Global Warming Revisited

The Second Law of Thermodynamics may help provide explanation for the global warming over the last 250 years.

Learning Objective

Describe effect of the heat dumped into the environment on the Earth’s atmospheric temperature

Key Points

As heat engines operate, a great deal of heat is lost to the environment due to inefficiencies.
Even in a Carnot engine, which is the most efficient theoretical engine, there is a heat loss determined by the ratio of temperature of the engine and its environment.
As more heat is dumped into the environment, Earth's atmospheric temperature will increase.

Terms

absolute zero
The coldest possible temperature: zero on the Kelvin scale and approximately -273.15°C and -459.67°F. The total absence of heat; the temperature at which motion of all molecules would cease.
Carnot cycle
A theoretical thermodynamic cycle. It is the most efficient cycle for converting a given amount of thermal energy into work.

Full Text

The Second Law of Thermodynamics may help provide explanation for why there have been increases in Earth's temperatures over the last 250 years (often called "Global Warming"), and many professionals are concerned that the entropy increase of the universe is a real threat to the environment .

Global Land-Ocean Temperature

Global mean land-ocean temperature change from 1880 – 2012, relative to the 1951 – 1980 mean. The black line is the annual mean and the red line is the five-year running mean. The green bars show uncertainty estimates.

As an engine operates, heat flows from a heat tank of greater temperature to a heat sink of lesser temperature. In between these states, the heat flow is turned into useful energy with the help of heat engines. As these engines operate, however, a great deal of heat is lost to the environment due to inefficiencies. In a Carnot engine, which is the most efficient theoretical engine based on Carnot cycle, the maximum efficiency is equal to one minus the temperature of the heat sink (T_c) divided by the temperature of the heat source (T_h).

$(Eff_c = 1 - \frac{T_c}{T_h})$.

This ratio shows that for a greater efficiency to be achieved there needs to be the greatest difference in temperature available. This brings up two important points: optimized heat sinks are at absolute zero, and the longer engines dump heat into an isolated system the less efficient engines will become.

Unfortunately for engine efficiency, day-to-day life never operates in absolute zero. In an average car engine, only 14% to 26% of the fuel which is put in is actually used to make the car move forward. This means that 74% to 86% is lost heat or used to power accessories. According to the U.S. Department of Energy, 70% to 72% of heat produced by burning fuel is heat lost by the engine. The excess heat lost by the engine is then released into the heat sink, which in the case of many modern engines would be the Earth's atmosphere. As more heat is dumped into the environment, Earth's atmospheric (or heat sink) temperature will increase. With the entropy of the environment constantly increasing, searching for new, more efficient technologies and new non-heat engines has become a priority.

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