15.1 Evidence and causes of climate change
5 min read•july 31, 2024
Climate change is reshaping our world, and the evidence is overwhelming. From rising temperatures to melting ice caps, the signs are clear. Scientists have gathered data from various sources, painting a picture of a warming planet that's changing faster than ever before.
The causes of climate change are both natural and human-made. While the Earth's climate has always fluctuated, human activities like burning fossil fuels and are now the main drivers. Understanding these factors is crucial for tackling the climate crisis and predicting future impacts.
Evidence for Climate Change
Temperature and Atmospheric Records
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- Long-term temperature records from multiple sources (surface stations, satellites, ocean buoys) demonstrate clear warming trend over past century
- Ice core samples provide detailed record of past atmospheric composition and temperature revealing significant recent changes compared to historical patterns
- Isotopic analysis of atmospheric carbon dioxide reveals increasing proportion of carbon from fossil fuel sources
- Satellite measurements of Earth's energy imbalance demonstrate more energy trapped in climate system than escaping to space
- Measured through instruments like NASA's Clouds and the Earth's Radiant Energy System (CERES)
- Shows Earth absorbing about 0.5 to 1 Watt per square meter more than it emits to space
Sea Level and Cryosphere Changes
- measurements from tide gauges and satellites show accelerating increase in global mean sea level
- Rate of sea level rise has increased from ~1.4 mm/year in 20th century to ~3.6 mm/year from 2006-2015
- Observations of , Arctic sea ice decline, and changes in plant and animal distributions provide visible evidence of climate change impacts
- Arctic sea ice extent decreased by ~13% per decade since 1979
- Mountain glaciers retreating worldwide (Alps, Himalayas, Andes)
- Shifts in plant hardiness zones and animal migration patterns
Paleoclimate and Proxy Data
- Paleoclimate data from tree rings, sediment cores, and coral reefs offer context for current climate changes relative to natural variability
- Tree rings provide annual temperature and precipitation records going back centuries to millennia
- Lake and ocean sediment cores can span millions of years, recording temperature, CO2 levels, and other climate indicators
- Proxy data sources help reconstruct past climates
- Pollen records indicate vegetation changes linked to climate shifts
- Stalagmites in caves record changes in rainfall patterns over thousands of years
Causes of Climate Change
Anthropogenic Factors
- Anthropogenic greenhouse gas emissions, primarily carbon dioxide from fossil fuel combustion and land-use changes, drive recent climate change
- CO2 concentrations increased from ~280 ppm in pre-industrial times to over 410 ppm today
- Deforestation and changes in land use alter Earth's albedo and carbon storage capacity, contributing to climate forcing
- Tropical deforestation releases stored carbon and reduces Earth's capacity to absorb CO2
- Urban expansion increases albedo in some areas, decreases it in others
- Industrial processes release various pollutants and aerosols with both warming and cooling effects on climate
- Sulfate aerosols from industrial emissions have a cooling effect
- Black carbon (soot) has a warming effect, especially when deposited on snow and ice
Natural Climate Forcings
- Natural climate forcings include variations in solar radiation, volcanic eruptions, and changes in Earth's orbit (Milankovitch cycles)
- Solar radiation varies on 11-year sunspot cycle and longer-term cycles
- Major volcanic eruptions (Mount Pinatubo 1991) can cause short-term cooling
- Milankovitch cycles operate on timescales of tens to hundreds of thousands of years
- Internal climate variability influences short-term climate patterns
- El Niño-Southern Oscillation (ENSO) affects global temperatures and precipitation patterns
- Pacific Decadal Oscillation (PDO) influences climate over decades
- Changes in ocean circulation patterns significantly impact regional and global climate
- Atlantic Meridional Overturning Circulation (AMOC) transports heat from tropics to North Atlantic
Greenhouse Gases and Climate
Greenhouse Effect Mechanisms
- (CO2, methane, water vapor) absorb and re-emit longwave radiation, trapping heat in Earth's atmosphere
- CO2 absorbs infrared radiation in 15-micron wavelength band
- Methane absorbs in 7.7-micron band
- Greenhouse effect maintains Earth's habitable temperature, but enhanced greenhouse gas concentrations intensify this effect
- Natural greenhouse effect warms Earth by ~33°C compared to no atmosphere
- Enhanced greenhouse effect causes additional warming
Greenhouse Gas Properties
- Different greenhouse gases have varying potencies and atmospheric lifetimes, quantified by Potential (GWP)
- Methane has GWP of 28-36 over 100 years
- Nitrous oxide has GWP of 265-298 over 100 years
- Carbon cycle regulates atmospheric CO2 concentrations through exchanges between atmosphere, biosphere, hydrosphere, and lithosphere
- Oceans absorb ~25% of anthropogenic CO2 emissions
- Terrestrial biosphere currently acts as net carbon sink
Climate Feedbacks and Forcings
- Positive feedback loops accelerate warming trends
- Methane release from thawing permafrost amplifies initial warming
- Ice-albedo feedback: melting ice exposes darker surfaces, increasing absorption of solar radiation
- Radiative forcing quantifies change in Earth's energy balance due to greenhouse gases and other climate forcings
- Measured in Watts per square meter (W/m²)
- Total anthropogenic radiative forcing in 2011 estimated at 2.29 W/m² ( AR5)
- Atmospheric water vapor content increases with temperature, amplifying greenhouse effect through positive feedback loop
- Water vapor feedback roughly doubles the warming from CO2 alone
Factors Influencing Climate Change
Attribution Studies and Modeling
- Attribution studies use climate models and statistical analyses to determine relative impact of different forcings on observed climate changes
- Fingerprinting techniques identify patterns of change associated with specific forcings
- Optimal fingerprinting methods quantify contribution of each forcing to observed changes
- IPCC regularly assesses and synthesizes scientific evidence to quantify contributions of various factors to climate change
- Comprehensive reports released every 5-7 years
- Special reports address specific topics (oceans and cryosphere, land use)
Natural vs. Anthropogenic Contributions
- Natural forcings alone cannot explain observed rate and magnitude of recent global warming
- Solar variability and volcanic activity insufficient to account for observed warming
- Anthropogenic greenhouse gas emissions estimated responsible for more than 100% of observed warming since 1950, partially offset by aerosol cooling effects
- Some anthropogenic aerosols (sulfates) have cooling effect, masking some greenhouse warming
- Regional climate changes may differ from global trends due to local factors
- Urban heat island effect can amplify warming in cities
- Land-use changes affect local temperature and precipitation patterns
Uncertainty and Climate Projections
- Detection and attribution techniques distinguish between natural variability and signals in observational data
- Statistical methods assess significance of observed trends
- Pattern-based approaches identify fingerprints of different forcings
- Uncertainty in climate projections arises from various sources, necessitating probabilistic approaches in climate impact assessments
- Future depend on socioeconomic factors and policy decisions
- Model limitations in representing complex Earth system processes
- Natural variability can mask or amplify anthropogenic trends on short timescales
Key Terms to Review (18)
Anthropogenic climate change: Anthropogenic climate change refers to the alterations in Earth's climate caused primarily by human activities, especially the burning of fossil fuels, deforestation, and industrial processes. These activities increase concentrations of greenhouse gases like CO2 and methane in the atmosphere, leading to global warming and significant shifts in climate patterns. Understanding this term is crucial for identifying the evidence and causes of climate change and addressing its impacts on the environment and society.
Biodiversity loss: Biodiversity loss refers to the decline in the variety and variability of life forms within a given ecosystem, including the reduction in species populations and extinction of species. This phenomenon is closely linked to environmental changes driven by human activities, such as habitat destruction, pollution, and climate change, which disrupt ecosystems and diminish their resilience.
Carbon footprint: A carbon footprint is the total amount of greenhouse gases, primarily carbon dioxide (CO₂), that are emitted directly and indirectly by an individual, organization, event, or product, usually measured in equivalent tons of CO₂. This measure helps to assess the environmental impact of various activities and lifestyle choices on climate change, as it quantifies how much each contributes to the greenhouse gas emissions that drive global warming.
Climate modeling: Climate modeling is the use of mathematical representations to simulate and predict climate behavior and changes over time based on various factors such as greenhouse gas emissions, land use, and solar radiation. These models help scientists understand past climate patterns, assess current climate conditions, and forecast future climate scenarios, providing critical insights into evidence and causes of climate change.
Deforestation: Deforestation is the large-scale removal of trees and forests, often resulting in damage to the quality of the land. This process disrupts the balance of Earth's energy budget, contributes to increased greenhouse gas concentrations, and affects various climate-related factors such as albedo. Additionally, deforestation plays a significant role in climate change by altering weather patterns and increasing the severity of extreme events, highlighting its profound impact on our environment.
Emission scenarios: Emission scenarios are projections of future greenhouse gas emissions based on various factors such as economic growth, technological advancements, and policy changes. These scenarios help in understanding the potential range of climate change impacts by illustrating how different choices can lead to different levels of atmospheric concentration of greenhouse gases, which is crucial for assessing future climate conditions and impacts.
Extreme weather events: Extreme weather events are significant and unusual atmospheric phenomena that cause severe impacts on the environment, human life, and infrastructure. These events can include hurricanes, tornadoes, floods, droughts, and heatwaves, often intensified by climate change. Their frequency and severity have been linked to global warming, which alters traditional weather patterns and leads to unpredictable climatic conditions.
Glacial retreat: Glacial retreat refers to the process by which glaciers lose mass and shrink in size due to melting, calving, and sublimation, particularly in response to rising temperatures. This phenomenon serves as a clear indicator of climate change, reflecting both local and global warming trends as glaciers respond to environmental changes and fluctuations in temperature.
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 warming impacts various components of the climate system, including atmospheric composition, energy balance, and climate classifications, resulting in significant changes to weather patterns and extreme events.
Greenhouse gases: Greenhouse gases are gases in the Earth's atmosphere that trap heat, contributing to the greenhouse effect, which warms the planet. These gases include carbon dioxide, methane, nitrous oxide, and water vapor, and they play a crucial role in regulating Earth's temperature by absorbing and re-emitting infrared radiation. Understanding greenhouse gases is essential because they directly influence the planet's energy balance, climate change, and overall weather patterns.
IPCC: The Intergovernmental Panel on Climate Change (IPCC) is a scientific body established by the United Nations to assess the science, impacts, and potential responses to climate change. It plays a crucial role in providing policymakers with comprehensive and authoritative information about climate change, including evidence of its causes and the projections for future climate scenarios.
James Hansen: James Hansen is a prominent climate scientist known for his work in understanding climate change and its impacts. His research has played a significant role in raising awareness about the urgent need for climate action and has influenced public policy related to environmental issues. Hansen's work links directly to the evidence and causes of climate change, climate modeling, and the associated impacts on weather patterns and extreme events.
Kyoto Protocol: The Kyoto Protocol is an international treaty that commits its parties to reduce greenhouse gas emissions, based on the premise that global warming exists and human-made CO2 emissions have caused it. Adopted in 1997 and entering into force in 2005, it represents a significant step in global efforts to address climate change and highlights the need for countries to take collective action to mitigate its effects.
Natural climate variability: Natural climate variability refers to the inherent fluctuations in climate that occur due to natural processes, such as volcanic eruptions, solar radiation changes, and ocean-atmosphere interactions. These variations can happen over various timescales, from months to millennia, and play a significant role in shaping the Earth's climate. Understanding natural climate variability is crucial for distinguishing between long-term climate trends and short-term changes caused by external factors.
Ocean acidification: Ocean acidification refers to the process by which the ocean becomes more acidic due to increased levels of carbon dioxide (CO2) in the atmosphere. When CO2 is absorbed by seawater, it reacts with water to form carbonic acid, leading to a decrease in pH levels. This phenomenon is closely linked to climate change, as rising global temperatures and increased CO2 emissions are driving both ocean acidification and broader environmental shifts.
Paris Agreement: The Paris Agreement is a landmark international accord adopted in 2015 aimed at combating climate change by limiting global warming to well below 2 degrees Celsius above pre-industrial levels, with efforts to limit the temperature increase to 1.5 degrees Celsius. It represents a collective commitment from nearly every country in the world to reduce greenhouse gas emissions, enhance resilience to climate impacts, and promote sustainable development, making it vital for addressing the evidence and causes of climate change, improving air quality, and implementing effective mitigation and adaptation strategies.
Sea level rise: Sea level rise refers to the increasing elevation of the ocean's surface, primarily driven by climate change through two main processes: the melting of glaciers and ice sheets and the thermal expansion of seawater as it warms. This phenomenon has significant implications for coastal regions, ecosystems, and human populations, highlighting the urgency of understanding its causes and effects on the planet's climate systems.
Temperature anomalies: Temperature anomalies refer to deviations from a long-term average temperature for a specific location and time period, often expressed in degrees Celsius or Fahrenheit. Understanding these anomalies is crucial as they help scientists track climate change and assess the impacts of rising global temperatures on various systems, including weather patterns, ecosystems, and human societies.