Global warming – Lessons from the Second Law of Thermodynamics

Posted on September 5, 2011


Every physical and biological process is a ‘heat-engine’ of sorts:

The maximum efficiency of a heat-engine is determined solely by the initial and final temperatures of the heat. 

This is another way of looking at the Second Law of Thermodynamics, says Jennifer Coopersmith, physicist and author.

Our atmosphere, rivers, lakes and seas, ‘the very Earth itself’, are all ‘heat sinks’ for our human activities.

With global warming, as those ‘sinks’ become warmer, the efficiency of every single ‘engine’, every single process on Earth, will be compromised.

‘Take photosynthesis,’ Dr Coopersmith explains. ‘Gases, in the presence of chlorophyll and sunlight, are tamed and made to reside quietly in large and complicated plant molecules.’

Surely the total disorder has thus decreased?  No, says Dr Coopersmith, a researcher at La Trobe University’s Bendigo campus and author of ‘Energy, the Subtle Concept’ published by Oxford University Press last year.

‘What ensures that entropy will always increase is that, at the same time as plant material is being formed, waste heat is thrown out in the form of (chaotic) evaporated water molecules. This is called “plant-cooling”. Up to 500 times as much water must be lost to the atmosphere for plant-cooling as is required for plant growth.

Photosynthesis at its limit

‘But, in tropical regions, the ambient temperature is already so high, the air already so humid, that plant-cooling and hence photosynthesis is at the limit of efficiency. Any increase in temperature and plant growth will decrease or stop.’

So why hasn’t this devastating consequence caused alarm bells to ring, she asks? ‘One reason is that, as so many of our man-made engines operate very inefficiently, any reduction in the theoretical maximum efficiency doesn’t show up.

Dr Coopersmith: The efficiency of every process on Earth will be compromised

‘Take a typical internal combustion engine.  It operates at a mere 13 per cent efficiency. Most of the fuel heats the engine block rather than moving the car from A to B. The car should really be called a “cooker-on-wheels”!’

She admits that the reduction in ideal efficiency is very small (less than 0.5 per cent for a two degree rise).

‘But, of course, globally, this still adds up to a lot. For example, a change in the interest rate of only 0.5per cent still causes a stir in financial systems.’

‘Make no mistake,’ Dr Coopersmith warns, ‘an increase in the background temperature is a bad thing. Even wasteful car engines run better when the ambient temperature is low.’

With global warming, the efficiency of every single process on Earth will be compromised, she says – except for one that will work better: ‘The Earth itself will lose its excess heat to the cold of outer space at a faster rate than before!’

Dr Coopersmith’s book has been described by the former President of the Royal Society, Nobel laureate Sir Aaron Klug, as ‘illuminating’ and ‘full of surprises’. By making connections one never sees in books on thermodynamics, he says ‘it makes one think about the subject in a new way’. 

Her work has been featured on the ABC Radio National ‘Science Show’ and ‘Ockham’s Razor’. You can also read Dr Coopersmith’s commentary on Global Warming and the Second Law of Thermodynamics here.