On the issue of global warming, I am always on the lookout for simple explanations from the scientific community that can be used to communicate to hoi polloi why global warming is occurring primarily because of human activity (especially emissions of heat trapping greenhouse gases like carbon dioxide - CO2).
RealClimate is of course a good site to start to look for such arguments (after you read through the scientific details :-)).
A recent post by Corinne Le Quéré of the University of East Anglia is worth reading because she tries to explain why we know global warming comes from human activity in a simple manner:
Read the whole post for details.
This question keeps coming back, although we know the answer very well: all of the recent CO2 increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming. There is a lot of evidence to support this statement which has been explained in a previous posting here and in a letter in Physics Today . However, the most convincing arguments for scientists (based on isotopes and oxygen decreases in the atmosphere) may be hard to understand for the general public because they require a high level of scientific knowledge. I present simpler evidence of the same statement based on ocean observations, and I explain how we know that not only part of the atmospheric CO2 increase is due to human activities, but all of it.
On time-scales of ~100 years, there are only two reservoirs that can naturally exchange large quantities of CO2 with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon (carbon is the "C" in CO2) must be conserved. If the atmospheric CO2 increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs.
In summary, we know that the rise in atmospheric CO2 is entirely caused by fossil fuel burning and deforestation because many independent observations show that the carbon content has also increased in both the oceans and the land biosphere (after deforestation). If the oceans or land had contributed to the rise in atmospheric CO2, they would hold less carbon. Their response to warming may be real, but it is less than their response to increasing CO2 and other climate changes for the moment.