Since 1750, the atmospheric concentration of carbon dioxide has increased by about 32% (from about 280 to 376 parts per million in 2003), primarily due to the combustion of fossil fuels and land use changes. Approximately 60% of that increase (60 parts per million) has taken place since 1959.

Data: Millennium Ecosystem Assessment, 2005. Ecosystems and Human Well-being: Synthesis.
Island Press, Washington, DC.

NOAA (April 2, 2009) released new information regarding Arctic sea ice based on research at the University of Washington and the Joint Institute for the Study of the Atmosphere and Ocean in Seattle-

-Arctic summers may well be ice-free in 30 years, an update to previously released projections that such an event would occur by the end of the century.

- Summer sea ice is expected to decline from its current 4.6 million square kilometers (about 1.8 million square miles) to about 1 million square kilometers (about 390,000 square miles)

-This represents a loss of an area two-fifths the size of the continental U.S.