Past Changes in earths temperature

The Earth has passed through many stages of warming and cooling. Some of these give an indication as to how the temperature can, once it starts to rise due to excess greenhouse gases, rise quickly due to feedback mechanisms which accelerate the process.

Global warming event that kick started mammal diversity 55 million years ago

55 million years ago (Paleocene - Eocene Thermal Maximum ref 1 p202) global temperatures rose quickly by 5 deg. C. Over a period of about 10,000 years 4.5 trillion tons of carbon dioxide (equiv to 570 ppm) had entered the earths atmosphere, probably from undersea volcanism which set off gas release from dead plant and animal matter, oil and gas. It took 200,000 years for the earth to stabilize at lower temperatures. Now we are releasing CO2 thirty times faster than at that time.

65 million years ago (end of cretaceous period)

Earth heated up by 4 to 5 deg C 18% land vertebrate families and 16% marine families died off including dinosaurs. This was as a result of greenhouse gases due to a giant meteor (see also event 250 million years ago.)

Antarctic was ice free 100 million years ago

Evidence from sediments and fossilized shells collected from the tropical North Atlantic off Surinam indicate that 100 million years ago water temperatures ranged from 33 to 42 deg C and the sea level was over 200 metres above today's levels (ref 1 p 203). The Antarctic continent had vegetation and animals living on it at that time.

250 million years ago warming released methane to accelerate the process with a 10 deg C rise.

250 million years ago (end of permian period) nearly all life vanished on earth (95% marine species and 70% land species became extinct). A meteor struck the earth in the (present day) arctic, debris and gas blotted out the sun for a few years and much of the plant and animal life on land became extinct. (See comment on ocean currents below.) Eventually after some time the earth warmed up by 5 deg C due to the greenhouse gasses that had been released from volcanism after the impact and most life in the oceans also died out. With the rise of temperature it is believed that frozen methane hydrates melted and the release caused more global warming and a further rise of 5 deg C making 10 deg in all. Much of the remaining life also died out at this stage. It took 200 thousand years before the temperature restored to that which was normal to sustain life.

Six petrified rain forests of huge club moss trees, horsetails and ferns were discovered in coal mines deep beneath Kentucky and Illinois. These indicate that the rain forest collapsed quite quickly at that time.

Earth was mostly frozen 500 million years ago followed by the greatest global warming event a swing from minus to plus 50 c.

600 million years ago the planet's land mass had moved towards the south pole and ice built up over thousands of years, with glaciers reaching to near the equator. As it built up the ice reflected more energy back and this accelerated the process. The planet's average temperature fell to minus 50 C (today's average is plus 15 C). When the land mass moved and broke up, meltdown took place over a long time period and a major factor was increase in volcanism (ref 163) releasing CO2. The earths average temperature swung to plus 50 C and subsequently stabilized at a lower value.

Relevance of past - the same could happen due to CO2 rise today.

The rise in greenhouse gases today could be of the same order as in these past events, but this time it is man made and happening much faster. Any recovery could take thousands of years.

Simulations show also that a rise in CO2 to 500 ppm would be sufficient to cause a rise of temperature the result which would be the death of the algae in the oceans, the change being quick due to feedback effects (ref 2 "The revenge of Gaia" book by James Lovelock.)

The map pages show how land would eventually be flooded. These are based on a rise in sea level of 67 metres and ignores the expansion of the sea due to warming and changes in land levels. In the past the sea has been 200 metres higher, although the land was flatter at that time.

The areas of the sea where life exists could be very much reduced.

Algae require a water temperature below 10 deg. C in which to live. Much of the sea has a top layer between 30 to 100 metres depth which is above this temperature and when not disturbed is therefore clear and dead. Below 10 deg. C the sea is rich in nutrients and today these areas are found toward the north and south of the planet, or deep down.

Desert areas could spread

Water is difficult to retain on land above 20 deg. C. Below 10 deg. water stays in the ground. In the tropics the ecosystems sustain clouds and rain above the forest canopy which replenishes the water with rain, but if temperatures were to rise another 4 deg. C this would destabilize and tropical forest would turn to scrub or desert.

Other causes of climate change, orbit, tilt, wobble, sunspots.

There are other events that affect temperature changes by varying the amount of sunlight that hits the earth.

The earths orbit varies on a 100,000 year cycle (and a 400,00 year cycle) with the center offset slightly, from elliptical to more circular taking the earth nearer or farther from the sun and these have become associated with the ice ages, the more elliptical orbit causing more sun and summer melting.

The earths axis tilts on its axis on a 41,000 year cycle. Less tilt results in cooler summers, less melting and thus promotes ice, more tilt results in more summer melting.

The earth also wobbles so its axis-of-spin shifts on a 11,000 year cycle. Thus the date and month of mid winter, darkest day in the northern hemisphere, will vary through the 12 month period in 11,000 years.

Volcanic activity varies and sunspot activity varies.

Forests absorb CO2 by photo synthesis, so as forests are cut down less is absorbed. Where destruction of forests leads to soil erosion, regeneration could take an impossibly long time to be effective.

Four Ice ages - changes to warmer periods quick due to feedback.

There have been 4 ice ages in the last 400,000 years and we are now well into the fourth intermediate warm period (ref 103). The Pleistocene - Holocene change 11,000 years ago took place in just a few decades and caused the extinction of mammoths and many other species. Sea levels then rose by 120 metres.

What seems to happen is that these changes start slowly and then reach a point, or threshold, where the change accelerates (refer to feedback notes below.)

There has over the last 400,000 years been a close correlation between the level of CO2 in the air and global temperature, until the last few years when the CO2 level has broken away and increased sharply. Air bubble analysis in the ice cores taken from the Antarctic and Greenland confirm that the CO2 levels today are the highest ever reached in 440,000 years (ref 102).

Recent short fluctuations

There seems to be a shorter cycle of world temperature variation also. It was warmer in North Europe during the Viking era when the Vikings populated part of Greenland. It then turned colder from 1400 to 1900, and the Greenland farmers died off due to failure of agriculture. In these latter times the river Thames in England froze over completely. These shorter variations are put down to sunspot changes. When cosmic rays from exploding supernovae hit water vapor, cloud is formed that reflect the suns rays. Solar wind drives the cosmic rays away lowering cloud formation and thus permitting more warming.

The evidence of warming today.

A NASA backed computer study has predicted that the summer sea ice in the arctic will have vanished by 2040, except for some coastal ice off Greenland and Ellesmere island. (By Aug 2008, 1/4 of the coastal ice off Ellesmere island became detached and broke up.) Their model puts this down entirely to the increased level of CO2 in the air. The melting will be quite rapid after 2024. In the last 40 years, the arctic ice thickness has reduced by 40% (ref 148).

Today's evidence of warming is that glaciers in the Alps have shrunk, the west side of the Antarctic ice which rests on rock under sea level is showing signs of being undermined and ice has broken away, Greenland is loosing 200 cubic metres of ice a year, summers are hotter and storms more violent (ref 133).

Effect of time on the relation between CO2 and global Temperature.

The effect of increased CO2 and temperature depends on time also. The increase in CO2 now will have an accumulative effect in future years and today's emission level even if stabilised will therefore effect future generations to a much greater extent than today.

The Ocean currents and Gulf Stream could be halted by sudden melt and temperatures worldwide affected.

The Gulf Stream flows from the West Indies into the northern Atlantic. It conveys substantial heat and thus warming of the climate in the UK, Ireland, Norway and Northern Europe and to a lesser extent the East coast of North America. The Gulf Stream flow then falls, as the salt concentrates when ice is formed and the sea becomes heavier, and returns at deep ocean level to the tropics. Without it the UK and Northern Europe temperatures instead of being well above those of a similar latitude such as Hudson's Bay and Moscow, would have a much colder winter at these latitudes.

A sudden melt of Greenland ice due to global warming could slow or halt the gulf stream. A mass of melted fresh water would in these circumstances dilute the salt in the ocean water which would become lighter and thus halt the fall to lower levels in the north Atlantic.

The Gulf Stream is part of a global system of currents that cool some areas and warm others around the world. If flow ceased the effect would be to cease the transfer of heat from the tropics to the north Atlantic. Our winters in northern Europe would then be colder and this may counteract the warming effect in the North Atlantic. This heat would not be transferred away from the tropics which would make these areas even hotter.

The global system of currents transfers oxygen from the ocean surface and distributes it around the ocean and this is essential for life in the oceans. A further effect if oxygen is not distributed is that the oceans would stagnate and toxic substances such as Hydrogen Sulphide could build up and eventually affect life on earth. It is suggested that these two events were the cause of mass extinctions in the warm up 250 million years ago (ref 164).

In a worse case of a sudden flow of a large amount of trapped water from Greenland the Gulf stream could cease for some years and this would have a cooling effect where the feedback systems go into reverse with lower temperatures & increased ice in the northern hemisphere for a long period (ref 134).

Feedback systems

There are several (positive) feedback systems which tend to accelerate the process of global warming once the process is under way:-

1) Ice reflects most of the suns rays back to space. Thus if the edges of the ice sheets begin to melt due to warming, rock and earth is exposed which is dark and reflects less heat. With more heat retained by the earth more ice melts, the reflective surface is reduced and more energy is absorbed (Budyko effect).

2) Tropical forests must have sufficient water or water replacement to exist. Land will not retain water over 20 deg. C due to rapid evaporation. Thus as the land warms, tropical forest reduces and turns to scrub or desert areas. The land that replaces lost forest areas lacks the tropical cooling mechanism so it becomes warmer and thus less forest area is available. In other words tropical forests melt away. Clearing tropical forests will of course also accelerate this effect.

3) The forests in Canada and Siberia are dark and heat absorbing. Where these spread due to warming and cover land that absorbs heat less well, there will be more heat retained.

4) Above 4 deg. C water expands with warmth and is lighter. A layer 30 to 100 metres deep in the oceans forms above 10 deg. C in which the photosynthetic algae use up all the nutrients and the water becomes clear with little life. As the sea warms, less CO2 is therefore absorbed and less stratus cloud forms which reflects the sun.

5) As forest and algae die, more CO2 and methane is released causing more warming.

The amount of methane in the atmosphere has risen over 200 years from 0.6 to 1.7 parts per million (ppm) although small this is now equivalent to 20 times the effect of CO2, ie 34 ppm equivalent. Methane is produced by landfills, cultivation of rice paddies and animal digestion (wind).

6) Perhaps the most worrying feedback process is due to the large amounts of methane (CH4) are held in ice crystals within molecular sized voids known as Clathrates (146). These are stable at high pressure or at sufficiently low temperature. The Clathrates could melt near to land with higher temperature releasing methane. If very large amounts are released suddenly this could become quite catastrophic. (Generally, a feedback process would work in reverse but not this one.)

7) Methane is formed in lakes as the permafrost melts and ancient vegetation decomposes. These are held in ice bubbles which are released when the ice melts each year. In northern Siberia there is an area of these lakes the size of the USA. The potential release of methane from this source is 10 times the existing level (ref 163). The greenhouse effect of this release would therefore be the same as the existing CO2 level of 338 ppm.

8) Global warming leads to more storms and thus high winds. High winds stir up deep water which hold more CO2 thus less CO2 is absorbed by the oceans (effect reported by University of East Anglia and the British Antarctic survey May 2007 on research in the southern oceans. )

9) As the height of an ice cap melts and lowers, the warmer air at the lower height will help to accelerate the process.