This paper addresses the question of where we now stand with respect to det
ection and attribution of an anthropogenic climate signal. Our ability to e
stimate natural climate variability, against which claims of anthropogenic
signal detection must be made, is reviewed. The current situation suggests
control runs of global climate models may give the best estimates of natura
l variability on a global basis, estimates that appear to be accurate to wi
thin a factor of 2 or 3 at multidecadal timescales used in detection work.
Present uncertainties in both observations and model-simulated anthropogeni
c signals in near-surface air temperature are estimated. The uncertainty in
model simulated signals is, in places, as large as the signal to be detect
ed. Two different, but complementary, approaches to detection and attributi
on are discussed in the context of these uncertainties.
Applying one of the detection strategies, it is found that the change in ne
ar-surface, June through August air temperature field over the last 50 year
s is generally different at a significance level of 5% from that expected f
rom model-based estimates of natural variability. Greenhouse gases alone ca
nnot explain the observed change. Two of four climate models forced by gree
nhouse gases and direct sulfate aerosols produce results consistent with th
e current climate change observations, while the consistency of the other t
wo depends on which model's anthropogenic fingerprints are used. A recent i
ntegration with additional anthropogenic forcings (the indirect effects of
sulfate aerosols and tropospheric ozone) and more complete tropospheric che
mistry produced results whose signal amplitude and pattern were consistent
with current observations, provided the model's fingerprint is used and det
ection carried out over only the last 30 years of annually averaged data. T
his single integration currently cannot be corroborated and provides no opp
ortunity to estimate the uncertainties inherent in the results, uncertainti
es that are thought to be large and poorly known. These results illustrate
the current large uncertainty in the magnitude and spatial pattern of the d
irect and indirect sulfate forcing and climate response. They also show det
ection statements depend on model-specific fingerprints, time period, and s
easonal character of the signal, dependencies that have not been well explo
red.
Most, but not all, results suggest that recent changes in global climate in
ferred from surface air temperature are likely not due solely to natural ca
uses. At present it is not possible to make a very confident statement abou
t the relative contributions of specific natural and anthropogenic forcings
to observed climate change. One of the main reasons is that fully realisti
c simulations of climate change due to the combined effects of all anthropo
genic and natural forcings mechanisms have yet to be computed. A List of re
commendations for reducing some of the uncertainties that currently hamper
detection and attribution studies is presented.