About 7000 years ago, it was the last ice age which end abruptly and marked the beginning of the modern climate era, and of human civilization.
Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives. But the difference nowadays is that this climate warming is created by human and not by small variations in Earth's orbit.
Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth’s climate responds to changes in greenhouse gas levels. They also show that in the past, large changes in climate have happened very quickly, geologically-speaking : in tens of years, not in millions or even thousands.
- The sea level rise, which rose about 17 centimeters in the last century.
- The global temperature rise : the Earth has warmed since 1880, and most of this warming has occurred since the 1970s.
- The warming of oceans, indeed we can notice a warming of 0,302 degrees Fahrenheit since 1969 in the top 700 meters.
- The shriking of ice sheets, for exemple, Greenland lost 150 to 250 cubic kilometers of ice per year between 2002 and 2006.
- The declining of Arctic sea ice : it becomes thicker and smaller very rapidly since the last several decades.
- The glacial retreat (Alpes, Himalaya, Andes, Rockies, Alaska, and Africa).
- Events become more extreme, with for exemple the U.S. which witnessed increasing numbers of intense rainfall events.
- The acidification of oceans, because since the beginning of the Industrial Revolution, the acidity of surface ocean waters has increased by about 30 percent, and the amount of carbon dioxide absorbed by the upper layer of the oceans is increasing by about 2 billion tons per year.
- The decrease of snow cover.
Most climate scientists agree the main cause of the current global warming trend is human expansion of the "greenhouse effect"— warming that results when the atmosphere traps heat radiating from Earth toward space.
Certain gases in the atmosphere block heat from escaping, such as water vapor, nitrous oxide, methane, and carbone dioxide.
The most important impact on climate is because of water vapor, which keep the most the warm from the earth : the water vapor increases as the Earth's atmosphere warms.
The carbone dioxide is a minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO2 concentration by a third since the Industrial Revolution began. This is the most important long-lived "forcing" of climate change.
The methane is produced both through natural sources and human activities, including the decomposition of wastes in landfills, agriculture, and especially rice cultivation, as well as ruminant digestion and manure management associated with domestic livestock.
The nitrous oxide is produced by soil cultivation practices, especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric acid production, and biomass burning.
So the human activity play a big role in this warming : the industrial activities that our modern civilization depends upon have raised atmospheric carbon dioxide levels from 280 parts per million to 400 parts per million in the last 150 years.
We could think that the sun could have an impact on this global warming, as it is the first warm source of the Earth, but several lines of evidence show that current global warming cannot be explained by changes in energy from the sun:
- Since 1750, the average amount of energy coming from the sun either remained constant or increased slightly.
- If the warming were caused by a more active sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere. That's because greenhouse gasses are trapping heat in the lower atmosphere.
- Temperatures will continue to rise, because human-induced warming is superimposed on a naturally varying climate.
- Frost-free season and growing season will lenghten : these seasons have been increasing since the 1980s, affecting ecosystems and agriculture.
- Changes in precipitation patterns : projections of future climate suggest that the recent trend towards increased heavy precipitation events will continue.
- More droughts and heat wave, which are projected to become more intense, and cold waves less intense everywhere.
- Hurricanes will become stronger and more intense, but the relative contributions of human and natural causes to these increases are still uncertain.
- Sea level will continue to rise of 1 to 4 feet by 2100 : this is the result of added water from melting land ice and the expansion of seawater as it warms.
- Arctic will become more and more ice-free, it is expected to become essentially ice free in summer before mid-century.
Source : http://climate.nasa.gov/