Climate varies
From a geological perspective - over millions of years - climate on Earth has been like a roller coaster, changing from warm and humid to ice ages.

Climate variations thousands of years ago can be read in ice core boring, tree rings, corals and stalagmites through 'proxy data'.

Ice core borings can inform us in detail about climate up to more than half a million years ago. Climate change is nothing new.

  1. Geological layers and fossils can give us some information at a rougher scale.
    At times solar activity has changed, the polar axis may have tilted more or less towards the sun, our planet may have had a more or less elliptic circle on its journey around the sun, changing solar input.

    Volcanoes are usually cooling
    Big volcanic eruptions have increased albedo by throwing sulphuric aerosols high up in the air for several years, at times bringing on new ice ages.
    In some instances volcanoes have been huge cracks in the earth's surface emitting enormous amounts of greenhouse gases and lava but hardly any aerosols. The amount of atmospheric greenhouse gases 400 million years ago could thus be 10 times as high as today. At those times, solar radiation was considerably weaker, so the effect would not be directly comparable.

    Volcanic aerosols, more ice, snow and deserts increase albedo, have a cooling effect.
    Increased solar input and more GHGs have a warming effect.
    These natural climate change forces have always been a part of the Earth's ecology.

    The last 500 millions years nature has drawn carbon from the air to the ground through photosynthesis.
    Carbon can e.g. be stored for a long time in dead trees and plants at the bottom of swamps, thereby withdrawing carbon from the air.
  1. Over the last 400,000 years the Earth's climate has been unstable, with very significant temperature changes, going from a warm climate to an ice age in as rapidly as some centuries.
    These rapid changes suggest that climate may be quite sensitive to internal or external climate forcings and feedbacks.

    As can be seen from the blue curve in the graphic below, temperatures have been less variable during the last 10 000 years.
    This is the era called Holocene.
    Based on the evidence available, it is unlikely that global mean temperatures have deviated by more than 1°C up or down during this period.

    It is during these last 10 000 years that agriculture appeared, being the precondition for building civilisations.
    In a way you can argue that the first 'modern' human beings appeared somewhere in the Middle East at this time, when the first human beings transformed from being nomadic or semi-nomadic to settling down to become agriculturalists. The Biblical story about Adam and Eve seen metaphorically, fits well with this very radical event in human history.

    The information presented on this graph indicates that in the Holocene period there has been a strong correlation between carbon dioxide content in the atmosphere and temperature.

    In earlier times this correlation has been less clear, with CO2 emissions to air as a rule coming as a feedback response centuries after the initial radiative forcing. - When the main radiative forcing has been the Milankovich cycles, the climate change would have started by a slight increase in solar forcing, raising the temperature maybe 0,01 centigrade. This slight temperature rise would in turn lead to a slightly smaller ice cover in the Arctic, entailing a slight reduction in albedo, in turn leading to a small increase in radiative forcing.

    The main radiative forcing factors have always been solar radiation and albedo. The process of slow warming could go on for centuries or even millennia before the oceans would be warm enough to start giving small amounts of carbondioxide and methane to the atmosphere, thereby enhancing the temperature rise in feedback loops. The warming loops of interaction between greenhouse gases and albedo would stop by e.g. a big volcanic eruption cooling down the systems with large amounts of aerosols (particles) blown up in the stratosphere, thereby overwhelming the warming with the sudden increase in albedo.

    The climate system is sensitive. This was clearly demonstrated by the slightly cooler period in the 1950's and 1960's with the increased albedo from clouds of sulphur particles belched out by an environmentally reckless industrialisation; heavy smokestacks and coalfired powerplants, especially in the Soviet union and China.

    The current aerosol albedo from fossil fuel combustion, increased sandstorms, the Asian brown haze and forest fires is still significant, and masks the real GHG radiative forcing to some extent. However, the increasing energy in the atmosphere and in the oceans has an impact that can be observed. And every decade the last forty years has been warmer than the preceding one, in spite of record low solar activity except a short solar burst in 1998, and in spite of the high aerosol albedo. The solar activity the last ten years is the lowest in 110 years. If solar forcing were the only radiative forcing factor, we should now experience a 'little ice age' like in the 1800's. Instead we register the warmest decade ever recorded, in spite of colder spells in the temperate zone.

    The timelag between a temperature rise and increase of atmospheric GHGs that was common in previous times is no longer there. We now have the unusual phenomenon of greenhouse gases being the main radiative forcing factor rather than just being a feedback mechanism.

    A possible current scenario: anthropogenic emissions of GHGs could bring the climate to a state where it reverts to the highly unstable climate of the pre-ice age and the Eocen period.
    Rather than following a linear evolution, the climate may follow a non-linear path with sudden and dramatic surprises when GHG levels reach an as-yet unknown tipping point.

    Graphics;: Vital Climate Graphics updated (UNEP/GRID-Arendal)