Anthropogenic aerosols are short-lived and mostly produce negative radiative
- The major sources of anthropogenic aerosols are fossil fuel and biomass
burning. These sources are also linked to degradation of air quality and acid
- Since the SAR, significant progress has been achieved in better characterising
the direct radiative roles of different types of aerosols. Direct radiative
forcing is estimated to be -0.4 Wm-2 for sulphate, -0.2 Wm-2
for biomass burning aerosols, -0.1 Wm-2 for fossil fuel organic
+0.2 Wm-2 for fossil fuel black carbon aerosols. There is much
less confidence in the ability to quantify the total aerosol direct effect,
and its evolution over time, than that for the gases listed above. Aerosols
also vary considerably by region and respond quickly to changes in emissions.
- In addition to their direct radiative forcing, aerosols have an indirect
radiative forcing through their effects on clouds. There is now more evidence
for this indirect effect, which is negative, although of very uncertain magnitude.
Natural factors have made small contributions to radiative forcing over the
Confidence in the ability of models to project future climate has increased.
- The radiative forcing due to changes in solar irradiance for the period
since 1750 is estimated to be about +0.3 Wm-2, most of which occurred
during the first half of the 20th century. Since the late 1970s, satellite
instruments have observed small oscillations due to the 11-year solar cycle.
Mechanisms for the amplification of solar effects on climate have been proposed,
but currently lack a rigorous theoretical or observational basis.
- Stratospheric aerosols from explosive volcanic eruptions lead to negative
forcing, which lasts a few years. Several major eruptions occurred in the
periods 1880 to 1920 and 1960 to 1991.
- The combined change in radiative forcing of the two major natural factors
(solar variation and volcanic aerosols) is estimated to be negative for the
past two, and possibly the past four, decades.
Complex physically-based climate models are required to
provide detailed estimates of feedbacks and of regional features. Such models
cannot yet simulate all aspects of climate (e.g., they still cannot account fully
for the observed trend in the surface-troposphere temperature difference since
1979) and there are particular uncertainties associated with clouds and their
interaction with radiation and aerosols. Nevertheless, confidence in the ability
of these models to provide useful projections of future climate has improved due
to their demonstrated performance on a range of space and time-scales.
Figure 4: Simulating the Earth’s temperature variations, and
comparing the results to measured changes, can provide insight into the
underlying causes of the major changes.
A climate model can be used to simulate the temperature changes that occur
both from natural and anthropogenic causes. The simulations represented
by the band in (a) were done with only natural forcings: solar variation
and volcanic activity. Those encompassed by the band in (b) were done with
anthropogenic forcings: greenhouse gases and an estimate of sulphate aerosols,
and those encompassed by the band in (c) were done with both natural and
anthropogenic forcings included. From (b), it can be seen that inclusion
of anthropogenic forcings provides a plausible explanation for a substantial
part of the observed temperature changes over the past century, but the
best match with observations is obtained in (c) when both natural and anthropogenic
factors are included. These results show that the forcings included are
sufficient to explain the observed changes, but do not exclude the possibility
that other forcings may also have contributed. The bands of model results
presented here are for four runs from the same model. Similar results to
those in (b) are obtained with other models with anthropogenic forcing.
[Based upon Chapter 12, Figure
There is new and stronger evidence that most of the warming observed over
the last 50 years is attributable to human activities.
The SAR concluded: “The balance of evidence suggests a discernible human
influence on global climate”. That report also noted that the anthropogenic
signal was still emerging from the background of natural climate variability.
Since the SAR, progress has been made in reducing uncertainty, particularly with
respect to distinguishing and quantifying the magnitude of responses to different
external influences. Although many of the sources of uncertainty identified in
the SAR still remain to some degree, new evidence and improved understanding support
an updated conclusion.
- Understanding of climate processes and their incorporation in climate models
have improved, including water vapour, sea-ice dynamics, and ocean heat transport.
- Some recent models produce satisfactory simulations of current climate without
the need for non-physical adjustments of heat and water fluxes at the ocean-atmosphere
interface used in earlier models.
- Simulations that include estimates of natural and anthropogenic forcing
reproduce the observed large-scale changes in surface temperature over the
20th century (Figure 4). However, contributions from
some additional processes and forcings may not have been included in the models.
Nevertheless, the large-scale consistency between models and observations
can be used to provide an independent check on projected warming rates over
the next few decades under a given emissions scenario.
- Some aspects of model simulations of ENSO, monsoons and the North Atlantic
Oscillation, as well as selected periods of past climate, have improved.
- There is a longer and more closely scrutinised temperature record and new
model estimates of variability. The warming over the past 100 years is very
to be due to internal variability alone, as estimated by current models. Reconstructions
of climate data for the past 1,000 years (Figure
1b) also indicate that this warming was unusual and is unlikely7
to be entirely natural in origin.
- There are new estimates of the climate response to natural and anthropogenic
forcing, and new detection techniques have been applied. Detection and attribution
studies consistently find evidence for an anthropogenic signal in the climate
record of the last 35 to 50 years.
- Simulations of the response to natural forcings alone (i.e., the response
to variability in solar irradiance and volcanic eruptions) do not explain
the warming in the second half of the 20th century (see for example Figure
4a). However, they indicate that natural forcings may have contributed
to the observed warming in the first half of the 20th century.
- The warming over the last 50 years due to anthropogenic greenhouse gases
can be identified despite uncertainties in forcing due to anthropogenic sulphate
aerosol and natural factors (volcanoes and solar irradiance). The anthropogenic
sulphate aerosol forcing, while uncertain, is negative over this period and
therefore cannot explain the warming. Changes in natural forcing during most
of this period are also estimated to be negative and are unlikely7
to explain the warming.
- Detection and attribution studies comparing model simulated changes with
the observed record can now take into account uncertainty in the magnitude
of modelled response to external forcing, in particular that due to uncertainty
in climate sensitivity.
- Most of these studies find that, over the last 50 years, the estimated rate
and magnitude of warming due to increasing concentrations of greenhouse gases
alone are comparable with, or larger than, the observed warming. Furthermore,
most model estimates that take into account both greenhouse gases and sulphate
aerosols are consistent with observations over this period.
- The best agreement between model simulations and observations over the last
140 years has been found when all the above anthropogenic and natural forcing
factors are combined, as shown in Figure 4c). . These
results show that the forcings included are sufficient to explain the observed
changes, but do not exclude the possibility that other forcings may also have
In the light of new evidence and taking into account the remaining uncertainties,
most of the observed warming over the last 50 years is likely7
to have been due to the increase in greenhouse gas concentrations.
Furthermore, it is very likely7
that the 20th century warming has contributed significantly to the observed
sea level rise, through thermal expansion of sea water and widespread loss of
land ice. Within present uncertainties, observations and models are both consistent
with a lack of significant acceleration of sea level rise during the 20th century.