(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: The sun is the source of warmth on earth. Any increase in temperature is likely due to changes in solar radiation.
Answer: It’s true that the earth is warmed, for all practical purposes, entirely by solar radiation, so if the temperature is going up or down, the sun is a reasonable place to seek the cause.
Turns out it’s more complicated than one might think to detect and measure changes in the amount or type of sunshine reaching the earth. Detectors on the ground are susceptible to all kinds of interference from the atmosphere — after all, one cloud passing overhead can cause a shiver on an otherwise warm day, but not because the sun itself changed. The best way to detect changes in the output of the sun — versus changes in the radiation reaching the earth’s surface through clouds, smoke, dust, or pollution — is by taking readings from space.
This is a job for satellites. According to PMOD at the World Radiation Center there has been no increase in solar irradiance since at least 1978, when satellite observations began. This means that for the last thirty years, while the temperature has been rising fastest, the sun has not changed.
There has been work done reconstructing the solar irradiance record over the last century, before satellites were available. According to the Max Planck Institute, where this work is being done, there has been no increase in solar irradiance since around 1940. This reconstruction does show an increase in the first part of the 20th century, which coincides with the warming from around 1900 until the 1940s. It’s not enough to explain all the warming from those years, but it is responsible for a large portion. See this chart of observed temperature, modeled temperature, and variations in the major forcings that contributed to 20th century climate.
RealClimate has a couple of detailed discussions on what we can conclude about solar forcing and how science reached those conclusions. Read them here and here.