The anthropogenic factors that have increased the amount of CO2 and other greenhouse gases since the 18th century include burning of fossil fuels, forest clearing and industrial processes. The CO2 concentration in the atmosphere has risen from 270 ppm to 370 ppm in the past two hundred and fifty years (Figure 2.3), mainly due to combustion of fossil fuels. This exceeds the natural variation (established through ice cores) over the past 650,000 years (180–300 ppm) (Jansen et al., 2007). The average Figure 2.4: Atmospheric concentrations of carbon annual growth rate in CO2 dioxide, methane and nitrous oxide over the concentration between 1995 and last 10,000 years (large panels) and since -1 2005 was 1.9 ppm y , which is 1750 (inset panels) significantly higher than the 40year average since 1960 (1.4 ppm y-1), when the continuous record of atmospheric measurements began (Forster et al., 2007). Radiative forcing There is a balance between incoming solar radiation and outgoing terrestrial radiation. Any process that alters the energy balance of the earth-atmosphere system is known as radiative forcing (RF). Some of the major causes that may trigger radiative forcing include variation in the earth’s orbit, solar radiation, volcanic activity and atmospheric composition (Forster et al., 2007). Figure 2.4 depicts the radiative forcing by the atmospheric concentrations of CO2, CH4 and N2O over the last 10,000 years (large panels) and since 1750 (inset panels). Measurements are shown from ice cores (symbols with different colours for different studies) and atmospheric samples (red lines). In Figure 2.5, AR4 provides global average radiative How do you forcing estimates differentiate between climate variability and and ranges in 2005 for climate change? anthropogenic agents and mechanisms, together with the typical geographical extent (spatial scale) of the forcing and the assessed level of scientific
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Source: IPCC, 2007a IWRM as a Tool for Adaptation to Climate Change
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