Summary

In this chapter we have learnt that:

• Records of temperature, atmospheric composition and sea level from ice cores from Greenland and Antarctica, from ocean and lake sediment cores and other proxy records have provided a wealth of information about past climates over much of the past million years.

• The current levels of carbon dioxide and methane concentrations in the atmosphere and their rate of increase is unprecedented in the palaeoclimate record over the last half million years and probably for much longer.

• It is likely that the 50-year period of the second half of the twentieth century was the warmest northern hemisphere period in the last 1300 years.

• It is very likely that the warming of 4 to 7 °C since the last glacial maximum 18 000 years ago occurred at an average rate about ten times slower than the warming of the twentieth century.

• The main trigger mechanism for the series of ice ages over the last million years or more has been the variations in the distribution of solar radiation especially in the polar regions arising from the regular variations in parameters of the Earth's orbit around the Sun - called Milankovitch cycles. Variations in greenhouse gases have served to add a positive feedback to this forcing. These orbital variations are well understood and the next ice age is not expected to begin for at least 30 000 years.

• For the first half of the last interglacial period (~130 000-123 000 years ago), a large increase in summer solar radiation due to orbital forcing led to higher temperatures in polar regions 3 to 5 °C warmer than today and melting in polar regions that led to 4 to 6 metres higher sea level than today.

• Some of the abrupt events during the last 100 000 years that have been identified in the records from ice core and other data may have been associated with large fresh water inputs into the ocean due for instance to large ice discharges that resulted in large-scale changes to the ocean circulation.

Having now in these early chapters set the scene, by describing the basic science of global warming, the greenhouse gases and their origins and the current state of knowledge regarding past climates, I move on in the next chapter to describe how, through computer models of the climate, predictions can be made about what climate change can be expected in the future.

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