From the Royal Society, a fellowship of the world's most eminent scientists, the oldest scientific academy in existence.
Aspects of climate change on which there is wide agreement:
The Royal Society Climate change: a summary of the science I September 2010 I 5
Changes in atmospheric composition
25. Global-average CO2 concentrations have been observed to increase from levels of around 280 parts per million (ppm) in the mid-19th century to around 388 ppm by the
end of 2009. CO2 concentrations can be measured in “ancient air” trapped in bubbles in
ice, deep below the surface in Antarctica and Greenland; these show that present-day
concentrations are higher than any that have been observed in the past 800,000 years,
when CO2 varied between about 180 and 300 ppm. Various lines of evidence point
strongly to human activity being the main reason for the recent increase, mainly due to
the burning of fossil fuels (coal, oil, gas) with smaller contributions from land-use
changes and cement manufacture. The evidence includes the consistency between
calculations of the emitted CO2 and that expected to have accumulated in the
atmosphere, the analysis of the proportions of different CO2 isotopes, and the amount
of oxygen in the air.
26. These observations show that about half of the CO2 emitted by human activity since the
industrial revolution has remained in the atmosphere. The remainder has been taken up
by the oceans, soils and plants although the exact amount going to each of these
individually is less well known.
27. Concentrations of many other greenhouse gases have increased. The concentration of
methane has more than doubled in the past 150 years; this recent and rapid increase is
unprecedented in the 800,000 year record and evidence strongly suggests that it arises
mainly as a result of human activity.
http://royalsociety.org/climate-change-summary-of-science/