1. Technically, or if you want to say naturally, we are living in the interglacial stage of an ice age. Human activities of unprecedented scale have started to have an impact on the planet, particularly on the gas-mix in our atmosphere.
Until now, hot (interglacial) and icy periods have been following each other during the last millions of years (see graphic). This is also called Milakovitch cyclicity, after the scientist who discovered it. Wikipedia writes:“ Milankovitch cycles are the collective effect of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković. The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns, resulting in 100,000 year ice age cycles of the Quaternary glaciation over the last few million years. The Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time, the elliptical orbit rotates, more slowly, leading to a 21,000 year cycle between the seasons and the orbit. In addition, the angle between Earth's rotational axis and the normal to the plane of its orbit changes from 21.5 degrees to 24.5 degrees and back again on a 41,000 year cycle. Currently, this angle is 23.44 degrees and is decreasing. The Milankovitch theory of climate change is not perfectly worked out; in particular, the largest observed response is at the 100,000 year time scale, but the forcing is apparently small at this scale, in regard to the ice ages. Various feedbacks (from carbon dioxide, or from ice sheet dynamics) are invoked to explain this discrepancy.”

Interestingly, cold and hot periods can be directly correlated to the C02 partial pressure in the atmosphere, though the picture may look complicated if viewed over a short time period. Under normal circumstances, one would expect planet earth to return to ice-age conditions, given the demonstrated climatic cycles of the past. Large volcanic eruptions (dust, S02 in the high atmosphere) are also known to affect (= cool) the global climate for a while, and are believed to trigger cold spells that can last for years or even perhaps several hundreds of years. A collision of our planet with planetary objects (comets, astroids, etc) would certainly produce climatic change, though dimension and time scale of such changes remain poorly understood.


2. The Keeling Curve. Let me cite Wikipedia: “ The Keeling Curve is a graph showing the variation in concentration of atmospheric carbon dioxide since 1958. It is based on continuous measurements taken at the Mauna Loa Observatory in Hawaii under the supervision of Charles David Keeling. Keeling's measurements showed the first significant evidence of rapidly increasing carbon dioxide levels in the atmosphere. Many scientists credit Keeling's graph with first bringing the world's attention to the effects that human activity were having on the Earth's atmosphere and climate.
Charles Keeling of the Scripps Institution of Oceanography was the first person to make frequent regular measurements of the atmospheric carbon dioxide (CO2) concentration, taking readings at the South Pole and in Hawaii from 1958 onwards. Due to funding cuts in the mid-1960s, Keeling was forced to abandon continuous monitoring efforts at the South Pole, but he scraped together enough money to maintain operations at Mauna Loa, which have continued to the present day.
The measurements collected at Mauna Loa show a steady increase in mean atmospheric CO2 concentration from about 315 parts per million by volume (ppmv) in 1958 to over 380 ppmv by the year 2006. This increase in atmospheric CO2 is considered to be largely due to the combustion of fossil fuels, and has been accelerating in recent years. This is supported by measurements of carbon dioxide concentration in ancient air bubbles trapped in polar ice cores, which show that mean atmospheric CO2 concentration was between 275 and 280 ppmv for several thousand years but started rising sharply at the beginning of the nineteenth century. Since carbon dioxide is a greenhouse gas, this has significant implications for global warming.
Though Mauna Loa is an active volcano, Keeling and collaborators made measurements on the incoming ocean breeze and above the thermal inversion layer to minimize local contamination from volcanic vents. In addition, the data is normalized to negate any influence from local contamination. Measurements at many other isolated sites have confirmed the long-term trend shown by the Keeling Curve, though no sites have a record as long as Mauna Loa.
The Keeling Curve also shows a cyclic variation of about 5 ppmv in each year corresponding to the seasonal change in uptake of CO2 by the world's land vegetation. Most of this vegetation is in the Northern hemisphere, since this is where the majority of the land is located. The level decreases from northern spring onwards as new plant growth takes carbon dioxide out of the atmosphere through photosynthesis and rises again in the northern fall as plants and leaves die off and decay to release the gas back into the atmosphere."

Comparing and pondering the above data it is obvious that we have departed from the typical range of the hot interglacial climate, given C02 is seen rising to levels not known for the last few millions of years. Are we seeing a ‘run-away’ greenhouse, a one-way and irreversible heating-up of the planet? If this were the case, all polar ice will melt and the global sea level might rise by as much as the frequently quoted 80 m – this would certainly eliminate global human society as we know it. Once Earth is dominated by an aquatic realm, climate might stabilize again given a good heat exchange in the oceans, and probably a very dense cloud cover, that will reflect sunlight and hence lower the energy input for the planet.

Science fiction? Not necessarily! It has happened a long time ago during intervals of the Cretaceous period, known as the Cenomanian transgression.
During that time, all low-lying lands on earth were flooded, with only a few islands emerging, a mellow, temperate and probably quite foggy and boggy water world in brief.

Actually, the reasons of climate change, and what to do about it, seem two pairs of different shoes. Too much discussion, in my opinion, is wasted arguing what caused global warming and climate change. It's happening, and we're the guinea pigs of an uncontrolled large-scale experiment. Instead, discussion should focus on what can be done to constrain the planet’s fever curve and to bring it back to a healthy and sustainable CO2 level. Short-term considerations (economy, job market, national agendas etc) will need to be balanced with possible long-term consequences (habitat loss, flooding, drinking water availability, mass-migration and crop failure/ hunger). Let's hope there will be enough wisdom, courage and leadership to enact necessary changes in time!

© 2007 by Franz L Kessler

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