Global warming was NEVER said to be caused ONLY by humans.
In fact, past global climate changes have been studied in detail and they provide evidence of CO2-linked warming:
Climate Model Links Warmer Temperatures to Permian Extinction
http://www.physorg.com/news6003.html
"The CCSM indicated that ocean temperatures warmed significantly at higher latitudes because of rising atmospheric levels of carbon dioxide, a greenhouse gas."
There are a couple of different theories for why Mars may be warming, but none of the theories published in peer-reviewed scientific papers involve increased solar radiation, since that is known to have been decreasing recently.
"Wobbles in the orbit of Mars are the main cause of its climate change in the current era," Oxford's Wilson explained.
(Related: "Don't Blame Sun for Global Warming, Study Says" [September 13, 2006].)
http://news.nationalgeographic.com/news/2006/09/060913-sunspots.html
According to researcher, Lori Fenton:
"Our Nature article had some press coverage, and some who heard the news would like to attribute our modeled warming (of 0.65° C or 1.2° F) over the 20 year period to changes in solar output, but our work doesn't involve any such changes. The warming we have modeled is caused by changes on the surface of Mars, rather than by any changes in the Sun. Furthermore, the climate forcing process we have identified is caused by the redistribution of bright dust over a darker surface, a process which does not occur on Earth (at least not to such a magnitude that it influences the global climate system). Earth does undergo albedo changes that impact the climate, but the controlling processes are quite different -- such as changes in cloud cover, sea ice, and vegetation. Thus the warming we have modeled is caused by a process that is unique to Mars, and nothing in our work can be used to make inferences about climate change on Earth. I hope this clears up any confusion to the layperson."
http://humbabe.arc.nasa.gov/~fenton/