Energy Flows, Bathtubs and Climate

Posted on September 27, 2013


Father Theo Goes to Climate School 2

Climate is all about energy flow.  Energy flows into Earth’s climate system from the Sun, warms the air, water and soil, then escapes again into space.  Conditions on Earth affect the flow.  They can either facilitate or slow the escape of energy into space, which has consequences to the climate.  The faster energy escapes, the less is trapped in Earth’s climate system, the cooler it becomes on Earth.  The opposite is true as well.  If the escape of energy is slowed, the amount of energy remaining in Earth’s climate system rises.

Think of a bathtub standing in for Earth’s climate system.  The energy entering in is represented by water flowing from a tap.  The energy escaping into space is represented by the water going out the drain.  In a stable system, the outflow equals the inflow, and the water (or energy budget) remains at the same level.  However, if you turn the tap a little higher, the amount of energy trapped in the system rises.  The tub rises.  Things heat up.  If you turn the tap a little lower, the energy flowing out exceeds the energy flowing in.  The tub sinks, and things cool down.

You can achieve comparable results by expanding or shrinking the size of the drain.

Variations in solar output can affect how much energy the Earth is receiving at any given time, however the evidence is against such variations having an influence on the current bout of climate warming.  We’ve just passed through the warmest decade in the human record during a period of an unusually cool Sun.  A cool Sun should not be making us hotter.  Turning down the tap should not make the water level rise.

For another explanation, we have to look to other influences on the Earth’s energy flow, the two main one’s being albedo and greenhouse gases.  Both influence the escape of energy into space.  They widen or narrow our drain.

albedo feedback effect

albedo feedback effect

Albedo is why it’s cooler to wear a white t-shirt under a summer sun than a black one.  Albedo can also have effects on a planetary scale.  Energy that is reflected back into space by albedo cools us.  Energy that is absorbed warms us.

Albedo is important in ArcticSea ice because the ice, which is now rapidly shrinking and disappearing, used to reflect back into space energy which the darker sea surface beneath it is now absorbing.  Albedo is important in regard to black carbon, which, in settling on surfaces such as glaciers, greatly accelerates their melting.

Thick clouds are reflective.  Sometimes particles (known as aerosols) sent into the air by volcanic eruptions or industry are also reflective.  Plants can either absorb or reflect varying amounts of energy.

Greenhouse gases warm the planet by trapping infrared energy of certain wavelengths before it can leave the atmosphere.  They effectively slow the energy drain into space.

The most important greenhouse gases are water vapour, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone.  (Ozone is best known in the context of the ozone layer in the upper atmosphere which protects the planet from ultraviolet light.  Don’t be confused by that.  The ozone layer has nothing to do with climate change.)

The concentration of greenhouse gases in the atmosphere determines how strong or weak the warming effect is.  Thus it’s important to understand the kinds of things which can affect greenhouse gas concentrations.  Climate science thus also studies the motion of materials—particularly carbon—back and forth from soil and rock, ocean, atmosphere and biosphere, the Four Reservoirs discussed in Part 1.


Acknowledgements to the course I recently participated in through Coursera and the University of British Columbia:  Climate Literacy:  Navigating Climate Change Conversations, taught by Sarah Burch and Sara Harris.


Posted in: Climate School