With the question of global warming always to the fore, scientists have become adept at finding new, more accurate, and easier ways of monitoring the average temperature of Earth. The latest is to measure the rate of rotation of the Earth exactly; as the planet grows warmer, the days, it seems, become ever so slightly longer than they were before.
It all has to do with a principle known as the "Conservation of Angular Momentum". The angular momentum of a rotating body depends on its rate of spin and on the distance of the various parts of the object from its axis of rotation. It is a cardinal principle of physics that in the absence of any external influences, angular momentum is "conserved": it remains constant. The principle can be seen at work in the case of a swimmer somersaulting from a high diving board; she can increase her rate of spin by doubling up her body. Likewise, a ballet dancer doing a pirouette with arms outstretched can spin more rapidly by drawing in his limbs, thereby decreasing the distance of several parts of his body from his axis of rotation.
As a first approximation, the earth-atmosphere system can be regarded as free from external influence - and therefore its angular momentum must remain constant. But the rate of movement of the atmosphere is not constant. Apart from day-to-day changes in wind strength, the average speed of the atmosphere varies significantly over the years; and an increase in the speed of the wind implies an increase in the angular momentum of the atmosphere about the Earth's axis of rotation. If, therefore, the angular momentum of the whole earth-atmosphere system is to remain constant, there must be a compensatory change elsewhere; the Earth itself must lose momentum, and slow down.
Now over the past 50 years, the Earth's average temperature has been increasing at a rate equivalent to 0.8 Celsius per century. An increase in temperature implies an increase in atmospheric energy, so it is reasonable to assume that the average speed of the world's winds have also increased, and studies have found this to be the case.
Moreover, accurate measurements of the Earth's speed of rotation during the same period have shown that, as the principle of the conservation of angular momentum in these circumstances would predict, it is spinning slightly less rapidly upon its axis. By this means it has been possible to show that global warming is causing the Earth to slow down at a rate of about 0.56 milli-seconds per century. So accurate monitoring the average length of day provides a rough measure of any increase in the average global temperature.