Introduction to climate dynamics and climate modelling

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Technical Note

5.5.3 The last century

In the period 1906-2005, the global mean surface temperature rose by 0.75 ± 0.18°C (Fig. 5.30). Moreover, the rate of warming increased sharply, with the increase in the last 50 years being almost double that in the last 100 years. This warming, which has lead to the highest mean temperatures in at least several centuries (Fig. 5.26), is clear at global and hemispheric scale as well as over all the continents except Antarctica (Fig. 5.32).

**Figure 5.30:** Global and hemispheric annual combined land-surface air temperature and SST anomalies (°C) (red) from 1850 to 2006 relative to the 1961 to 1990 mean, along with the 5 to 95% error bars (from HadCRUT3, adapted from Brohan et al., 2006). The smooth blue curves show decadal variations. Figure 3.6 from Trenberth et al. (2007), reproduced with permission from IPCC.

The surface temperature of the oceans has also increased, although generally more slowly than that of the continents. Some oceanic areas, e.g. close to the Southern tip of Greenland, even display a slight cooling during the 20^th century (Fig. 5.31). The warming is associated with clear modifications of the cryosphere, such as a retreat of the large majority of glaciers, the permafrost and seasonally frozen ground, as well as a decline in the snow cover over land, especially in spring. In the Arctic, the sea ice extent has declined by about 3% per decade since 1978. The decrease in the extent of the sea ice is even larger in summer, at a rate of about 8% per decade. By contrast, the sea ice in the Southern Ocean appears to have been relatively stable over the last 30 years.

**Figure 5.31:** Linear trend of annual temperatures between 1901 and 2005 (°C per century). Areas in grey have insufficient data to produce reliable trends. The minimum number of years needed to calculate a trend value is 66 years between 1901 and 2005. An annual value is available if there are 10 valid monthly temperature anomaly values. Figure 3.9 from Trenberth et al. (2007), reproduced with permission from IPCC.

Over the period 1993–2003, the sea level rose at a rate of about 3.1 mm per year. The thermal expansion of the ocean and the melting of land ice (from glaciers and ice sheets) contributed about equally to these recent changes, the former being probably the larger contributor. This indicates an acceleration of the rise in sea levels compared to the period 1870–1990, when the rate was about 1.5 mm per year.

When driven by natural forcings only, climate models cannot reproduce the observed warming. By contrast, if anthropogenic forcings are included, the results are compatible with the observed changes. The dominant anthropogenic forcing is the increase in greenhouse gas concentrations in the atmosphere. This is associated with a strong radiative forcing of about 2 Wm^-2 in 2000 compared to preindustrial conditions (see section 4.1.2). Although the effect of sulphate-aerosol emissions by human activities is less precisely known, one of its more robust effects is a net negative radiative forcing that partly compensates for the warming due to the greenhouse gases. Humanity has also strongly affected the land use, in particular through deforestation. This has an impact on the chemical composition of the atmosphere, for instance when the wood is burned and releases CO₂. This also modifies the physical characteristics of the surface such as the albedo, roughness and water availability. While most of the human-induced changes in greenhouse gas concentrations and in sulphate aerosols have occurred in the last 150 years, land-use modifications started thousands of years ago in some regions, and certainly had an impact on climate at regional scale and perhaps at the global one.

The large changes in climate observed recently thus appear to be outside the range of natural variability on decadal to centennial timescales, but these changes are compatible with those predicted by models including anthropogenic forcings. This has led the IPCC to conclude, in its 4^th assessment report, that: "It is very likely that anthropogenic greenhouse gas increase caused most of the observed increase in global average temperature since the mid-20^th century. Without the cooling effect of atmospheric aerosols, it is likely that greenhouse gases alone would have caused a greater global mean temperature rise than observed during the last 50 years" (Solomon et al. 2007). "Very likely" in this sentence means a likelihood higher than 90 %, while "likely" corresponds to the 66% level. In the future, additional changes are expected, as discussed in the next chapter.

**Figure 5.32:** Comparison of observed continental- and global-scale changes in surface temperature with results simulated by climate models using natural and anthropogenic forcings. The decadal averages of the observations are shown for the period 1906 to 2005 (black line) plotted against the centre of the decade and relative to the corresponding average for 1901 to 1950. Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5% to 95% range for 19 simulations from 5 climate models using only the natural forcings due to solar activity and volcanoes. Red shaded bands show the 5% to 95% range for 58 simulations from 14 climate models using both natural and anthropogenic forcings. Figure TS22 from Solomon 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.