Climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations. Recent climate changes, cannot be explained by natural causes alone. It is especially warming since the mid-20^th century. It is extremely likely that human activities have been the dominant cause of that warming.

Earth´s temperature depends on the balance between energy entering and leaving the planet´s system. When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun´s energy is reflected back into space, Earth avoids warming. When absorbed energy is released back into space, Earth cools.

Many factors, both natural and human, can cause changes in Earth´s energy balance, including:

• Variations in the sun´s energy reaching Earth
• Changes in the reflectivity of Earth´s atmosphere and surface
• Changes in the greenhouse effect, which affects the amount of heat reained by Earth´s atmosphere.

Climate change is a change in the statistical distribution of weather patterns when that change lasts for an extended period of time: decades to millions of years.

Climate change is caused by:

• factors such as biotic processes, variations in solar radiation received by Earth,

• plate tectonics, and volcanic eruptions
• certain human activities have also been identified as significant causes of recent climate change, often referred to as global warming.

Are used observations and theoretical models, based on geological evidence from:

• Borehole temperature profiles

• Cores removed from deep accumulations of ice, floral and faunal records

• Glacial and periglacial processes

• Stable-isotope and other analysis of sediment layers

• Records of past sea levels

PHYSICAL EVIDENCE

• Historical and archaeological evidence: oral history and historical documents can offer insights into past changes in the climate. Climate change effects have been linked to the collapse of various civilizations.
• Glaciers: are considered among the most sensitive indicators of climate change. Their size is determinate by a mass balance between snow input and melt output. As temperature warm, glaciers retreat unless snow precipitation increases to make up for the additional melt. The most significant climate processes since the middle to late Pliocene are the glacial and interglacial cycles. The present interglacial period has lasted about 11.700 years.
• Arctic sea ice loss: Satellite observations show that Arctic sea ice is now declining at a rate of 13.3 percent per decade, relative to the 1891 to 2010 average.
• Vegetation: a change in the type, distribution and coverage of vegetation may occur given a change in the climate. Some changes in climate may result in increases precipitation and warmth. Vegetation stress, rapid plant loss and desertification in certain circumstances are radical changes.
• Pollen analysis: different groups of plants have pollen with distinctive shapes and surface textures. Changes in the type of pollen found in different layers of sediment in lakes, bogs, or river deltas indicate changes in plant communities.
• Cloud cover and precipitation: scientists have plublished evidence o increased cloud cover over polar regions, as predicted by climate models. Estimated global land precipitation increases by approximately 2% over the course of the 20^th century.
• Dendroclimatology: is the analysis of tree ring growth patterns to determine past climate variations.
• Ice cores: The air trapped in bubbles in the ice can also reveal the CO2 variations o the atmosphere from the distant past, well before modern environmental influences. The changes in CO2 over may millennia, and continues to provide valuable information about the differences between ancient and modern atmospheric conditions.
• Animals: different species o beetles tend to be found under different climatic conditions. Knowledge of the present climatic range of the different species, and the age of the sediments in which remains are found, past climatic conditions may be inferred.
• Sea level change: in the early Pliocene, global temperatures were 1-2ºC warmer than the present temperature, yet sea level was 15-25 meters higher than today.

CAUSES

Energy is received from the Sun the rate at which it is lost to space determine equilibrium temperature and climate of Earth. This energy is distributed around the globe by winds, ocean currents, and other mechanisms to affect the climates of different regions.

Forcing mechanisms can be “internal” o “external”. Internal forcing mechanisms are natural processes within the climate system itself. External forcing mechanisms can be natural or anthropogenic.

1. Internal forcing mechanisms: scientists generally define the five components of earth´s climate system to include atmosphere, hydrosphere, cryosphere, lithosphere and biosphere.

Ocean variability: the ocean is a fundamental part of the climate system, some changes in it occurring at longer timescales than in the atmosphere, as it has hundreds of times more mass and thus very high thermal inertia. Short-term fluctuations such as the Niño-Southern Oscillation, the Pacific decadal oscillation, the North Atlantic oscillation, and the Arctic oscillation, represent climate variability rather than climate change. On longer time-scales, alterations to ocean processes such as thermohaline circulation play a key role in redistributing heat by carrying out a very slow and extremely deep movement of water and the long-term redistribution of heat in the world´s oceans.

Life: affects climate through its role in the carbon and water cycles and through such mechanisms as albedo, evapotranspiration, cloud formation, and weathering.

2. External forcing mechanisms:

Orbital variations: variations in Earth´s orbit lead to changes in the seasonal distribution of sunlight reaching the Earth´s surface and how it is distributed across the globe. The three types of orbital variations are variations in Earth´s eccentricity;

• Changes in the tilt angle o Earth´s axis of rotation

• Precession of Earth´s axis

Combined together, these produce Milankovitch cycles, which have a large impact on climate and are notable por their correlation to glacial and interglacial periods.

Solar output: variations in solar activity during the last several centuries based on observations o sunspots and beryllium isotopes. Three to our billion years ago, the Sun emitted only 70% as much power as it does today. Over the following approximately 4 billion years, the energy output of the Sun increases and atmospheric composition changed. The Great Oxygenation Event-oxygenation of the atmosphere around 2.4 billion years ago.

Volcanism: the eruptions considered to be large enough to affect the Earth´s climate on a scale of more than I year the ones that inject over 100.000 tons of SO2, into the stratosphere (sulfuric acid), causing cooling, blocking the transmission of solar radiation to the Earth´s surface for a period o a few years. Volcanoes are also part of carbon cycle. Over very long time periods, they release carbon dioxide from the Earth´s crust and mantle, counteracting the uptake by sedimentary rocks and other geological carbon dioxide sinks.

Plate tectonics: the motion o tectonic plates reconfigures global land and ocean areas and generates topography. The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The size of continents is also important. Because of the stabilizing effect of the oceans on temperature, yearly temperature variations are generally lower in coastal areas than they are inland. A larger supercontinent will therefore have more area in which climate is strongly seasonal than will several smaller continents or islands.

Human influences: in the context of climate variation, anthropogenic factors are human activities which affect the climate. The scientific consensus on climate change is “that climate is changing and these changes are in large part caused by human activities. Of most concern in these anthropogenic factors are:

• Increase in CO2 levels due to emissions from fossil fuel combustion

• Aerosols

• CO2 from cement manufacture

• Ozone depletion

• Animal agriculture and deforestation.

Radiative Forcing: is a measure of the influence of a particular factor on the net change in Earth´s energy balance. On average, a positive radiative forcing tends to warm the surface of the planet, while a negative forcing tends to cool the surface. GHGs have a positive forcing because they absorb energy radiating from Earth´s surface, rather than allowing it to be directly transmitted into space. Aerosols, can have a positive or negative radiative forcing, depending on how they absorb and emit heat or reflect light. GHGs has increased between 1990 and 2009 a 27,5%, carbon dioxide (CO2) is responsible for 80%., also there are contribution of metano (CH4) and chlorofluorocarbons (CFCs).

The greenhouse effect: greenhouse gases like water vapor (H2O), carbon dioxide (CO2), and methane (CH4), absorb energy, slowing or preventing the loss of heat to space. GHGs act like a blanket, making Earth warmer than it would otherwise be.

Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2, and other heat-trapping gases to the atmosphere. These greenhouse gas emissions have increased the greenhouse effect and caused Earth´s surface temperature to rise. The most important GHGs directly emitted by humans include carbon dioxide (CO2), methane (CH4), nitrous oxide (N20), and several others.

Carbon dioxide is the primary greenhouse gas that is contributing to recent climate change. CO2 is absorbed and emitted naturally as part of the carbon cycle: most of the chemicals that make up living tissue contain carbon. Carbon enters atmosphere as carbon dioxide from respiration and combustion. Carbon dioxide is absorved by producers to make carbohydrates in photosynthesis.

Respiration CH2O + O2—————CO2 + H20

Photosynthesis CO2 + H2O + LUZ—CH2O + O2

Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide formed during respiration. The animals and plants eventually die, carbon in their bodies is returned to the atmosphere as carbon dioxide. When descomposition is blocked. The plant and animal material may then be available as fossil fuel in the future combustion.

Feedbacks: climate feedbacks amplify o reduce direct warming and cooling effects. Positive feedbacks that amplify changes, feedbacks that counteract changes are called negative feedbacks; feedbacks are associated with changes in surface reflectivity, clouds, water vapor, and the carbon cycle. The melting of Arctic sea ice is another example of a positive climate feedback. Some types of clouds cause a negative feedback (warming temperatures can increase the amount or reflectivity of these clouds, reflecting more sunlight back into space, cooling the surface of the planet). Other types of clouds, however, contribute a positive feedback.

Bibliography:

• Wikipedia

• NASA, “Climate change and global warming”
• Arthur N. Strahler (1970), “Physical geography”, Ed. Jhon Wikey and Sons. INC
• America´s Climate Choices (2010), “Advancing the Science of Climate Change”, Washington, D.C.: The National Academies Press.
• Katie Peek (2016), “How will Climate change us”, September Scientific American

Relation links:

• NASA

http://www.nasa.gov/press-release/nasa-releases-detailed-global-climate-change-projections/

• CSIC

http://digital.csic.es/handle/10261/33470

• Greenpeace

  http://www.greenpeace.org/eu-unit/en/campaigns/Climate/

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