“We know that with global warming we’ll get more evaporation of the oceans,” said Frédéric Laliberté, a research associate at U of T’s physics department and lead author of a study published this week in Science. “But circulation in the atmosphere is like a heat engine that requires fuel to do work, just like any combustion engine or a convection engine.”
The atmosphere’s work as a heat engine occurs when an air mass near the surface takes up water through evaporation as it is warmed by the sun and moves closer to the equator. The warmer the air mass is, the more water it takes up. As it reaches the equator, it begins to ascend through the atmosphere, eventually cooling as it radiates heat out into space. Cool air can hold less moisture than warm air, so as the air cools, condensation occurs, which releases heat. When enough heat is released, air begins to rise even further, pulling more air behind it producing a thunderstorm. The ultimate “output” of this atmospheric engine is the amount of heat and moisture that is redistributed between the equator and the North and South Poles.
“By viewing the atmospheric circulation as a heat engine, we were able
to rely on the laws of thermodynamics to analyze how the circulation
would change in a simulation of global warming,” said Laliberté. “We
used these laws to quantify how the increase in water vapour that would
result from global warming would influence the strength of the
The researchers borrowed techniques from oceanography and looked at
observations and climate simulations. Their approach allowed them to
test global warming scenarios and measure the output of atmospheric
circulation under warming conditions.
“We came up with an improved technique to comprehensively describe how
air masses change as they move from the equator to the poles and back,
which let us put a number on the energy efficiency of the atmospheric
heat engine and measure its output,” said Laliberté.
The scientists concluded that the increase in water vapour was making
the process less efficient by evaporating water into air that is not
already saturated with water vapour. They showed that this inefficiency
limited the strengthening of atmospheric circulation, though not in a
uniform manner. Air masses that are able to reach the top of the
atmosphere are strengthened, while those that can not are weakened.
“Put more simply, powerful storms are strengthened at the expense of
weaker storms,” said Laliberté. “We believe atmospheric circulation will
adapt to this less efficient form of heat transfer, and we will see
either fewer storms overall or at least a weakening of the most common,
The findings are reported in the paper “Constrained work output of the
moist atmospheric heat engine in a warming climate” published January 30
. The work was supported by grants from the Natural Sciences and Engineering Research Council of Canada.
By: Sean Bettam