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Climate changes over time. In addition to seasonal changes during the year, long-term changes also occur over much longer periods of time that are harder to detect but which have much more significant environmental effects. These changes may be caused by changes to the rotation and cycling of the Earth of changes to sun radiation and brightness.
Although measuring long-term climatic change is difficult, researchers believe the burning of fossil fuels and the destruction of forest has resulted in a 0.6°C increase in average world temperature. Many predict an increase in the average world temperature by 2.5°C by the mid-twenty-first century.
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There are four seasons:
- Summer
- Autumn (also known as "fall")
- Winter
- Spring
These are the result of a 23.5° slant of the Earth. Because of this, different parts of the world are at different distances from the sun and this changes during the year. Different levels of sunshine are received by countries around mid-latitudes and countries with higher latitudes to the north or south. Equatorial countries have more variation in rainfall during the year and less variation in temperature, while for many countries the opposite applies.
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"Climate Change" is generally recognised as a significant departure over time from seasonal and absolute weather patterns that most people consider to be "normal".
The earth has undergone several periods of significant change throught the millenia, including several Ice Ages that have covered significant portions of the globe in permanent areas of ice, snow and sub-zero temperatures.
To understand climate change one needs to also study climate history. Later sections in this category will give overviews of these historical periods and then lead up to current day occurences.
Given these historical periods of the earth cooling and warming this section will concentrate here on more recent history and then look at current trends.
Since around 1750, after the first Industrial Revolution, human activities may have added to the amount of greenhouse gases in the atmosphere. Greenhouse gases also occur naturally and are in fact required to preserve the earth's atmosphere but there appears to have been a significant increase in Carbon Dioxide concentration since 1750 and the rate of that concentration appears to be increasing.
The reasons for this increase are mainly thought to be due to the increased burning of fossil fuels and living matter such as forests and vegetation, plus the use of aerosols. These gases and aerosols result in emissions that both absorb and emit heat and that also reflect light.
The addition of increased greenhouse gases and aerosols may have changed the composition of the atmosphere and may have influenced temperatures, rainfall, weather patterns, storms, cyclones, hurricanes and even sea levels.
Again it must be stated that some of these changes can be due to natural phenomena so establishing what level of Climate Change is due to human activity is challenging.
The International Panel on Climate Change (IPCC) is an intergovernmental panel of scientists and climatologists and it represents the most authoritative scientific view on this subject. Their reports and findings will be covered in future sections of this category.
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Since the Industrial Revolution around 1750 human activities have substantially added to the amount of heat-trapping greenhouse gases in the atmosphere. The burning of fossil fuels and biomass (living matter such as vegetation) has also resulted in emissions of aerosols that absorb and emit heat, and reflect light.
The addition of greenhouse gases and aerosols has changed the composition of the atmosphere. The changes in the atmosphere have likely influenced temperature, precipitation, storms and sea level. However, these features of the climate also vary naturally, so determining what fraction of climate changes are due to natural variability versus human activities is challenging.
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The greenhouse effect is real and helps to regulate the temperature of our planet. It is essential for life on Earth and is one of Earth's natural processes.
It is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they effectively 'trap' heat in the lower atmosphere) and re-radiation downward of some of that heat.
Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C).
So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect.
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Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point.
Pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 parts per million by volume (ppmv), and current levels are about 370 ppmv.
The concentration of CO2 in our atmosphere today, has not been exceeded in the last 420,000 years, and likely not in the last 20 million years.
According to the IPCC Special Report on Emission Scenarios (SRES), by the end of the 21st century, we could expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppm (75-350% above the pre-industrial concentration).
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Yes. Global surface temperatures have increased about 0.6°C (plus or minus 0.2°C) since the late-19th century, and about 0.4°F (0.2 to 0.3°C) over the past 25 years (the period with the most credible data). The warming has not been globally uniform.
Some areas (including parts of the southeastern U.S.) have, in fact, cooled over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Warming, assisted by the record El Niño of 1997-1998, has continued right up to the present, with 2001 being the second warmest year on record after 1998.
Linear trends can vary greatly depending on the period over which they are computed. Temperature trends in the lower troposphere (between about 2,500 and 26,000 ft.) from 1979 to the present, the period for which Satellite Microwave Sounding Unit data exist, are small and may be unrepresentative of longer term trends and trends closer to the surface. Furthermore, there are small unresolved differences between radiosonde and satellite observations of tropospheric temperatures, though both data sources show slight warming trends. If one calculates trends beginning with the commencement of radiosonde data in the 1950s, there is a slight greater warming in the record due to increases in the 1970s. There are statistical and physical reasons (e.g., short record lengths, the transient differential effects of volcanic activity and El Niño, and boundary layer effects) for expecting differences between recent trends in surface and lower tropospheric temperatures, but the exact causes for the differences are still under investigation (see National Research Council report "Reconciling Observations of Global Temperature Change").
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 49,000-79,500ft.) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger 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 high and low daily temperatures) over about 50% of the global land mass since the middle of the 20th century. Cloud cover has increased in many of the areas with reduced diurnal temperature range. The overall positive trend for maximum daily temperature over the period of study (1950-93) is 0.1°C/decade, whereas the trend for daily minimum temperatures is 0.2°C/decade. This results in a negative trend in the DTR of -0.1°C/decade.
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 glacier length 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 alititudes that weather stations allowing us 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 Arctic sea ice has decreased between 1973 and 1996 at a rate of -2.8 +/- 0.3%/decade. Although this seems to correspond to a general increase in temperature over the same period, there are lots of quasi-cyclic atmospheric dynamics (for example the Arctic Oscillation) which may also influence the extent and thickness of sea-ice in the Arctic. 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.
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