Here’s excerpts from another
realclimate.org piece. I think this one begins to address a recurring theme in this thread.
Natural
Variability and Climate Sensitivity
In this commentary, I will discuss the question "If somebody were to
discover that climate variations in the past were stronger than previously thought, what would be the implications
for estimates of climate sensitivity?" Pick your favorite time period – Little ice age, Medieval Warm Period, Last
Glacial Maximum or Cretaceous – the issues are the same. In considering this question, it is important to keep in
mind that the predictions summarized in the IPCC reports are not the result of some kind of statistical fit to past
data. Thus, a revision in our picture of past climate variability does not translate in any direct way into a change
in the IPCC forecasts. These forecasts are based on comprehensive simulations incorporating the best available
representations of basic physical processes. Of course, data on past climates can be very useful in improving these
representations. In addition, past data can be used to provide independent estimates of climate sensitivity, which
provide a reality check on the models. Nonetheless, the path from data to change in forecast is a subtle
one.
Climate doesn't change all by itself. There's always a reason, though it may be hard to ferret out.
Often, the proximate cause of the climate change is some parameter of the climate system that can be set off from
the general collective behavior of the system and considered as a "given," even if it is not external to the system
strictly speaking. Such is the case for CO2 concentration. This is an example of a climate forcing. Other climate
forcings, such as solar variability and volcanic activity, are more clearly external to the Earth's climate system.
In order to estimate sensitivity from past climate variations, one must identify and quantify the climate forcings.
A large class of climate forcings can be translated into a common currency, known as radiative forcing. This is the
amount by which the forcing mechanism would change the top-of-atmosphere energy budget, if the temperature were not
allowed to change so as to restore equilibrium. Doubling CO2 produces a radiative forcing of about 4 Watts per
square meter. The effects of other well-mixed greenhouse gases can be accurately translated into radiative forcings.
Forcing caused by changes in the Sun's brightness, by dust in the atmosphere, or by volcanic aerosols can also be
translated into radiative forcing. The equivalence is not so precise in this case, since the geographic and temporal
pattern of the forcing is not the same as that for greenhouse gases, but numerous simulations indicate that there is
enough equivalence for the translation to be useful.
Thus, an estimate of climate sensitivity from past data
requires an estimate of the magnitude of the past climate changes and of the radiative forcings causing the changes.
Both are subject to uncertainties, and to revisions as scientific techniques improve.
...
The Last
Glacial Maximum (i.e. the most recent "ice age", abbreviated LGM) probably provides the best opportunity for using
the past to constrain climate sensitivity. The climate changes are large and reasonably well constrained by
observations. Moreover, the forcing mechanisms are quite well known, and one of them is precisely the same as will
cause future climate changes. During the LGM, CO2 dropped to 180 parts per million, as compared to pre-industrial
interglacial values of about 280 parts per million. Depending on just what you assume about cloud and water vapor
distributions, this yields a radiative forcing of about -2.5 Watts per square meter. Global mean temperatures
dropped by about 7°C at the LGM. Does this mean that the true climate sensitivity is (7/2.5) = 2.8°C per (Watt per
square meter)? That would indicate a terrifying 11.2 °C warming in response to a doubling of CO2. Fortunately, this
alarming estimate is based on faulty reasoning, because there is a lot more going on at LGM time than just the
change in CO2. Some of these things are feedbacks like water vapor, clouds and sea-ice, which could be reasonably
presumed to the future as well as the past. Other forcings, including the growth and decay of massive Northern
Hemisphere continental ice sheets, changes in atmospheric dust, and changes in the ocean circulation, are not likely
to have the same kind of effect in a future warming scenario as they did at glacial times. In estimating climate
sensitivity such effects must be controlled for, and subtracted out to yield the portion of climate change
attributable to CO2. Broadly speaking, we know that it is unlikely that current climate models are systematically
overestimating sensitivity to CO2 by very much, since most of the major models can get into the ballpark of the
correct tropical and Southern Hemisphere cooling when CO2 is dropped to 180 parts per million. No model gets very
much cooling south of the Equator without the effect of CO2. Hence, any change in model physics that reduced climate
sensitivity would make it much harder to account for the observed LGM cooling. Can we go beyond this rather vague
statement and use the LGM to say which of the many models is most likely to have the right climate sensitivity? Many
groups are working on this very question right now. Progress has become possible only recently, with the
availability of a few long-term coupled atmosphere-ocean simulations of the LGM climate. Time will tell how
successful the program will turn out...
However that shakes out, if somebody were to wake me up in the middle
of the night tomorrow and tell me that the LGM tropical temperatures were actually 6°C colder than the present,
rather than 3C as I currently think, my immediate reaction would be "Gosh, the climate sensitivity must be much
greater than anybody imagined!" That would be the correct reaction, too, because the rude awakener didn't suggest
anything about revisions in the strength of the forcing mechanisms.
...
Now, how about the Holocene –
including the Little Ice Age and Medieval Warm Period that seem to figure so prominently in many skeptics' tracts ?
This is a far harder row to hoe, because the changes in both forcing and response are small and subject to large
uncertainties (as we have discussed in connection with the "Hockey Stick"). What we do know is that the proposed
forcing mechanisms – solar variability and mean volcanic activity – are small. Indeed, the main quandary faced by
climate scientists is how to estimate climate sensitivity from the Little Ice Age or Medieval Warm Period, at all,
given the relative small forcings over the past 1000 years, and the substantial uncertainties in both the forcings
and the temperature changes. The current picture of Holocene climate variations is based not just on tree ring data,
but on glacial mass balance and a wide variety of other proxy data. If this state of knowledge were to be revised in
such a way as to indicate that the amplitude of the climate variations were larger than previously thought, that
could very well call for for an upward revision of climate sensitivity
Indeed, quantitative studies of the
Holocene climate variations invariably support this notion (e.g. Hegerl et al, Geophys. Res. Lett 2003, or Andronova
et al Geophys. Res. Lett 2004.). Such studies can reasonably account for the observed variations as a response to
solar and volcanic forcing (and a few secondary things) with energy balance climate models tuned to have a climate
sensitivity equivalent to 2.5C per doubling of CO2. If the estimates of observed variations were made larger, a
greater sensitivity would then be required to fit the data. Ironically, even arch-skeptics Soon and Baliunas, who
would like to lay most of the blame for recent warming at the doorstep of solar effects, came to a compatible
conclusion in their own energy balance model study. Namely, any model that was sensitive enough to yield a large
response to recent solar variability would yield an even larger response to radiative forcing from recent (and
therefore also future) CO2 changes. As a result, their "best fit" of climate sensitivity for the twentieth century
is comfortably within the IPCC range.
http://www.realclimate.org/index.php?p=229
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