The previous sections have been mainly devoted to anthropogenic forcings. However, natural forcings such as those associated with explosive volcanoes and changes in total solar irradiance (TSI), also affect the Earth's climate. Precise measurements of the TSI are available from satellites for the last 30 years. They clearly show an 11-year cycle, associated with the well-known periodicity of solar activity. However the long term trend since 1980 is very slow (Fig. 4.6). Over this period, the amplitude of the changes in TSI has been of the order of 0.1 %, corresponding to a peak to peak change in TSI of about 1 Wm–2. Dividing by four to take into account the geometrical properties of the system (see Eq. 2.3), gives a value for the radiative forcing between high and low solar activity of about 0.25 Wm–2.
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Various methods have been used to extend our estimates of the changes in TSI back in time. The number of sun spots (observed since around 1610) and the production of the cosmogenic isotopes 14C and 10Be are both known to be related to solar activity. However, additional work is still required before we can make a clear quantitative link between these indirect measurements and variations in TSI. As a consequence, the magnitude of the changes in TSI before the availability of satellite measurements is not well known (see section 5.5.2.1). However, the most recent estimates agrees on relatively small changes between present-day and pre-industrial times corresponding to an increase in TSI of the order of 0.1% since 1750 (e.g., Wang et al. 2005).
Major volcanic eruptions have a dramatic local impact causing fatalities and damage to properties, crops, forest etc. The ash produced can travel hundreds of kilometres, altering atmospheric properties for days or weeks and modifying the characteristics of the Earth's surface after its deposition. Explosive volcanic eruptions can even have an influence on a larger spatial scale, affecting the whole of the Earth's climate significantly. Indeed, explosive eruptions can transport aerosols (mainly sulphates) directly to the stratosphere where they remain for a few years and affect nearly all regions (Fig. 4.7). As discussed for anthropogenic aerosols (section 4.1.2.2), the presence of sulphate aerosols in the stratosphere induces both a local warming in the stratosphere (mainly because of the enhanced absorption of solar radiation) and a cooling below (associated with the scattering of some radiation back to space). For major eruptions, the net radiative forcing reaches an average of several Wm–2 across the globe in the year following the eruption, and takes a few years to decrease to nearly zero.
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