...is that the earth is not a closed system. It receives energy from the sun, and radiates waste heat into space. Any interruption or change in either of these processes and it would get very unfortable here in no time flat. The earth does have ways of temporarily accumulating energy (as in placing high-density solar energy into concentrated fossil fuels) so that they are more accessible than as raw sunlight, and waste heat can be temporarily stored in the oceans and atmosphere, where a small portion of it is available to do useful work in maintaining the system as a whole. There is also considerable possible variation at the system frontiers to help increase or inhibit the flow of energy in or out (albedo).

But these temporary sinks and boundaries serve mainly as a shock absorber to absorb minor transients in the energy budget and smooth out variations. This is vital to create time lags in the energy processing and allow feedback mechanisms the opportunity to moderate things. It is remarkable that a physical system has evolved to do this, and that it is so effective at it. It is even more amazing how important the role of living things has been in this process. But one must remember this is all from the long-term point of view; this adaptability has been able to keep the earth's surface around the liquid point of water, a very narrow range. Changes in the budget are either small enough or slow enough for the system to adjust.

Very rapid (geologically speaking) stresses on the system may very well induce more extravagant adjustments (such as ice ages, warming events, or drastic sea-level changes), which although may still resist runaway excursions like on Venus, would be very catastrophic to finely balanced and vulnerable human industrial and agricultural systems and settlement patterns.

Perhaps someday, planetary engineers will be able to monitor environmental conditions and be able to correct them and even optimize them for our purposes. But right now, and for the forseeable future, we don't have a clue what we're doing and if we're smart we'll keep our hands off the controls and let the autopilot do it its job. And if we're smart, we won't let our unruly kids and deadbeat relatives have unrestricted access to the engine room.