Global greenhouse to icehouse and back again: The origin and future of the Boreal Forest biome

FROM GREENHOUSE TO ICEHOUSE

Icehouse and greenhouse worlds

Plate tectonics drives the slow shift of the continents across the globe, shifting from a supercontinent to fragmented continents and then back again. The supercontinent Rodinia formed about 1.1 billion years ago and broke up roughly 750 million years ago. One of the fragments included large parts of the continents we now find in the Southern Hemisphere. Plate tectonics brought the fragments of Rodinia back together in a different configuration about 300 million years ago, forming the best-known supercontinent, Pangaea. Pangaea subsequently broke up into the northern and southern supercontinents of Laurasia and Gondwana, about 200 million years ago. Both of these supercontinents have continued to fragment over the last 100 million years. Icehouse climates form when the continents are moving together. The sea level is low due to lack of seafloor production.

The climate becomes cooler and more arid, because of the reduction in rainfall due to the strong rain shadow effect of large super plateaus. Greenhouse climates, on the other hand, are formed as the continents disperse, with sea levels high due to the high level of sea floor spreading. There are relatively high levels of carbon dioxide in the atmosphere, possibly over three times the current levels, due to production at oceanic rifting zones. This produces a warm and humid climate.

The formation and break up of these supercontinents had a huge effect on evolution. Supercontinents are extremely bad for life. First, there is a massive reduction in the amount of shelf sea areas, where we think multi-cellular life may have started. Second, the interior of continents are very dry and global climate is usually cold. A number of key mass extinctions are correlated with the formation of supercontinents. For example it is estimated that up to 96 per cent of all marine species and 70 per cent of terrestrial vertebrate species became extinct during the Permian–Triassic extinction event 250 million years ago, which is nicknamed the ‘mother of all mass extinctions’. It is also not surprising the explosion of complex, multi-cellular organisms occurred during the Cambrian period about 550 million years ago, following the breakup of the Rodinia supercontinent.

Snowball Earth

Prior to about 650 million years ago there is an idea that the surface of the Earth became entirely frozen at least once-the so-called Snowball Earth hypothesis. It is a way to explain the sedimentary deposits found in the tropics, which show glacial features that suggest there must have been a lot of ice in the tropics. Opponents of the idea suggest that the geological evidence does not suggest a global freezing. Moreover there is difficulty in getting the whole ocean to become ice- or even slush covered. There is also the difficulty of seeing how the world, once in a snowball state, would subsequently escape the frozen condition. One answer is that this would occur through the slow build up of atmospheric carbon dioxide and methane, which would eventually reach a critical concentration, warming the atmosphere enough to start the melting process. There are a number of unanswered questions, including whether the Earth was a full snowball or a ‘slushball’ with a thin equatorial band of open water. But what is particularly interesting is the idea that the evolution of complex life put an end to the possibilities of ever having a snowball Earth again. Professor Andy Ridgwell at Bristol University has suggested that the evolution of marine micro-organisms that form calcite shells now buffers the oceans’ carbonate system so much that the extreme variation in atmospheric carbon dioxide needed to plunge the world into or out of a snowball or slush ball condition could not now occur.

Our modern climate system is a product of the slow movements of the continents across the face of the Earth. We are currently in an ‘icehouse world’, as we have continents on or surrounding each pole. The reduction of atmospheric carbon dioxide has allowed the growth of permanent ice sheets on Antarctica and Greenland. This has produced a very strong Equator–pole temperature gradient of at least 60°C, which drives a very vigorous climate system. The current arrangements of longitudinal continents and ocean gateways, has produced strong deep-water formation in the North Atlantic Ocean and Antarctica. The location of modern mountain ranges and plateaus controls where the major deserts and monsoonal systems of the world are located. The movement of continents has also profoundly affected global and regional climates, which have in turn influenced evolution. Our modern climate is ultimately a product of plate tectonics and the random location of the continents.

Global climate cooling

Fifty million years ago the Earth was a very different place. The world was both warmer and wetter, with rainforest extending all the way up to northern Canada and all the way down to Patagonia. So how did we go from this lush, vibrant Earth to the ice-locked, cool planet we have today. What caused the beginning of the great ice ages? If you compare a map of the world 50 million years ago with one of the world today they seem to be the same, until you look in detail. We saw that movements of the continents around the face of the planet are very slow, but minor changes in location have had a profound effect on global climate. Over the last 50 million years these small changes have moved the Earth’s climate from a being greenhouse to an icehouse world.

The last 100 million years

For the last 100 million years Antarctica has sat over the South Pole and the Americas and Asian continent have surrounded the North Pole. But only for the last 2.5 million years have we cycled in and out of the great ice ages, the so-called glacial–interglacial cycles. There must, therefore, be additional factors controlling the temperature of the Earth. In particular you need a means of cooling down the continents on or surrounding a pole. In the case of Antarctica the ice did not start building up until about 35 million years ago. Prior to that Antarctica was covered by lush, temperate forest: bones of dinosaurs have been found there dating from before they went extinct 65 million years ago. What changed 35 million years ago was a culmination of minor tectonic movements. Slowly South America and Australia are moving away from Antarctic. About 35 million years ago the ocean opened up between Tasmania and Antarctica. This was followed about 30 million years ago by the opening of the Drake Passage between South America and Antarctica, one of the most feared stretches of ocean. This allowed the Southern Ocean to start circulating around Antarctica. The Southern Ocean acts very much like the fluid in your refrigerator at home. It takes heat from Antarctica as it flows around the continent and then releases it into the Atlantic, Indian, and Pacific Oceans, into which it mixes. Opening up these seemingly small ocean gateways between the continents produced an ocean that can circulate around Antarctica completely, continually sucking out heat from the continent. So efficient is this process that there is now enough ice on Antarctica that if all of it melted the global sea level would rise over 65 metres-high enough to cover the head of the Statue of Liberty. This tectonic cause of the glaciation of Antarctic is also the reason that scientists are confident that global warming will not cause the eastern Antarctic ice sheet melt-if it were to melt it would cause an approximate 60 metre rise in sea level. The same cannot be said of the unstable western Antarctic ice sheet.

The ice-locked Antarctica of 30 million years ago did not, however, last long. Between 25 and 10 million years ago Antarctica ceased to be completely covered with ice. The question is why did the world start to cool all over again 10 million years ago and why did the ice start building up in the Northern Hemisphere? Palaeoclimatologists believe that relatively low levels of atmospheric carbon dioxide are essential if you are to maintain a cold planet. Computer models have shown that if you have high levels of atmospheric carbon dioxide you cannot get ice to grow on Antarctica even with the ocean heat extractor. So what caused the carbon dioxide to get lower and why did the ice start growing in the north?

What caused the big freeze?

In 1988 Professor Bill Ruddiman and his then graduate student Maureen Raymo while at the Lamont-Doherty Earth Observatory wrote an extremely influential paper. They suggested that global cooling and the build up of ice sheets in the Northern Hemisphere were caused by uplift of the Tibetan-Himalayan and Sierran-Coloradan regions. As we saw in Chapter 5 huge plateaus can alter the circulation of the atmosphere and they argued this cooled the Northern Hemisphere, allowing snow and ice to build up. However, what they did not realize at the time was most of the Himalayan uplift occurred much earlier between 20 and 17 million years ago and thus it was too early to have been the direct cause of the ice in the north. But Maureen Raymo then came up with a startling suggestion that this uplift may have caused a massive increase in erosion that uses up atmospheric carbon dioxide in the process. This is because when you make a mountain range you also produce a rain shadow. So, one side of the mountain has a lot more rain on it as the air is forced up and over the mountain. This is also why mountains erode much faster than gentle rolling hills. She argued that this extra rainwater and carbon dioxide from the atmosphere form a weak carbonic acid solution, which dissolves rocks.

But interestingly only the weathering of silicate minerals makes a difference to atmospheric carbon dioxide levels, as weathering of carbonate rocks by carbonic acid returns carbon dioxide to the atmosphere. As much of the Himalayas is made up of silica rocks there was a lot of rock that could lock up atmospheric carbon dioxide. The new minerals dissolved in the rainwater are then washed into the oceans and used by marine plankton to make shells out of the calcium carbonate. The calcite skeletal remains of the marine biota are ultimately deposited as deep sea sediments and hence lost from the global carbon cycle for the duration of the lifecycle of the oceanic crust on which they have been deposited. It’s a fast track way of getting atmospheric carbon dioxide out of the atmosphere and dumping it at the bottom of the ocean. Geological evidence for long-term changes in atmospheric carbon dioxide does support the idea that it has dropped significantly over the last 20 million years. The only problem scientists have with this theory is what stops this process. With the amount of rock in Tibet that has been eroded over the last 20 million years all the carbon dioxide in the atmosphere should have been stripped out. So there must be other natural mechanisms which help to maintain the balance of carbon dioxide in the atmosphere as the long-term concentration of carbon dioxide in the atmosphere is the result of a balance between what is removed by weathering and deposition in the deep ocean and the amount recycled by suduction zones and emitted by volcanoes.

With atmospheric carbon dioxide levels dropping between 10 and 5 million years ago the Greenland ice sheet started to build up. Interestingly Greenland started to glaciate from the south first. This is because you must have a moisture source to build ice with. So by 5 million years ago we had huge ice sheets on Antarctica and Greenland, very much like today. The great ice ages when huge ice sheets waxed and waned on North America and Northern Europe did not start until 2.5 million years ago, however there is intriguing evidence suggesting that around 6 million years ago these big ice sheets did start to grow. Rock fragments from the continent, eroded by ice and then dumped at sea by icebergs have been found in the North Atlantic Ocean, North Pacific Ocean, and Norwegian Sea at this time. This seems to have been a failed attempt to start the great ice ages and could be because of the Mediterranean Sea.