12.1 Causes and evidence of global climate change
4 min read•august 7, 2024
Global climate change is a pressing issue driven by human activities, primarily the emission of greenhouse gases. This topic explores the causes of climate change, including anthropogenic sources of CO2, , and other greenhouse gases that trap heat in the atmosphere.
Evidence of climate change is seen in rising temperatures, sea levels, and changes in weather patterns. We'll examine temperature and sea level observations, as well as proxies like ice cores and that reveal Earth's climate history.
Greenhouse Gases and Emissions
Anthropogenic Greenhouse Gas Sources
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- Greenhouse gases trap heat in the atmosphere by absorbing and emitting infrared radiation
- emissions have increased dramatically since the industrial revolution due to burning fossil fuels (coal, oil, natural gas)
- Methane is a potent greenhouse gas released from agricultural activities (livestock, rice cultivation), landfills, and natural gas production
- Methane has a shorter atmospheric lifetime than CO2 but a higher potential per molecule
- reduces the Earth's capacity to absorb CO2 through photosynthesis and releases the carbon stored in trees back into the atmosphere
- Industrial revolution marked the beginning of large-scale fossil fuel use and a rapid increase in atmospheric CO2 concentrations
- Pre-industrial CO2 levels were around 280 ppm, now over 400 ppm
- measures the change in Earth's energy balance due to factors like greenhouse gases
- Positive radiative forcing from anthropogenic emissions is driving global warming
Other Greenhouse Gases and Their Sources
- is released from agricultural soil management (fertilizers), industrial processes, and combustion
- in the lower atmosphere acts as a greenhouse gas and is formed by chemical reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight
- Major sources include vehicle exhaust and industrial emissions
- is the most abundant greenhouse gas but its atmospheric concentration is not directly affected by human activities
- However, as the Earth warms, more water evaporates leading to a loop
- (CFCs) are powerful greenhouse gases that also deplete the ozone layer
- Now regulated by the Montreal Protocol, but some of their replacements () are also strong greenhouse gases
Evidence of Climate Change
Temperature and Sea Level Observations
- Global warming refers to the long-term trend of rising average global temperatures due to the greenhouse effect
- Earth's average surface temperature has increased by about 1°C since the pre-industrial era
- is caused by of ocean water as it warms and the melting of land-based ice (glaciers, ice sheets)
- Global mean sea level has risen about 21-24 cm since 1880
- show how much warmer or cooler a region is compared to a long-term average
- The past five years (2016-2020) have been the warmest on record globally
Paleoclimate Proxies
- provide a record of past atmospheric composition and temperature
- Air bubbles trapped in ice cores allow direct measurement of past CO2 levels
- The ratio of oxygen isotopes in the ice can be used to infer past temperatures
- Tree rings are annual growth rings that are wider during warm, wet years and narrower during cold, dry years
- The width and density of tree rings can be used to reconstruct past climate conditions
- Older trees (like bristlecone pines) can provide climate records going back thousands of years
Climate History and Feedbacks
Earth's Climate History
- Paleoclimate refers to past climate conditions and the study of how Earth's climate has changed over geologic time scales
- Earth has experienced natural climate variations due to factors like changes in solar output, volcanic eruptions, and shifts in Earth's orbit ()
- are an example of natural climate change, characterized by colder global temperatures and the growth of continental ice sheets
- However, the current rate and magnitude of warming is unprecedented in at least the last 2,000 years and cannot be explained by natural factors alone
- The scientific consensus attributes the observed warming to human activities, primarily greenhouse gas emissions
Climate Feedbacks
- Feedback loops in the climate system can amplify (positive feedback) or dampen () the initial change
- Examples of positive feedbacks that accelerate warming:
- : As Arctic sea ice melts, it exposes darker ocean water that absorbs more sunlight, causing further warming and ice loss
- : Warmer air can hold more water vapor (a greenhouse gas), which traps more heat and causes additional warming
- An example of a negative feedback that slows warming:
- : As Earth's surface warms, it emits more infrared radiation to space, helping to cool the planet
- The net effect of all feedbacks will determine the ultimate magnitude of climate change for a given increase in greenhouse gases
Key Terms to Review (25)
Carbon dioxide: Carbon dioxide (CO2) is a colorless, odorless gas that is naturally present in Earth's atmosphere and plays a crucial role in various biogeochemical processes. It is produced by the respiration of animals and plants, combustion of fossil fuels, and decomposition of organic matter, making it a key player in ocean chemistry, atmospheric conditions, climate regulation, and the carbon cycle.
Chlorofluorocarbons: Chlorofluorocarbons (CFCs) are man-made chemical compounds that contain chlorine, fluorine, and carbon. They were primarily used as refrigerants, propellants in aerosol sprays, and solvents. These compounds are significant because they have been linked to ozone layer depletion and play a role in climate change by trapping heat in the atmosphere, thus contributing to global warming.
Climate feedback loops: Climate feedback loops are processes that can either amplify or dampen the effects of climate change by interacting with the climate system. These loops involve changes in the environment that lead to additional changes, creating a cycle of response that can either accelerate or slow down climate change impacts. Understanding these feedback loops is essential as they connect various elements like ocean chemistry, the carbon cycle, and the broader implications of climate change on Earth systems.
Deforestation: Deforestation is the large-scale removal of trees and forested areas, often resulting in damage to the quality of the land. This process can lead to various environmental issues, including soil degradation, loss of biodiversity, and alterations in the carbon and water cycles, impacting global climate patterns and ecological balance.
Fossil fuel combustion: Fossil fuel combustion is the process of burning fossil fuels, such as coal, oil, and natural gas, to release energy for various uses, including electricity generation, heating, and transportation. This process produces carbon dioxide and other greenhouse gases, which play a significant role in climate regulation and global warming.
Global warming: Global warming refers to the long-term increase in Earth's average surface temperature due to human activities, primarily the emission of greenhouse gases like carbon dioxide and methane. This rise in temperature has significant implications for atmospheric chemistry, influencing air quality and weather patterns, while also affecting interactions among Earth's spheres, including the atmosphere, hydrosphere, and biosphere. The consequences of global warming are evident in the changes to climate systems and environmental conditions across the globe.
HFCs: HFCs, or hydrofluorocarbons, are a class of synthetic compounds commonly used as refrigerants, aerosol propellants, and in foam-blowing applications. While they were introduced as a replacement for ozone-depleting substances, HFCs are potent greenhouse gases that contribute significantly to global warming, highlighting the complex relationship between industrial practices and climate change.
Ice ages: Ice ages are prolonged periods in Earth's history characterized by significant global cooling and the expansion of continental ice sheets and glaciers. These periods, driven by changes in Earth's orbit, atmospheric composition, and tectonic activity, have profound impacts on global climate, sea levels, and ecosystems.
Ice core samples: Ice core samples are cylindrical sections of ice drilled from glaciers or ice sheets, which contain trapped air bubbles and layers of snow that have accumulated over thousands of years. These samples provide critical evidence of past climate conditions, atmospheric composition, and greenhouse gas levels, revealing insights into the causes and effects of global climate change over time.
Ice-albedo feedback: Ice-albedo feedback is a climate mechanism where a reduction in ice and snow cover leads to decreased reflectivity (albedo) of the Earth's surface, resulting in increased absorption of solar energy and further warming. This process creates a self-reinforcing cycle that accelerates climate change, especially in polar regions.
Methane: Methane is a colorless, odorless gas that is the primary component of natural gas and a potent greenhouse gas with a global warming potential significantly higher than that of carbon dioxide over a short time frame. It is produced both naturally and through human activities, such as agriculture, fossil fuel extraction, and waste management. Methane plays a critical role in the greenhouse effect, contributing to climate change and influencing atmospheric chemistry and the carbon cycle.
Milankovitch cycles: Milankovitch cycles are long-term variations in Earth's orbit and axial tilt that affect the distribution of solar energy received by the planet, leading to changes in climate over thousands of years. These cycles are primarily driven by three astronomical factors: eccentricity, axial tilt, and precession. They play a crucial role in explaining past glacial and interglacial periods and provide insight into the natural causes of climate change.
Negative Feedback: Negative feedback is a process in which a system responds to a change by initiating actions that counteract that change, helping to maintain equilibrium or stability. This self-regulating mechanism is crucial in various natural processes, ensuring that systems remain balanced despite external influences.
Nitrous Oxide: Nitrous oxide (N2O) is a colorless, non-flammable gas with a slightly sweet odor, commonly known as laughing gas. It plays a significant role in the greenhouse effect, as it is a potent greenhouse gas that contributes to climate change. Additionally, nitrous oxide is an important component of the nitrogen cycle, impacting soil health and ecosystem productivity.
Ozone: Ozone is a molecule composed of three oxygen atoms, often found in two layers of the atmosphere: the stratosphere and the troposphere. In the stratosphere, it forms the ozone layer, which protects life on Earth by absorbing harmful ultraviolet (UV) radiation from the sun. However, in the troposphere, ozone acts as a greenhouse gas and contributes to global climate change by trapping heat in the atmosphere.
Paleoclimate: Paleoclimate refers to the climate of the Earth at different points in geological history, which can be understood by examining geological and biological evidence. By studying paleoclimate, scientists can learn about past climate conditions, including temperature, precipitation patterns, and atmospheric composition, which are crucial for understanding how climate has changed over millions of years and the natural variability of Earth's climate system.
Planck Feedback: Planck feedback refers to the process by which the Earth's temperature changes in response to variations in energy balance due to greenhouse gas emissions. Specifically, it describes how an increase in temperature leads to increased infrared radiation emitted by the Earth, which helps regulate the planet's energy balance. This feedback mechanism plays a crucial role in understanding the sensitivity of the climate system to warming and is fundamental in the study of global climate change.
Positive Feedback: Positive feedback refers to a process where an initial change in a system causes further changes that amplify the original effect, leading to greater and often accelerated impacts. This concept is crucial in understanding how different components of the Earth system interact, especially when it comes to climate dynamics and ecological responses.
Radiative forcing: Radiative forcing is a measure of how much energy in the form of radiation is being added to or taken away from the Earth’s atmosphere due to various factors. It plays a crucial role in understanding climate change, as it directly influences the balance between incoming solar radiation and outgoing infrared radiation. By quantifying the effects of greenhouse gases, aerosols, and land use changes on the energy balance, radiative forcing helps scientists assess the potential impacts on global temperatures and climate patterns.
Sea Level Rise: Sea level rise refers to the increase in the average level of the ocean's surface due to various factors, primarily linked to climate change. This phenomenon is a direct result of thermal expansion of seawater as it warms and the melting of glaciers and ice sheets, contributing to significant changes in coastal ecosystems and human settlements. Understanding sea level rise is crucial for assessing impacts on Earth’s integrated systems, the effects of climate change, and the evidence supporting global climate change.
Temperature anomalies: Temperature anomalies refer to the deviation of the Earth's surface temperature from a long-term average or baseline temperature over a specific period. These anomalies are crucial for understanding climate change, as they indicate how much warmer or cooler a given area is compared to historical norms, helping scientists assess the impacts of climate change on global and regional scales.
Thermal Expansion: Thermal expansion refers to the increase in volume or length of a substance as its temperature rises. This phenomenon occurs because the particles within a material move more vigorously at higher temperatures, leading to greater spacing between them. Thermal expansion is important for understanding various natural processes, including how rocks break apart in physical weathering and how climate change can affect ocean levels and land structures.
Tree rings: Tree rings are the concentric circles found in the cross-section of a tree trunk, which represent the annual growth of the tree. Each ring corresponds to one year of growth, allowing scientists to gather information about past environmental conditions, including climate changes, natural disasters, and ecological events.
Water vapor: Water vapor is the gaseous state of water that is present in the atmosphere and plays a crucial role in weather, climate, and energy transfer. It is a key component of the Earth's hydrological cycle, influencing various processes such as precipitation and evaporation. As a greenhouse gas, water vapor absorbs and re-emits infrared radiation, contributing significantly to the greenhouse effect and climate regulation.
Water vapor feedback: Water vapor feedback is a climate process where an increase in atmospheric temperature leads to higher water vapor levels, which then amplifies warming because water vapor is a potent greenhouse gas. This interaction creates a self-reinforcing cycle that significantly influences climate change and its impacts on the environment and weather patterns.