Are humans directly responsible for global warming?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

19 December 2010

Humans are warm blooded animals with a body temperature of about 36 degrees celcius. Therefore, they radiate heat energy to the surroundings all the time. The question then arises: what is the total amount of heat emitted by all the people in the world and can it affect the average temperature of the Earth’s atmosphere?

Knowing the normal body temperature and the surface area of the skin of a human being, it is possible to estimate the rate at which heat energy is radiated. I tackled that problem in this column many years ago (August 2004) and came out with a result of about 200 watts (W).

Now that was an estimate for an adult. The figure for children and babies would be much less since their bodies have smaller surface area (even though they also maintain the same 36 degrees temperature). Taking account of this, I further estimate that the average rate of radiation by a human being (not adult; not baby) is about 100W.

The total heat radiated by, say, all the 40 million people in Kenya is 4 billion watts! Let’s put that number in perspective: One billion watts are equal to 1,000,000kilowatts (kW) or 1,000 megawatts (MW). Therefore 4 billion watts is equivalent to 4,000,000kW or 4,000MW.

For comparison, Kenya’s total installed electricity generation capacity is about 1,200MW. Clearly, we are radiating a lot of heat.

Using a global population of 6.8 billion, the total radiation rate for all the human beings comes to about 680 billion watts; or 680,000MW. This figure, however, is dwarfed by the approximately 5,000,000MW of total installed electricity generation capacity in the world.

Still, what effect does 680,000MW have on the temperature of the planet’s atmosphere? To get the answer, we need some additional data.

The total mass of all the air in the atmosphere is estimated at about 5 million billion tonnes, that is the number 5 followed by 15 zeroes. Now, if one kilogram of air is heated by a one kW heater for one second, the temperature would rise by about one degree celcius.

Therefore, if one tonne (1,000kg) is heated in the same manner, the temperature would rise by only one thousandth of a degree. But if the heating rate was increased to one kilowatt (1kW = 1,000W), now the temperature would go up by one degree in one second again.

What about one megawatt (1MW=1,000kW)? This would raise the temperature of one tonne of air by 1,000 degrees in one second. Following this series of proportional increments, it turns out that, at 680,000MW, all the humans in the world raise the temperature of the 5million billion tonnes of the atmosphere by a ten-millionth of a degree every second.

With about 30 million seconds in a year, it means that humans can heat up the atmosphere by three degrees every 12 months. This of course assumes that the atmosphere doesn’t radiate heat out into space. That assumption is obviously wrong; otherwise, the Earth would be uninhabitable.