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A number of important uncertainties remain. These include:
- Discrepancies between the vertical profile of temperature change in the
troposphere seen in observations and models. These have been reduced as more
realistic forcing histories have been used in models, although not fully resolved.
Also, the difference between observed surface and lower-tropospheric trends
over the last two decades cannot be fully reproduced by model simulations.
- Large uncertainties in estimates of internal climate variability from models
and observations, though as noted above, these are unlikely (bordering on
very unlikely) to be large enough to nullify the claim that a detectable climate
change has taken place.
- Considerable uncertainty in the reconstructions of solar and volcanic forcing
which are based on proxy or limited observational data for all but the last
two decades. Detection of the influence of greenhouse gases on climate appears
to be robust to possible amplification of the solar forcing by ozone/solar
or solar/cloud interactions, provided these do not alter the pattern or time
dependence of the response to solar forcing. Amplification of the solar signal
by these processes, which are not yet included in models, remains speculative.
- Large uncertainties in anthropogenic forcing are associated with the effects
of aerosols. The effects of some anthropogenic factors, including organic
carbon, black carbon, biomass aerosols, and changes in land use, have not
been included in detection and attribution studies. Estimates of the size
and geographic pattern of the effects of these forcings vary considerably,
although individually their global effects are estimated to be relatively
- Large differences in the response of different models to the same forcing.
These differences, which are often greater than the difference in response
in the same model with and without aerosol effects, highlight the large uncertainties
in climate change prediction and the need to quantify uncertainty and reduce
it through better observational data sets and model improvement.
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.
- 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
unlikely to be due to internal variability alone, as estimated by current
models. Reconstructions of climate data for the past 1,000 years also indicate
that this warming was unusual and is unlikely 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. 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 unlikely to explain
- 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. 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.
In the light of new evidence and taking into account the remaining uncertainties,
most of the observed warming over the last 50 years is likely to have been due
to the increase in greenhouse gas concentrations.
Furthermore, it is very likely 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.