Let's look at another indicator, "More fossil fuel carbon (carbon-12) in the air." Carbon atoms come in two primary forms (isotopes). One of these is carbon—14 which is formed in the atmosphere on a natural basis (by cosmic rays) and is radioactive. It has a half-life of about 5,730 years and is formed and decays so that the amount in the air at any one time is normally constant. This is no longer the case.
Stay with me now.
When scientists measure the different isotopes of carbon in the atmosphere today, they find that the relative amount of carbon-14 compared to carbon-12 is decreasing. Since fossil fuels have been buried in the earth for millions of years, all of the carbon-14 isotope has decayed. Thus when these fuels are combusted they release the stable form of carbon (carbon-12) into the air. These emissions dilute the levels of carbon-14 normally present, which tells us that the increased amount of carbon dioxide in the atmosphere is coming from human activities.
Climate science and science in general, is not like a house of cards and is not based on a single line of evidence. There are many, many lines of evidence and data that collectively point to a single, consistent answer: namely, that rising carbon dioxide levels from fossil fuel burning is the main driver behind global warming.
The data continue to come in. The graph here of "Arctic Sea Ice Extent" is current through March 22, 2011. Sea ice extent normally reaches its maximum in the period from Feb. 18 to March 31. This year the maximum extent, so far, was reached on March 7 and at 5,650,000 square miles is 463,000 square miles below the 1979-2000 average. This reduction in ice extent equates to an area larger than the states of California and Texas combined. That is a lot less ice.