Global Warming : Science of Global Warming Has the Climate Changed Already?

Historical records show that the climate has changed in the last one hundred years. Not only are long-term averages changing, but the shorter-term variability in climate, from year to year and decade to decade, is also beginning to display changing patterns. Among the established trends and changes are the following:

Temperature increase. Global mean surface temperature has increased about 1.1°F (0.6°C) since the beginning of the 20th century, with night-time minimums increasing more than day-time maximums. While the warming record shows significant spatial and temporal variability, the global upward trend is unambiguous. Most of the warming in
All stations/trends displayed regardless of statistical significance

Source: National Climatic Data Center/NESDIS/NOAA

* Source: National Climatic Data Center NESDIS/NOAA

Green = increasing, Brown = decreasing
All stations/trends displayed regardless of statistical significance

Glaciers. There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century. Northern hemisphere sea-ice extent has decreased 10-15% since the 1950s, and Arctic summer sea-ice thickness is likely to have declined by 40%.

Sea-level rise. Global sea level has risen between 3.9 and 10 in. (10 - 25 cm) largely due to thermal expansion of the oceans, and to a lesser extent due to the melting of land-based glaciers. The rate of sea-level rise during the 20th century was about 10 times higher than the average rate during the last 3,000 years. Global ocean heat content has also increased since the late 1950s.

El Niño. More frequent, persistent and intense El Niños have been observed in recent decades. The persistent 1990 to mid-1995 warm phase of the El Niño-Southern Oscillation event was exceptional in the 120-year record of the phenomenon. The 1997 El Niño event appears to have been the strongest on record. While scientists cannot yet say with certainty that this observed change is due to human activity, this observation is consistent with some climate model predictions. With climate change, they suggest a warming pattern in the Pacific ocean similar to what occurs during an El Niño event now. Thus, against this background warming, future El Niño events might occur more frequently and be more intense.

In considering the direct and indirect historical record of climate observations from across the globe, the IPCC concluded in 2001, "an increasing body of observations gives a collective picture of a warming world."