The IPCC WG1 Second Assessment Report (IPCC, 1996) (hereafter SAR) concluded,
“the balance of evidence suggests that there is a discernible human influence
on global climate”. It noted that the detection and attribution of anthropogenic
climate change signals can only be accomplished through a gradual accumulation
of evidence. The SAR authors also noted uncertainties in a number of factors,
including the magnitude and patterns of internal climate variability, external
forcing and climate system response, which prevented them from drawing a stronger
conclusion. The results of the research carried out since 1995 on these uncertainties
and other aspects of detection and attribution are summarised below.
A longer and more closely scrutinised observational record
Three of the five years (1995, 1996 and 1998) added to the instrumental record
since the SAR are the warmest in the instrumental record of global temperatures,
consistent with the expectation that increases in greenhouse gases will lead
to continued long-term warming. The impact of observational sampling errors
has been estimated for the global and hemispheric mean surface temperature record
and found to be small relative to the warming observed over the 20th century.
Some sources of error and uncertainty in both the Microwave Sounding Unit (MSU)
and radiosonde observations have been identified that largely resolve discrepancies
between the two data sets. However, current climate models cannot fully account
for the observed difference in the trend between the surface and lower-tropospheric
temperatures over the last twenty years even when all known external influences
are included. New reconstructions of the surface temperature record of the last
1,000 years indicate that the temperature changes over the last 100 years are
unlikely to be entirely natural in origin, even taking into account the large
uncertainties in palaeo-reconstructions.
New model estimates of internal variability
Since the SAR, more models have been used to estimate the magnitude of internal
climate variability. Several of the models used for detection show similar or
larger variability than observed on interannual to decadal time-scales, even
in the absence of external forcing. The warming over the past 100 years is very
unlikely to be due to internal variability alone as estimated by current models.
Estimates of variability on the longer time-scales relevant to detection and
attribution studies are uncertain. Nonetheless, conclusions on the detection
of an anthropogenic signal are insensitive to the model used to estimate internal
variability and recent changes cannot be accounted for as pure internal variability
even if the amplitude of simulated internal variations is increased by a factor
of two or more. In most recent studies, the residual variability that remains
in the observations after removal of the estimated anthropogenic signals is
consistent with model-simulated variability on the space- and time-scales used
for detection and attribution. Note, however, that the power of the consistency
test is limited. Detection studies to date have shown that the observed large-scale
changes in surface temperature in recent decades are unlikely (bordering on
very unlikely) to be entirely the result of internal variability.
New estimates of responses to natural forcing
Fully coupled ocean-atmosphere models have used reconstructions of solar and
volcanic forcings over the last one to three centuries to estimate the contribution
of natural forcing to climate variability and change. Including their effects
produces an increase in variance on all time-scales and brings the low-frequency
variability simulated by models closer to that deduced from palaeo-reconstructions.
Assessments based on physical principles and model simulations indicate that
natural forcing alone is unlikely to explain the increased rate of global warming
since the middle of the 20th century or changes in vertical temperature structure.
The reasons are that the trend in natural forcing has likely been negative over
the last two decades and natural forcing alone is unlikely to account for the
observed cooling of the stratosphere. However, there is evidence for a detectable
volcanic influence on climate. The available evidence also suggests a solar
influence in proxy records of the last few hundred years and also in the instrumental
record of the early 20th century. Statistical assessments confirm that natural
variability (the combination of internal and naturally forced) is unlikely to
explain the warming in the latter half of the 20th century.
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