Kalmus - Global warming, the science

Peter Kalmus, "Being the Change...", Chapter 3.

At the topmost level, climate science has one thing to teach in regards to the well-being of our species and the rest of the biosphere: to curtail global warming, stop burning fossil fuels.

I hope to clarify two time scales of our predicament: speed of onset and duration. I also hope that this brief tour of Earth science will enrich you with a deeper understanding of your relationship with this beautiful planet.

 


Read Chapter 3 of "Being the Change". The first assignment deals with the beginning of the chapter through Peak temperature: Why mitigation is crucial.

We basically have three choices, mitigation, adaptation, and suffering. We're going to do some of each.

The question is what the mix is going to be. The more mitigation we do, the less adaptation will be required, and the less suffering there will be.

-John Holdren, 2007, Director of the White House Office of Science and Technology Policy, Assistant to the President [Obama] for Science and Technology

Mitigation

...means addressing the roots of a problem.

In the case of the climate crisis this might include:

  • Reducing greenhouse gas emissions.
  • Investing in new energy-generation technology.
  • Conservation.
  • Training for jobs in longer-term sustainable industries.

Radiative forcing

Energy from the sun arrives through Earth's atmosphere as electro-magnetic radiation = "light".

When parking lots / trees / lakes / anything is warm, it gives off infrared radiation, a longer wavelength light, that we can't see.

When radiation arriving = radiation leaving, Earth's average temperature remains roughly constant.
100% = 23% + 7% + 49% + 9% + 12% in the picture below.

But many things can happen to change that balance:

  • Volcanoes erupt, throwing sulfur-dioxide particles high into the atmosphere. These bright particles reflect sunlight. (negative RF)
  • White ice reflects a lot of sunlight. When ice in the Arctic melts, darker ocean water is exposed to the sun, and less sunlight is reflected. (positive RF)
  • If extra $CO_2$ is added to the atmosphere, more of the infrared radiation emitted by the surface is absorbed in the atmosphere, and less is emitted to space. (positive RF)

We can compare all these different processes by calculating for each the "radiative forcing": $$\text{radiative forcing}=\text{(total radiation arriving) - (total radiation leaving)}$$ This has units of energy / area: Watts / meter.

If this is out of balance for a long time, the temperature close to Earth's surface will:

  • increase for positive radiation forcing.
  • decrease for negative radiation forcing.

When any object gets hotter it radiates *more*. So if the RF is positive, a new equilibrium will be reached, but at a higher temperature, and vice versa.


Temperature appears to have been fairly stable before about 1750 (the beginning of the Industrial Revolution. So if we say that Earth's atmosphere was balanced in 1750, the RF was by definition 0 in 1750.

What will the Radiative Forcing be in the future?

That depends, now, mostly on what we humans do. So, the IPCC has defined four emissions scenarios, or "Representative Concentration Pathways (RCPs)" in terms of what the eventual, stable Radiative Forcing *might* be.

The point of these scenarios is to let different scientific groups put the same emission scenario into their different models, and see how their predictions compare.

Tables from IPCC AR5 (2013) by way of climate.gov.