Climate Change 2001:
Working Group I: The Scientific Basis
Other reports in this collection

12.2.3.2 Climatic response to natural forcing

Response to volcanic forcing
The climate response to several recent volcanic eruptions has been studied in observations and simulations with atmospheric GCMs (e.g., Robock and Mao, 1992, 1995; Graf et al., 1996; Hansen et al., 1996; Kelly et al., 1996; Mao and Robock, 1998; Kirchner et al., 1999). The stratosphere warms and the annual mean surface and tropospheric temperature decreases during the two to three years following a major volcanic eruption. A simulation incorporating the effects of the Mt. Pinatubo eruption and observed changes in stratospheric ozone in addition to anthropogenic forcing approximately reproduces the observed stratospheric variations (Figure 12.4; Bengtsson et al., 1999). It shows stratospheric warming after the volcanic eruption, superimposed on a long-term cooling trend. Although the surface temperature response in the Northern Hemisphere warm season following a volcanic eruption is dominated by global scale radiative cooling, some models simulate local warming over Eurasia and North America in the cold season due to changes in circulation (e.g., Graf et al., 1996; Kirchner et al., 1999). Variability from other sources makes assessment of the observed climate response difficult, particularly as the two most recent volcanic eruptions (Mt. Pinatubo and El Chichon) occurred in El Niño-Southern Oscillation (ENSO) warm years. Simulations with simple models (Bertrand et al., 1999; Crowley and Kim, 1999; Grieser and Schoenwiese, 2001) and AOGCMs (Tett et al., 1999; Stott et al., 2001) produce a small decadal mean cooling in the 1980s and 1990s due to several volcanic eruptions in those decades. Some simulations also produce global warming in the early 20th century as a recovery from a series of strong eruptions around the turn of the 20th century. It is unclear whether such a long-term response is realistic.

Response to solar forcing
Since the SAR, there have been new modelling and observational studies on the climate effects of variations in solar irradiance. The surface temperature response to the 11-year cycle is found to be small (e.g., Cubasch et al., 1997; White et al., 1997; North and Stevens, 1998; Crowley and Kim, 1999; Free and Robock, 1999). Low-frequency solar variability over the last few hundred years gives a stronger surface temperature response (Cubasch et al., 1997; Drijfhout et al., 1999; Rind et al., 1999; Tett et al., 1999; Stott et al., 2001). Model results show cooling circa 1800 due to the hypothesised solar forcing minimum and some warming in the 20th century, particularly in the early 20th century. Time-dependent experiments produce a global mean warming of 0.2 to 0.5°C in response to the estimated 0.7 Wm-2 change of solar radiative forcing from the Maunder Minimum to the present (e.g., Lean and Rind, 1998, Crowley and Kim, 1999).

Ozone changes in the Earth's atmosphere caused by the 11-year solar cycle could affect the temperature response in the free atmosphere. A relation between 30 hPa geopotential and a solar index has been shown over nearly four solar cycles by Labitzke and van Loon (1997). Van Loon and Shea (1999, 2000) found a related connection between upper to middle tropospheric temperature and a solar index over the last 40 years, which is particularly strong in July and August. Variations in ozone forcing related to the solar cycle may also affect surface temperature via radiative and dynamical processes (see discussion in Chapter 6; Haigh, 1999; Shindell et al., 1999, 2001), but observational evidence remains ambiguous (e.g., van Loon and Shea, 2000). The assessment of ozone-related Sun-climate interactions is uncertain as a result of the lack of long-term, reliable observations. This makes it difficult to separate effects of volcanic eruptions and solar forcing on ozone. There has also been speculation that the solar cycle might influence cloudiness and hence surface temperature through cosmic rays (e.g., Svensmark and Friis-Christensen, 1997; Svensmark, 1998). The latter effect is difficult to assess due to limitations in observed data and the shortness of the correlated time-series.

As discussed earlier in Section 12.2.3, differences between the response to solar and greenhouse gas forcings would make it easier to distinguish the climate response to either forcing. However, the spatial response pattern of surface air temperature to an increase in solar forcing was found to be quite similar to that in response to increases in greenhouse gas forcing (e.g., Cubasch et al., 1997). The vertical response to solar forcing (Figure 12.5) includes warming throughout most of the troposphere. The response in the stratosphere is small and possibly locally negative, but less so than with greenhouse gas forcing, which gives tropospheric warming and strong stratospheric cooling. The dependence of solar forcing on wavelength and the effect of solar fluctuations on ozone were generally omitted in these simulations. Hence, the conclusion that changes in solar forcing have little effect on large-scale stratospheric temperatures remains tentative.


Figure 12.4: (a) Observed microwave sounding unit (MSU) global mean temperature in the lower strato sphere, shown as dashed line, for channel 4 for the period 1979 to 97 compared with the average of several atmosphere-ocean GCM simulations starting with different atmospheric conditions in 1979 (solid line). The simulations have been forced with increasing greenhouse gases, direct and indirect forcing by sulphate aerosols and tropospheric ozone forcing, and Mt. Pinatubo volcanic aerosol and stratospheric ozone variations. The model simula-tion does not include volcanic forcing due to El Chichon in 1982, so it does not show stratospheric warming then. (b) As for (a), except for 2LT temperature retrievals in the lower troposphere. Note the steady response in the stratosphere, apart from the volcanic warm periods, and the large variability in the lower troposphere (from Bengtsson et al., 1999).

Figure 12.5: (a) Response (covariance, normalised by the variance of radiance fluctuations) of zonally averaged annual mean atmospheric temperature to solar forcing for two simulations with ECHAM3/LSG. Coloured regions indicate locally significant response to solar forcing. (b) Zonal mean of the first EOF of greenhouse gas-induced temperature change simulated with the same model (from Cubasch et al., 1997). This indicates that for ECHAM3/LSG, the zonal mean temp-erature response to greenhouse gas and solar forcing are quite different in the stratosphere but similar in the troposphere.

The different time-histories of the solar and anthropogenic forcing should help to distinguish between the responses. All reconstructions suggest a rise in solar forcing during the early decades of the 20th century with little change on inter-decadal time-scales in the second half. Such a forcing history is unlikely to explain the recent acceleration in surface warming, even if amplified by some unknown feedback mechanism.

Continues on next page



Other reports in this collection