1) Blackbody radiation feedback results in a significant increase in energy output for a rise in temperature (T^4), it is a negative feedback.
2) a and d are correct. The scatter incoming sunlight, and impact cloud chemistry. They also somewhat block outgoing IR radiation so can act as a greenhouse gas, but they do not preferentially scatter that back into space, and their overall impact is a negative forcing.
3) c is not a true statement about thermohaline circulations. Salty water is denser than fresh water.
4) 2.5 degrees including the effects of feedbacks
5) The percentage rise in CO2 concentrations is 42% (280ppm to 400ppm). This question was somewhat ambiguously phased as CO2 emissions rather than CO2 concentrations, although the vast majority of people got it right. We will investigate if any mark changes are necessary.
8) a) Models predicting temperatures over recent history do not have uncertainties in CO2 levels which are well constrained (and changing by only modest ammounts) over these time periods.