What could cause the environment to change, other than humans?
There are many things that can change climate, besides humans.
In the news reports about the major earthquakes in Haiti, Chile, and Japan, a common point was that the jolt was so violent, it altered the planet's orbit. In all these cases the alteration was minute and temporary, but a series of powerful earthquakes could make a big enough impact to change how our orbit and planetary tilt cycle the seasons.
Closely related to Earthquakes are Volcanoes. In the 18th Century, an Icelandic volcanic eruption increased concentrations of Sulfur Dioxide in the atmosphere, and is speculated to have been tied to a rash of respiratory illnesses and deaths in Europe a few months later. The eruption of Krakatoa in the 1890s turned sunsets a fiery crimson around the world for weeks. The eruption of Mt. Pinatubo in the 1990s decreased the Earth's average temperature by about .1C for a year from the ash reducing the sunlight that reached Earth's surface.
Volcanoes also do more than change atmospheric contents. Volcanic magma is so hot it will ignite anything combustible on contact. Anything that isn't combustible will melt or vaporize on contact. Something that hot underwater will heat the water. Inside this fiery material are bubbles of Sulfur Dioxide and Carbon Dioxide. These gases react with water to form Sulfuric and Carbonic acids. These acids change the acidity of the ocean water.
Changes in solar activity can also affect the climate. If solar activity diminishes, less energy reaches the Earth, and so the Earth's climate grows colder. If solar activity increases, more energy reaches the Earth, and warms the climate. The fact that the Earth, Mars and Jupiter have all increased 1C over the past 150 years indicates that the Sun is probably in a cycle of increased activity.