On 7/6/12 9:55 AM, Tunderbar wrote:
> If it did raise the temps, it was in trace amounts. And the global
> average temps have been rising since the last glaciation period,
> 15,000 years ago before SUVs. It isn't anomalous, it is what climate
> does.
An enhanced greenhouse effect from CO2 has been confirmed by multiple
lines of empirical evidence. Satellite measurements of infrared spectra
over the past 40 years observe less energy escaping to space at the
wavelengths associated with CO2. Surface measurements find more downward
infrared radiation warming the planet's surface. This provides a direct,
empirical causal link between CO2 and global warming.
The greenhouse gas qualities of carbon dioxide have been known for over
a century. In 1861, John Tyndal published laboratory results identifying
carbon dioxide as a greenhouse gas that absorbed heat rays (longwave
radiation). Since then, the absorptive qualities of carbon dioxide have
been more precisely quantified by decades of laboratory measurements
(Herzberg 1953, Burch 1962, Burch 1970, etc).
The greenhouse effect occurs because greenhouse gases let sunlight
(shortwave radiation) pass through the atmosphere. The earth absorbs
sunlight, warms then reradiates heat (infrared or longwave radiation).
The outgoing longwave radiation is absorbed by greenhouse gases in the
atmosphere. This heats the atmosphere which in turn re-radiates longwave
radiation in all directions. Some of it makes its way back to the
surface of the earth. So with more carbon dioxide in the atmosphere, we
expect to see less longwave radiation escaping to space at the
wavelengths that carbon dioxide absorb. We also expect to see more
infrared radiation returning back to Earth at these same wavelengths.
Satellite measurements of outgoing longwave radiation
In 1970, NASA launched the IRIS satellite that measured infrared spectra
between 400 cm-1 to 1600 cm-1. In 1996, the Japanese Space Agency
launched the IMG satellite which recorded similar observations. Both
sets of data were compared to discern any changes in outgoing radiation
over the 26 year period (Harries 2001). The resultant change in outgoing
radiation was as follows:
Figure 1: Change in spectrum from 1970 to 1996 due to trace gases.
'Brightness temperature' indicates equivalent blackbody temperature
(Harries 2001).
What they found was a drop in outgoing radiation at the wavelength bands
that greenhouse gases such as carbon dioxide (CO2) and methane (CH4)
absorb energy. The change in outgoing radiation is consistent with
theoretical expectations. Thus the paper found "direct experimental
evidence for a significant increase in the Earth's greenhouse effect".
This result has been confirmed by subsequent papers using more recent
satellite data. The 1970 and 1997 spectra were compared with additional
satellite data from the NASA AIRS satellite launched in 2003 (Griggs
2004). This analysis was extended to 2006 using data from the AURA
satellite launched in 2004 (Chen 2007). Both papers found the observed
differences in CO2 bands matching the expected changes from rising
carbon dioxide levels. Thus we have empirical evidence that increased
CO2 is causing an enhanced greenhouse effect.
Surface measurements of downward longwave radiation
A compilation of surface measurements of downward longwave radiation
from 1973 to 2008 find an increasing trend of more longwave radiation
returning to earth, attributed to increases in air temperature, humidity
and atmospheric carbon dioxide (Wang 2009). More regional studies such
as an examination of downward longwave radiation over the central Alps
find that downward longwave radiation is increasing due to an enhanced
greenhouse effect (Philipona 2004).
Taking this a step further, an analysis of high resolution spectral data
allows scientists to quantitatively attribute the increase in downward
radiation to each of several greenhouse gases (Evans 2006). The results
lead the authors to conclude that "this experimental data should
effectively end the argument by skeptics that no experimental evidence
exists for the connection between greenhouse gas increases in the
atmosphere and global warming."
Figure 2: Spectrum of the greenhouse radiation measured at the surface.
Greenhouse effect from water vapor is filtered out, showing the
contributions of other greenhouse gases (Evans 2006).
Conservation of Energy
Huber and Knutti (2011) published a paper in Nature Geoscience,
Anthropogenic and natural warming inferred from changes in Earth’s
energy balance. They take an approach in this study which utilizes the
principle of conservation of energy for the global energy budget using
the measurements discussed above, and summarize their methodology:
"We use a massive ensemble of the Bern2.5D climate model of intermediate
complexity, driven by bottom-up estimates of historic radiative forcing
F, and constrained by a set of observations of the surface warming T
since 1850 and heat uptake Q since the 1950s....Between 1850 and 2010,
the climate system accumulated a total net forcing energy of 140 x 1022
J with a 5-95% uncertainty range of 95-197 x 1022 J, corresponding to an
average net radiative forcing of roughly 0.54 (0.36-0.76)Wm-2."
Essentially, Huber and Knutti take the estimated global heat content
increase since 1850, calculate how much of the increase is due to
various estimated radiative forcings, and partition the increase between
increasing ocean heat content and outgoing longwave radiation. The
authors note that more than 85% of the global heat uptake (Q) has gone
into the oceans, including increasing the heat content of the deeper
oceans, although their model only accounts for the upper 700 meters.
Figure 3 is a similar graphic to that presented in Meehl et al. (2004),
comparing the average global surface warming simulated by the model
using natural forcings only (blue), anthropogenic forcings only (red),
and the combination of the two (gray).
Figure 3: Time series of anthropogenic and natural forcings
contributions to total simulated and observed global temperature change.
The coloured shadings denote the 5-95% uncertainty range.
In Figure 4, Huber and Knutti break down the anthropogenic and natural
forcings into their individual components to quantify the amount of
warming caused by each since the 1850s (Figure 4b), 1950s (4c), and
projected from 2000 to 2050 using the IPCC SRES A2 emissions scenario as
business-as-usual (4d).
Figure 4: Contributions of individual forcing agents to the total
decadal temperature change for three time periods. Error bars denote the
5–95% uncertainty range. The grey shading shows the estimated 5–95%
range for internal variability based on the CMIP3 climate models.
Observations are shown as dashed lines.
As expected, Huber and Knutti find that greenhouse gases contributed to
substantial warming since 1850, and aerosols had a significant cooling
effect:
"Greenhouse gases contributed 1.31°C (0.85-1.76°C) to the increase, that
is 159% (106-212%) of the total warming. The cooling effect of the
direct and indirect aerosol forcing is about -0.85°C (-1.48 to -0.30°C).
The warming induced by tropospheric ozone and solar variability are of
similar size (roughly 0.2°C). The contributions of stratospheric water
vapour and ozone, volcanic eruptions, and organic and black carbon are
small."
Since 1950, the authors find that greenhouse gases contributed 166%
(120-215%) of the observed surface warming (0.85°C of 0.51°C estimated
surface warming). The percentage is greater than 100% because aerosols
offset approximately 44% (0.45°C) of that warming.
"It is thus extremely likely (>95% probability) that the greenhouse gas
induced warming since the mid-twentieth century was larger than the
observed rise in global average temperatures, and extremely likely that
anthropogenic forcings were by far the dominant cause of warming. The
natural forcing contribution since 1950 is near zero."
Conclusion
There are multiple lines of empirical evidence that increasing carbon
dioxide causes an enhanced greenhouse effect. Laboratory tests show
carbon dioxide absorbs longwave radiation. Satellite measurements
confirm less longwave radiation is escaping to space at carbon dioxide
absorptive wavelengths. Surface measurements find more longwave
radiation returning back to Earth at these same wavelengths. The result
of this energy imbalance is the accumulation of heat over the last 40 years.
See:
http://www.skepticalscience.com/empirical-evidence-for-co2-enhanced-greenhouse-effect.htm