The main causes of climate change are the amount of greenhouse gases in Earth's atmosphere, the amount of Sun's energy reaching the Earth, and the amount of reflectivity of the Earth's surface.
The greenhouse effect allows Earth to be a habitable planet, maintaining a suitable temperature. Greenhouse gases in the atmosphere trap the infrared radiation Earth emits to space from the incoming solar radiation, effectively creating a blanket around our planet. There are records dating back a few hundred thousand years, tying the global temperature with the amount of carbon dioxide (and other greenhouse gases) in the atmosphere. The general trend shown in the data is that temperature increases as the amount of carbon dioxide increases, and temperature decreases as the amount of carbon dioxide decreases. However, due to human activity, particularly the burning of fossil fuels, the amount of carbon dioxide in the atmosphere (along with other fossil fuels such as methane and nitrous oxide) has increased to record highs, leading to a warming and changing climate.
The amount of solar energy reaching Earth also fluctuates, due to changes in solar intensity, Earth's tilt and orbital position. These changes are able to affect Earth's climate, but operate on much longer timescales and therefore probably are not the root of the changes we see in our present climate.
Reflectivity has to do with the amount of solar radiation that Earth reflects or absorbs. Naturally, Earth has an albedo of about 30%, meaning about 30% of solar radiation is reflected back to space by reflective surfaces such as clouds, certain aerosols, or surface ice. However, the less reflective surfaces Earth has, the lower the albedo and therefore more solar energy is absorbed by the Earth.
The causes of climate change are not isolated, but rather interact with one another in a series of complicated feedback loops. Many other factors are involved with understanding climate change, as all of Earth's systems are interdependent and interconnected.