"Several indicators stand out most conspicuously in the picture of surface climate variations and changes in the U.S. over the past century. These include the rather steady increase in precipitation derived from extreme 1-day precipitation events; the systematic decrease in the day-to-day variations of temperature; and the increased frequency of days with precipitation. Trends in other indicators of climate change are now neither sufficiently large nor persistent enough to be considered as strongly suggestive of systematic change, even though it remains a likely explanation. These include the increase of total precipitation and the related increase in cloud amount, as well as an overall increase in mean temperature. The area of the country that has experienced an increase in mean temperature has risen while the proportion of the country with much below normal mean minimum temperatures has decreased. Many of these indicators appear to have undergone significant change during the late 1970s and have more or less remained at these levels to the present. In contrast, other surface climate change indicators (such as the frequency of tropical cyclones) reflect the kind of climatic variability that is completely consistent with the premise of a stable or unchanging climate.
The increase in temperature across the U.S. in this century is slightly smaller, but of comparable magnitude to the increase of temperature that has characterized the world as a whole. The increase in minimum temperature and the related increase in area affected by much above normal minimum temperatures are also found in many other countries of the northern hemisphere. Worldwide precipitation over land has changed little through the twentieth century; increases noted in high latitudes have been balanced by low-latitude decreases. By comparison, the change in precipitation in the U.S. is still relatively moderate compared to some of the increases and decreases at other latitudes. Decreases in the day-to-day differences of temperature observed in the U.S. are also apparent in China and Russia, the only other large countries analyzed as of this date. The persistent increase in the proportion of precipitation derived from extremely heavy precipitation has not been detected in these other countries.
Introduction in Global warming
“Global warming” has been introduced by the scientific community and the media as the term that encompasses all potential changes in climate that result from higher average global temperatures. Hundreds of scientists from many different countries are working to understand global warming and have come to a consensus on several important aspects. In general, Global warming will produce far more profound climatic changes than simply a rise in global temperature. A recent study by an international panel of scientists suggested that if trends in current emissions of greenhouse gases and aerosols continue, the globe may warm by an average of 2°C by the year 2100. The average rate of warming would probably be greater than any seen in the last 10,000 years An analysis of temperature records shows that the Earth has warmed an average of 0.5°C over the past 100 years. This is consistent with predictions of global warming due to an enhanced greenhouse effect and increased aerosols. Yet, it could also be within acceptable limits for natural temperature variation. The twelve warmest years of the twentieth century have occurred since 1980. The Earth’s warmest years since 1861 have been: 1981, 1983, 1987, 1988, 1989, 1990, 1991, 1994, 1995, 1996, 1997 and 1998. 1997 and 1998 were the two warmest years recorded during that period. This lends support to the assumption that the Earth’s climate is warming. However, it may take another decade of continued increases in global temperatures to provide conclusive evidence that the world’s climate is warming as a result of the enhanced greenhouse effect. Global surface air temperature in 1997 was warmer than any previous year this century, marginally exceeding the temperature of 1995. Part of the current global warmth is associated with the tropical El NiЯo, without which a record global temperature would probably not have occurred. Global surface temperatures in 1998 set a new record for the period of instrumental measurements, report NASA/GISS researchers who analyzed data collected from several thousand meteorological stations around the world. The global temperature exceeded that of the previous record year, by such a wide margin that the 1998 calendar year is certain to also set a new record. The United States experienced in 1998 its warmest year in the past several decades. As for the Russia, global surface air temperatures in 1997-98 were not warmer than previous years.
Реферат опубликован: 11/04/2010