How Far the Sun?
Mars is too cold. Venus too hot. Earth is just right in terms of temperature for us to live. How does this all affect climate change? The notion that scientists understand how changes in Earth's orbit affect climate well enough for estimating long term natural climate trends is challenged in a recent study.
Scientists have long studied how variations in Earth's orbit relate to ice ages, cycles of glacier building and retreat, and even mass extinctions.
The idea that cyclical variations in Earth's orbit can cause major climate changes was first proposed by astrophysicist Milutin Milankovitch. The main variables are eccentricity, obliquity and precession.
Eccentricity refers to the changing shape of Earth's orbit around the Sun, which varies from nearly circular to elliptical over a cycle of about 100,000 years. Obliquity refers to the angle at which Earth's axis is tilted with respect to the plane of its orbit, varying between 22.1 degrees and 24.5 degrees over a 41,000-year cycle. Precession is the gradual change in the direction Earth's axis is pointing, a 21,000-year cycle during which the axis carves out an imaginary cone shape.
The three orbital variations together affect the total amount of sunlight received by the Earth, and distribution of that sunlight at different latitudes and at different times. With time periods measured in tens of thousands of years, one would expect that changes in climate as a result of orbital variations would occur over similar time periods. Indeed, the Milankovitch theory of climate change has been used to explain the global climate of the last 2 million years, with changes between warmer interglacial periods and colder Ice Ages occurring over a 100,00 year cycle, as predicted by the Milankovitch theory of climate change.
The current research was conducted by a team led by Professor Rohling of the University of Southampton's School of Ocean.
"Understanding how climate has responded to past change should help reveal how human activities may have affected, or will affect, Earth's climate. One approach for this is to study past interglacials, the warm periods between glacial periods within an ice age," said Rohling.
The team compared the current warm interglacial period with one of 400,000 years ago.
Many aspects of the Earth-Sun orbital configuration during period were similar to those of the current interglacial. For this reason, this period is often considered as a similar period for future climate development in the absence of human influence.
Previous studies had used the analogy to suggest that the current interglacial should have ended 2 to 2.5 thousand years ago. So why has it remained so warm?
According to the 'anthropogenic hypothesis', long term climate impacts of man's deforestation activities and early methane and carbon dioxide emissions have artificially held us in warm interglacial conditions, which have persisted since the end of the Pleistocene, about 11,400 years ago.
To address this issue, the researchers used a new high resolution record of sea levels, which reflect ice volume. This record, which is continuous through both interglacials, is based on the 'Red Sea method' developed by Rohling.
Water passes between the Red Sea and the open ocean only through the shallow Strait of Bab-el-Mandab, which narrows as sea levels drop, reducing water exchange. Evaporation within the Red Sea increases its salinity, or saltiness, and changes the relative abundance of stable oxygen isotopes.
By analyzing oxygen isotope ratios in tiny marine creatures called foraminiferans preserved in sediments that were deposited at the bottom of the Red Sea, the scientists reconstructed past sea levels, which were corroborated by comparison with the fossilized remains of coral reefs.
The researchers found that the current interglacial has lasted some 2 to 2.5 millennia longer than predicted by the current theory for the way in which orbital effects control the ice age cycles. This theory is based on the intensity of solar radiation reaching the Earth at latitude 65 degrees North on 21 June, which is the northern hemisphere Summer solstice.
However, a different theory looks at the amount of solar energy reaching the Earth at the same latitude during the summer months. Under this theory, sea levels could remain high for another two thousand years or so, even without greenhouse warming.
Future research should narrow down which theory is correct. Once this is determined the effect so of humanity activities on climate can be better determined.
For further information: http://www.space.com/scienceastronomy/planetearth/orbit_climate_010212.html or http://en.wikipedia.org/wiki/Milankovitch_cycles