I think about things that maybe most people wouldn't worry about. We live in a violent universe. Earth recently had a near-miss with a bus-sized asteroid that could have wiped out a major city with a ten megaton blast. However, chances are it would have just exploded over an ocean someplace. It is unlikely to hit a city. However, there are also more asteroids out there.
The last time the supervolcano in Yellowstone National Park erupted 640,000 years ago, it destroyed life in several nearby states. By comparison, this makes the asteroid look like small potatoes. These explosions are so big they create their own weather over hundreds of miles. We are "overdue" for another eruption, which could wipe out half the country, but the experts say that it will not happen any time soon. I heard that NASA is trying to find a way to relieve some of the pressure below the park.
In 535 AD, multiple volcanic eruptions, and possibly a supervolcano, blackened out the sky everywhere on earth, creating an 18-month winter.
A couple of times in Earth's history the whole planet froze solid with a layer of ice a mile thick.
Over hundreds of millions of years, there have been several mass extinctions on planet Earth. At least one may have been caused by a gamma-ray burst. Gamma-ray bursts are massive amounts of deadly radiation from space given off by black hole formation. Although such events hitting Earth are extremely rare, they have the potential to wipe out all life on Earth.
The Earth was hit by an object the size of the planet Mars 4.5 billion years ago. This is how we got the Moon, which is made of material from the Earth's crust.
These catastrophic events are fortunately very rare.
We have been technically living in an ice age for 2.5 million years. There have been several periods of massive glaciation in human history. These usually had devastating effects on the human population. The human race was almost wiped out 50,000 years ago. All of human civilization, such as farming, writing, working with metals, building cities, occurred during a "brief" 10,000 year warm period after the last period of glaciation. We have been fortunate to live in a "brief" time of very stable climate. No matter what humans do with CO2, and we are going to run out of fossil fuels in 100 years anyway, we expect another period of glaciation 10,00 years from now.
The Earth's orbit around the sun is not entirely stable. The slow precession of the orbit causes dramatic effects on the climate.
Although you could argue that rising CO2 is an issue in the short run, over the long term the decline of CO2 has been very dramatic and looks very bleak. Over the last 40 million years atmospheric CO2 levels have been in a nosedive. This is because natural processes sequester CO2 in the ground. During the last period of glaciation, CO2 levels got down to a record low of 180 parts per million, which is just barely above the level where all terrestrial plants die off from a lack of CO2. If humans are around for another 10,000 years then we will have to deal with this problem.
