Lecture 13

• over the last 650,000 years there have been:

  • NH summer insolation reduction (due to changes in Earth's orbit) causes ice sheets to grow
  • colder oceans hold more CO2 so lower atmospheric concentrations
  • warmer interglacial periods and glacial periods
• CO2 during last glacial/interclacials (10,000-22,000 years)


  atmospheric CO2 increases from about 190 ppmv to 265 ppmv. lag temperature change by a few centuries due to deep oceans. methane represents wetness in the tropics. when the tropics are wet CH4 emissions and atmospheric concentrations are higher. CH4 increases from about 350 ppbv to 650 ppbv
• mechanisms of recent Holocene climate variability

  • internal climate variability = year-to-year or decade-to-decade variability (generated within the climate systems e.g. ENSO - much of this variability is unpredictable more than a year ahead
  • forced variability can drive climate variability e.g. orbital changes, changes in solar insolation, volcanic eruptions
• models are used for forecasting future climate or climate change are similar to models used for weather forecasting
• parameterisation:

  • solve equations of motion (= conservation of mass, momentum, heat and moisture)
  • many processes occur below the grid resolution (~100 km) e.g. cloud formation, convection, flow over mountains,...
  • these processes need to have their effects represented in terms of the large-scale (resolved) flow
  • uncertainty in how these processes are represented leads to uncertainty in model feedbacks and in climate sensitivity
• external influences on climate:

  • changes in the frequency of volcanic eruptions
  • changes in solar radiation
  • changes in greenhouse gases

  if you put these into climate models, they reproduce key features but also climate variability generated within the climate system
• solar contribution small - episodic cooling from volcanoes
• human = GHGs and aerosols
• Temperatures exceeding 34°C recorded in 7 out of the last 10 years compared to 7 times between 1961 to 2010
• Change in the number of days per year with maximum temperatures exceeding 35ºC
• Large changes globally under high emissions scenario
• • Increases and decreases in annual mean rainfall for a strong global warming scenario by the end of the century
  • Increases in high latitudes, Equatorial Pacific, and monsoon regions; decreases in sub-tropics
  • Extreme rainfall generally increases; drying regions become drier