From rising air and ocean temperatures to shrinking glaciers and widespread melting of snow and ice, evidence of a changing global climate is all around us. The Intergovernmental Panel on Climate Change has linked many of these changes in climate to an increase greenhouse gas emissions from human activities, which was documented in a 2007 assessment report compiled by thousands of scientists over decades of research and debate.
Regardless of their causes – whether you believe in anthropogenic drivers, like fossil fuels from power plants and cars or not — the observed changes in climate are scientific facts that have grave implications for the future of natural and human systems.
Global temperature trends estimated by four different research groups all show a warming of the Earth over the past century, with particularly rapid increases over the past few decades.
Since 1901, global average surface temperatures have risen at an average rate of 0.13ºF per decade. The United States has warmed at nearly twice the global rate since the 1970s.
The years between 1995 and 2006 rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).
Average Arctic temperatures have increased at almost twice the global average rate in the past 100 years.
Since 1978, the Arctic Sea ice shrunk on average 2.7% per decade, with larger decreases in the summer of about 7.4%. Arctic summer sea ice has decreased by roughly 34% since 1979.
Satellite images show the three lowest minimum extents of Arctic Sea ice were reached in 2007, 2008 and 2009. The 2007 total reflected a loss of 490,000 square miles of sea ice—an area larger than Texas and California combined. The Arctic summer could be ice-free by mid-century, according to a study by the National Oceanic and Atmospheric Administration.
Since 1960, glaciers worldwide have lost more than 2,000 cubic miles of water, contributing to observed changes in sea level rise.
Since 1900, seasonally frozen ground has decreased by 7% (up to 15% in the spring) in the Northern Hemisphere. The time that lakes stay frozen has generally decreased at an average rate of one to two days per decade since the mid-1800s.
The portion of North America covered by snow has generally decreased since 1972. The average extent for the 1970s (1972 to 1979) was 3.43 million square miles, compared with 3.3 million for the 1980s, 3.21 million for the 1990s, and 3.18 million from 2000 to 2008.
Since 1901, global precipitation has increased at an average rate of 1.9% per century (specifically in eastern parts of North and South America, northern Europe, and northern and central Asia), while precipitation in the lower 48 states has increased at a rate of 6.4% per century. Climate change will also cause some areas to experience decreased precipitation such as in the Sahel, the Mediterranean, southern Africa, and parts of southern Asia.
Sea surface temperatures have been higher during the past three decades than at any other time since large-scale measurement began in the late 1800s. From 1901 through 2009, sea surface temperatures rose at an average rate of 0.12 degrees per decade. Over the last 30 years, temperatures have risen more quickly at a rate of 0.21 degrees per decade.
Three different studies show that ocean heat content (the amount of energy the ocean absorbs) has increased substantially since 1955. Ocean heat content not only determines sea surface temperature, but also affects sea level and currents.
Between 1993 and 2005 sea level rose, on average, 3mm (0.1 inches) per year, attributed to an increase in melting ice and thermal expansion as the ocean absorbs excess energy.
During the 20th century, sea level rose an average of 7 inches after 2,000 years of relatively little change. The 2007 IPCC report conservatively predicts that sea levels could rise 10 to 23 inches by 2100 if current warming patterns continue.
Intensity of cyclones (hurricanes, tropical storms, and other intense rotating storm) has risen noticeably over the past 20 years. Six of the 10 most active years have occurred since the mid-1990s, likely caused by increasing sea surface temperatures.
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