THE CLIMATE CHANGE Weather vs. Climate The difference between weather and climate is primarily a matter of time and geography. Weather refers to the conditions of the atmosphere over a short period of time, such as hours or days, and typically for a local area. Climate refers to the behavior of the atmosphere over a longer period of time, and usually for a large area. Familiar examples of weather characteristics include the daily temperature, humidity, or the amount of precipitation produced by a storm. Weather also includes severe weather conditions such as hurricanes, tornadoes, and blizzards. Because of the dynamic nature of the atmosphere, it is not possible to predict weather conditions in a specific location months or years in advance. Climate is typically defined based on 30-year averages of weather. [4] Climate represents our expectations for the weather. For example, climate tells us how warm we expect a typical summer to be, how much rainfall would correspond to a wetter-than-average spring, or how frequently we expect a snowy winter to occur. Scientists can compare recent and long-term observations of the climate to detect the influence of greenhouse gases on climate conditions. The CO₂ that humans have added to the atmosphere has caused an increase in the atmospheric concentration of CO₂. The graph shown here, called a Keeling Curve after the scientist who first developed it, represents the record of increased global atmospheric carbon dioxide concentration. Phisical Evidences Evidence for climatic change is taken from a variety of sources that can be used to reconstruct past climates. Reasonably complete global records of surface temperature are available beginning from the mid-late 19th century. For earlier periods, most of the evidence is indirect—climatic changes are inferred from changes in proxies, indicators that reflect climate, such as vegetation, ice cores,[64] dendrochronology, sea level change, and glacial geology. Temperature measurements and proxies The instrumental temperature record from surface stations was supplemented by radiosonde balloons, extensive atmospheric monitoring by the mid-20th century, and, from the 1970s on, with global satellite data as well. The 18O/16O ratio in calcite and ice core samples used to deduce ocean temperature in the distant past is an example of a temperature proxy method, as are other climate metrics noted in subsequent categories. Historical and archaeological evidence Main article: Historical impacts of climate change Climate change in the recent past may be detected by corresponding changes in settlement and agricultural patterns.[65] 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.[65] Decline in thickness of glaciers worldwide over the past half-century Glaciers Glaciers are considered among the most sensitive indicators of climate change.[66] Their size is determined by a mass balance between snow input and melt output. As temperatures warm, glaciers retreat unless snow precipitation increases to make up for the additional melt; the converse is also true. Glaciers grow and shrink due both to natural variability and external forcings. Variability in temperature, precipitation, and englacial and subglacial hydrology can strongly determine the evolution of a glacier in a particular season. Therefore, one must average over a decadal or longer time-scale and/or over many individual glaciers to smooth out the local short-term variability and obtain a glacier history that is related to climate. A world glacier inventory has been compiled since the 1970s, initially based mainly on aerial photographs and maps but now relying more on satellites. This compilation tracks more than 100,000 glaciers covering a total area of approximately 240,000 km2, and preliminary estimates indicate that the remaining ice cover is around 445,000 km2. The World Glacier Monitoring Service collects data annually on glacier retreat and glacier mass balance. From this data, glaciers worldwide have been found to be shrinking significantly, with strong glacier retreats in the 1940s, stable or growing conditions during the 1920s and 1970s, and again retreating from the mid-1980s to present.[67] The most significant climate processes since the middle to late Pliocene (approximately 3 million years ago) are the glacial and interglacial cycles. The present interglacial period (the Holocene) has lasted about 11,700 years.[68] Shaped by orbital variations, responses such as the rise and fall of continental ice sheets and significant sea-level changes helped create the climate. Other changes, including Heinrich events, Dansgaard–Oeschger events and the Younger Dryas, however, illustrate how glacial variations may also influence climate without the orbital forcing. Natural causes alone cannot explain recent changes Line graph with a line that show the observed temperature increases, a blue band that show how the temperature would have changed over the past century due to only natural forces, and a red band that shows the combined effects of natual and human forces. The blue band that shows natural forces starts and ends the 20th century just above 56 degrees Fahrenheit. The actual observed global average temperatures closely follows the model projections that use both human and natural forces - beginning in 1900 at just above 56 degrees Fahrenheit and ending in 2000 around 58 degrees Fahrenheit. Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming. Source: USGRCP 2009 Line graph with three trend lines that represent concentration of carbon dioxide, methane, and nitrous oxide over time. The three gasses follow a very similar pattern that starts low and is steady from year zero until sometime around 1900 where the concentration of all three gasses start increasing dramatically. In the first 1900 years carbon dioxide and nitrous oxide remain at levels around 280 parts per million and 250 parts per billion, respectively. By 2000, carbon dioxide measures around 380 parts per million and nitrous oxide measures around 320 parts per billion. Similarly methane remains at approximately 700 parts per billion until around 1800. By 2000, methane concentrations measure close to 2000 parts per billion. This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air. Source: USGCRP 2009 Natural processes such as changes in the sun's energy, shifts in ocean currents, and others affect Earth's climate. However, they do not explain the warming that we have observed over the last half-century. [1] Human causes can explain these changes Most of the warming of the past half century has been caused by human emissions of greenhouse gases. [1] Greenhouse gases come from a variety of human activities, including: burning fossil fuels for heat and energy, clearing forests, fertilizing crops, storing waste in landfills, raising livestock, and producing some kinds of industrial products. Greenhouse gas emissions are not the only way that people can change the climate. Activities such as agriculture or road construction can change the reflectivity of Earth's surface, leading to local warming or cooling. This effect is observed in urban centers, which are often warmer than surrounding, less populated areas. Emissions of small particles, known as aerosols, into the air can also lead to reflection or absorption of the sun's energy. Learn more about past and present climate trends and their causes. Climate will continue to change unless we reduce our emissions During the 21st century, global warming is projected to continue and climate changes are likely to intensify. Scientists have used climate models to project different aspects of future climate, including temperature, precipitation, snow and ice, ocean level, and ocean acidity. Depending on future emissions of greenhouse gases and how the climate responds, average global temperatures are projected to increase worldwide by 2°F to 11.5°F by 2100. [1] Learn more about the projections of future climate change. Climate change impacts our health, environment, and economy Climate change affects our environment and natural resources, and impacts our way of life in many ways. For example: Warmer temperatures increase the frequency, intensity, and duration of heat waves, which can pose health risks, particularly for young children and the elderly. Rising sea levels threaten coastal communities and ecosystems. Changes in the patterns and amount of rainfall, as well as changes in the timing and amount of stream flow, can affect water supplies and water quality and the production of hydroelectricity. Changing ecosystems influence geographic ranges of many plant and animal species and the timing of their lifecycle events, such as migration and reproduction. Increases in the frequency and intensity of extreme weather events, such as heat waves, droughts, and floods, can increase losses to property, cause costly disruptions to society, and reduce the availability and affordability of insurance. We can prepare for some of the likely climate change impacts to reduce their effect on ecosystem and human well-being. Making such preparations is known as adaptation. Examples of adaptation include strengthening water conservation programs, upgrading stormwater systems, developing early warning systems for extreme heat events, and preparing for stronger storms through better emergency preparation and response strategies. Natural causes alone cannot explain recent changes Line graph with a line that show the observed temperature increases, a blue band that show how the temperature would have changed over the past century due to only natural forces, and a red band that shows the combined effects of natual and human forces. The blue band that shows natural forces starts and ends the 20th century just above 56 degrees Fahrenheit. The actual observed global average temperatures closely follows the model projections that use both human and natural forces - beginning in 1900 at just above 56 degrees Fahrenheit and ending in 2000 around 58 degrees Fahrenheit. View enlarged image Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming. Source: USGRCP 2009 Line graph with three trend lines that represent concentration of carbon dioxide, methane, and nitrous oxide over time. The three gasses follow a very similar pattern that starts low and is steady from year zero until sometime around 1900 where the concentration of all three gasses start increasing dramatically. In the first 1900 years carbon dioxide and nitrous oxide remain at levels around 280 parts per million and 250 parts per billion, respectively. By 2000, carbon dioxide measures around 380 parts per million and nitrous oxide measures around 320 parts per billion. Similarly methane remains at approximately 700 parts per billion until around 1800. By 2000, methane concentrations measure close to 2000 parts per billion. View enlarged image This graph shows the increase in greenhouse gas (GHG) concentrations in the atmosphere over the last 2,000 years. Increases in concentrations of these gases since 1750 are due to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion molecules of air. Source: USGCRP 2009 Natural processes such as changes in the sun's energy, shifts in ocean currents, and others affect Earth's climate. However, they do not explain the warming that we have observed over the last half-century. [1] Human causes can explain these changes Most of the warming of the past half century has been caused by human emissions of greenhouse gases. [1] Greenhouse gases come from a variety of human activities, including: burning fossil fuels for heat and energy, clearing forests, fertilizing crops, storing waste in landfills, raising livestock, and producing some kinds of industrial products. Greenhouse gas emissions are not the only way that people can change the climate. Activities such as agriculture or road construction can change the reflectivity of Earth's surface, leading to local warming or cooling. This effect is observed in urban centers, which are often warmer than surrounding, less populated areas. Emissions of small particles, known as aerosols, into the air can also lead to reflection or absorption of the sun's energy. Climate will continue to change unless we reduce our emissions During the 21st century, global warming is projected to continue and climate changes are likely to intensify. Scientists have used climate models to project different aspects of future climate, including temperature, precipitation, snow and ice, ocean level, and ocean acidity. Depending on future emissions of greenhouse gases and how the climate responds, average global temperatures are projected to increase worldwide by 2°F to 11.5°F by 2100. [1] Learn more about the projections of future climate change.