The extreme worst case scenarioss - fortunately not very likely (?)
  1. Worst case Scenario 1:
    Rapid warming in "leaps". Development might not be linear.

    Increasingly warmer global climate may lead to passing unknown tipping points.
    If the ice now floating on the polar seas melts, this may lead to various "feedback"-mechanisms impacting on other areas: More open water and less ice in polar areas reduce albedo (reflections) mean more solar energy being absorbed in the oceans.

    Warmer oceans will release gases such as CO2 dissolved in the water, and will absorb less CO2 from the air, thereby reducing the effect of the "ocean sink" .

    There are indications that pockets of the strong greenhouse gas methane and of CO2 that are now stored in frozen swamps in the Siberian permafrost areas and in pockets and methane hydrates at the bottom of the sea and in lakes might be released.
    With sudden release of such gas pockets when ice and permafrost lids disappear, there might be a substantial increase of atmospheric greenhouse gases, and a leap in average global temperatures.
    With a warmer atmosphere, oceans also become warmer.

    Warmer water expands, leading to rising water levels world wide.
    Warmer seas also lead to more rapid movements in glaciers on land, with massive slides of glacier ice into the water.
    Sea levels may rise rapidly by several meters

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  1. In addition, more water will evaporate to the air, giving more heavy precipitation, storms and flooding leading to erosion and land degradation.

    Warmer oceans also mean less coral reef production, an important storage of carbon.

    Warmer air means much more water may be absorbed in the air without condensing to clouds. (Clouds entail increased albedo, and may have a negative forcing effect, while water vapor in the air has a strong positive forcing effect.)

    This again may lead to another leap in temperature, melting more permafrost etc.

    Warmer weather implies more forest fires releasing soot and CO2 into the atmosphere.
    Black carbon on snow and ice will increase melting significantly.
    More CO2 absorbed by the oceans will make it more acidic.
    If the water turns too acidic, biological production will increasingly suffer, and absorb correspondingly less CO2 .

    In turn this might trigger other tipping points of no return, leading to a run away greenhouse effect.

    Theoretically, the climate of the Earth may approach that of Venus, and may eventually lead to the destruction of most life on Earth as we know it

  1. Second extreme scenario: New Ice age.
    Theoretically, and conversely, the greenhouse effect may lead to a new and rapidly forming ice age.

    Due to the greenhouse effect, the Greenland glaciers melt quickly, while precipitation over Siberia increases, leading to bigger rivers pouring freshwater into the Arctic Seas.

    More freshwater in the Arctic Seas may weaken the halocline, the line between the salty and warm Gulf stream water and the cold water in the Arctic.

    A weakened halocline may disrupt the thermohaline circulation (THC) in the ocean which make cooled, salty water containing a lot of dissolved gases such as CO2 "fall" deep into the sea - an important driving force in the "ocean conveyor belt". A disruption of the thermohaline circulation may eventually halt the Gulf stream which brings huge amounts of salty, warm water to Northern Europe and the North Pole.
    Without this enormous surface sea current, climate in Northern Europe would become approximately 10 centigrades colder.

    Feedback mechanisms speed up the process:
    Freshwater weighs less than salt water and remains on top. If solar activity diminishes slightly, or volcanic activity increases the amount of aerosols and thereby albedo in the atmosphere, huge parts of the northern oceans may freeze in winter, again increasing albedo. In addition the last years' solar activity has been unusually low. Since 2003 until now (2011) the sunspot activity has been as low as it was in the 1800. At that time we still had a climate that some call "the little ice age". , bringing very cold winters to especially the Northern hemisphere. The global average temperature has been estimated to have dropped as much as 0,3ºC in the coldest period, called the "Maunder minimum".

    Higher average temperatures in the Caribbean due to global warming will bring more warm and moist air with the Gulf stream towards Europe. A warmer Southern Europe may also entail that more precipitation may reach northern parts of the continent.
    A new and comprehensive ice age may get a flying start.

    Svalbard. Photo: L. Kullerud
  1. The last years, winters in western parts of northern Europe and eastern parts of the USA have been unusually cold, in spite of these last years also being among the warmest on record globally. Unfortunately this might be a sign of a quite dramatic shift in climate. It seems that since there is less ice covering the seas around the North Pole, a climate shift takes place.

    Since 1970, the summer polar sea ice has been reduced by more than 11% per decade. In autumn, the warm sea water from the Gulf stream meets the rim of the polar sea ice further north and over a wider area than usual when the winter sea ice is forming. This means that more energy than previously rise to the atmosphere, since the area of open sea is so much bigger. Unusually much moist and relatively warm air rises up to 8000 meters or more, warming the upper troposphere in the Arctic. This warm air pushes the colder air in the stratosphere upwards, which in turn disturbs and slows down the lower stratospheric jetstreams at around 10 000 meters, resulting in high air pressure and jet stream deviations. Increased air pressure in the north-east (a high) and a lower air pressure further south-west, entail less chances of westerly winds towards Scandinavia, but may push these winds further south in Europe and in addition give easterly cold winds towards the US East coast. This is a situation called a negative North Atlantic Oscillation (NAO) or Negative Arctic Oscillation.

    As a consequence, Arctic areas, parts of Siberia and Canada experience relatively warm winters, while temperate zones in Europe and USA receive the cold Arctic winds and people may perceive that a new ice age is coming, despite of - or because of - global warming. Strong La Ninas and NAOs combined with much energy in the atmosphere will give cold winters and "freaky" weather over parts of the globe.


    A persistent negative Arctic oscillation combined with La Nina, a permanent weakening of the Gulf stream and warmer seas in the south west Atlantic due to global warming may give quite dramatic impacts on the weather in Northern Europe.

    Photo: Snow storm in Southern Norway. Åke Bjørke
  1. Between these two extreme scenarioes, there are several others more likely.
    The UN Panel of Climate Change has tried to describe some of the more likely ones.

    Main scenario: "Warmer, drier, wetter, wilder". A warmer air means more evaporation, and will keep more water longer before it condenses to clouds and precipitation. Longer dry spells interrupted by heavy rainy periods are inevitable. As a rule of thumb the weather is chaotic and impossible to predict for longer periods. But there will be tendencies or trends. Areas that usually receive much precipitation are likely to receive more. Areas that are dry now, are likely to become even drier. This means that e.g. parts of Scandinavia and Northeastern USA will receive more precipitation, especially in wintertime. As long as the temperatures are below 0 centigrades this precipitation will come in the form of snow. Due to an increasingly open Arctic ocean in summer seasons it is more likely than previously that for some years to come, negative NAO conditions may prevail in early winter periods, bringing cold weather and snow to Western Europe and Eastern USA, but milder winters to Canada, Greenland and parts of Siberia. Due to massive melting of the Greenland glaciers, more freshwater will be found on the ocean surface in the Arctic. Since freshwater freezes quicker than salty water, winter sea ice is likely to expand despite rising temperatures. An increasingly more open polar summer ocean combined with sea ice expansion during winter time will disturb normal weather patterns in the polar and temperate zones for years to come.

    The weather is likely to become increasingly more unpredictable, more "aggressive", and most previous weather records will be broken at an accellerating pace. - Nothing new, but the pace and intensity. The "hundred-year-flooding" will come every second year instead of every hundred year. Global average temperatures will rise by 0.2-0.4 centigrades per decade, with significant variations. Polar areas, especially the Arctic, are likely to have a more rapid temperature rise than other places.

    When the average global temperatures have increased by one centigrade, it is likely that Southern Europe, Southern Africa, parts of Asia, the Middle East and South West USA will experience an increasingly tougher freshwater stress and desertification. When global average temperatures reach 16 centigrades (about 1,3 centigrades more than now), it is likely that parts of the Amazon rainforest will dry up and burn, adding significant amounts of carbondioxide to the atmosphere.

    When the global average temperature rise to 17 centigrades, it is likely that parts of the Siberian and Canadian tundras melt, releasing massive amounts of methane and carbondioxide and enhance the greenhouse effect. Mountain glaciers worldwide will shrink significantly, and after temporarily increasing, several main rivers will partly dry out during summertimes. Storms and hurricanes might not necessarily become more frequent, but probably more violent. The oceans will be too warm to be able to absorb more GHGs, and will probably start releasing them to the atmosphere instead. Most coral reefs in the tropics and subtropics will bleach and die.

    Worst case scenario is that the climate systems reach an unknown tipping point, where there is a sudden shift to a new and unkown paradigm. This scenario is described by the US military in this report:
    "National Security and the Threat of Climate Change" US top military assessment on climate change and trigger points

    Dr James Lovelock, known for the "Gaia theory" thinks it is likely that most of our planet will be uninhabitable if current GHG emissions and unsustainable consumption continue.

    Photo: Desertification in Egypt. Mohamed Tawfic