Climate Change: Reprise

Back in September I wrote a post entitled "Climate Change: Yes, But Why?". My premise was that, while "climate change" is clearly occurring, the issue of causality needs more attention.

After that post, I asked some members of the Oak Ridge National Lab staff (including one of ORNL's corporate fellows, Dr. Tom Wilbanks, who was part of the Nobel-winning Intergovernmental Panel on Climate Change) to share their perspectives. Tom was able to point me in the right direction regarding infrared absorption of various molecules (e.g., H2O, CO2, etc.).

This month's WIRED (16.06) has a great point-counterpoint cover story (which, in the URL, is referenced as "heresies"). Among their proposed "heresies" are Embrace Nuclear Power, Carbon Trading Doesn't Work and China Is The Solution. I think these are great points -- points that hopefully will shift the dialog from impassioned proselytizing to rational decisionmaking. (However, a cursory glance at the WIRED comments section shows this to be a vain hope....)

The WIRED piece has motivated me to renew my own research into the causal relationships between solar radiation (which has been on a cyclical increase, with a corresponding decrease in sunspot number), natural phenomena, and human influence. I finally found a graphic that I find compelling:

This graphic (h/t Thomas Everth's GreenBlog in New Zealand) shows total atmospheric absorption of various frequencies of electromagnetic radiation, as well as individual absorption spectra for ozone (O3), water (H2O) and carbon dioxide (CO2).

It also overlays the distribution of incident solar radiation (most of which is between 200nm and 2μm wavelength, covering the visual spectrum of light as well as the near-infrared band) as well as the Earth's outgoing thermal radiation (covering the infrared-C band from 3 to 80 μm wavelength). Note that the x-axis (wavelength) is a logarithmic scale, going from 0.1 to 1, 10 then 100.

The red line shows the absorption of O2 + O3 (ozone). As expected, the left side of ozone's absorption spectra (wavelength below 200nm, in the ultraviolet spectrum) shows nearly 100% absorption. Our natural defense against harmful ultraviolet radiation is the ozone layer (which is depleted by chlorofluorocarbons, but that's for another post).

The blue line shows the absorption spectrum of water. While some have claimed that water vapor is the real culprit behind global warming (usually with some reference to how much warmer we are when there's a cloud layer), this plot shows that water's absorption spectra drops where the outgoing thermal radiation increases. Therefore, water is nearly transparent to the infrared-C radiation between 5 and 20 μm wavelength. This too is expected -- otherwise Forward Looking Infrared (FLIR) wouldn't work on humid nights or with a high water vapor content in the air.

What I find compelling is the green line showing CO2's absorptive properties. In particular, note the spike in absorption between 10 and 20 μm -- where CO2 absorbs nearly 100% of incident IR-C radiation.

When I first considered the absorption and scattering of radiation by molecules in our atmosphere, I initially discarded it as a cause for concern. My rationale was that the absorbed radiation would be randomly scattered, or re-radiated -- and therefore would balance itself out.

While this is true for incoming radiation from external sources (e.g., the sun), what I failed to consider is that we live exothermic lives on Earth. When we convert fuels into energy, we're also creating heat -- so the outbound radiation becomes the key consideration.

By adding the outbound radiation to the equation, with nearly half of the absorbed radiation being scattered back to the Earth, we can conclude that carbon dioxide is a major contributor to climate change.

What can we do about this? Invest in alternative, renewable energy sources (such as solar, which is getting more efficient in capturing the 1.3kW per square meter peak incidence from the sun; wind, with some models generating more than 6MW per turbine; and better batteries to store energy). Travel less (thanks, Internet). And save.