Even though the global climate has been changing for billions of years by natural causes, it is clear that human activities are also presently changing the global climate, primarily through the introduction of greenhouse gases such as CO2 into the atmosphere while cutting down tropical rain forests that act as sinks for the CO2 and put oxygen back into the atmosphere. The time scale of observation of these human, or anthropogenic, changes is short, but the effect is clear, with a nearly one degree change in global temperature measured for the past few decades. The increase in temperature will lead to more water vapor in the atmosphere, and since water vapor is a greenhouse gas, this will lead to a further increase in temperature. Many computer-based climate models are attempting to predict how much global temperatures will rise as a consequence of anthropogenic input of gases to the atmosphere and what effects this temperature rise will have on melting of the ice sheets, sea level rise, and greenhouse temperature rise.
Climate changes are difficult to measure, partly because the instrumental and observational records go back only a couple of hundred years in Europe. From these records, global temperatures have risen about one degree since 1850, most notably between 1890 and 1940, and again since 1970. This variation, however, is small compared to some of the other variations induced by natural causes, and some scientists argue that it is difficult to separate anthropogenic effects from the background natural variations. Rainfall patterns have also changed in the past 50 years, with declining rainfall totals over low latitudes in the Northern Hemisphere, especially in the Sahel, which has experienced major droughts and famine, and in the Mediterranean, United States southwest, and Gobi Desert. However, high-latitude precipitation has increased in the same time period. These patterns all relate to a general warming and shifting of the global climate zones to the north.
Research by Dr. Zaito Pan's group at St. Louis University has shown that with global warming, some regions known as warming holes will actually become wetter and slightly colder than at present. In the United States, the central plains centered on the Missouri River Basin represent a warming hole, formed by the interaction between the convergence of water vapor leading to increased rainfall, accumulation of water in thick soils, and evaporation enhancing cooling. Other warming holes are predicted to grow in eastern China around Beijing and over the Amazon Basin of South America.
The increased precipitation is significant; for instance, a 21 percent increase in precipitation for the Missouri/Mississippi River basins is predicted in the central United States region, along with an alarming 51 percent increase in the amount of water flowing through the major rivers. River flood levels will be higher and floods will be more frequent and potentially devastating. The consequences of these climate changes are not appreciated and desperately need to be understood by the people who live in the region, developers, insurance underwriters, urban planners, politicians, and the federal government. \_/
Some of the warming caused by increases in greenhouse gases may be counteracted by an increase in aerosols, small airborne solid or liquid particles, which may have a cooling effect. Aerosols include particles such as sulphate, organic carbon, black carbon, nitrates, and dust. As the climate warms, more and more dust is being picked up from regions that are undergoing increased aridness and desertification such as the fringes of the Gobi and Sahara Deserts. This dust gets emplaced high into the atmosphere where it may reside some time and may actually have a small cooling effect.
comparison of Short-term climate changes with the Medium-term paleoclimate Record
It can be very difficult and complex to separate the effects of short-term human-induced climate changes from natural variations on longer-term time scales. The present day global warming is unusual for the climate record of the past 1,300 years, but has counterparts induced by natural causes about 125,000 years ago and in the older geological record. The
last time (125,000 years ago) climates warmed as significantly as the planet is now experiencing, loss of polar ice led to sea level rise of 13-20 feet (4-6 m), suggesting that the world's coastlines are in grave danger of moving inland to higher ground. Ice core data show that temperatures in Greenland were 4-7°F (3-5°C) hotter than present, a level that many models predict will be reached by the end of this century. The last 50 years appear to be the hottest in the past 1,300 years, but significant fluctuations have occurred.
The measured increases in anthropogenic greenhouse gases can more than account for the measured temperature rise of the surface of Earth in the past 50-100 years. The less-than-expected warming is probably related to lowering of the temperature by aerosols from volcanic eruptions and dust from desert environments. These measurements strongly suggest that the present-day global warming is being forced by the human-induced injection of greenhouse gases into the atmosphere, not by other long-term climate-forcing mechanisms that have controlled other global warming and cooling events in past geological times.
The measured surface warming is nearly global in scale, with the exception being Antarctica, which is sheltered from parts of the global atmosphere/ocean system. Climate models are consistent with the global warming being produced by anthropogenic causes. Many local variations exist, such as "warming holes," where local atmospheric effects are stronger than the global changes (see sidebar on page 51).
The global warming is also likely affecting wind patterns, the most extreme hot and cold nights, extratropical storm patterns, and causing an increase in heat waves. Effects are stronger in the Northern than in the Southern Hemisphere.
Global sea levels are currently rising as a result of the melting of the Greenland and Antarctica ice sheets and thermal expansion of the world's ocean waters due to global warming. We are presently in an interglacial stage of an ice age, and sea levels have risen nearly 400 feet (130 m) since the last glacial maximum 20,000 years ago, and about six inches (15.25 cm) in the past 100 years. The rate of sea level rise seems to be accelerating and may presently be as much as an inch (2.5 cm) every 8-10 years. If all the ice on both ice sheets were to melt, global sea levels would rise by 230 feet (70 m), inundating most of the world's major cities and submerging large parts of the continents under shallow seas.
The coastal regions of the world are densely populated and are experiencing rapid population growth. Approximately 100 million people presently live within three feet (1 m) of the present day sea level. If sea level were to rise rapidly and significantly, the world would experience an economic and social disaster of a magnitude not yet experienced by the civilized world. Many areas would become permanently (on human time scales) flooded or subject to inundation by storms, beach erosion would be accelerated, and water tables would rise.
The Greenland and Antarctic ice sheets have some significant differences that cause them to respond differently to changes in air and water temperatures. The Antarctic ice sheet is about ten times as large as the Greenland ice sheet, and since it sits on the South Pole, Antarctica dominates its own climate. The surrounding ocean is cold even during summer, and much of Antarctica is a cold desert with low precipitation rates and high evaporation potential. Most meltwater in Antarctica seeps into underlying snow and simply refreezes, with little running off into the sea. Antarctica hosts several large ice shelves fed by glaciers moving at rates of up to a thousand feet (300 m) per year. Most ice loss in Antarctica is accomplished through calving and basal melting of the ice shelves, at rates of about 10-15 inches (25-38 cm) per year.
In contrast, Greenland's climate is influenced by warm North Atlantic currents and its proximity to other land masses. Climate data measured from ice cores taken from the top of the Greenland ice cap show that temperatures have varied significantly in cycles of years to decades. Greenland also experiences significant summer melting, abundant snowfall, has few ice shelves, and its glaciers move quickly at rates of up to miles (several km) per year. These fast-moving glaciers are able to drain a large amount of ice from Greenland in relatively short amounts of time.
The Greenland ice sheet is thinning rapidly along its edges, losing an average of 15-20 feet (4.5-6 m) in the past decade. In addition, tidewater glaciers and the small ice shelves in Greenland are melting an order of magnitude faster than the Antarctic ice sheets, with rates of melting between 25-65 feet (7.6-20 m) per year. About half of the ice lost from Greenland is through surface melting that runs off into the sea. The other half of ice loss is through calving of outlet glaciers and melting along the tidewater glaciers and ice shelf bases. If just the Greenland ice sheet melts, the water released will contribute another 23 feet (7 m) to sea level rise, to a level not seen since 125,000 years ago.
These differences between the Greenland and Antarctic ice sheets lead them to play different roles in global sea level rise. Greenland contributes more to the rapid short-term fluctuations in sea level, responding to short-term changes in climate. In contrast, most of the world's water available for raising sea level is locked up in the slowly changing Antarctic ice sheet. Antarctica contributes more to the gradual, long-term sea level rise.
Data released by the Intergovernmental Panel on Climate Change in 2007 points clearly to a cause of the recent melting of the glaciers. Most data suggests that the current melting is largely the result of the recent warming of the planet in the past 100 years through the effects of greenhouse warming. Greenhouse gases have been increasing at a rate of more than 0.2 percent per year, and global temperatures are rising accordingly. The most significant contributor to the greenhouse gas buildup is CO2, produced mainly by the burning of fossil fuels. Other gases that contribute to greenhouse warming include carbon monoxide, nitrogen oxides, methane (CH4), ozone (O3) and chlorofluorocar-bons. Methane is produced by gas from grazing animals and termites, whereas nitrogen oxides are increasing because of the increased use of fertilizers and automobiles, and the chlorofluorocarbons are increasing from release from aerosols and refrigerants. Together the greenhouse gases have the effect of allowing short-wavelength incoming solar radiation to penetrate the gas in the upper atmosphere, but trapping the solar radiation after it is re-emitted from Earth in a longer wavelength form. The trapped radiation causes the atmosphere to heat up, leading to greenhouse warming. Other factors also influence greenhouse warming and cooling, including the abundance of volcanic ash in the atmosphere and solar luminosity variations, as evidenced by sunspot variations.
Measuring global (also called eustatic) sea level rise and fall is difficult because many factors influence the relative height of the sea along any coastline. These vertical motions of continents are called epeiro-genic movements, and may be related to plate tectonics, to rebound from being buried by glaciers, or to changes in the amount of heat added to the base of the continent by mantle convection. Continents may rise or sink vertically, causing apparent sea level change, but these sea level changes are relatively slow compared to changes induced by global warming and glacial melting. Slow, long-term sea level changes can also be induced by changes in the amount of seafloor volcanism associated with seafloor spreading. At some times in Earth history, seafloor spreading was particularly vigorous, and the increased volume of volcanoes and the mid-ocean ridge system caused global sea levels to rise.
Steady winds and currents can mass water against a particular coastline, causing a local and temporary sea level rise. Such a phenomenon is associated with the El Nino-Southern Oscillation (ENSO), causing sea levels to rise by 4-8 inches (10-20 cm) in the Australia-Asia region. When the warm water moves east in an ENSO event, sea levels may rise may 4-20 inches (10-50 cm) across much of the North and South American coastlines. Other atmospheric phenomena can also change sea level by centimeters to meters locally, on short time scales. Changes in atmospheric pressure, salinity of sea waters, coastal upwelling, onshore winds, and storm surges all cause short term fluctuations along segments of coastline. Global or local warming of waters can cause them to expand slightly, causing a local sea level rise. It is even thought that the extraction and use of ground water and its subsequent release into the sea might be causing sea level rise of about 0.05 inches (.13 cm) per year. Seasonal changes in river discharge can temporarily change sea levels along some coastlines, especially where winter cooling locks up large amounts of snow that melt in the spring.
It is clear that attempts to estimate eustatic sea level changes must be able to average out the numerous local and tectonic effects to arrive at a globally meaningful estimate of sea level change. Most coastlines seem to be dominated by local fluctuations that are larger in magnitude than any global sea level rise. Recently, satellite radar technology has been employed to precisely measure sea surface height and to document annual changes in sea level. Radar altimetry is able to map sea surface elevations to the sub-inch scale and to do this globally, providing an unprecedented level of understanding of sea surface topography. Satellite techniques support the concept that global sea levels are rising at about .1 inch per decade.
Even though Earth's climate has been changing between warm and cold periods for the past four and a half billion years, it is clear that humans have been rapidly increasing the amount of greenhouse gases in the atmosphere for the past 200 years. These gases, primarily carbon dioxide, have resulted in a new climate-forcing mechanism, whereby human or anthropogenic changes have forced the average global temperature to rise by nearly one degree in the past 200 years, and the temperature increase is continuing. The average land and sea surface temperatures have both increased, most notably over northern high latitudes, and the world's mountain glaciers and permafrost are receding and melting. Sea level is now rising at .12 inch per year (.31 cm/yr.), as a result of melting glaciers and thermal expansion of warming waters. Sea level is expected to continue to rise at least at this rate and may accelerate rapidly and rise by as much as 230 feet (70 m) if both the Greenland and Antarctic ice sheets were to melt. It is likely that the Greenland ice sheet will melt in the next few centuries, but the Antarctic ice sheet appears more stable and should survive longer. If only the Greenland ice sheet melts then sea levels will rise by an additional 23 feet (7 m), still placing the 100 million people who live within three feet (1 m) of present sea level in immediate danger.
Was this article helpful?