Has climate changed over the last century?
Here is a brief overview of how climate has changed over the 20th century, as presented in the latest report from the Intergovernmental Panel on Climate Change (IPCC) which came out a few months ago:

• The global average surface temperature has increased over the 20th century by about 0.6°C. 
• The 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861.  The increase in temperature in the 20th century is likely to have been the largest of any century during the past 1000 years. 
• Nights are warmer (night-time daily minimum air temperatures over land increased by about 0.2°C per decade.  This is about twice the rate of increase in day-time daily maximum air temperatures (0.1°C per decade).
• Snow cover has decreased since the 1960s.  Widespread retreat of mountain glaciers in non-polar region during the 20th century. Sea-ice extent and thickness have decreased.
• Global-average sea level rose between 0.1 and 0.2 meters during the 20th century.  Global ocean heat content has increased since the late 1950s.
• It is very likely that precipitation has increased (0.5-1 percent per decade in the 20th century over most mid- and high-latitudes of the Northern Hemisphere continents, and 0.2-0.3 percent per decade over the tropical (10°N to 10°S) land areas. 
• In the mid- and high-latitudes of the Northern Hemisphere over the latter half of the 20th century, it is likely that there has been a 2 to 4 percent increase in the frequency of heavy precipitation events [thunderstorms and large-scale storm activity].
• It is likely that there has been a 2% increase in cloud cover over mid- and high-latitude land areas during the 20th century.
• In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades.

Are these changes in climate in fact due to changes in the levels of CO2 and other man-made greenhouse gases?  
The main conclusion of the IPCC report is that "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities".  This conclusion is based on:

• Examination of past temperature record over the last 1000 years.  The warming of the past 100 years in very unlikely to be due to internal variability alone, and is unlikely to be due to entirely natural sources.
• Computer models trying to calculate the changes in temperature over past 140 years cannot match the observations by using only natural forcings (volcanic eruptions, solar variability).  The last 50 years can only be explained if increasing levels of greenhouse gases and aerosols are included in the models in addition to the natural forcings. 

With increasing levels of greenhouse gases in the atmosphere, how will climate respond in the future?  
This is one of the hardest questions to address as illustrated in the NOVA video we will be watching in class next week.  The only tools we have to examine this question are computer models of climate.  These climate models must incorporate a description of the atmospheric circulation and composition, oceanic circulation, hydrological cycle, clouds, etc... To look at past and future changes in temperature, these models need to consider: (1) anthropogenic emissions of greenhouse gases and aerosols,  (2) external sources of climate variability such as volcanoes and solar variabiliy, and (3) internal feedback processes that amplify the effects of increasing greenhouse gases, such as cloud-albedo feedbacks, ice-albedo feedbacks, water vapor feedback.  

Current state-of-the art climate models predict the some of the following (again from the IPCC report):

• Increase in global surface temperature by 1.4-5.8C over the period from 1990 to 2100. Larger rate of temperature increase than at any other point over the last 10,000 years.
• Increasing levels of precipitation, and larger annual variability over northern mid- and high-latitudes in winter.
• Increasing frequency of extreme events such as heat waver, high precipiration events, droughts, tropical cyclones.
• Further decreases in Northern Hemisphere snow and ice (snow cover, sea-ice extent, glaciers and ice-caps).
• Increase in global mean sea level by 0.09 to 0.88 meters between 1990-2100, due to thermal expansion of the oceans and melting glaciers and ice caps.

 Last Updated:
11/30/2001