I think it only fitting to repeat Joshua Feldman's posts on the evidence.
1) The Earth is warming - as corroborated by a host of evidence.
2) This warming is caused by greenhouse gases, as supported by a wide constellation of evidence.
3) The greenhouse gas that has changed is CO2 and human fossil fuel emissions are driving the atmospheric CO2 concentration increase according to carbon isotope evidence.
I'll introduce each of these points with a separate post, starting with 1 below:
1) We have a conclusive array of evidence that the Earth is warming. This evidence comes from a range of direct measurements, but is also corroborated by a host of other phenomena. On the measurement side, we have ocean surface, ocean depth (XBT & buoy) data sets, surface land measurements, and satellite data sets (such as the MSU data sets from RSS & UAH). All these data sets are in agreement. On the physical phenomena side we have hundreds of studies documenting the global retreat of the cryosphere (ice caps, glaciers, sea ice, and ice shelves). These studies feature a number of high profile satellite studies showing overall ice mass loss in southern and peripheral Greenland, the Antarctic Peninsula and Western Antarctica. There is excellent satellite data showing a precipitous loss in Arctic sea ice extent. Finally there are thousands of studies from the field of biology documenting seasonal ecosystem changes modifying such things as animal and fish migration patterns and timing, timing of spring blooms, autumn seed bearing, and other biospheric phenomena that corroborate climate. All of these streams of data corroborate each other and tell a consistent story - the Earth is warming.
Surface Temps (instrument record):
Satellite microwave sounding unit temperature data sets:
Sea ice extent data sets:
Sea ice volume (area times thickness):
Antarctic losing ice mass:
Greenland losing ice mass:
Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE - I. Velicogna
Retreat of glaciers:
Corroborating evidence of changes to the natural world (the field of "phenology") - see 3rd post down from here.