Global Warming and Climate Change

Averaged over all land and ocean surfaces, temperatures on Earth have warmed about 1.35°F (0.75ºC) over the last century. Much of this warming-about 0.72°F (0.4°C)-has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 0.7ºC or 1.26°F since 1979), especially over the Northern Hemisphere.

Global warming is thus the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades and its projected continuation. Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. The uncertainty in this range results from both the difficulty of predicting the amount of future greenhouse gas emissions and uncertainties regarding climate sensitivity.

This relatively small change in temperature may seem unremarkable or even insignificant, but in actuality it doesn't take much to make an impact (think of how an increase of a few degrees in human body temperature can lead to fever and serious repercussions if the temperature rise goes unchecked). Even a change of less than one degree Celsius is enough to cause changes in weather patterns, rainfall and sea levels. And it is worth noting that global warming doesn't simply mean that it gets hotter everywhere; in fact, disruptions to weather patterns could mean that normally warm regions experience unprecedented cold or snow, while normally cold climates have green Christmases and a much shorter ski season than they're used to.

An increase in global temperatures can cause other changes, including a rising sea level and changes in the amount and pattern of precipitation. These changes may increase the frequency and intensity of extreme weather events, such as floods, droughts, heat waves, hurricanes, and tornados. Other consequences include higher or lower agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors. Some scientists see phenomena such as an earlier spring arrival, plant and animal range shifts and population changes and coral reef bleaching as harbingers of impacts likely to become more frequent and widespread with continued warming.

Recent Observations

Here are some key points and observations in global temperature and weather activity in the past decade, according to the BBC Weather Center:

• Four out of five of the warmest years ever recorded were in the 1990s
• The 1990s was the warmest decade of the last millennium with 1998 being the warmest year globally since records began in 1861
• Sea levels are rising globally, arctic sea ice is thinning and rainfall is becoming heavier in some parts of the world; average global sea levels have increased by between 0.1 and 0.2 meters over the last 100 years
• Snow cover in the Northern Hemisphere has declined since the late 1960s by around 10%; mountain glaciers in non polar regions have retreated significantly during the 20th century
• In Asia and Africa there has been an increased frequency and intensity of droughts in the last few decades