[MUSIC] Hello, and welcome to this week's lecture of origins. It is entitled from greenhouse to ice house, climate change over the past 50 million years. The lecture will give you a view into some important events in earth youngest history. Here we are dealing with the persistent drift from subtropical to temperate and arctic conditions in the mid and high latitudes that took place over the past 50 million years. Over this time span, plate tectonic events such as mountain building, and reduced volcanism were the ruling factors in long-term climate change. These events would lead to the reduction of greenhouse gases in the atmosphere, and the rearrangement of ocean, and atmospheric circulation causing global cooling. Over this long period of time, plants and animals had to adapt to these climatic challenges. Over the past 3 to 2 million years, rhythmic changes between glacial and interglacial episodes have ruled the Earth's climate. The pacemaker of the short-term flicker in climate has been caused by regular changes in the incoming radiation received by Earth called solar forcing. This led to rapid environmental changes that favor the biological evolution which eventually led to the arrival of our own species Homo sapiens on the global scene. My name is Michael Houmark-Nielsen, I'm employed at the Natural History Museum and I study land forms made by glaciers in sedimentary records from the Pleistocene ice ages and the intervening mild episodes. I work in the research group called GeoGenetics which explore climate change and its impact on the physical and biological environments on several time scales. In my lecture, I will focus on the North Atlantic Region. But I would give proper reference to the global geological events, that played a decisive role on the route from greenhouse to ice house. The origin of our present day glacially dominated climate is primarily to be found in the plate tectonic evolution of the Cenozoic, which is the youngest era in earth history. This evolution that began about 50 million years ago caused a steady drift away from warm greenhouse climate in the Eocene Epoch, towards cool ice house conditions in the present time. Then in the early Eocene, ocean floor spreading was very active. The atmospheric dose of greenhouse gases such as CO2 was high and global climate was warm. As a consequence, high latitude continents experienced subtropical environments. The continents were old and worn down and global sea level was high. 50 million years later in the Pleistocene Epoch, Earth's relief had risen dramatically, the amount of green-house gases had been seriously reduced, and sea level had fallen drastically, and global temperatures were low. Thus climate on Earth had steadily been cooling, particularly in polar and high latitude regions. Because continents from the beginning of the Cenozoic Era were already close to their present positions, long-term climate change must have been caused by other factors than simple drift to higher latitudes over the past 50 million years. Looking at Earth today, we see a planet with oceans and continents arranged in a particular north-south setting. In the high latitudes, north and south, climate is cold, the oceans are often covered with sea ice, and the land masses are permanently frozen and covered with glaciers. The mid latitudes has temperate climates where precipitation rules the distribution of forests and deserts. In the near equatorial latitudes we find the tropics with hot climates and continents are dressed in rain forests and surrounded by warm oceans. This was not always how the face of the earth would appear. The record with it's fossils tells us that not long ago ideological terms, earth was in a quite different and much warmer climatic mode. We detect these gradual cooling from geological strata namely the Climate Archives. Climate archives are lessons composed on undisturbed geological strata that can be dated and from which fossils and geochemical data related to climate and environment can be extracted. The fossil record is used to reconstruct temperatures through time provided we have good knowledge on the demand of individual species to its habitat. From the Circum-Arctic Region fossil flora assemblages indicate that subtropical broad leaved forests and oceans free of sea ice in the Eocene gave way for temperate forests in the Miocene, which survived as open forests in Northernmost Greenland intil the end of the Pliocene about two and a half million years ago. Here after, shrub tundra vegetation and permafrost has ruled the Arctic. In Central Europe, the vegetation changed from subtropical to northern temperate broad leafed forests, indicating a decline in regional temperatures from more than 20 degrees centigrade to less than 5 degrees centigrade on an annual average. Before we proceed, we will take a closer look at the mechanisms that drives the earth's climate. Plate tectonic rearrangement operates by spreading and subduction of lithosphere plates. In the long-term, this will affect the rise and subsidence of the ocean floor and the uplift and down earing of mountains. On tectonic time scales, ocean basin volumes change with the rise and subsistence of mid-oceanic ridges and submarine volcanic plateaus which is controlled by sea floor spreading rates and the migration of hot mantle plumes. This leads to long-term changes in the global sea level in the order of several hundred meters. Age dated evidence of world wide marine flooding, or regression, allows us to give estimates on the changes in global sea level through time. We have learned that the path of global ocean and atmospheric circulation systems depend on the plate tectonic setting. However, the strength of the systems depends on the amount of energy received by Earth from solar radiation and the ability of Earth to keep the energy from escaping back into space. Greenhouse gases including carbon dioxide and methane in the atmosphere are responsible for the Earth's energy household In the way that high amounts of CO2 will turn global climate into a warm greenhouse mode, while low concentrations will twist climate into a cool icehouse mode. Global plate tectonic development, over the past 50 million years, also includes the rising of worldwide mountain belts. Together with the reduced ocean flow spreading, these large plate tectonic reorganizations had a decisive effect on the ocean-continent configuration. Plate collisions caused uplift of large land masses, for instance, the plateaus of Colorado, the Andies, Tibet, meaning that enormous volumes of rock became exposed to weathering. Physical weathering increases the area of the rock bodies that can be attacked by chemical breakdown. Chemical breakdown such as hydrolysis removes greenhouse gasses from the atmosphere. CO2 will bind up with slightly acid water to form hydrogen carbonate that slowly dissolve silicate rocks, especially in moist and warm climates. This causes crystalline rocks to be transformed into clay minerals and irons that end up being transported by rivers to the ocean. Here, carbon and oxygen are trapped in calcium carbonate shells on microorganisms which eventually settle on the ocean floor. This process probably was responsible for the larger part of the reduction of atmospheric CO2. This significantly enhanced global cooling and furthermore reduced CO2 emissions to the atmosphere from volcanic eruptions may have speeded up cooling further. This was caused by a reduction in the ocean floor spreading which began to slow down after the Eocene. Because the Earth's climate is highly dependent on the amount of atmospheric green house gases, detecting CO2 changes throughout the past have been a first order proxy for quantifying global temperature changes. From isotopic studies of plantonic for mini shells, it is suggested that atmospheric CO2 was about four times higher during the Eocene and today and it has been steadily declining until recent centuries. In short, it can be demonstrated that Earth's greenhouse gas budget depend on the input of CO2 to the atmosphere from volcanic activity and removal from the atmosphere by weathering of continental rocks. [MUSIC]
