1,22 × 1017 W. Emitted terrestrial radiation = 4π Re

2 σ Te4.

σ = 5,6697 10-8 W/m2K4 is the Stefan-Boltzmann constant.

Because the mean temperature of the Earth is neither increasing nor decreasing, the total terrestrial flux radiated to space must balance the solar radiation absorbed by the Earth:

4e

2e0

2e TR4H)1(R σπ=α−π

6

Note that this is a definition of emission temperature Te, which is the temperature one would infer looking back at Earth if a black body curve was fitted to the measured spectrum of outgoing radiation.

C18K2554

)1(HT 4 0

e °−==σ

α−= .

Note that the radius of the Earth, Re, has cancelled out: Te depends only on albedo and the distance of the Earth from the Sun. Putting in numbers we find that the Earth has an effective temperature of 255 K. Table 2 lists the various parameters for some of the planets and compares approximate measured values (Tm), with Te computed from upper equation. The agreement is very good, except for Jupiter where it is thought that approximately half of energy input comes from the gravitational collapse of the planet. However, as can be seen from Table 2, the effective temperature of Earth is 33 K cooler than the globally averaged observed surface temperature which is Ts = 288 K.

R [109 m] H0 [W/m2] α Te [K] Tm [K] Ts [K]

Venus 108 2632 0,77 227 230 760 Earth 150 1367 0,30 255 250 288 Mars 228 589 0,24 211 220 230 Jupiter 780 51 0,51 103 130 134 Table 2: Properties of some of the planets. H0 is the solar constant at a distance r from the Sun, α is the planetary albedo, Te is the emission temperature computed from upper equation, Tm is the measured emission temperature and Ts is the global mean surface temperature.

We saw that the atmosphere is rather opaque to IR, so we cannot think of terrestrial radiation as being radiated into space directly from the surface. Much of the radiation emanating from the surface will be absorbed by H2O and CO2 before passing through the atmosphere. On average, the emission to space will emanate from some level in the atmosphere (typically about 5 km) such that the region above that level is mostly transparent to IR. It is this region of the atmosphere, rather than the surface, that must be at the emission temperature. Thus radiation from the atmosphere will be directed downward, as well as upward, and hence the surface will receive not only the net solar radiation, but also IR from the atmosphere, which is called counterradiation. Because the surface feels more incoming radiation than if the atmosphere were not present (or were completely transparent to IR) it becomes warmer than Te. This has become known as the greenhouse effect. Temperatures diverts also because of fluid motions (air currents) carry heat both vertically and horizontally.

Picture 5: Absorption of IR by greenhouse gasses

7

We will now look at distribution of the outgoing long-wave radiation:

• 114 % of insolation is leaving the surface as ground radiation, • 23 % is lost as latent heat in water vapor through evaporation from oceans and soil, • 7 % is lost as sensible heat through conduction and convection.

So, the surface is losing 144 % out of 100 % of insolation. This is possible because of s counterradiation. Recall that the surface receives 49 % of insolation. In addition to that, counterradiation provides another 95 %. So together that makes 144 %, which shows that energy flows for the surface are balanced.

Picture 6: Distribution of outgoing long-wave radiation.

In a similar way, we can also balance flows for the atmosphere. Atmosphere gains:

• 20 % of direct solar energy absorption by molecules, dust and clouds, • 102 % of absorption of ground radiation, • 23 % of transfer of latent heat from the surface, • 7 % of transfer of sensible heat from the surface,

a total of 152 %. Atmosphere loses: • 95 % of counterradiation • 57 % of radiation to space

again, a total of 152%. 2.4 Greenhouse Model Since the atmosphere is thin, let us simplify things by considering a planar geometry, in which the incoming radiation per unit area is equal to the average flux per unit area striking the Earth. This average incoming solar flux, or insolation, per unit area of the entire Earth’s surface is

2

02e

2e0 m/W342H

41

R4RH

area surface Earthsradiation incoming dintercepteflux solar average ==

ππ

== .

At the surface, the average incoming short-wave flux is 0H)1(41

α− .

We will represent the atmosphere by a single layer of temperature Ta. The atmosphere is at the emission temperature, because it is this region that is emitting to space. We also assume that it is completely transparent to short-wave solar radiation. On the other hand, the atmosphere has

8

absorptivity ε, i.e., a fraction of the IR upwelling from the surface is absorbed within the atmosphere.

Terrestrial radiation being emitted to space, per unit area is 4aTA σ↑= .

Down welling flux from atmosphere is the same, 4aTAA σ↑=↓= .

The flux radiating upward from the ground is 4sTS σ↑= ,

where Ts is the surface temperature.

Picture 7: The greenhouse model, comprising a surface at temperature Ts, and an atmospheric layer at

temperature Ta, subject to incoming solar radiation H0/4. The atmosphere absorbs only a fraction, ε, of the terrestrial radiation up welling from the ground.

Since the whole Earth atmosphere system must be in equilibrium (on average), the net flux into the system must vanish:

↑ε−+↑=α− S)1(AH)1(41

0 .

Net flux at the ground must in equilibrium also be zero

↓+α−↑= AH)1(41S 0 or 4

a04s TH)1(

41T σ+α−=σ .

Combining those equations we obtain

a

4/1

s T2T ⎟⎠⎞

⎜⎝⎛

ε= .

In the limit, when ε → ∞ atmosphere acts as transparent, and temperatures of Earth and atmosphere are equal, Ts = Ta. In the limit of ε → 1 atmosphere is completely opaque to IR, i.e., it absorbs all the IR radiating up from the ground and the same layer is “seen” by the ground as is emitting to space. Surface temperature is

Ts = 21/4 Ta.

9

So the presence of an absorbing atmosphere increases the surface temperature by a factor 21/4 = 1,19. Applying this factor to our calculated value Te = 255 K, we predict Ts = 303 K = 30 °C. This is closer to the actual mean surface temperature of 288 K, but is now an overestimate! For one thing, not all the solar flux incident on the top of the atmosphere reaches the surface. Typically, some 20-25 % is absorbed within the atmosphere (including by clouds). For another, we saw that IR absorption is incomplete. In these cases, the greenhouse effect will be less effective, and Ts will be less that the value implied above. This analyse shows us that the atmosphere partial transmits IR. So in general, values are

0 < ε < 1: Ta < Ts < 21/4 Ta.

So, of course, partial transparency of the atmosphere to IR radiation (a “leaky” greenhouse) reduces the warming effect. Note, however, that the atmosphere is always cooler than the ground.

3 Zonal Models 3.1 Theoretical Background Energy balance models are typically one-dimensional, that dimension being latitude. The aim is to calculate the temperature at the surface or Ts:

Ts = Ta + dT, where dT is the greenhouse increment. As we saw in previous chapter, the greenhouse increment at this time is about 33 K, and is a function of the efficiency of the infrared absorption. All this would be fairly simple except for the fact that most energy balance models are not global models, but zonal or latitudinal models. As such, we must have some equation or part of an equation that accounts for the flow of energy from one latitudinal zone to the next. In this schematic the equation has now become:

radiation energy in = radiation energy out + transport into another zone.

As in previous model, for opaque atmosphere (ε = 1) we can propose the equation: 4

a0 TH)1( σ=α− , where, as before, α is the planetary albedo, H0 is the solar constant, and σ is the Stefan-Boltzmann constant. We also need to consider the fact that not all zones receive the same amount of incoming radiation, since the incident angle of the Sun to a particular zone varies. The larger the angle and the longer the day, the more radiation a place receives. Length of path through atmosphere is also important which is also dependent on the angle of incidence, so it effectively reinforces latitudinal differences.

Picture 8: Areas at Equator gets more solar

energy than poles.

Picture 9: Weather moves energy from the Equator to the poles.

The global energy budget is, of course, neatly balanced, that is, net radiation (incoming - outgoing) is equal to zero. This is not so for parts of the globe. Indeed, since insolation is distributed by latitude, one would expect that net radiation would also vary by latitude. If we average energy flows over an entire year, it becomes clear that low latitudes have positive energy balance (energy surplus), while high latitudes have negative energy balance (energy deficit). However, over the years low and high latitudes experience the same average temperatures, i.e. they don't progressively heat or cool. This situation can only be possible if energy is continuously transported from equatorial and tropical areas into middle and high latitudes. Two mechanisms are responsible for pole ward heat transport:

1) circulation of waters in the ocean and 2) global atmospheric circulation.

There exists some ratio, γi, that corrects the average incoming radiation for a given zone. We can then calculate the incoming solar radiation for a given zone as:

incoming radiation for zone i = 4

H0iγ .

We are also concerned with the surface albedo of each zone. These are dependent upon the ratio between land and water and the type of land covering. Each zone also has its own zonal surface temperature, which usually includes the greenhouse increment.

Picture 10: In zonal one-dimensional energy balance model we take into account transport between the

zones.

There exists a critical temperature, Tc, which is that temperature below which the land becomes ice covered. The typical literature value for this temperature is Tc = –10,0 °C. For a

11

given zone, the surface albedo depends on the surface temperature of that zone. If the surface temperature is below the critical temperature, the albedo for the zone is the albedo of ice, or 0,68. For temperatures above the critical temperature, several equations are used. The most typical is:

⎩⎨⎧

<≥α⋅ϕ+ϕ−

=α=αci

cicloudsiiii TT62,0

TT1)T( ,

where: αi is the surface albedo for the zone,

ϕi is fractional cloud cover in each zone, αclouds is albedo of the clouds.

Cloud albedo is typically specified as being 0,5. Fractional cloud cover values are a typical input for a standard energy balance model. However, in our model we can further simplify this equation:

⎩⎨⎧

<≥

=αci

cii TT62,0

TT31,0,

where 0,31 is average albedo for land, not covered with snow or ice, as seen in Table 1. McGuffie and Henderson-Sellers present a relatively simple one-dimensional Earth balance model. Interactive model is provided from The Shodor Education Foundation, Inc., and is accessible on internet page http://www.shodor.org/master/environmental/general/energy/. There exist more complicated methodologies, but this equation represents the fundamental parameters in an energy balance model. Again, the purpose of the energy balance model is to calculate the temperature of the zone. A final equation for calculating temperatures is as follows:

CB

B5ACT)1(4

H

Tisi

0i

i +

ϕ+−+α+γ=

where: Ti is the average temperature of the zone, γi is the ratio correction of zone to incoming radiatior, H0 is the solar constant, αi is the surface albedo for the zone, ϕi is fractional cloud cover in each zone, Ts is the mean global surface temperature, A = 204 W/m2 is a constant that defines the long-wave radiation loss, B = 2,17 W/m2C is another constant that defines the long-wave radiation loss, C = 3,80 W/m2C is the transport coefficient.

This equation is iterated until an equilibrium value is reached, determined by some user-defined tolerance value. In this model, the incoming and outgoing energy for the individual "zones" are calculated, and the individual temperatures and global mean temperature are calculated.

12

3.2 Calculations If we put into the model present value of solar constant and present values for zonal annual radiation and zonal initial temperatures we get global mean temperature of 14,8 °C. Altitudes from 70° to 90° have temperatures below critical and are covered with ice.

Temperatures, calculated with Zonal model

-60

-50

-40

-30

-20

-10

0

10

20

30

85756555453525155

Calculated Zone

Tem

petra

ture

[°C

]

Initial Temp

Ho

0,94 Ho

0,82 Ho

0,81 Ho

Picture 11: Initial and final zonal temperatures at several different solar constant. The red line indicates

critical temperature, below which the land becomes ice covered.

First, it is interesting to observe what result this model gives at value of solar constant of the time when snowball Earth occurred. Solar output was then 6 % less than today, so our input is 94 % of present solar constant H0. The global mean temperature for these values is lower, 6,1 °C. In our model drops in temperatures are well expressed but far from icy planet. Only altitudes from 70° to 90° are covered with ice, as before. Our conclusion is, that we did not take into account all the contributors, which add to climate changes. Our goal is to determine at which value of solar constant Earth’s system would become unstable. At that point all Earth would become covered with ice in this model. With aiming we can find out the breaking value. At solar constant of 0,82 H0 the global mean temperature is –14,5 °C and altitudes from 90° to 40° degrees are covered with ice and have surface albedo 0,62. This option is extreme but still far from snowball Earth. But if we reduce solar constant for just a percentage more (0,81 H0), the Earth’s climate becomes unstable and sooner or later the whole Earth becomes so called snowball with global mean temperature –44,4 °C. These values for solar constant (0,81 H0) are, off course, unrealistic. Our aim was only to show that there exist instabilities in Earth’s climate system. When conditions reach so called breaking point, the system would collapse.

13

3.3 Stable and Unstable Ranges

Picture 12: Stable and unstable ranges at various solar fluxes and concentrations of CO2.

On Picture 12 there are presented ice-line latitudes (at sea level) as a function of the effective solar flux (Es on the Picture 12 or H0 in our previous notation), or equivalent pCO2 (for Es = 1,0), based on a simple energy-balance model of the Budyko-Sellers type (after Caldeira and Kasting, 1992; Ikeda and Tajika, 1999). Of three possible stable points for Es = 1,0, the Earth actually lies on the partially ice-covered branch at point 1. An instability due to ice albedo feedback drives any ice-line latitude < 30° onto the ice-covered branch. A pCO2 = 0,12 bar is required for deglaciation of an ice-covered Earth, assuming the planetary albedo is 0,6 and Es = 1,0 (Caldeira and Kasting, 1992). The snowball Earth hypothesis is qualitatively predicated on these findings. In the 1960s, Budyko was concerned with the small icecap instability, which predicts a possible switch to the ice-free branch (e.g. disappearance of Arctic sea ice) due to anthropogenic global warming. 3.4 Climate Forcing with GISS GCM For the end we can compare our calculations and explorations of climate behaviour to advanced climate forcing with GISS GCM simulations developed by authors Linda E. Sohl and Mark A Chandler. The simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings, alone and in combination: 6 % solar luminosity decrease, four atmospheric CO2 scenarios (1260 ppm, 315 ppm, 140 ppm and 40 ppm), a 50 % increase and 50 % decrease in ocean heat transports (OHT), and a change in obliquity to 60°. None of the forcings, individually, produced year-round snow accumulation on low-latitude continents, although the solar insolation decrease and 40 ppm CO2 scenarios allowed snow

14

and ice to accumulate at high and mid-latitudes. Combining forcings further cools the climate: when solar luminosity, CO2 and ocean heat transports were all decreased, annual mean freezing and snow accumulation extended across tropical continents. No simulation would have initiated low-latitude glaciations without contemporaneous glaciations at higher latitudes, a finding that matches the distribution of glacial deposits, but which argues against high obliquity as a cause of the Varanger ice age. Low-level clouds increased in most scenarios, as did surface albedo, while atmospheric water vapor amounts declined; all are positive feedbacks that drive temperatures lower. In the most severe scenario, global snow and ice cover increased to 68 %, compared to 12 % under modern conditions, and water vapor dropped by 90 %. These results do not necessarily preclude a snowball Earth climate scenario for the Varanger glacial interval. However, either more severe forcings existed, or radical changes occurred in the ocean/atmosphere system that is unaccounted for by the GCM. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, owing to an increasingly dry atmosphere. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, owing to an increasingly dry atmosphere.

Picture 13: Surface air temperature

15

4 Frozen and Fried 4.1 Snowball Earth Prologue

Image: DAVID FIERSTEIN

Picture 14: First stage of snowball Earth. Around 760 million years ago Rhodinia, that included most of the world's land, started to split apart, forming a new ocean basin that was a forerunner to today's Pacific. As this rift tore, it created new beachfront property as well as new coastal waters hospitable to plankton. Life bloomed in these regions. Rampant photosynthesis pulled billions of tons of carbon dioxide out of the air, and this carbon got quickly buried on the continental shelves as calcium carbonate sediments.

Picture 15: The location of Earth's continents during the younger Varanger glaciation, which took place

650 million years ago.

Formerly landlocked areas were also closer to oceanic sources of moisture. Increased rainfall scrubbed even more heat-trapping carbon dioxide out of the air. Consequently, global temperatures fell, and large ice packed form in the polar oceans. The white ice reflects more solar energy than does darker seawater, driving temperatures even lower. This feedback cycle triggered an unstoppable cooling effect that engulfed the planet in ice within a millennium.

16

4.2 Snowball Earth at Its Coldest

Image: DAVID FIERSTEIN

Picture 16: Second stage; snowball Earth at its coldest. Average global temperature fell dawn to -50 °C. The relatively small amount of heat escaping from the Earth's interior is sufficient to prevent the oceans from freezing to the bottom, but would still allow a kilometre thick cap of sea ice to form. Most organisms died, but seafloor hot springs supported few microbes that thrive on chemicals rather than sunlight. Survival prospects seem even rosier for psychrophilic (cold-loving) organisms of the kind living today in the intensely cold and dry mountain valleys of East Antarctica. Carbon dioxide moving into the oceans from volcanoes is about 1 % C13; the rest is C12. Autotrophic organisms, such as algae and microbes, tend to extract the lighter form, C12, when they turn carbon dioxide into sugars through photosynthesis. Today (and over most of the last 500 million years), approximately 20 % of the carbon entering the ocean is removed as organic matter, which requires that modern calcium carbonate is enriched in C13 by approximately 0,5 % relative to the volcanic source. Inorganic carbon burial: 2332 SiOCaCOCaSiOCO +↔+ Organic carbon burial: 2222 OOCHOHCO +↔+

Picture 17: Carbon isotope fractionation.

Variations in carbon isotope in carbonate rocks represent times of mass extinction, but none was as large or as long-lived as the one caused by snowball Earth glaciation. These extreme glaciations occurred just before a rapid diversification of multicellular life, culminating in the so-called Cambrian explosion between 575 and 525 million years ago. Ironically, the long periods of isolation and extreme environments would most likely lead to evolutionary burst by prolonged genetic isolation and selective pressure.

17

Image: HEIDI NOLAND

Picture 18: Phylogenetic trees indicate how various groups of organisms evolved from one another, based on their degrees of similarity.

Several examples of Neoproterozoic glacial deposition in marine waters are unusually rich in sedimentary ore deposits called banded iron-formation or BIF, which is composed largely of ferric iron oxide (Fe2O3) and silica (SiO2). Most BIFs are older than 1850 million years and are attributed to low levels of free oxygen in the deep ocean at that time. Modern seawater contains less than one part per billion of iron because iron in its oxidized form (Fe3+) is quite insoluble. Joseph L. Kirschvink, a geobiologist at the California Institute of Technology, reasoned that during the millions of years of ice-covered oceans, the amount of gas exchange between the ocean and atmosphere would be reduced, and the deep ocean would quickly become anoxic, allowing reduced iron (Fe2+) to build up to high concentrations. Once the glaciation ended, the ocean would quickly become oxidized, and the iron would precipitate out in close association with the deposits of sediment-laden icebergs.

Picture from Scientific American.

Picture 19: Iron formation.

18

4.3 Hothouse Aftermath

Image: DAVID FIERSTEIN

Picture 20: Last Stage; hothouse aftermath.

In 1992 Kirschvink pointed out that during a global glaciation shifting tectonic plates would continue to build volcanoes and to supply the atmosphere with carbon dioxide. At the time of snowball Earth, the liquid water needed to erode rocks and bury the carbon would be trapped in ice. Furthermore, the population of plants, which consume the gas for photosynthesis, was severally reduced. With nowhere to go, carbon dioxide would collect to incredibly high levels to heat the planet and end the global freeze. Around 700 million years ago, the Sun shone about 6 % less brightly than it does today due to stellar evolution. Overcoming the runaway freeze would require roughly 350 times (120.000 ppm) the present-day concentration of carbon dioxide to raise temperatures to the melting point at the Equator. In fact, the scientists think that there would have been around 1000 times more carbon dioxide than in today's atmosphere. Assuming volcanoes of the Neoproterozoic belched out gases at the same rate as they do today, the planet would have remained locked in ice for up to tens of millions of years. A snowball Earth would be not only the most severe conceivable ice age but also the most prolonged. Once melting begins, the ice-albedo feedback is reversed and combines with the extreme greenhouse atmosphere to drive surface temperatures upward. The warming proceeds rapidly because the change in albedo begins in the tropics, where insolation and surface area are maximal. As tropical oceans thaw, seawater evaporated and worked along with carbon dioxide to produce even more intense greenhouse conditions, bringing global average temperature up to 50 °C. All the ice would disappear within 100 – 200 years after melting began. Swollen rivers washed bicarbonate and other ions into the oceans and combined with abundant bicarbonate ions to form post-glacial carbonate layers around the globe.

Image: GALEN PIPPA HALVERSON

Picture 21: Rocky cliffs along Namibia's Skeleton Coast have provided some of the best evidence for the snowball earth hypothesis.

19

5 Conclusion If Earth was so prone to freezing in Neoproterozoic times, why haven't total ice ages engulfed the planet anytime since then? The Sun deserves some credit, having grown steadily stronger since the Neoproterozoic. Another potential explanation for the rare occurrence of snowball events in Earth history is a continental configuration. When most continents are close to the Equator, the Earth is deprived of a mechanism that keeps the amount of carbon dioxide in the atmosphere above a critical level. For the last million years, the Earth has been in its coldest state since the Neoproterozoic. Some evidence suggests that each successive glaciation over the last several cycles has been getting stronger and stronger, but still far from the critical threshold needed to plunge the Earth into a snowball state. But could such a state be in our future? Certainly over time scales of hundreds to thousands of years, we are more concerned with anthropogenic effects on climate, as the Earth heats up in response to emissions of carbon dioxide. But only time will tell where the Earth’s climate will drift over millions of years.

6 References [1] Freeze-fry from the snowball Earth (online). February 2000 (citied on 20 February

2004). Available on address: http://www.geotimes.org/feb00/newsnotes/snowball.html [2] Hoffman, P. F. and Schrag, D. P. Snowball Earth. Sci Am 282 (1): 68 - 75, January

2000. [3] Hoffman, P. F. and Schrag, D. P. The Snowball Earth Hypothesis: Testing the Limits of

Global Change. Terra Nova 14 (3): 129 – 155, June 2002. [4] Monasterky, R. The king of all ice ages may have spurred animal evolution (online). 22

August 1998 (citied on 20 February 2004). Available on address: http://www.sciencenews.org/sn_arc98/8_29_98/bob1.htm.

[5] Portree, D. S. F. Snowball Earth (online). Pennsylvania State University, 22 March 2001 (citied on 20 February 2004). Available on address: http://www.earthsky.com/2001/es010322.html.

[6] Sohl, L. E. and Chandler, M. A. Climate Forcings and the Initiation of Low-Latitude Ice Sheets During the Neoproterozoic Varagner Glacial Interval. J Geophis Res – Atmos 150 D16: 20737 - 20756, 27th August 2000.

[7] Sohl, L. E. and Chandler, M. A. Did the Snowball Earth Have a Slushball Ocean? (online). (citied on 20th February 2004). Available on address: http://www.giss.nasa.gov/research/intro/sohl_01/.

[8] Testing the Snowball Earth Hypothesis. EPS 8 Lab IV. 5th March 2002. [9] Welcome to Energy Balance Modeling! (online). The Shodor Education Foundation, Inc,

1998 (citied on 20 February 2004). Available on address: http://www.shodor.org/master/environmental/general/energy/application.html.