Introduction to climate dynamics and climate modelling

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Technical Note

5.5.2.1 Hemispheric-scale changes

The last millennium is certainly the period in the past for which we have the greatest number of proxy records. Tree rings, lake and marine sediments, ice cores, etc, all provide very useful information on past climate changes. Nevertheless, the uncertainty over temperature changes is still significant (Fig. 5.26). All the available reconstructions show relatively mild conditions around 1000 AD (the so-called Medieval Warm Period), followed by a cooling that culminated around the 17-19^th centuries (the so-called Little Ice Age). However, the amplitude and the exact time of the changes vary strongly between the different reconstructions. However, all the reconstructions reach their absolute maximum during the 20^th century.

**Figure 5.26:** Reconstructions of Northern Hemisphere temperature variations over the last 1300 years using multiple climate proxy records and the instrumental record for the last 150 years. All series have been smoothed to remove fluctuations on time scales less than 30 years. All temperatures represent anomalies (^oC) from the 1961 to 1990 mean. Figure 6.10 from Jansen et al. (2007) using a modified legend, published in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, copyright IPCC 2007. Reproduced with permission. See Jansen et al. (2007) for a full reference to all the reconstructions.

When analysing climate changes over periods of the order of several millennia, orbital forcing is generally dominant. However, for shorter periods, such as the last 1000 years, variations in insolation at the top of the atmosphere due to changes in orbital parameters are relatively weak and other forcings have thus to be taken into account. On these time scales, the two dominant natural forcings are changes in total solar irradiance (TSI) and large volcanic eruptions (see section 4.1.2.4). The majority of the volcanic eruptions have a dramatic local impact but only a weak large-scale influence on climate. By contrast, some major eruptions can transport large amounts of aerosols into the stratosphere where they can stay for a few years. Those aerosols modify the radiative properties of the atmosphere, decreasing the solar irradiance at the surface and thus lowering the temperature, in particular in summer. In addition, the volcanic aerosols have an impact on the atmospheric circulation and tend to favour a positive phase of the North Atlantic Oscillation. As a consequence, a major volcanic eruption is often followed by a warm winter over Europe, a characteristic of a positive NAO index.

In contrast to the orbital forcing whose time development is very well known, we are still uncertain are present for both solar (TSI) and volcanic forcings. On the one hand, the volume and characteristics of the aerosols released by the volcanic eruptions have to be derived indirectly from the measurements of sulphate loads in ice cores (Fig. 5.27). On the other hand, we have precise measurement of TSI from satellites over the last 30 years. For earlier times, measurement of the concentration of cosmogenic isotopes, such as ¹⁰Be and ¹⁴C, in ice cores can be used as a proxy for TSI. When solar activity is low, the shielding of the Earth from energetic cosmic rays is weaker, and there is an increase in the production of those isotopes. However, the link between the concentration of cosmogenic isotopes in ice cores, solar activity and solar forcing is far from simple. It has even been suggested that some of the widely used reconstructions (e.g. Fig. 5.27) overestimates the long term changes in TSI by up to a factor five.

The last millennium is an ideal test case for climate models to compare natural and human induced changes. Whether driven by solar and volcanic forcings as well as by anthropogenic forcings (increase in greenhouse gas concentration, sulphate aerosol load, land use changes, see section 5.5.3), the simulated temperatures are within the range provided by the reconstructions. This gives us some confidence in the validity of models. Furthermore, simulations can be used to analyse the causes of the observed changes. In particular, the cold periods during the Little Ice Age correspond well to times with a relatively low TSI and frequent volcanic eruptions.

**Figure 5.27:** Radiative forcings and simulated temperatures during the last 1100 years. The global mean radiative forcing (W m^-2) used to drive climate model simulations due to (a) volcanic activity, (b) solar irradiance variations and (c) all other forcings (which vary between models, but always include greenhouse gases, and, except for those with dotted lines after 1900, tropospheric sulphate aerosols). (d) Annual mean Northern Hemisphere temperature ( ^oC) simulated under the range of forcings shown in (a) to (c), compared to the concentration of overlapping Northern Hemisphere temperature reconstructions. All forcings and temperatures are expressed as anomalies from their 1500 to 1899 means and then smoothed with a Gaussian-weighted filter to remove fluctuations on time scales less than 30 years. Figure 6.13 from Jansen et al. (2007) using a modified legend, published in: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, copyright IPCC 2007. Reproduced with permission. See Jansen et al. (2007) for a full reference to all the time series.