By Luisa Cristini, PhD, University of Hawaii at Manoa.
[Note from the editor: This is the twelfth in a series of blog entries that will focus on introductory topics in climate dynamics and modeling, and will serve to provide insight into the current understanding of the science.]
The study of the climate of the far past must rely on indirect estimates and climate reconstructions; these are often modified as new information becomes available. The evidence for the climate of the early Earth is particularly scarce. When Earth was formed about 4.6 billion years ago, the solar irradiance was about 30% lower than at present. If the conditions (albedo, composition of the atmosphere, distance between the Earth and the Sun, etc) then had been the same as they are now, the average surface temperature would be 30ºC (86ºF) below today’s. During the first 700-800 million years, the repeated bombardment by meteorites would certainly have warmed the climate. Nevertheless, in such conditions, the Earth would have been frozen during a large part of its history. This contrasts with geological evidence for a liquid ocean at least 4 billion years ago. The apparent discrepancy is called the “faint early Sun paradox”. The main cause is thought to be that there was a much stronger greenhouse effect during Earth’s early lifetime. The atmosphere was very different from today, with a much higher carbon dioxide (CO2) concentration (probably more than 100 times the present value) and nearly no oxygen. With time, the atmospheric composition modified, in particular because of the oxygen liberated by photosynthesis. This accumulated in the atmosphere, leading to a large increase in the atmospheric oxygen concentration 2.2. to 2.4 billion years ago as well as to the formation of an ozone layer in the stratosphere.
Several large climate fluctuations have occurred during the Precambrian eon (from Earth formation to 542 million years ago). One of the best documented of these events is a glaciation that took place around 600 to 750 million years ago. It was so severe that the whole Earth was covered by ice. At that time, all the continents were grouped close to the South Pole and this may have initiated a cooling of the continents, probably during a time when the orbital configuration favored the growth of ice sheets (see upcoming article on orbital parameters). After an initial cooling period, the ice-albedo feedback (described in the part 10 of this series) was strong enough to generate an additional temperature decrease leading to a progression of ice towards the Equator and eventually covering the whole Earth (the “snowball Earth hypothesis”). This condition could have ceased due to the high input of CO2 from volcanoes, resulting in a gradual warming of the planet.
On timescales of millions of years, the carbon cycle is mainly controlled by the exchanges between rocks and surface reservoirs (ocean, atmosphere, biosphere). This long-term carbon cycle determines the concentration of atmospheric CO2. During the Phanerozoic eon (the last 542 million years), tectonic activity became intense and high production rates of oceanic crust at the mid-ocean ridges resulted in more buoyant oceanic plates that pushed seawater upward. This resulted in flooding of the low-lying parts of the continents. The production rate of oceanic crust by tectonic activity played a particularly important role since the relatively large divergences that followed the break-up of the super-continents around 200 million years ago (super-continent Pangaea) and 550 million years ago (super-continent Pannotia) are associated with significant increases in CO2 concentration. Furthermore, the periods of low CO2 concentration generally correspond with recorded glaciation. However, the link between CO2 and global temperature cannot, on its own, explain all the past climate variations, in particular on the regional scale. Other factors, such as the location of the continents must also be taken into account.
Over the last 65 million years, during a period called the Cenozoic, the global atmospheric CO2 concentration has gradually decreased from more than 1000 ppmv (parts per million in volume) to less than 300 ppmv. This long-term decrease was partly due to fewer volcanic emissions and to changes in the rate of weathering of silicate rocks. The decline in the CO2 concentration was associated with a cooling from the warm conditions of the early Eocene climatic optimum, between 52 and 50 million years ago. This shift is often referred to as a transition from a greenhouse climate to a (pre-Industrial Age!) icehouse, in which ice sheets are present over Antarctica (starting around 35 million years) and over Greenland (starting around 3 million years ago).
Sixty million years ago, the location of the continents was quite close to that of the present-day. However, a relatively large seaway was present between North and South America while Antarctica was still connected to South America. The uplift of Panama and the closure of the Central America seaway modified the circulation in the Atlantic Ocean, possibly influencing the glaciation over Greenland. More importantly, the opening, deepening and widening of the Drake Passage (between South America and Antarctica) and the Tasmanian Passage (between Australia and Antarctica) allowed the formation of the intense Antarctic Circumpolar Current that isolates Antarctica from the influence of milder mid-latitudes and increased the cooling there. Finally, the uplift of the Himalayas and the Tibetan Plateau strongly modified the atmospheric circulation in these regions. These examples illustrate the strength of the driving force associated with the changes in boundary conditions due to plate tectonics.
Relatively brief events are also recorded in the geological archives. One of the most spectacular is the large meteorite impact that occurred 65 million years ago at the boundary between the Cretaceous and Paleogene periods (or K-Pg boundary). This cataclysm could have caused the extinction of many plant and animal species, including the dinosaurs, but its climatic impact is not well known and its long-term influence is not clear.
In the last million years Earth’s climate has been characterized by glacial and interglacial cycles, regulated by Earth’s orbital parameters. The periodicity of the three parameters – eccentricity (how close the orbit is to a circle), obliquity (the degree Earth’s rotational axis is tilted from perpendicular to its orbital plane) and precession (the rotation of Earth’s orbit) – determines the amount and distribution of solar radiation in different regions of the Earth. The information recorded in ice cores documents the alternation between long glacial periods (or Ice Ages) and relatively brief interglacials over the last 800 thousand years. We are currently living in the latest of these interglacials: the Holocene. The glacial period that is the best known is the latest one, which peaked around 21 thousand years ago and is referred to as the Last Glacial Maximum (LGM). At that time, the ice sheets covered the majority of the continents at high northern latitudes, as far south as 40ºN. Because of the accumulation of water in the form of ice over the continents, the sea level was lower by around 120 m (394 ft), exposing new land to the surface.
Greenhouse gas concentrations have varied nearly synchronously with temperature and ice volume over at least the last 600 thousand years. The difference between interglacial and glacial periods reached about 80 ppmv for carbon dioxide and 300 ppbv (parts per billion in volume) for methane. This corresponds to a strong amplifying mechanism for cooling during glacial periods. However, as mentioned in the latest IPCC report, “the qualitative and mechanistic explanation of these CO2 variations remains one of the major unsolved question in climate research”.
References and further resources
Cristini L., K. Grosfeld, M. Butzin, and G. Lohmann (2012). Influence of the opening of the Drake Passage on the Cenozoic Antarctic Ice Sheet: A modeling approach. Palaeogeography, Palaeoclimatology, Palaeoecology. http://linkinghub.elsevier.com/retrieve/pii/S0031018212002271.