Someone of your class says, that climate change is a natural process and that we pay too much attention on that problem. This is what Climate Change 2007, the Fourth Assessment Report (AR4) of the United Nations Intergovernmental Panel on Climate Change (IPCCv) says (The report is the largest and most detailed summary of the climate change situation ever undertaken, involving thousands of authors from dozens of countries): "Warming of the climate system is unequivocal (=eindeutig)." "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." "Carbon dioxide, methane, and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values." The amount of carbon dioxide and methane in the atmosphere in 2005 exceeds by far the natural range of the last 650,000 years. 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. Cold days, cold nights, and frost events have become less frequent. Hot days, hot nights, and heat waves have become more frequent. Eleven of the twelve years in the period (1995-2006) rank among the top 12 warmest years in the instrumental record (since 1850, towards the end of the Little Ice Age). 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. "Average Arctic temperatures increased at almost twice the global average rate in the past 100 years." The SPM documents increases in wind intensity, decline of permafrost coverage, and increases of both drought and heavy precipitation events. "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 warming causes seawater to expand, which contributes to sea level rising. Sea level rose at an average rate of about 1.8 mm/year during the years 1961-2003. The rise in sea level during 1993-2003 was at an average rate of 3.1 mm/year. It is not clear whether this is a long-term trend or just variability.