The 3 factors: sun, albedo and atmospheric chemistry.
The only factor that has changed significantly the last 100 years is the atmospheric chemistry.There are three main factors directly influencing the energy balance of our planet:
- The total solar influx, depending on distance from the sun and solar activity
- Albedo, or reflections of solar rays from the Earth and back into space.
- The chemical composition of the atmosphere.
The earth system is continually changing. Temperatures on our planet has changed much and quickly before. These changes can be regional: a temperature increase in the northern hemisphere can have a corresponding lower temperature in the southern part of the world. The global temperature may vary due to solar activity, like the temperature rise during the first part of last century. The rapid temperature rise during the last decades cannot be explained by increased solar activity. The only factor of the three mentioned above where we have clear evidence of change is the atmospheric chemical composition. During the last 200 years, the amount of CO2 in the atmosphere has increased by more than 30%, atmospheric methane has increased by more than 150% while atmospheric N2O has increased by 17% and tropospheric ozone by 35%."For the last half of the century the positive forcing due to well-mixed greenhouse gases has increased rapidly over the last four decades, while in contrast the sum of natural forcings has been negative over the past two and possibly even four decades." (IPCC: Climate Change 2001- Synthesis report)