Human activities can alter the climate system by changing the chemistry of the atmosphere and the character of the earth's surface.  Weather records over the last 100 years or so tell us that both global and regional climate has changed.  Scientists are also able to reconstruct climate from hundreds to thousands of years ago (known as paleoclimate) - many of these studies indicate that climate changes since the Industrial Revolution are unusual. 

Temperature (Global):  Global surface temperatures (land and ocean) are approaching a 1 degree Celsius increase since the year 1880.  (If that doesn't sound like much, consider that global temperature during the last ice age, when much of the United States was covered with ice sheets, was just 5 degrees below average.)  And there is evidence that the warming has recently accelerated:  the five warmest years on record are 1998, 2002, 2003, 2004 and  2001 respectively. 

source:  National Climatic Data Center, Ashville, NC

Temperature (United States).  According to the National Climate Data Center, "the last five 5-year periods (2000-2004, 1999-2003, 1998-2002, 1997-2001, 1996-2000), were the warmest 5-year periods (i.e. pentads) in the last 110 years of national records, illustrating the anomalous warmth of the last decade" for the United States.

Precipitation (Global).  As the atmosphere warms, evaporation rates increase and the hydrological cycle becomes more active.  Therefore, precipitation is expected to increase, as is precipitation variability (floods and droughts).   So far, it is estimated that precipitation has increased 0.5 to 1% in the northern hemisphere.  During the 20th century, cloud cover has also increased by about 2% over land areas in the mid and high latitudes. 

Snow Cover and Ice.  Changes in snow and ice cover are a response to increased temperatures.  Nearly allmountain glaciers have retreated over the last 100 years, and overall snow cover has declined about 10% since the 1960's.  Warm season sea ice has decreased by 10 to 15% since the 1950's, while summer Arctic sea ice has shrunk by nearly 40%.  Finally, a widespread decrease in the number of frost days (mornings that dip below freezing) has been reported over the United States.  More information on changes in snow and ice cover is available from the National Snow and Ice Data Center in Boulder, CO. 

Sea level.  Global average sea level has risen by nearly .2 meters over the last 100 years.  Due to thermal expansion and melting ice caps, sea level is predicted to rise between 0.09 and 0.88 meters by the year 2100, partly depending on future greenhouse gas emissions.  Visit the United States Geological Survey's excellent summary on sea level and climate.  

Oceans.  The global oceans have absorbed a large amount of heat over the last century, increasing in temperature between .1 and .5 degrees.  This excess heat, which will eventually be transferred to the atmosphere, may account for a lag in the global warming response to increased greenhouse gases.  It will also contribute to future sea level rise for decades to come.  More on ocean warming here and here.

Extremes.  Climate scientists have reported increases in the number of very hot days over global land areas,  more intense precipitation events, increased minimum (nighttime) temperatures, and a decrease in the number of very cold days.  There no evidence to date of increases in hurricane, storm and tornado activity.  Click here for excellent information on changes in weather extremes in the United States. 

Past Climates.  Clues about past weather conditions can be retrieved from tree-ring growth cycles, from the chemistry of sediments buried deep below oceans and lakes, and from air bubbles trapped within glaciers.  The IPCC(2001) reports that "the magnitude of the warming over the 20th century is likely to have been the largest of any century in the last 1000 years".  Paleoclimate studies also show that the climate system is capable of very rapid change, particularly during transition periods between ice ages.  

National Climatic Data Center

The NCDC's climate monitoring page has excellent, up-to-date information on both global and U.S. climate

Climate Hot Map

Early warning signs and fingerprints of global warming

Climate Diagnostics Center

Understanding how climate varies and changes

National Academies Report on Climate Change

Top U.S. scientists review the issue

IPCC Climate Change 2001: The Scientific Basis  The latest summary report on observed and predicted climate changes from the Intergovernmental Panel on Climate Change (they release comprehensive reports on all climate research about every 5 or 6 years).