What level of concern about climate change is justified? Part One.

This post is about the motivation for this blog. So it's an important post. Perhaps this post should have come at the start. Anyway, diving in... what are the key facts of the matter about climate change?

Firstly, we know there is an important natural greenhouse effect on Earth. The sun, which is hot (at an effective temperature of 5000C) emits electromagnetic radiation at a high energy (high frequency; low wavelength - mostly in the visible and ultra-violet spectra). The Earth, which is warm (about 14C) re-emits electromagnetic radiation mostly at low energies (in the infrared spectra). Some of this infra red radiation is absorbed by certain gases in the atmosphere (water vapour and carbon dioxide) and then re-radiated. Half of this re-radiating heat radiation goes back down to earth, leading to a higher equilibrium surface temperature. We can easily calculate what the temperature of the earth would be without greenhouse gases - about minus 15Celsius. So we know that the temperature of the Earth is about 30Celsius higher than it would otherwise be, due to the effect of these gases. It seems that water vapour and carbon dioxide are the two most important of these gases, although other gases such as methane are also important.

Water vapour is the most important component of the greenhouse effect, but the concentration of water vapour in the atmosphere depends on temperature. The second most important gas carbon dioxide had an atmospheric concentration of about 280 parts per million by volume (ppmv) before human industrialisation (and after the ice ages where it dropped to 180 ppmv).

The most basic climate model w0uld suggest that a 100% effective greenhouse would raise (absolute) temperatures by 2^(1/4) or about 20%*250K=50 Celsius. This can be compared to the observed temperature increase of about 30 Celsius.

Simple models (similar to those of Arrhenius) suggest that the doubling of carbon dioxide concentrations from 280 to 560ppmv would raise global average temperatures by approximately 5C. Complex (General Circulation) models, and an analysis of the forcings in the ice ages (see previous post) suggest an average temperature rise of 3C for a doubling of CO2 concentrations, but with a probability range between about 1.2C - 6C (although there is non-zero chance of temperature rises above the maximum in this range).

Whilst there are arguments why the temperature might be less than the 3C, there are also arguments why it might be higher than this. How do observations of the whole world compare?

Next post I will discuss the observational record and what the estimates of climate sensitivity mean for us.


Anton Howes said...

Hmm why is CO2 the most important?
I know it contributes around 50% to global warming (I'm using my IB textbook here), but methane is 30 times more potent (18% contribution), and Nitrous oxides are 150 times more potent (6%), Ozone is a whopping 2000 times more potent (12%), and CFCs are an incredible 10,000-25,000 times more potent (14%).

Shouldn't we be concentrating more on reducing the levels of the other gases (except maybe Ozone!) first, particularly as they are generally from artificial sources, whereas CO2 and methane are also the product of organic, natural processes (livestock passing wind disturbingly comes to mind)?

I don't deny that carbon dioxide has a massive effect, partly due to its volume however. Then again, I'm using a secondary school textbook for my info!

...and if the effectiveness of water vapour as a heat trapper depends on temperature, does this mean either that as temperatures rise there will be an accelerating effect, or that it partly cancels out the effect?

(P.S. typo towards end of 2nd para)
(P.P.S. have added you to my blog roll)

TheClimatePhilosopher said...

CO2 is the most significant of the forcings that you mention, and it's concentration is also rising quickly. Of course it may be cost-effective to reduce the others too.

The increase in CO2 comes mostly from the combustion of fossil fuels and from deforestation. Both of these processes are manifestly under our control.

..."if the effectiveness of water vapour as a heat trapper depends on temperature, does this mean either that as temperatures rise there will be an accelerating effect, or that it partly cancels out the effect?"

There's more water vapour at higher temperatures. There is a positive feedback process, which is not great enough to be 'accelerating', but it does accentuate the effect of CO2. (It's a bit like the 'multiplier' in Keynesian economics).

Thanks for the typo correction; I'll update now. I've added yours to blogs I'm following...

Chris said...

Hi, I don't think I ever pointed out that Arrhenius ultimately estimated a figure of 2.1C for the global temperature rise due to a doubling of CO2. It has since been updated on Wiki:
"Arrhenius estimated that halving of CO2 would decrease temperatures by 4 - 5 °C (Celsius) and a doubling of CO2 would cause a temperature rise of 5 - 6 °C[4]. In his 1906 publication, Arrhenius adjusted the value downwards to 1.6 °C (including water vapour feedback: 2.1 °C)."

TheClimatePhilosopher said...

Great, thank you, Chris; that is extremely helpful.