The Gaia hypothesis keeps cropping up in fiction and popular science, doubtless to make a comeback in the context of climate change.
There are various articulations of Gaia, which proposes the Earth constitutes a complex regenerative system that always returns the environment to a life-sustaining equilibrium. One version maintains that Earth and its atmosphere and environments supports life and constantly brings it back into balance; another draws into the equation the Earth's biomass (the totality of life on the planet), to say that the full system constitutes a self-balancing (homeostatic) system.
Gaia was proposed in the 1960s by James Lovelock, with Dian Hitchcock. Working for NASA, they were charged with researching the atmosphere on Mars, for signs of life. Finding the Martian atmosphere to be in a deadly state of equilibrium, they contrasted this with Earth's atmosphere, in a relative state of flux (between oxygen and carbon dioxide in particular). Their Gaia proposal grew out of that.
Yet Lovelock's background was in chemistry and medical research rather than environmental science, and he was employed by NASA to develop equipment to analyse Mars' atmosphere. Hitchcock's background was philosophy, and she was to test his logic.
After the initial formulation, Lovelock's main collaborator has been Lynn Margulis, a biologist who couched the theory in more careful terms: of trends rather definitive equilibrium. (Margulis' reputation, however, is built on much more significant work, on the origin of organelles in eukaryotic cells: that is, that the organs of cells with nuclei emerged through symbiosis of separate entities). Her contribution to Gaia allows that no species has guaranteed passage through the bottlenecks of time.
But a major criticism of Gaia is its teleological nature: that is, that it implies some intention or purpose behind the planet's formation.
Recently, Peter Ward, an American biology professor, wrote a book that proposes the opposite: the Medea hypothesis, which says that life is constantly trying to kill itself and its own environment (The Medea Hypothesis: Is life on Earth ultimately self-destructive?). His overview in New Scientist is worth reading: it contains much background information about Earth's environmental changes.
My concern is that none of this is saying anything in particular. There is no guarantee that life in any form will survive a major disaster such as global nuclear war or a sufficiently large meteor impact. In fact, current projections are that the Earth will become totally lifeless within 500 to 1,000 million years, purely through the expansion of the sun - and that compares to the 3.8 billion years it has taken to develop to this point.
Behind both Lovelock's and Ward's articulation is the fact that Earth's environment and atmosphere has changed quite drastically over its history, causing mass extinction - several times, and life itself is the frequent culprit, due to cumulative changes in chemical composition of atmosphere and oceans. One such event was the evolution of photosynthesis 2.3 billion years ago. This entailed the absorption of carbon dioxide, and the emission of oxygen: a double whammy. On the one hand, oxygen was pure poison to most life at the time. On the other hand, over the course of 200 million years, the sucking out of carbon dioxide froze the oceans: this (first) snowball earth lasted 100 million years.
(although we credit the most well-known extinction event with an external cause - the meteor 65 million years ago that wiped out non-avian dinosaurs - most were due to events of local origin, pointing to imbalances that build up over time, usually from biological causes.)
There is no guarantee that the planet would return to a life-sustaining balance. Nor is there viable evidence that life deliberately tries to kill itself, or will ever succeed. Yet what it does say is that the variety of life is such that it has survived a number of cataclysmic changes. Pretty much all environmental niches that we can identify have corresponding life forms that could survive it (albeit most of the extreme cases are microbial).
On an immediate level, if we make the planet inhospitable for ourselves, other life forms will surely survive. For what it's worth. But human intervention has been nothing like any previous climate change bar the meteor: all others have been far more gradual. In terms of our lifetime, it's slow, but on a less anthropocentric scale, we are inducing a real shock to the planet. Yes, climate change is natural, but not in the framework in which we live. And the ride will be somewhere between bumpy and catastrophic, depending on our capacity to move forward together on the issue quickly enough.