Ice cores from Antarctica have kept a record of past atmospheric conditions going back hundreds of thousands of years; shown here are four glacial cycles. Carbon dioxide (CO2), methane (CH4) and temperature always went up and down in step... until the 20th century, when the levels of CO2 and, still more, CH4 soared. There is good reason to expect a temperature rise must follow promptly.

.       Source: Hansen (2005), Climatic Change 68: 269.