🌡️Intro to Climate Science Unit 7 – Climate Forcings and Feedbacks

Key Concepts and Definitions

• Climate forcings are factors that alter Earth's energy balance and drive changes in global temperature
• Positive forcings warm the climate system while negative forcings cool it
• Radiative forcing measures the change in energy flux at the top of the atmosphere due to a specific factor
• Climate feedbacks are processes that amplify or dampen the initial response to a forcing
• Positive feedbacks enhance the initial change (ice-albedo feedback) while negative feedbacks counteract it (blackbody radiation)
• Climate sensitivity quantifies the temperature change in response to a doubling of atmospheric CO2 concentration
  • Equilibrium climate sensitivity (ECS) refers to the long-term change after the system has reached a new equilibrium
  • Transient climate response (TCR) measures the change at the time of CO2 doubling, before the system has fully adjusted
• Tipping points are thresholds beyond which the climate system undergoes rapid, irreversible changes (melting of the Greenland ice sheet)

Types of Climate Forcings

• Greenhouse gases (CO2, methane, water vapor) absorb and re-emit infrared radiation, trapping heat in the atmosphere
• Aerosols are tiny particles suspended in the atmosphere that can have cooling (sulfates) or warming (black carbon) effects
• Changes in solar irradiance, such as variations in the 11-year sunspot cycle, affect the amount of energy reaching Earth
• Volcanic eruptions release sulfur dioxide, which forms reflective aerosols that temporarily cool the climate
• Land use changes, like deforestation, alter surface albedo and evapotranspiration rates
• Variations in Earth's orbit (Milankovitch cycles) cause long-term changes in the distribution of solar energy
  • Eccentricity: shape of Earth's orbit around the sun (100,000-year cycle)
  • Obliquity: tilt of Earth's axis relative to its orbital plane (41,000-year cycle)
  • Precession: wobble of Earth's axis (23,000-year cycle)

Natural vs. Anthropogenic Forcings

• Natural forcings operate independently of human activities and include solar variability, volcanic eruptions, and orbital changes
• Anthropogenic forcings result from human activities, primarily the burning of fossil fuels and land use changes
• Greenhouse gas emissions from human activities have become the dominant forcing since the Industrial Revolution
  • Atmospheric CO2 levels have risen from ~280 ppm in pre-industrial times to over 410 ppm today
  • Methane concentrations have more than doubled due to agriculture, livestock, and fossil fuel extraction
• Anthropogenic aerosols, while having a net cooling effect, contribute to air pollution and have adverse health impacts
• Deforestation and urbanization have altered surface properties and regional climate patterns
• The rate and magnitude of anthropogenic forcings are unprecedented in Earth's recent history

Major Climate Feedback Mechanisms

• Water vapor feedback: warmer air holds more moisture, enhancing the greenhouse effect
• Ice-albedo feedback: melting ice and snow expose darker surfaces, absorbing more solar radiation
• Cloud feedback: changes in cloud cover and properties can have both warming and cooling effects
  • Low, thick clouds reflect sunlight (cooling) while high, thin clouds trap heat (warming)
  • The net effect of cloud feedback remains a significant source of uncertainty in climate models
• Lapse rate feedback: as the atmosphere warms, the rate of temperature decrease with height (lapse rate) changes, affecting heat transfer
• Carbon cycle feedbacks: warming can release CO2 from oceans and permafrost, amplifying the initial forcing
  • Permafrost thaw exposes organic matter to decomposition, releasing CO2 and methane
  • Warmer oceans absorb less CO2 and can become a net carbon source
• Vegetation feedbacks: climate-induced changes in plant growth and distribution can affect albedo, evapotranspiration, and carbon storage

Radiative Forcing and Energy Balance

• Earth's energy balance is determined by the difference between incoming solar radiation and outgoing infrared radiation
• Radiative forcing is expressed in watts per square meter (W/m^2) and quantifies the change in energy balance due to a specific factor
• Positive radiative forcing (greenhouse gases) means more energy is retained in the Earth system, leading to warming
• Negative radiative forcing (reflective aerosols) means more energy is reflected back to space, leading to cooling
• The current net anthropogenic radiative forcing is estimated to be around +2.3 W/m^2, primarily due to greenhouse gas emissions
• Radiative forcing can be used to compare the relative contributions of different climate drivers
  • CO2 has the largest radiative forcing (+1.68 W/m^2), followed by methane (+0.97 W/m^2) and halocarbons (+0.18 W/m^2)
  • Aerosols have a net negative forcing (-0.27 W/m^2 for direct effect, -0.55 W/m^2 for indirect cloud effects)

Climate Sensitivity and Tipping Points

• Equilibrium climate sensitivity (ECS) is estimated to be between 1.5°C and 4.5°C for a doubling of atmospheric CO2
  • This range reflects uncertainties in climate feedbacks, particularly cloud feedback
  • A higher ECS means more warming for a given increase in greenhouse gas concentrations
• Transient climate response (TCR) is lower than ECS because it doesn't allow the deep ocean to fully adjust
  • TCR is estimated to be between 1.0°C and 2.5°C for a doubling of CO2
  • TCR is more relevant for near-term climate projections (decades to a century)
• Tipping points are critical thresholds beyond which the climate system undergoes abrupt, often irreversible changes
  • Examples include the collapse of the West Antarctic Ice Sheet, the shutdown of the Atlantic Meridional Overturning Circulation (AMOC), and the dieback of the Amazon rainforest
  • Tipping points are difficult to predict and represent high-impact, low-probability events
• The risk of crossing tipping points increases with higher levels of global warming
  • Limiting warming to 1.5°C or 2°C, as outlined in the Paris Agreement, reduces the likelihood of triggering major tipping points

Measuring and Modeling Forcings and Feedbacks

• Radiative forcings can be measured directly (solar irradiance) or estimated from changes in atmospheric composition (greenhouse gases)
  • Satellites (CERES) measure the Earth's energy balance at the top of the atmosphere
  • Ice cores provide a record of past greenhouse gas concentrations and climate forcings (volcanic sulfates)
• Climate models simulate the complex interactions between forcings, feedbacks, and the climate system
  • Models range from simple energy balance models to comprehensive Earth System Models (ESMs)
  • ESMs couple the atmosphere, ocean, land surface, and sea ice components, and include biogeochemical cycles
• Models are evaluated against observations and paleoclimate records to assess their performance and reliability
  • The Coupled Model Intercomparison Project (CMIP) coordinates climate model experiments and projections
  • The latest generation of models (CMIP6) includes more detailed representations of forcings and feedbacks
• Uncertainty in climate projections arises from internal variability, model differences, and future emission scenarios
  • Internal variability refers to natural fluctuations in the climate system (El Niño-Southern Oscillation)
  • Model differences reflect the range of climate sensitivities and feedback strengths across models
  • Emission scenarios (Representative Concentration Pathways) explore different pathways of future greenhouse gas concentrations

Implications for Future Climate Change

• The magnitude and rate of future climate change depend on the interplay between forcings, feedbacks, and human actions
• Continued greenhouse gas emissions will lead to further warming and increase the likelihood of severe, irreversible impacts
  • Global temperature is projected to rise by 2.6°C to 4.8°C by 2100 under a high emission scenario (RCP8.5)
  • Sea level could rise by 0.3 to 1.0 meter by 2100, threatening coastal communities and infrastructure
• Positive feedbacks, such as the ice-albedo feedback and permafrost thaw, could amplify warming and accelerate climate change
  • The Arctic is warming twice as fast as the global average, with summer sea ice projected to disappear by mid-century
  • Permafrost thaw could release up to 150 billion tons of carbon by 2100, equivalent to 15 years of current human emissions
• Tipping points, if crossed, could lead to abrupt, large-scale changes in the climate system
  • The collapse of the West Antarctic Ice Sheet could raise sea level by several meters over centuries to millennia
  • The shutdown of the AMOC could disrupt global ocean circulation and weather patterns
• Mitigating climate change requires reducing greenhouse gas emissions and enhancing carbon sinks
  • The Paris Agreement aims to limit warming to well below 2°C, with efforts to reach 1.5°C
  • Achieving these targets requires rapid decarbonization of the energy system and large-scale deployment of negative emission technologies (afforestation, bioenergy with carbon capture and storage)
• Adapting to the impacts of climate change is necessary to reduce risks and build resilience
  • Adaptation measures include coastal protection, water resource management, and climate-resilient infrastructure
  • Nature-based solutions, such as ecosystem restoration and green infrastructure, can provide multiple benefits for adaptation and mitigation