Much debate in the last five years about the greenhouse effect has centered on interpreting temperature numbers generated at weather stations all over the world. The data from these thermometers are averaged and plotted in attempts to determine just how fast the earth has heated up since the measurements began. There is now no doubt the world is getting warmer. The thermometers show that the world is warmer now than at any time since the measurements started. The year 1990 was the hottest year in the last century. Together with 1991, the years of 1983, 1987, 1988, and 1989, have been measured to be the warmest 6 years in the last hundred years. 1991 was the second warmest year of the past century, perhaps due to the eruption of Mt. Pinatubo during that year. The ash from the volcano in the upper atmosphere blocks some sunlight to earth, and is expected to generate a temporary two or three year cooling effect. After that time, most ash particles will have settled back to earth, and most scientists expect to see the global warming trend continue.
According to scientists, we can with "99% confidence conclude that current temperatures represent a real warming trend rather than a chance fluctuation over the 30-year period." Most scientists agree that the planet's temperature has risen 0.5 degrees Celsius since 1900, and will continue to increase at an increasing rate. The environment is responding to this warming. For instance, a study of mountain plants in the Alps (Europe), shows that some cold-loving plants are starting to move to higher and cooler altitudes. That is a possible response to increasing temperatures.
CURRENT TRENDS.
The global effects of the greenhouse effect cannot be directly predicted simply because we do not have enough knowledge in the subject. However, we have been able to draw direct connections between certain natural phenomenon that supports the idea that something is changing.
Global warming has great effect on crops and weather conditions around the world. The northern hemisphere contains more land area than the southern hemisphere, and conversely, a lower percentage of the world's oceans. Since oceans absorb more heat than land areas, it is not surprising that most climate models predict faster heating over the northern hemisphere than the global average. In addition, models predict faster temperature increases at higher latitudes. If global warming trends continue, high temperatures everywhere in the US may reduce US agricultural productivity. Northern continental areas are projected to have drier summer soils, due in part to earlier snow melts in the spring, and hotter, more cloudless summers, causing extensive evaporation of ground moisture. In addition, if the inland areas of the northern hemisphere are expected to receive less moisture, then, lake and river levels will be lower. Some reports predict the level of the Great Lakes will drop between 2 and 8 feet. River flows in the western US may be very vulnerable to increase temperatures expected as result of the greenhouse effect.
When many people think of global warming, their first concern is the possible rise of sea levels. With a large number of the world's cities in coastal areas, this is a significant problem. There are two major causes of rising sea levels. First, extra water is produced when ice melts. Secondly, the natural expansion of sea water as it becomes warmer. The range of sea ice around both poles continues to shrink, as it melts. Even with the level of greenhouse gases present today, the earth may warm enough in the next 50 years or so to completely melt the sea ice located on the poles.
Damage from rising seas is very diverse. Buildings and roads close to the water could be flooded and they could suffer damage from hurricanes and tropical storms. "There are good physical reasons to suggest that more intense storms (hurricanes) could result from global warming." Warmer oceans cause more intense storms. Experts believe that global warming could increase the intensity of hurricanes by over 50 percent. Hurricane Andrew's devastation in 1992 set new records. According to the National Hurricane Center in Miami, the 1990 season was the most active year on their records for combined Atlantic and Pacific hurricanes. Damage caused by future hurricanes to populated areas will be more severe since higher sea levels are predicted for the next century. In addition, as the sea rises, beach erosion takes place, particularly on steep banks. Wetlands are lost as sea levels rise. Another serious problem is the threat of salt water intruding into underground fresh water reserves in coastal areas.
In 1992, a report was published by the United Nations, which proposes that if CO2 and other greenhouse gas emissions continue with present trends (which is the case), the coastal plains of Bangladesh and the Netherlands will flood by the year 2100. Furthermore, the islands of the Maldives would completely disappear. This would happen if only a two foot increase in sea level occured.
Rate of climate warming.
since 2001 and the 10 warmGlobal surface temperatures have increased about 0.74°C (plus or minus 0.18°C) since the late-19th century, and the linear trend for the past 50 years of 0.13°C (plus or minus 0.03°C) per decade is nearly twice that for the past 100 years. The warming has not been globally uniform. Some areas (including parts of the southeastern U.S. and parts of the North Atlantic) have, in fact, cooled slightly over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Lastly, seven of the eight warmest years on record have occurred est years have all occurred since 1995.
An enhanced greenhouse effect is expected to cause cooling in higher parts of the atmosphere because the increased "blanketing" effect in the lower atmosphere holds in more heat, allowing less to reach the upper atmosphere. Cooling of the lower stratosphere (about 49,000-79,500 ft.) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger in the radiosonde data likely due to uncorrected errors in the radiosonde data.
Relatively cool surface and tropospheric temperatures, and a relatively warmer lower stratosphere, were observed in 1992 and 1993, following the 1991 eruption of Mt. Pinatubo. The warming reappeared in 1994. A dramatic global warming, at least partly associated with the record El Niño, took place in 1998. This warming episode is reflected from the surface to the top of the troposphere.
There has been a general, but not global, tendency toward reduced diurnal temperature range (DTR: the difference between daily high or maximum and daily low or minimum temperatures) over about 70% of the global land mass since the middle of the 20th century. However, for the period 1979-2005 the DTR shows no trend since the trend in both maximum and minimum temperatures for the same period are virtually identical; both showing a strong warming signal. A variety of factors likely contribute to this change in DTR, particularly on a regional and local basis, including changes in cloud cover, atmospheric water vapor, land use and urban effects.
Indirect indicators of warming such as borehole temperatures, snow cover, and glacier recession data, are in substantial agreement with the more direct indicators of recent warmth. Evidence such as changes in glacial mass balance (the amount of snow and ice contained in a glacier) is useful since it not only provides qualitative support for existing meteorological data, but glaciers often exist in places too remote to support meteorological stations. The records of glacial advance and retreat often extend back further than weather station records, and glaciers are usually at much higher altitudes than weather stations, allowing scientists more insight into temperature changes higher in the atmosphere.
Large-scale measurements of sea-ice have only been possible since the satellite era, but through looking at a number of different satellite estimates, it has been determined that September Arctic sea ice has decreased between 1973 and 2007 at a rate of about -10% +/- 0.3% per decade. Sea ice extent for September for 2007 was by far the lowest on record at 4.28 million square kilometers, eclipsing the previous record low sea ice extent by 23%. Sea ice in the Antarctic has shown very little trend over the same period, or even a slight increase since 1979. Though extending the Antarctic sea-ice record back in time is more difficult due to the lack of direct observations in this part of the world.
Are El Niños related to Global Warming?
El Niños are not caused by global warming. Clear evidence exists from a variety of sources (including archaeological studies) that El Niños have been present for thousands, and some indicators suggest maybe millions, of years. However, it has been hypothesized that warmer global sea surface temperatures can enhance the El Niño phenomenon, and it is also true that El Niños have been more frequent and intense in recent decades. Whether El Niño occurrence changes with climate change is a major research question.
Is the climate becoming more variable or extreme?
Examination of changes in climate extremes requires long-term daily or even hourly data sets which until recently have been scarce for many parts of the globe. However these data sets have become more widely available allowing research into changes in temperature and precipitation extremes on global and regional scales. Global changes in temperature extremes include decreases in the number of unusually cold days and nights and increases in the number of unusually warm days and nights. Other observed changes include lengthening of the growing season, and decreases in the number of frost days.
Global temperature extremes have been found to exhibit no significant trend in interannual variability, but several studies suggest a significant decrease in intra-annual variability. There has been a clear trend to fewer extremely low minimum temperatures in several widely-separated areas in recent decades. Widespread significant changes in extreme high temperature events have not been observed. There is some indication of a decrease in day-to-day temperature variability in recent decades.
In areas where a drought or excessive wetness usually accompanies an El Niño or La Niña, these dry or wet spells have been more intense in recent years. Further, there is some evidence for increasing drought worldwide, however in the U.S. there is no evidence for increasing drought.In some areas where overall precipitation has increased (ie. the mid-high northern latitudes), there is evidence of increases in the heavy and extreme precipitation events. Even in areas such as eastern Asia, it has been found that extreme precipitation events have increased despite total precipitation remaining constant or even decreasing somewhat. This is related to a decrease in the frequency of precipitation in this region.
Many individual studies of various regions show that extra-tropical cyclone activity seems to have generally increased over the last half of the 20th century in the northern hemisphere, but decreased in the southern hemisphere. Furthermore, hurricane activity in the Atlantic has shown an increase in number since 1970 with a peak in 2005. It is not clear whether these trends are multi-decadal fluctuations or part of a longer-term trend.
Rising of sea level by increasing global warming.
Global mean sea level has been rising at an average rate of 1.7 mm/year (plus or minus 0.5mm) over the past 100 years, which is significantly larger than the rate averaged over the last several thousand years. Depending on which greenhouse gas increase scenario is used (high or low) projected sea-level rise is projected to be anywhere from 0.18 (low greenhouse gas increase) to 0.59 meters for the highest greenhouse gas increase scenario. However, this increase is due mainly to thermal expansion and contributions from melting alpine glaciers, and does not include any potential contributions from melting ice sheets in Greenland or Antarctica. Larger increases cannot be excluded but our current understanding of ice sheet dynamics renders uncertainties too large to be able to assess the likelihood of large-scale melting of these ice sheets.
No comments:
Post a Comment