On 7/10/12 11:48 PM, Will Janoschka wrote:
> CO2 in any amount, can account for to no more than 0.15% of any
> Earth warming or cooling. Check my numbers. -will-
See:
http://edu-observatory.org/olli/IPCC_SPM.2.png
The Nature commentary by Penner et al. on which this argument is based
actually says that on top of the global warming caused by carbon
dioxide, other short-lived pollutants (such as methane and black carbon)
cause an additional warming approximately 65% as much as CO2, and other
short-lived pollutants (such as aerosols) also cause some cooling.
However, claiming that CO2 has only caused 35% of global warming is a
gross misinterpretation and misunderstanding of the paper.
In August 2010, Nature published a commentary by Penner et al. which
mainly focused on the uncertainty regarding the effect short-lived
pollutants (such as aerosols and black carbon) have on the climate. As
is often the case, many in the blogosphere misinterpreted and
misunderstood the statements and conclusions in the commentary. Not
surprisingly, the biggest misinterpretation related to the contribution
of anthropogenic greenhouse gases to global warming. Below is the most
misunderstood quote, with emphasis on the key word.
"Of the short-lived species, methane, tropospheric ozone and black
carbon are key contributors to global warming, augmenting the radiative
forcing of carbon dioxide by 65%. Others � such as sulphate, nitrate and
organic aerosols � cause a negative radiative forcing, offsetting a
fraction of the warming owing to carbon dioxide."
Numerous blogs have (mis)interpreted this statement to mean that carbon
dioxide is only causing 35% as much global warming as previously
believed. A more accurate reading of the quote is that certain
short-lived pollutants cause warming in addition to carbon dioxide -
quantitatively, approximately 65% as much warming as CO2. And certain
other short-lived species cause a cooling effect which offsets some of
this warming.
This is not a new conclusion. The IPCC puts the radiative forcing from
CO2 at 1.66 W/m^2, compared to the forcing from other greenhouse gases,
black carbon, and tropospheric ozone at approximately 1.4 W/m^2.
Similarly, the negative forcing from aerosols is approximately -1.2 W/m^2.
Figure 1: Radiative forcing estimates from the IPCC FAR
Thus if anything, the 65% figure is an underestimate of the
contributions of short-lived pollutants to global warming, but this
contribution does not change the 1.66 W/m2 radiative forcing from CO2 or
the amount of global warming it has caused.
Much ado has also been made about another quote from the commentary:
"Warming over the past 100 years is consistent with high climate
sensitivity to atmospheric carbon dioxide combined with a large cooling
effect from short-lived aerosol pollutants, but it could equally be
attributed to a low climate sensitivity coupled with a small effect from
aerosols. These two possibilities lead to very different projections for
future climate change."
This statement gets to the main point of the commentary - that there
remains significant uncertainty regarding the effect of these
short-lived pollutants on the global climate. However, estimates of the
planetary climate sensitivity to increasing atmospheric CO2 and other
radiative forcings are not solely based on the change in the mean global
temperature over the past 100 years. In fact, the climate sensitivity
parameter has been estimated through many different methods, including:
o climate models
o recent responses to large volcanic eruptions
o recent responses to solar cycles
o paleoclimate data
o data from the last Glacial Maximum
o and yes, data from the instrumental period
All of these different methods show strong agreement, overlapping in
the IPCC climate sensitivity range of 2 to 4.5�C for a doubling of
atmospheric CO2 (2xCO2).
Figure 2: Distributions and ranges for climate sensitivity from
different lines of evidence. The circle indicates the most likely value.
The thin colored bars indicate very likely value (more than 90%
probability). The thicker colored bars indicate likely values (more than
66% probability). Dashed lines indicate no robust constraint on an upper
bound. The IPCC likely range (2 to 4.5�C) and most likely value (3�C)
are indicated by the vertical grey bar and black line, respectively
(Knutti and Hegerl 2008)
Interestingly, Penner et al. find that whether the climate sensitivity
parameter is on the low or high end, reducing anthropogenic emissions of
the short-lived warming pollutants would achieve a significant reduction
in global warming over the next 50-100 years. In the red lines in the
Figure 3, they employ a climate model with a sensitivity of 5�C for
2xCO2, slightly outside the IPCC likely range. The blue line is a
climate model with a sensitivity of 2�C for 2xCO2, on the lower end of
the IPCC range. Note that even with the lower climate sensitivity, the
model shows the planet warming 3�C by 2100 in this emissions scenario
(see the figure caption for further details).
Figure 3: Global mean temperature measurements (black) and projections
based on an IPCC scenario with high emissions (A2) for a climate
sensitivity parameter of 5�C (upper red) and 2�C (upper blue). Linearly
decreasing the total anthropogenic radiative forcing owing to methane,
tropospheric ozone and black carbon � starting in 2010 and achieving
pre-industrial levels by 2050 � results in significant near-term climate
mitigation (lower blue and red curves) (Penner 2010)
Unfortunately, reducing the short-lived cooling pollutants such as
aerosols would cause a warming effect of similar magnitude, and so CO2
remains the primary pollutant of concern. Coincidentally, a group of
scientists from NASA GISS just published a paper in Science entitled
Atmospheric CO2: Principle Control Knob Governing Earth's Temperature.
Although it is important to reduce the remaining climate uncertainties,
such as the magnitude of the impacts of short-lived pollutants, it does
not change the fact that CO2 is very likely the driving force behind the
current global warming, or that if we double the amount of CO2 in the
atmosphere from pre-industrial levels, the planet will likely warm in
the range of 2 to 4.5�C.
See:
http://www.skepticalscience.com/co2-warming-35-percent.htm