far the natural range of the last 650,000 years (320 to 790 ppb). The primary source of the increase in carbon dioxide is fossil fuel use, but land-use changes also make a contribution. The primary source of the increase in methane is very likely to be a combination of human agricultural activities and fossil fuel use. How much each contributes is not well determined. Nitrous oxide concentrations have risen from a pre-industrial value of 270 ppb to a 2005 value of 319 ppb. More than a third of this rise is due to human activity, primarily agriculture. Warming of the planet Cold days, cold nights, and frost events have become less frequent. Hot days, hot nights, and heat waves have become more frequent. Additionally: Eleven of the twelve years in the period (1995–2006) rank among the top 12 warmest years in the instrumental record (since 1880). Warming in the last 100 years has caused about a 0.74 °C increase in global average temperature. This is up from the 0.6 °C increase in the 100 years prior to the Third Assessment Report. Urban heat island effects were determined to have negligible influence (less than 0.0006 °C per decade over land and zero over oceans) on these measurements. Observations since 1961 show that the ocean has been absorbing more than 80% of the heat added to the climate system, and that ocean temperatures have increased to depths of at least 3000 m (9800 ft). "Average Arctic temperatures increased at almost twice the global average rate in the past 100 years." It is likely that greenhouse gases would have caused more warming than we have observed if not for the cooling effects of volcanic and human-caused aerosols. See global dimming. Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years (including both the Medieval Warm Period and the Little Ice Age). Ice, snow, permafrost, rain, and the oceans The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drought and heavy precipitation events. Additionally: "Mountain glaciers and snow cover have declined on average in both hemispheres." Losses from the land-based ice sheets of Greenland and Antarctica have very likely (>90%) contributed to sea level rise between 1993 and 2003. Ocean