Climate change deniers cite Mars as an example that climate change is being caused by events outside of Earth's environment and not by manmade emissions. This, of course, belies their claim that global warming isn't occurring, but we'll ignore that point. What is important is that climate change on Mars does not have anything to do with what is happening on Earth. Recent images from the European Space Agency mission Mars Express illustrate this point.
Deniers point at images of the martian polar regions over a period of years as proof that manmade emissions are not causing global warming. What we see is that the carbon dioxide polar caps freeze in the martian winter, melt in the spring and then freeze again the following winter. However, comparison of some of these images shows that subsequent freezing does not reach the same amount as it did in the past. The carbon dioxide polar cap appears to be getting smaller each year. Since the CO2 polar cap needs really cold temperatures, the conclusion by some is that the polar regions on Mars are getting warmer. Since there are no manmade emissions on Mars, this warming must be due to other things, such as a warming Sun, and these same causes are responsible for global warming on Earth, not manmade emissions.
There are a lot of problems with this claim. The first is that there is no evidence to support it. The second is there is a mountain of evidence to deny it. But, let's not let reality get in the way.
The fact is that the climate changes on both Mars and Earth through natural cycles. These are well-known and are called Milankovitch cycles on Earth. The great part about these cycles in that deniers use the Milankovitch cycles as explanations for the current change in Earth's climate, but then ignore them on Mars. The natural cycles on Mars are much more extreme because it does not have a very large moon, like Earth does. The climate on Mars changes much more rapidly as a result.
Now, the ESA Mars Express images show a deep martian crater and the layers of sediments can be seen. A nearby crater that is not as deep does not show these deeper layers, meaning the layers are in the rock and were not subsequently formed in the larger crater. While we cannot be sure without actually going there and doing tests on the layers, it is believed these layers are caused by cycles in the martian climate, just as expected with the natural cycles.
And, what about those cycles on Earth? The scientist claiming the martian changes are evidence for natural causes here on Earth is Habibullo Abdussamatov, head of space research at St. Petersburg's Pulkovo Astronomical Observatory in Russia. He said, in 2007, that the Sun entered a period where it is dimmer, beginning in the 1990s, and that we would see a steep cooling of the climate as a result, 'within 15 to 20 years.' Well, it has been five years since he made that claim and what we have actually seen is that global warming has continued with the warmest years ever recorded occurring since then.
The evidence and track record don't seem to be supporting the deniers.