A method for checking the consistency between the residual variability in the observations after removal of externally forced signals and the natural internal
variability estimated from control simulations is also available.
6.4. Climate Forcings and Responses
There are several reasons why one should not expect a simple relationship between the patterns of radiative forcing and temperature response. First, strong feedbacks such
as those due to water vapour and sea-ice tend to reduce the difference in the temperature response due to different forcings. This is illustrated graphically by the
response to the simplified aerosol forcing used in early studies. The magnitude of the model response is largest over the Arctic in winter even though the forcing is
small, largely due to ice-albedo feedback. The large-scale patterns of change and their temporal variations are similar, but of opposite sign, to that obtained in
greenhouse gas experiments (Figure 6.6).
Specifically, latitude-month plot of radiative forcing and model equilibrium response for surface temperature are in Fig. 6.6. (a) Radiative forcing (W/m2 ) due
to increased sulphate aerosol loading at the time of CO2 doubling (b) Change in temperature due to the increase in aerosol loading (c) Change in temperature due to CO2
doubling. Note that the patterns of radiative forcing and temperature response are quite different in (a) and (b), but that the patterns of large-scale temperature
responses to different forcings are similar in (b) and (c).
Second, atmospheric circulation tends to smooth out temperature gradients and reduce the differences in response patterns. Similarly, the thermal inertia of the climate
system tends to reduce the amplitude of short-term fluctuations in forcing. Third, changes in radiative forcing are more effective if they act near the surface, where
cooling to space is restricted, than at upper levels, and in high latitudes, where there are stronger positive feedbacks than at low latitudes.
Summary: Different models may give quite different patterns of response for the same forcing, but an individual model may give a surprisingly similar response for
different forcings. The first point means that attribution studies may give different results when using signals generated from different models. The second point means
that it may be more difficult to distinguish between the response to different factors than one might expect given the differences in radiative forcing.
6.5. Climate Response to Natural Forcings
The climate response to several recent volcanic eruptions has been studied in observations and simulations with atmospheric GCMs. The stratosphere warms and the annual
mean surface and tropospheric temperature decreases during the 2-3 years following a major volcanic eruption. A simulation incorporating the effects of the Mount Pinatubo
eruption and observed changes in stratospheric ozone in addition to anthropogenic forcing approximately reproduces the observed stratospheric variations
(Figure 6.7). It shows stratospheric warming after the volcanic eruption, superimposed on a long-term cooling trend. Variability from
other sources makes assessment of the observed climate response difficult, particularly as the two most recent volcanic eruptions (Pinatubo and El Chichon) occurred in
ENSO warm years.
Figure 6.7: (a) Observed microwave sounding unit (MSU) global-mean temperature in the lower stratosphere, shown as dashed line, for channel 4 for the period
1979-97 compared to 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 Mount Pinatubo volcanic aerosol and
stratospheric ozone variations. The model simulation 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.
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. The
vertical response to solar forcing (Figure 6.8) 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. Hence, the
conclusion that changes in solar forcing have little effect on large-scale stratospheric temperatures remains tentative. 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 interdecadal timescales 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.
Figure 6.8: 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. This indicates that for ECHAM3/LSG, the zonal mean temperature response to greenhouse gas and solar forcing are quite
different in the stratosphere but similar in the troposphere.
Summary: We conclude that climate forcing by changes in solar irradiance and volcanism have likely caused fluctuations in global and hemispheric mean temperatures.
Qualitative comparisons suggest that natural forcings produce too little warming to fully explain the twentieth century warming (Figure 6.9
and Figure 6.10). The indication that the trend in net solar plus volcanic forcing has been negative in recent decades
makes it unlikely that natural forcing can explain the increased rate of global warming since the middle of the 20th century.
Figure 6.9: (a) Five-year running mean Northern Hemisphere temperature anomalies since 1850 (relative to the 1880-1920 mean) from an energy-balance model forced
by Dust Veil volcanic index and solar index. Two values of climate sensitivity to doubling CO2 were used; 3.0 K (thin solid line), and 1.5K (dashed line). Also shown
are the instrumental record (thick solid line) and a reconstruction of temperatures from proxy records (crosses). (b) As for (a) but for simulations with volcanic, solar
and anthropogenic forcing (greenhouse gases and direct and indirect effects of tropospheric aerosols).
Figure 6.10: Global mean surface temperature anomalies relative to the 1880-1920 mean from the instrumental record compared with ensembles of four simulations with
a coupled ocean-atmosphere climate model forced (a) with solar and volcanic forcing only, (b) with anthropogenic forcing including well mixed greenhouse gases, changes
in stratospheric and tropospheric ozone and the direct and indirect effects of sulphate aerosols, and (c) with all forcings, both natural and anthropogenic. The thick
line shows the instrumental data while the thin lines show the individual model simulations in the ensemble of four members. Note that the data are annual-mean values.
6.6. Climate Response to Anthropogenic Forcings
Well-mixed greenhouse gases make the largest and best-known contribution to changes in radiative forcing over the last century or so. There remains a large uncertainty
in the magnitude and patterns of other factors, particularly those associated with the indirect effects of sulphate aerosol. Models run with increases in greenhouse gases
alone give a warming which accelerates in the latter half of the century. When a simple representation of aerosol effects is included the rate of warming is reduced.
The global mean response is similar when additional forcings due to ozone and the indirect effect of sulphates are included.
Increases in greenhouse gases lead to a warming of the troposphere and a cooling of the stratosphere due to CO2 (IPCC 1995). Reductions in stratospheric ozone lead to a
further cooling, particularly in the stratosphere. Anthropogenic sulphate aerosols cool the troposphere with little effect on the stratosphere. When these three forcings
are included in a climate model albeit in a simplified way, the simulated changes show tropospheric warming and stratospheric cooling, as observed and as expected on
physical principles (Figure 6.11). Note this structure is distinct from that expected from natural (internal and external) influences.
Figure 6.11: Simulated and observed zonal mean temperature change as a function of latitude and height. The contour interval is 0.1 K. All signals are defined to
be the difference between the 1986-95 decadal mean and the 20 year 1961-80 mean. (a), increases in CO2 only; (b), as (a), but with a simple representation of sulphate
aerosols added; (c) , as (b), with observed changes in stratospheric ozone; (d), observed changes.
The spatial pattern of the simulated surface temperature response to a steady increase in greenhouse gases is well documented. The warming is greater over land than ocean
and generally small during the twentieth century over the southern ocean and northern North Atlantic where mixing extends to considerable depth. The warming is amplified
in high latitudes in winter by the recession of sea-ice and snow, and it is close to zero over sea-ice in summer. Despite the qualitative consistency of these general
features, there is considerable variation from model to model.
6.7. Detection and Attribution Studies
All new single pattern studies published since IPCC-1995 detect anthropogenic fingerprints in the global temperature observations, both at the surface and aloft. The
signal amplitudes estimated from observations and modelled amplitudes are consistent at the surface if greenhouse gas and sulphate aerosol forcing are taken into account,
and in the free atmosphere if ozone forcing is also included. Fingerprints based on smaller areas or on other variables yield more ambiguous results at present.
Example Study: This study estimated the magnitude of modelled 20th century greenhouse gas, aerosol, solar and volcanic signals in decadal mean data. Signals are
fitted by general linear regression to moving fifty year intervals beginning with 1906-56 and ending 1946-96. The signals are obtained from four ensembles of transient
change simulations, each using a different historical forcing scenario. Greenhouse gas, greenhouse gas plus direct sulphate aerosol, low frequency solar, and volcanic
forcing scenarios were used. Each ensemble contains four independent simulations with the same transient forcing. Two estimates of natural variability, one used for
optimisation and the other for the estimation of confidence intervals, are obtained from separate segments of a long control simulation.
Signal amplitudes estimated with multiple regression become uncertain when the signals are strongly correlated ("degenerate"). Despite the problem of degeneracy, positive
and significant greenhouse gas and sulphate aerosol signals are consistently detected in the most recent fifty year period (Figure 6.12
) regardless of which or how many other signals are included in the analysis. The residual variation that remains after removal of the signals is consistent with the
model's internal variability. In contrast, recent decadal temperature changes are not consistent with the model's internal climate variability alone, nor with any
combination of internal variability and naturally-forced signals, even allowing for the possibility of unknown processes amplifying the response to natural forcing.
Figure 6.12: Best-estimate contributions to global-mean temperature change. Reconstruction of temperature variations for 1906-1956 (a and b) and 1946-1995 (c and d)
for G and S (a and c) and GS and SOL (b and d). (G denotes the estimated greenhouse gas signal, S the estimated sulphate aerosol signal, GS the greenhouse gas / aerosol signal
obtained from simulations with combined forcing, SOL the solar signal). Observed (thick black), best fit (dark grey dashed), and the uncertainty range due to internal
variability (grey shading) are shown in all plots. (b) and (d) show contributions from GS (orange) and SOL (blue). (a) and (c) show contributions from G (red) and S (green).
All time series were reconstructed with data in which the 50-year mean had first been removed.
Summary: Results from optimal fingerprint methods indicate a discernible human influence on climate in temperature observations at the surface and aloft and over
a range of applications. All recent studies reject natural forcing and internal variability alone as a possible explanation of recent climate change. Analyses based on a
single anthropogenic signal focussing on continental and global scales indicate that:
- Changes over the past 30-50 years are very unlikely to be due to internal variability as simulated by current models.
- The combined response to greenhouse and sulphate forcing is more consistent with the observed record than the response to greenhouse gases alone.
- Inclusion of the simulated response to stratospheric ozone depletion improves the simulation of the vertical structure of the response.
Analyses based on multiple anthropogenic and natural signals indicate that:
- The combination of natural external forcing (solar and volcanic) and internal variability is unlikely to account for the spatio-temporal pattern of change over the
past 30-50 years, even allowing for possible amplification of the amplitude of natural responses by unknown feedback processes.
- Anthropogenic greenhouse gases are likely to have made a significant and substantial contribution to the warming observed over the second half of the 20th century,
possibly larger than the total observed warming.
- The contribution from anthropogenic sulphate aerosols is less clear, but appears to lie in a range broadly consistent with the spread of current model simulations.
A high sulphate aerosol forcing is consistently associated with a stronger response to greenhouse forcing.
- Natural external forcing may have contributed to the warming that occurred in the early twentieth century.
Results based on variables other than continental and global scale temperature are more ambiguous.
6.8. Conclusions
