The zero line represents the long term mean temperature from 1880-2001, and the red and blue bars are showing annual departures from that mean.
As is evident in the graph, 2001 was second only to 1998 in terms of global temperature, and the trend has been toward increasing temperatures at least since the beginning of the 20th century. Land temperatures have greater anomalies than the ocean, which is to be expected since land heats up and cools down faster than water.
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