Scientists have no concrete way of determining whether changes in the climate are due to natural or manmade causes. This would require the existence of a control earth in a parallel universe where man never evolved. Scientists could then compare the climates and temperatures of the 2 worlds to see what the differences would be. Because a control earth doesn’t exist, scientists must concoct models using knowledge of the known natural variations of earth’s paleoclimate. The models estimate what average temperatures and CO2 concentrations should be due to natural forcing. Scientists use these models and compare them with actual observed data. According to the almost unanimous consensus among climate scientists, earth’s climate is now warming due to both natural forcing and manmade production of greenhouse gases through the burning of fossil fuels. However, earth is currently in an unusual point of a 400,000 year cycle of orbital geometry that complicates climate scientist’s models and makes it even more difficult to discern the difference between natural and anthropogenic influence on the present day’s climate.
Illustration of earth’s orbital geometry. During Ice Ages earth’s orbit is more elliptical in shape, reducing the amount of solar radiation it receives. During Interglacials earth’s orbit is more round. On a 400,000 year cycle earth goes into a prolonged period of time in a round orbit. During the Stagell Interglacial (~420,000 BP-~395,000 BP) earth was in a prolonged round orbit. Earth’s orbital geometry for the past 11,000 years has been similar to the orbit that earth was in during the Stagell Interglacial, suggesting we will remain within an interglacial for tens of thousands of years with or without the influence of anthropogenic greenhouse gases.
Natural variations in earth’s climate are triggered by the Milankovitch cycles. The gravitational pull of Jupiter and Saturn causes eccentricities in earth’s orbit. During Ice Ages, earth’s orbit is more elliptical in shape reducing the amount of solar heat reaching earth. The orbit of earth is more round during interglacials. This variation in earth’s orbit goes through 100,000 year cycles. The degree of earth’s tilt also varies but on a 41,000 year cycle. The gravitational pull of moon and sun causes this variation in earth’s tilt. The 23,000 year cycle of earth’s precession is a third influence on the amount of solar heat (insolation) reaching earth. Precession is the time of year when either the southern or northern hemisphere is closest (perihelion) or farthest (apehelion) from the sun. All of these cycles cause the amount of insolation reaching the earth to vary slightly, but over long periods of time these small differences trigger the alternating cycles of cold Ice Ages and warm Interglacials.
Formerly, scientists thought Interglacial periods lasted on average 10,000 years while Ice Ages lasted for 100,000 years. More recent evidence suggests Interglacials average between 15,000-20,000 years. However, every 400,000 years, earth enters a prolonged period of time within a round orbit, resulting in a much longer interglacial. The Stagell Interglacial, known as the Hoxnian in Europe, occurred from ~420,000 BP-~395,000 BP when earth was in this prolonged round orbit. Some scientists estimate it lasted even longer from ~423,000 BP-~362,000 BP. The present day Interglacial, the Holocene, began about 11,000 years ago. The earth is currently in a similar orbital geometry as it was during the Stagell Interglacial, meaning we are not even halfway through the present day interglacial. Scientists are studying the Stagell Interglacial to predict what earth’s climate will be like in the future, specifically the next 50,000 years. Some climate models suggest that because of the current unique orbital geometry, earth won’t shift into an Ice Age again for another 40,000 years. The models predict a complete melting of the northern Ice Cap 30,000 years in the future, but if modern anthropogenic factors are considered, the northern Ice Cap will collapse in less than 200 years.
Chart of the past 600,000 years showing fluctuating carbon dioxide levels. Higher carbon dioxide levels are correlated with higher temperatures. The Stagell Interglacial is scientifically known as Marine Isotope Stage 11 (MIS 11). Note the length compared to other Interglacials. Interglacials are odd numbers. MIS 3 isn’t labeled on the chart and was an interstadial (a briefer warming period) rather than a full blown interglacial. One of the ways scientists determine past climates is by taking cores at the bottom of the ocean. The core layers are radiometrically dated, and fossils of microscopic organisms known as foraminifera are examined and analyzed. The various species of foraminifera live within strict temperatures ranges, so a general idea of the temperatures at the time of their deposition can be assumed simply based on species composition. However, exact average annual temperatures can even be determined based on the chemical isotopic compositions of the fossil foraminifera shells, thus the name–Marine Isotope Stages.
The Gubik geological formation along the coast of northwest Alaska provides evidence of high marine transgressions dating all the way back to the Pliocene. One of these layers includes a high sea stand from the Stagell Interglacial that was 23 meters above modern sea level.
A species of ostracod. Fossilized ostracods found in layered sediment on the sea bottom provide paleoclimatic data based on species and chemical isotopic composition.
Devil’s Hole, a cave in southern Nevada. Calcite veins below the water table here provide radiometrically datable isotopic evidence of past climates going back 800,000 years. It was evidence from this site that initially showed the Stagell interglacial was much longer than the average warm period. Evidence from this site also created a dilemma for scientists’ assumptions about the Milankovitch cycles. Isaac Winograd determined from data found here that interglacials were twice as long as previously thought. Some scientists think this evidence still supports the theory that Milankovtich cycles cause the alternating cycles between warm and cold stages, while others think the data is not consistent with and therefore debunks the theory.
Some of the best evidence of the Stagell Interglacial comes from the Gubik formation in northwest Alaska where in some places remnants of 6 former marine highstands, including layered fossil shorelines and barrier islands, are found superimposed (conveniently for geologists) from youngest to oldest. The marine highstand dating to the Stagell Interglacial is 23 meters above sea level, evidence of just how much polar ice must have melted.
Studies of average annual temperatures during the Stagell Interglacial are contradictory. Some studies suggest it was warmer than other interglacials, while others seem to indicate that though it was longer, it was not as warm as other interglacials. Evidence found at the bottom of the ocean is stark and dramatic. The Ice Age preceding the Stagell, known as Marine Isotope Stage 12, was a particularly cold one–it left a record amount of debris carried by icebergs during meltwater pulses. (Rocks are carried by icebergs out to sea. Icebergs break off glaciers during minor warm phases within Ice Ages and carry debris in events referred to as “ice rafting.” Eventually the icebergs melt and drop this debris, known as drop stones, in visible piles at the bottom of the ocean.) But during the next 23,000 years during the Stagell Interglacial, ice rafting debris is almost completely absent. This suggests the glaciers and ice caps in the northern hemisphere were completely or seasonally absent then.
The northern hemisphere ice caps may have completely collapsed during the most recent interglacial previous to the one we live in now. The Sangamonian Interglacial (~132,000 BP-~118,000 BP) reached modern day temperatures by ~132,000 BP and kept getting warmer until 130,000 BP. The highest sea level transgression occurred by ~125,000 BP. Unlike during the Stagell Interglacial, however, ice began to reform and grow back rapidly as earth’s orbit returned to a more elliptical geometry.
During the Stagell Interglacial most of southeastern North America was probably covered in forest, though paleobotanical evidence from this period of time is scare, if it exists at all. A Late Irvingtonian Land Mammal Age fauna was present. It was similar in species composition to the later Rancholabrean Land Mammal Age with the exception of bison which hadn’t crossed the Bering land bridge yet. Mastodon, deer, peccary, llamas, horses, tapirs, Armbruster’s wolves, and jaguars were likely common. Ancestral forms of saber-tooths and ground sloths (Smilodon gracilis and Megalonyx whealeyi respectively) lived in the region then. Giant tortoises must have been pretty widespread in the mild climate as well.
Carbon dioxide levels during the Sangamonian Interglacial were higher than those of the Holocene until the 20th century when man’s burning of fossil fuels artifically pushed CO2 levels above even Sangamonian levels. Carbon dioxide doesn’t cause higher temperatures. Instead it usually lags natural temperature increases by thousands of years. But it is correlated with temperature increases, and scientists believe it magnifies them in some kind of poorly understood feedback mechanism. Of course, it isn’t up to me, but I think humans need to stop treating earth’s atmosphere like it’s some kind of gigantic chemistry experiment before mankind creates yet another ecological catastrophe.
“Clue from MIS 11 to Predict the Future Climate–A Modelling Point of View”
Earth and Planetary Science Letters 212 (2003)
Poore, Richard: et. al.
“Marine Isotope Stage 11 and Associated Terrestrial Records”
U.S. Geological Survey open file report 00-312
Both of these references are available online as pdf. You can find them easily with a google search.