The Earth is on the verge of failure as a home for biological life.
How we got here
Life has persisted on the earth, in one form or another, for 3.4 billion years. 3.4 billion years ago, a younger and cooler sun sent about 120,000 Terawatts to a methane and CO2 shrouded early earth. That corresponds to a black body temperature of around 230 Kelvin, or -43 Celsius, at the "thermal top" of the atmosphere. The dense blanket of atmosphere, and the internal heat of the newly-formed earth, kept the bottoms of the oceans warm enough for liquid water chemistry. More-active plate tectonics created mid-ocean rifts with high volcanic and geothermal activity, producing chemical gradients that fueled the formation of early chemo-synthetic life.
Temperature gradients between the molten interior of the Earth, its sunlight-absorbing heated surface, and the cold of space have always been the ultimate power source for life. Energy flows from concentrated hot to diffuse cold, and life is powered by that flow of energy. Both intense heat, which destroys structure, and intense cold, which immobilizes chemical reactions, are the enemies of life as an active, self-preserving, self-replicating process. DNA-based life operates best over a narrow range of temperatures, and global homeostatic feedback mechanisms (which James Lovelock playfully dubbed "Gaia" to tweak the pompous) have kept some regions and altitudes at the right temperatures for life since it began - otherwise we would not be here to tell the tale.
As the sun heated up, the earth and the life on it evolved and adapted to changing conditions. The sun now sends 170,000 Terawatts to earth, and the thermal top of the atmosphere is now around -18 Celsius, 25 degrees hotter. Meanwhile, the surface of the earth averages about 15C today, with geologically recent excursions down to 9C ("Ice ages"). The surface has been much hotter, and it has been much colder than these gentle extremes, but rarely has it been so biologically productive. Billions of years of natural selection have made Earth life very good at maintaining its environment.
Peter Ward's "Under a Green Sky" describes a number of times in the Earth's history when the process almost failed, when "snowball earth" or "hydrogen sulfide" earth suffered mass extinctions, often induced by failures of homeostasis. Brownlee and Ward's "Rare Earth" tells us how seldom this long sequence of near misses continues long enough to produce intelligent life. There are many planets in the galaxy with the right mass and energy input to support life, and this has led some bold astronomers to boldly claim that the universe is teeming with life. But chemical composition, sequence, and dumb luck all play strong roles.
Physicist Enrico Fermi understood that the interstellar migration of intelligence, if possible, would rapidly fill and dominate the galaxy. Even if 99.99% of the hypothetical instances of intelligent life fail to do this, all it takes is one, sometime in the 14 billion year life of the galaxy, and the sky would be full of obvious signs of cosmic engineering, as would our own solar system. Earth life is close to the threshold of this expansion; humans are capable of great stupidity, but great tenaciousness, so it seems inevitable that our capabilities will cross this threshold in a geological eyeblink. Fermi asked "where are they?" Fermi knew the technical difficulties are surmountable. Is intelligent life rarer than we hope, or is extermination inevitable?
At least one form of optimism, either cosmically widespread life, or inevitable cosmic expansion, is unjustified. Possibly both. I am betting that life is very rare in our galaxy - and very precious. I am also betting that at least a few humans will mature past our childish hostilities to nature and each other, and begin seeding the rest of the galaxy with life. But the clock is ticking, and we have a very short time to mature. Even if we learn to life harmoniously with nature, nature's time may be very short. We have propensities to shorten that brief time, but we are capable of extending it far beyond "natural" bounds. What are those bounds, and how can we detect them?
The Glacial Ages
Geological evidence suggests five major "Ice Ages" since the beginning of life, periods with oscillating temperatures and major glacial activity, associated with major changes in the homeostatic mechanisms involving life. We are in the middle of the Pliocene-Quaternary glaciation, beginning about 2.6 million years ago, with dozens of temperature cycles so far. The previous "Karoo Ice Age" from 360 million years ago to 260 MYA, corresponded to the emergence of plant life on land, with major reductions of atmospheric CO2 as fires created charcoal and geological processes buried it. Land plants developed lignin, which allowed biomass to increase rapidly; this non-decomposable biomass accumulated until termites and new fungi appeared to decompose it, millions of years later. Many of these evolutionary "innovations" were potential show stoppers - the fact that life survived them, and went on to thrive, is not evidence that they were inevitably benign. Evolution has no foresight; every major turn can lead to disaster.
The Quaternary glaciation is probably a result of the evolution of grasses, which have "C4 metabolism" and can thrive at much lower CO2 levels than "C3" plants. Grasses help sequester carbon, and the associated drop in CO2, to spectacularly low levels of 180ppm or lower, helped trigger the recent glaciations, accumulating ice at the poles and lowering sea level, creating more land surface for the grasses to grow. Another "recent" innovation is tall conifer trees with highly flammable resin-soaked needle leaves in canopies, which fall to the ground and fuel frequent fires. The fires clear the way for new conifers. Conifers are C3 plants, and competitive with the grasses.
The oscillation between trees and grasses, ice accumulation and CO2 accumulation, has been increasing over the last million years or so, and shows no signs of steadying to a uniform level. This oscillating behavior resembles a phenomena in electronic oscillator circuits called "squegging" or sometimes "motorboating" (because of the sound it makes - "putt putt putt ... "). Squegging occurs when there is fast oscillation whose amplitude is controlled by a slow, poorly stabilized, and highly nonlinear feedback loop. Depending on the dynamics and the fragility of the components, squegging can evolve into "latch up" - the oscillator sticks at an extreme steady voltage level - or failure, as a component overheats and fails.
There is no guarantee that glaciation-age oscillations will not continue to increase, even in the absence of human exacerbations. While it is possible that the oscillations might settle down, it is also possible that they will reach catastrophic extremes. Remember, the sun is heating up. Even without the oscillations, in about 500 million years the solar input will be enough to heat the planet beyond the "DNA range". The homeostatic mechanisms will fail, CO2 will accumulate, the atmospheric heat blanket will thicken, the seas will boil, and the surface of the earth will become uninhabitable. One of the natural oscillatory extremes, perhaps within a million years, may bring about that doomsday far sooner than an inevitable gradual rise will. The earth will "latch up" - dead forever, like Venus.
William Calvin suggests that the rapid environmental variations required adaptations faster than the slow pace of evolution, and this led to the evolution of large learning brains, which can develop and accumulate adaptations much more rapidly than genetic variation can. Humans can migrate between temperature extremes, mountain and shore, changing diets and gathering methods in ways that other animals can't. Other megafauna were tightly coupled to particular environments, and evolutionarily fragile; humans ate them and moved on. Unfortunately, on the surface of this finite planet, we have run out of places to move on.
Have we made things worse?
Historically, as human numbers increased, culture adapted to crowding. We developed technologies to extract more food from the land, at least temporarily, while learning how to coexist without the intertribal warfare that used to be the dominating cause of death in prehistoric tribes. This knowledge allowed human populations to increase tenfold over the last century, leading to further crowding, of humans and of the diminishing wildlands. The crowding coexists with cultural oscillations, reversions to misanthropy, barbarism, and tribal warfare that occasionally destroy some human subcultures. But failed cultures are replaced by others; tragedies are forgotten without descendants. Cultures evolve by natural selection, just as species do. Surviving cultures aren't "good" in any moral sense, but they are tenacious and conservative. They protect their assets, and don't waste them in conflict. Individuals finding themselves in conflict-seeking or asset-destroying cultures are advised to move on.
Unfortunately, humans can be unaware of many assets and many conflicts. In the early days of World War Two, Berliners were blithely ignorant of the rapid destruction of international goodwill and the vengefulness accumulating in the vastly larger world around their tiny Reich; the bombs that soon rained down upon them were met with shocked indignation and disbelief, not guilt. 21st century humans in developed societies act entitled to an unending stream of goodies from a quite finite globe. Our food abundance rests on chemical fertilizers, which depend on a rapidly depleting supply of accessible phosphates, without which we would have to increase agriculture far beyond the remaining supply of potentially arable land. Species are disappearing; genetic material accumulated over billions of years is disappearing in decades. That information is vital to the growth of new biotechnologies, and possibly to our existence. While human-human conflict is on the wane, our warfare against nature has been exported to the rest of world, with apparent gains in the developed world matched by catastrophic losses in the developing countries that import our dirtier industries.
Hope for the future
However, we are more aware of what we are doing to the world than any culture or time in history. There are more of us, but we are capable of treading more lightly on the world than in any time in the past. Primitive herders and ignorant farmers have destroyed far more land than industrial farming has done, but our increased agricultural yields have reverted some land back to nature. By crowding into cities, both our fecundity and per-person footprint have been reduced. We cannot keep consuming nature in our gardens and megafarms as we did at any time in the past. As we replace material consumption with knowledge and skill, we don't need to.
Even without us, the biologically world may fail quite soon in geological terms. With us, the failure could come a few million years sooner. However, our wisdom, while slight and inadequate, is growing. Individual people and cultures may be stupid and doomed, but the future will grow out of the smart ones, the future-focused preservers of assets and developers of capability and cooperation. Evolution may be smarter than us, and has accumulated a vast treasury of tricks and adaptations and genetically stored knowledge, but it has zero foresight. Humans can look ahead, and while our attempts to predict and prepare for the future are almost always frustrated, they are sometimes successful. Out of those rare successes, multiplied by billions of potentially-capable individuals, long term planetary success becomes possible.
Gaia is 3.4 billion years old. Does that make her an old lady, or young girl suffering through a troublesome adolescence, about to become the successful mother of a galaxy filled with life? Are we humans the pathogens that will end her life, or the seeds of her explosive expansion? We are potentially both, and we are in a position to choose the outcome. There is no possible way Gaia can survive a billion years without intelligent, prediction-driven help. With human commitment to a long future, and with the vast resources of a solar system waiting to be brought to life, Gaia's children can fill our galaxy, and the galaxies as far as we can see.