The goldilocks planet, Jan Zalasiewicz & Mark Williams, 2012



The chemistry of those zircons suggests that the magma ocean cooled to form a kind of ‘proto-crust’, a planet wide shell made of rock types basalt and anorthosite (a distinctive pale rock formed of feldspar), in something like the way the moon did (perhaps 100 million years into the Earth’s history).

A dynamic climate machine  (liquid water, currents, plate tectonic) was in place very early in the life of Earth, and direct evidence of its action may be seen among some of the world’s oldest strata (3 to 3.8 billion years old).

About 3.8 billion years ago, the sun was faint, emitting about 75% of its present values of light and heat. Yet the planet was not icy, possibly thanks to the green house gas in high proportion in the atmosphere and the presence of water more than continents – so with lower albedo – and darker clouds reflecting less of the radiation.

Banded Iron Formations (3.8 billion years ago) were until recently considered as evidence for early oxygen release by photosynthesis from cyanobacteria (blue-green algae) as they combine carbon dioxide with water. The oxygen thus released combined with the giant stores of dissolved iron accumulated in the oceans and insoluble iron oxides sank to the sea bottom to form bands of iron. An alternative hypothesis considers that these iron-rich rocks could have form in a anoxic oceanic setting, by the action of microbes which were photosynthetic but which did not produce free oxygen. Those bacteria are regarded as the most ancient photosynthetic organisms, producing iron oxide and hydrogen.

Some 2.4 billion years ago, the great oxygenation event seems to mark the origin, within one group of microbes, of the molecular machinery that enabled oxygenic photosynthesis.  It was a catastrophe for the organisms of the Archean that had evolved in an oxygen-poor world. Oxygen was a poison, a gas as dangerous as chlorine is to us. They were gradually forced to the fringes of life in the low-oxygen zones of the oceans, or deep into the interior of rocks. Rusted sediments can still be seen today. By 2.4 billion years ago an oxygen-bearing atmosphere was achieved. Oxygenation enabled the formation of an ozone layer that blocks the harmful UV radiation. It changed the earth’s climate and it effect upon methane may have been key to the refrigeration of the world.

Circumstantial evidence of the correlation of the glaciations and of oxygenation is reasonably good. The first glaciations followed the great oxygenation event.

There is little evidence for another glaciation for nearly 1.5 billion years – global surface temperatures appear to have remained generally stable (this is 1/3 of the age of Earth).

Until 2.4 billion years ago, life was limited to simple prokaryotic cells, ancestors of modern bacteria. By 1.8 billion years ago this changes with the first fossil evidence of eukaryotic cells – cells with nucleus. The increase in size and complexity seen with the rise of cell cooperation, which ultimately led to multicellular organisms, seems to have coincided with increases in the amount of free oxygen during the Proterozoic (-2.5BY to -0.5BN). This suggest a strong feedback between life and its environment.

Snowball earth: some evidence exist from the later part of the Proterozoic, in rock strata that are between 740 and 580 million years old and which have witnessed 4 glaciations. Plate tectonic may have triggered intense glaciation. Early continents wandered across the planet rapidly (the mantle was hotter than today). By 800 million years ago, the supercontinent known as Rodinia started to break up facilitating the weathering of rocks by storms and rainfall. Rain fall through the atmosphere dissolves carbon diaoxide to form carbonic acid. Carbonic acid can then react with silicate (feldspar). The weathering reactions produce calcium and bicarbonate ions fixed by marine organisms to form limestone. Carbon is scrubbed from the atmosphere and buried at the sea.

With more carbon dioxide in the atmosphere, global temperature rise higher. More evaporated water falls as rain and more silicate weathering happens with more carbon removed from the atmosphere (this works over hundreds of thousands of years). When temperature is too low, there are less rainfall and slower silicate weathering. Perhaps this is the key process that maintained our Goldilocks planet. The breaking of Rodinia may have favor silicate weathering, carbon removal from atmosphere and glaciation ensued.

Snowball effect may have been complete – a theory only, highly controversial – hydrological cycle would have stopped, no cloud, no rain, erosion (and silicate weathering) would have stopped. With ice up to a km high, temperature would have plummeted to perhaps -50°. Budyko’s model suggests that such Snowball should last forever. Models shows that however that in the worst case scenario, temperature would have remained high enough to prevent a complete snowball effect, with the equator remaining free of ice, most likely.

In the Mc Murdo Dry Valleys of Antarctica there are lakes covered with ice up to 8 meters thick, below which there is liquid water.  The water is rich in mineral salts, sometimes 7 times saltier than sea water. In these lake the dense, saline deep waters do not mix with surface water and therefore receive almost no oxygen. But even there, there is life.

On a Snowball earth, glacial conditions may persist for millions of years. But volcanoes will continue to pour carbon dioxide – and ash, to drape over and darken icy surface. Over millions of years, carbon dioxide in that arid atmosphere can build up to levels perhaps 100 times above present level. At some point the greenhouse gas effect overcome the ice-albedo effect.

The molecular clocks say that the origins of many animal groups occurred in the late Proterozoic, probably around the time of glaciation 650 million years ago. The late Proterozoic glaciations may even have been a spur to evolution. Super fertilized oceans (melting ice precipitating large quantities of carbonate sediment on to the sea) would have helped algae and microplanckton thrive. They would grow and die, burying carbon with them, leaving oxygen to build up in the atmosphere.

The last half billion years show a world oscillating between greenhouse and icehouse states.

We live now in an icehouse world: it began some 33 million years ago.

The preceding 3 billion years had been a slimeworld of assorted microbes. After this long reign, there was a brief interval of a few tens of million years when the strange Ediacaran organisms appeared.

(200 millions years ago) as the warm tropical surface water of these ancient seas travelled north and south, evaporation at their surface made them become cooler and saltier – and so more dense – and they began to sink below the less dense polar waters. This was the start of the thermohaline circulation. This mechanism operates strongly when poles are cold (as the density contrast between tropical and polar waters is important). In an ice house climate like ours, this circulation is strong and the oceans are well mixed.

In rocks, sudden change from dark to grey, from barren to burrowed, signals oxygen on the seabed. The most effective mechanism to deliver oxygen to the deep sea is a speeding-up of thermohaline circulation. That in turn typically signals the spread of sea ice at high latitudes.

Carbon has three naturally occurring isotopes: 14 C (archeologist dating carbon); a lighter 12c easier for the plant to use as it takes less energy to pick up this isotope from the surrounding air/sea water and convert it to biomass than the heavier form 13C. So plant possess more 12C relative to 13C.

For 4 billion years, the land was essentially barren. For eons, dangerous ultraviolet prevented the development of any life on the land. Only after the atmosphere became oxygenated, and the ozone developed, was there a possibility of terrestrial life.

By 380 millions years ago, plants had developed strong and deep rooting systems. By carboniferous, ferns had evolved to become gigantic. The carbon sequestrated in their skeletons, and then buried as coal, would become a new force in climate history.  When the sea level rose, the freshwater swamps and the forests that grew on them were drowned and the buried under sediment. The constant interplay between these trends created the coal-bearing strata.

By the late Permian the continents on earth were reassembled into a single supercontinent, Pangaea. This interior of this supercontinent became dry, as the inland areas became too distant from sources of ocean moisture.

As Permian drew to a close, the earth’s mantle released extraordinary amounts of basaltic lava. The blanket of lava became, commonly, 2-3 kilometers thick. The associated out-gassing yielded vast quantities of carbon dioxide and sulphur dioxide too. Oxygen levels seemed to fall then. Between these killing mechanisms of poisoning, warming…life on earth underwent its greatest trial since the times of the Snowball Earth. 9/10th of all life extinguished.

On similarity at least for the most pronounced hyperthermal events (when a world already warm catch fever): the release of massive amounts of carbon into the atmosphere. The lessons emerging from this is that the progress of these events is complex, with poorly understood feedbacks acting to both slow down and speed up the rate of climate change at different times. We humans have started something that will likely go on to develop its own momentum.

Antartica is not simply a frozen waste land because it lies at the pole. In some places the Antarctic ice sheet is over 4 km thick and over 1 million years old.

Foraminifera are amoeba-like organisms whose fossil record extends back over 500 million years. Each foraminifer comprises just a single cell. It has a shell of different shapes and sizes. The most famous are the nummulites who lived 55 millions years ago (can be found in the limestone used to construct the pyramids at Giza).

The proportions of the oxygen isotopes that went into a limestone reflected the temperature of the water in which the limestone formed. As water cooled, more of the heavy isotope went in the limestone ( e.g. ratio of heavy oxygen (18O) to light oxygen (16O) would change).

The Antarctic circumpolar current effectively cut off the supply of warm water from lower latitudes. Antarctica was plunged deeper into cold.

The East Antarctic ice sheet has long been stable (the biggest accounting for about 90% of the Antarctic ice), then a different story emerges from West Antarctica: between 5 and 3 million years ago, large masses grew and collapsed repeatedly.

Pliocene really matters, because something like it may be where we are all heading. The journey to the past may be eerily similar to a journey into the future. It is the only time in the previous 3 million years that we have evidence for atmospheric carbon dioxide levels as high as 400ppmv.

Leaves bear stomata, tiny openings on the leaf that allow the transfer of gases and moisture between the plant and the air around it.  When carbon dioxide is high there are fewer stomata, and vice versa.

The small flickering in climate curve is the signal of rhythmic, astronomically driven climate change – a signal dominated, in this part of Earth history, by the 40,000 year change in the tilt of the Earth’s axis. (Astronomically driven cycle exist every 21,000 and 41,000 years (effect of tilt and elliptical orbit around the sun, varying from cumulative effect when both are at their lowest to the highest when both are at their highest); 100,000 years (short eccentricity when the orbit is almost circular) and 400,000 years (long eccentricity when the orbit is at its greater eccentricity).

The Gulf Stream is a marvelous machine. Throw enough melting ice and melt water into the north Atlantic and the salinity – hence density – of the surface waters will be lowered sufficiently to prevent sinking in the far north of the dense and salty (and colder) gulf stream water (that  then begin a long journey southwards along the sea floor). If the Gulf stream is shut off, north west Europe can be thrown into a deep freeze (5°C colder then it is).

The glacial intervals are long, lasting of the order of a 100,000 years or more. Interglacials are relatively brief by comparison – in the order of 10,000 years.

Temperature started to rise about 11,700 years ago, marking the beginning of the Holocene.  The oxygen isotopes suggest that the temperature rises were accomplished in a few decades – perhaps in one human lifetime.

Megafloods are linked to the melting of continental ice sheets, typically by the formation of a very large lake held in place by an ice barrier, that then drains catastrophically when the ice dam fails. The ensuing flood can change the world.

By 5000 years ago, the sea level had reached its current level.

A later failure of the Asian rains, 4,300 years ago, took place as humans were beginning to create settled agricultural civilizations. As the drought took place the Neolithic culture in China collapsed, as did those, it seems, in Mesopotamia and India.

The Little Ice age seems to have killed off the Viking colony in Greenland in the mid-fifteenth century. The little ice age is depicted via charming scenes of inhabitants of London and Amsterdam ice skating on the frozen Thames and Amstel rivers.

The change in solar radiation between sunspot highs and sunspot lows is very slight.

Between 1980 and 2002, the sun has held steady. In that time, global temperatures have undergone a steep climb, rising by 0.5 degree. The Sun does not seem responsible – or not largely responsible – for recent temperature change.

The best mixed air on earth, as far from local and human influence as it is possible to be, in the middle of the Pacific Ocean at the summit of Hawaii. The reading started at 315ppmv in 1950’s but each successive year the mean value was 1 ppmv higher. And so the pattern has gone on (reaching 400 ppmv now), inexorably. Humans are in the driving seat. And over the last couple of decades the rate has increased from 1 ppmv to 2 ppmv a year.

Since the Industrial revolution, the carbon in the atmosphere has also gone into the oceans, dissolving to produce carbonic acid. This has lower the average pH of the oceans by a tenth of a pH unit, from 8.2 to 8.1. pH scale is logarithmic, so there are now some 30% more hydrogen ions in the sea than in pre-industrial times. By the end of the century there may be 100% more hydrogen ions in the oceans. Acidification of the oceans, is likely bad news, for nothing like it has occurred for many millions of years. From 600 ppmv onward (in the 2050’s) coral reef is likely to stop growing and start dissolving.

1900-1940 +0.3°C, 1940-1970 -0.2°C, 1980 to 2000 +0.6°C. Antarctic peninsula average temperature have risen by +3°C in the last 50 years. 2005 and following years were the warmest years on record.

The oceans have absorbed most of the extra heat that has been put into the Earth system over the last 50 years, as the temperature of the upper several hundred meters has increased by a little more than half a degree.  The extra heat is equivalent to little more than 2 billion atom bombs. The energy makes the water molecules move more energetically to the extent that they move on average a little further apart. The whole ocean surface lift up, and thermal expansion is responsible for about half of the 20 cm sea level rise of the past century.