After a whole semester of getting hammered by all kinds of climate related courses, one develops a great respect for the Earth and for the sheer dumb luck that we call life.
For example, a little closer to the Sun, we might have gone the Venus way, a little farther we would have been like Mars. Even then, we had to have the right amount of CO2 in the atmosphere, a surface dominated by water and its wierd behavior to survive. Also interesting is the fact that we have as a major component of our atmosphere a very reactive gas called oxygen whose fraction has not varied for a couple of million years. A little more oxygen, everything would have burned to cinders long time ago. A little less, the diversity of life that we see might not have existed. How silly bacteria and plants have managed to maintain the fraction of oxygen for this long a time period is quite a mystery!!
The Earth’s climate is like the holy grail for students of nonlinear physics, and thousands make a living by showing that we still have much to learn about it. That humans will never be able to accurately predict it over very long times scales is a given, so everyone has a gala time building models of increasing complexity to see how good we can become at this game of prediction in the future. All that being said, the basic physics behind climate are well understood, just that to predict it will require crazily large amounts of measurements of a crazily large number of variables, which just may not happen anytime before we cook ourselves.
To a first approximation, the earth is very much like an engine: it takes energy from the sun, does some work with it, and rejects the waste heat back to space. Since it has been doing this for millions of years, it seems reasonable to assume that what comes in must go out, otherwise this energy would build up within the earth, eventually destroying it. This kind of state is called radiative equilibrium.
So, what kind of work does the earth do using this energy ? Well, it causes winds, oceans currents to develop, helps photosynthesis and eventually (though not necessarily) forms something interesting like Aishwarya Rai. Every source of energy available to humans today, from flowing water to coal to Red Bull, all are sunlight in different forms (except nuclear, of course).
So, the main purpose of the earth’s weather (If one can impute purpose to it) seems to be to distribute this energy to all corners of the earth. There are places like Antartica which really cannot be bothered by all this energy and reflect it back, other places like Kolkata and Chennai which seem to take in more than needed, but on the whole, the spread is quite even. The atmosphere seems to act like a piston, moving this way and that, taking excess energy from here and transferring it there. The poor atmosphere just wants to remain calm and steady, but somehow it manages to gain enough energy to become unstable. In the tropics where we live, the main mechanism by which it tries to regain equilibrium seem to be via rainfall – little wonder why it rains so much in the tropics!
The oceans take in quite a bit of this energy, keep it to themselves for a long time (unless something makes them throw it back out) and act like the flywheel – even if there is no much energy coming in from the sun (at night, say), they provide some energy back to the atmosphere, which is why coastal regions don’t really have too much variations in temperature.
But unlike human built engines, climate and weather don’t really bother about (probably due to lack of awareness) Carnot’s theorem. The source and sink temperatures for most of earth’s thermodynamic cycles is miniscule – the temperature difference for a good monsoon to happen is probably lesser than 10 degrees centigrade, amounting to an efficiency of almost zero! Makes you wonder if the Earth managed so well with zero efficiency thermodynamic cycles, why are we hell bent upon engines with huge contrasts in source and sink temperatures! Maybe we are just too impatient, and want to travel from Bangalore to Boston in less than a day. In contrast, ocean currents probably take years to do such a trip.
Come to think of it, huge gradients of anything are not really visible on earth, and where they exist they are quite destructive (think waterfall!). But even large gradients like waterfalls finally work to reduce the same gradient (these are called negative feedbacks). There are processes which enhance gradients (called positive feedbacks), and which are believed to have caused (and removed) the many Ice Ages that the earth has experienced, but on the whole this policy of keeping gradients small seems to have worked quite well for the Earth – Ice Age or otherwise, the Earth’s mean temperature does not seemed to have varied by more than 10 degree centigrade over millions of years. Individual areas might have huge variations, but as a whole it has been quite steady.
In contrast, a factory can have a variation of over 100 degrees over a single day! Obviously, factories cannot last millions of years. Any extreme event ( like human civilization ;) cannot sustain itself for long. But we seem to have made an ideal out of extremes : F1 racing, supersonic flights, George Bush, Bill Gates, Narendra Modi and so on, which does not seem to conform with sustainable behavior. One wonders whether sustainability addresses such issues. There is little doubt that our industrial civilization of the past 200 years will be a blip in the history of the Earth, but it remains to be seen (by who/whatever inhabits this planet millions of years from now) whether the Earth changes forever due to this extreme blip.