Part VIII: Climate Change Science
Observed changes, radiative forcing, carbon cycle dynamics, climate modeling, impacts, and mitigation pathways for anthropogenic climate change
1. Observed Climate Changes
The observational evidence for a changing climate is drawn from multiple independent measurement systems spanning surface thermometers, ocean profiling floats, satellite sensors, ice cores, and paleoclimate proxies. This chapter synthesizes the key lines of evidence as assessed by IPCC AR6 Working Group I (2021).
1.1 Global Temperature Records
Several independent research groups maintain global mean surface temperature (GMST) records. Despite using different raw data, homogenization methods, and spatial interpolation techniques, all datasets show consistent warming since the mid-19th century:
Major Temperature Datasets
Key Temperature Findings (IPCC AR6)
1.2 Ocean Heat Content and Deep Ocean Warming
The ocean is the primary reservoir for excess energy trapped by enhanced greenhouse forcing. Over 90% of the additional energy in the climate system since 1971 has been absorbed by the ocean. The fundamental relationship governing ocean heat uptake is:
$$\frac{dH}{dt} = N \cdot A$$
where \(H\) is ocean heat content (J), \(N\) is the net top-of-atmosphere (TOA) energy imbalance (W/m\(^2\)), and \(A\) is Earth's surface area (~5.1 x 10\(^{14}\) m\(^2\))
The ocean's thermal inertia introduces a crucial time lag: even if atmospheric greenhouse gas concentrations were stabilized today, the ocean would continue to warm for decades to centuries as it equilibrates with the new radiative forcing. This concept is central to understanding "committed warming."
1.3 Sea Level Rise
Global mean sea level (GMSL) is rising due to two principal mechanisms: thermal expansion of warming ocean water and addition of mass from melting ice sheets and glaciers. The total sea level budget can be expressed as:
$$\frac{d\eta}{dt} = \frac{1}{\rho_w A}\frac{dH}{dt}\cdot\frac{1}{c_p} \cdot \alpha + \frac{1}{\rho_w A}\frac{dM}{dt}$$
The first term is the thermosteric (thermal expansion) contribution and the second is the barystatic (mass addition) contribution, where \(\alpha\) is the thermal expansion coefficient, \(c_p\) is specific heat, \(\rho_w\) is water density, and \(M\) is ocean mass.
Observed Sea Level Budget (IPCC AR6)
1.4 Cryosphere Changes
The cryosphere -- encompassing sea ice, ice sheets, glaciers, snow cover, and permafrost -- is among the most sensitive and rapidly changing components of the climate system.
Arctic Sea Ice
September (minimum) Arctic sea ice extent is declining at approximately 13% per decade relative to the 1981-2010 average. Ice thickness has decreased by roughly 40% since submarine measurements of the 1980s. Multi-year ice (ice surviving at least one summer melt season) has decreased from ~30% of the Arctic Ocean in the 1980s to less than 5% today. Climate models project seasonally ice-free Arctic summers (defined as below 1 million km\(^2\)) as early as the 2040s under all emission scenarios. The loss of sea ice reduces surface albedo (ice-albedo feedback), amplifying Arctic warming.
Ice Sheets
Greenland: Mass loss has accelerated from ~34 Gt/yr (1992-2001) to ~280 Gt/yr (2012-2021), driven by both increased surface melt and calving of marine-terminating glaciers. Contains enough ice to raise GMSL by ~7.4 m if completely melted. Surface melt has expanded to higher elevations and further inland.
Antarctica: Net mass loss of ~150 Gt/yr (2012-2021), predominantly from West Antarctica where marine ice sheet instability (MISI) may be underway in the Thwaites and Pine Island glacier systems. East Antarctica has been roughly in balance or gaining mass from increased snowfall. Antarctica contains ~58 m of sea level equivalent.
Mountain Glaciers
Globally, glaciers have lost approximately 267 Gt/yr during 2000-2019 (Hugonnet et al., 2021). This is the largest contribution to sea level rise after thermal expansion. Retreat is essentially universal across all glacierized regions. Many small glaciers (below ~1 km\(^2\)) are projected to disappear entirely by 2100 regardless of emissions. Glacier loss threatens water security for approximately 2 billion people who depend on glacial meltwater for drinking, agriculture, and hydropower.
Permafrost
Permafrost temperatures have increased by 0.3-1.0°C between 2007 and 2019 at depths below seasonal influence. The active layer (seasonally thawing surface) is deepening. Northern Hemisphere permafrost contains an estimated 1,400-1,700 Gt of organic carbon -- roughly twice the atmospheric carbon pool. Thawing releases CO\(_2\) and CH\(_4\), constituting a positive feedback that is not yet fully incorporated into climate projections.
1.5 Precipitation and Hydrological Cycle Changes
The Clausius-Clapeyron relation dictates that the atmosphere's water vapor holding capacity increases at approximately 7%/°C. This thermodynamic constraint drives observed and projected changes in the hydrological cycle:
$$\frac{de_s}{dT} = \frac{L_v \, e_s}{R_v \, T^2}$$
Clausius-Clapeyron equation: saturation vapor pressure \(e_s\) increases exponentially with temperature \(T\)
1.6 Attribution of Observed Changes to Specific Forcings
Detection and attribution (D&A) methods use optimal fingerprinting to decompose observed climate changes into contributions from individual forcings. The approach uses general circulation model simulations driven by individual forcings to construct spatial-temporal fingerprint patterns, then employs total least squares regression to determine scaling factors:
$$\mathbf{y} = \sum_i \beta_i \mathbf{x}_i + \boldsymbol{\varepsilon}$$
where \(\mathbf{y}\) is the observed change vector, \(\mathbf{x}_i\) are model-simulated fingerprint patterns for forcing \(i\), \(\beta_i\) are scaling factors, and \(\boldsymbol{\varepsilon}\) is the residual (internal variability)
IPCC AR6 Key Attribution Findings
2. Radiative Forcing and Climate Sensitivity
Radiative forcing quantifies the change in Earth's energy budget due to an external perturbation. Climate sensitivity describes how much the climate system warms in response to that perturbation. Together, these concepts form the conceptual backbone of climate change science.
2.1 Radiative Forcing: Concept and Definition
Radiative forcing (RF) is defined as the net change in the energy flux at the tropopause (or top of atmosphere) caused by an imposed perturbation, after allowing stratospheric temperatures to readjust to radiative equilibrium but with surface and tropospheric temperatures held fixed. Units are W/m\(^2\). A positive RF warms the surface; a negative RF cools it.
IPCC AR6 introduced the concept of Effective Radiative Forcing (ERF), which additionally allows rapid adjustments in clouds, water vapor, and atmospheric temperature profiles to occur before computing the forcing. ERF better predicts the eventual temperature response than instantaneous RF because it captures fast tropospheric adjustments that modify the forcing before slow surface temperature feedbacks engage.
$$\text{ERF} = \text{RF}_{\text{inst}} + \text{rapid adjustments}$$
ERF includes rapid adjustments in tropospheric temperature, water vapor, clouds, and surface albedo that occur on timescales of days to weeks
2.2 Radiative Forcing from Greenhouse Gases
Simplified expressions for the radiative forcing from the principal well-mixed greenhouse gases have been derived from line-by-line radiative transfer calculations. The logarithmic dependence of CO\(_2\) forcing arises because the central absorption bands are nearly saturated, so additional CO\(_2\) primarily broadens the wings of absorption lines:
$$\Delta F_{CO_2} = 5.35 \ln\frac{C}{C_0}$$
where \(C\) is the current CO\(_2\) concentration (ppm) and \(C_0\) is the preindustrial reference (280 ppm). For a doubling, \(\Delta F_{2 \times CO_2} \approx 3.7\) W/m\(^2\)
$$\Delta F_{CH_4} = 0.036\left(\sqrt{M} - \sqrt{M_0}\right)$$
where \(M\) is CH\(_4\) concentration (ppb) and \(M_0\) is the preindustrial reference (~722 ppb). The square-root dependence reflects partial pressure broadening saturation.
$$\Delta F_{N_2O} = 0.12\left(\sqrt{N} - \sqrt{N_0}\right)$$
where \(N\) is N\(_2\)O concentration (ppb) and \(N_0 \approx 270\) ppb preindustrial
Total ERF Estimates (2019 relative to 1750, IPCC AR6)
2.3 Aerosol Forcing
Aerosols exert both direct and indirect effects on Earth's radiation budget. They represent the largest source of uncertainty in anthropogenic forcing:
Direct (Radiation-Aerosol Interaction, ERFari)
Aerosol particles scatter and absorb solar radiation. Sulfate aerosols are highly reflective (cooling effect), while black carbon absorbs strongly (warming). Net direct effect: approximately -0.3 W/m\(^2\).
Indirect (Cloud-Aerosol Interaction, ERFaci)
Aerosols act as cloud condensation nuclei (CCN), increasing cloud droplet number concentration and decreasing droplet size. This enhances cloud albedo (Twomey/first indirect effect) and may extend cloud lifetime (Albrecht/second indirect effect). Net indirect effect: approximately -0.8 W/m\(^2\), but with large uncertainty. This is the dominant source of total forcing uncertainty.
2.4 Equilibrium Climate Sensitivity (ECS)
Equilibrium Climate Sensitivity is defined as the equilibrium global mean surface temperature change following a sustained doubling of atmospheric CO\(_2\). It encapsulates the net effect of all climate feedbacks operating on timescales of decades to centuries:
$$\text{ECS} = -\frac{\Delta F_{2\times CO_2}}{\lambda}$$
where \(\Delta F_{2 \times CO_2} \approx 3.7\) W/m\(^2\) and \(\lambda\) is the net climate feedback parameter (W/m\(^2\)K\(^{-1}\)). Negative \(\lambda\) indicates a stable climate.
IPCC AR6 Assessment of ECS
2.5 Feedback Mechanisms
The net feedback parameter \(\lambda\) is the sum of the Planck response and individual feedback contributions:
$$\lambda = \lambda_0 + \sum_i \lambda_i = \lambda_{\text{Planck}} + \lambda_{\text{WV}} + \lambda_{\text{LR}} + \lambda_{\alpha} + \lambda_C$$
where \(\lambda_0 = \lambda_{\text{Planck}} \approx -3.2\) W/m\(^2\)K\(^{-1}\) is the reference Planck response (negative = stabilizing), and \(\lambda_i\) are individual feedbacks
Planck Response (\(\lambda_{\text{Planck}} \approx -3.2\) W/m\(^2\)K\(^{-1}\))
The fundamental stabilizing feedback. A warmer Earth emits more longwave radiation to space per Stefan-Boltzmann law. Without other feedbacks, a CO\(_2\) doubling would produce only ~1.2°C warming.
Water Vapor Feedback (\(\lambda_{\text{WV}} \approx +1.8\) W/m\(^2\)K\(^{-1}\))
The strongest positive feedback. Warming increases atmospheric water vapor (Clausius-Clapeyron), which is itself a potent greenhouse gas, trapping more outgoing longwave radiation. Well-understood and constrained by observations.
Lapse Rate Feedback (\(\lambda_{\text{LR}} \approx -0.6\) W/m\(^2\)K\(^{-1}\))
The tropical upper troposphere warms faster than the surface (moist adiabatic process), enhancing OLR emission to space -- a negative (stabilizing) feedback. Partially offsets water vapor feedback. At high latitudes, surface warms faster (surface-based inversions), contributing a positive feedback.
Ice-Albedo Feedback (\(\lambda_{\alpha} \approx +0.4\) W/m\(^2\)K\(^{-1}\))
Warming melts ice and snow, replacing high-albedo surfaces (reflectivity ~0.6-0.9) with low-albedo ocean or land (~0.06-0.3). More solar radiation is absorbed, amplifying the initial warming. Strongest at high latitudes.
Cloud Feedback (\(\lambda_C \approx +0.5\) W/m\(^2\)K\(^{-1}\))
The most uncertain feedback. Low clouds may decrease with warming (positive feedback via reduced reflection). High clouds may rise (positive feedback via enhanced greenhouse effect). IPCC AR6 assessed the net cloud feedback as positive but with a wide uncertainty range. Marine stratocumulus breakup (Schneider et al., 2019) represents a potential high-sensitivity scenario.
2.6 Transient Climate Response (TCR)
TCR is defined as the warming at the time of CO\(_2\) doubling in a 1%/year CO\(_2\) increase experiment. It is more policy-relevant than ECS because it reflects warming on multi-decadal timescales with ocean heat uptake still ongoing:
3. Carbon Cycle and Greenhouse Gases
Understanding the global carbon cycle is essential for projecting future atmospheric CO\(_2\) concentrations and climate change. The carbon cycle involves exchanges between four major reservoirs: the atmosphere, oceans, terrestrial biosphere, and lithosphere (geological).
3.1 Global Carbon Budget: Sources and Sinks
The Global Carbon Project publishes annual updates to the global carbon budget. For the decade 2013-2022:
Global Carbon Budget (2013-2022 average, Gt CO\(_2\)/yr)
The airborne fraction (fraction of emissions remaining in the atmosphere) has been remarkably stable at ~44% over the past six decades, implying that natural sinks have kept pace with growing emissions. However, this is not guaranteed to continue as the climate warms and carbon cycle feedbacks intensify.
3.2 The Keeling Curve and Seasonal Cycle
Continuous atmospheric CO\(_2\) measurements began at Mauna Loa Observatory, Hawaii, in 1958 under Charles David Keeling. The resulting "Keeling Curve" is one of the most iconic datasets in science, showing:
3.3 Ocean CO\(_2\) Uptake and Acidification
The ocean absorbs approximately 26% of anthropogenic CO\(_2\) emissions annually through physical and biological processes. When CO\(_2\) dissolves in seawater, it undergoes a series of chemical reactions that produce carbonic acid and lower ocean pH:
$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^- \rightleftharpoons 2H^+ + CO_3^{2-}$$
Ocean carbonate chemistry: dissolved CO\(_2\) forms carbonic acid, which dissociates to release hydrogen ions (lowering pH) and shifts the carbonate equilibrium
3.4 Terrestrial Carbon Sinks
The terrestrial biosphere currently absorbs approximately 28% of anthropogenic CO\(_2\) emissions. This sink is driven by two main effects:
3.5 Methane (CH\(_4\))
Methane is the second most important anthropogenic greenhouse gas. Its atmospheric concentration has risen from ~722 ppb (preindustrial) to ~1,920 ppb (2023), a 166% increase.
3.6 Global Warming Potential (GWP)
GWP provides a metric for comparing the climate impact of different greenhouse gases relative to CO\(_2\)over a specified time horizon (TH):
$$\text{GWP}_i = \frac{\int_0^{TH} a_i \cdot C_i(t)\,dt}{\int_0^{TH} a_{CO_2} \cdot C_{CO_2}(t)\,dt}$$
where \(a_i\) is the radiative efficiency (W/m\(^2\)/kg) of gas \(i\), \(C_i(t)\) is the time-dependent abundance after a pulse emission, and TH is the time horizon (typically 20 or 100 years)
GWP-100 Values for Key Greenhouse Gases (IPCC AR6)
| Gas | Lifetime | GWP-20 | GWP-100 |
|---|---|---|---|
| CO\(_2\) | Variable (centuries) | 1 | 1 |
| CH\(_4\) (fossil) | ~12 yr | 82.5 | 29.8 |
| N\(_2\)O | ~109 yr | 273 | 273 |
| SF\(_6\) | ~3200 yr | 18,300 | 25,200 |
| CFC-12 | ~100 yr | 11,200 | 12,500 |
3.7 Nitrous Oxide and Halocarbons
Nitrous Oxide (N\(_2\)O)
Halocarbons
3.8 Carbon Cycle Feedbacks
Climate-carbon cycle feedbacks can amplify or dampen future warming beyond what is predicted from emissions alone:
Permafrost Carbon Feedback
Thawing permafrost releases CO\(_2\) (aerobic decomposition) and CH\(_4\) (anaerobic, in waterlogged soils). Estimated potential release: 50-250 Gt C by 2100 under high-emission scenarios. This feedback is largely irreversible on human timescales and could add 0.1-0.4°C additional warming by 2100.
Wetland Methane Feedback
Warming increases microbial methanogenesis in wetlands. Tropical and boreal wetlands may expand under warming and increased precipitation. This constitutes a positive feedback that may already be contributing to the recent acceleration in atmospheric CH\(_4\) growth.
Weakening Ocean and Land Sinks
As the ocean warms, its CO\(_2\) solubility decreases. Increasing stratification reduces deep-water ventilation, limiting carbon sequestration at depth. On land, increased respiration rates under warming may eventually exceed gains from CO\(_2\) fertilization. Models suggest the combined sink efficiency will decline from ~55% to ~40-50% by 2100 under high-emission pathways.
4. Climate Models and Projections
Climate models are mathematical representations of the climate system that solve the fundamental equations of fluid dynamics, thermodynamics, and radiative transfer on a discretized grid. They are the primary tools for projecting future climate change.
4.1 Model Hierarchy
Climate models span a hierarchy of complexity, each suited to different questions:
Energy Balance Models (EBMs)
The simplest climate models. A zero-dimensional EBM treats Earth as a single point with one temperature:
$$C\frac{dT}{dt} = \Delta F - \lambda T$$
where \(C\) is effective heat capacity (J/m\(^2\)K), \(T\) is temperature anomaly, \(\Delta F\) is radiative forcing, and \(\lambda\) is the feedback parameter. At equilibrium, \(T_{eq} = \Delta F / \lambda\).
One- and two-dimensional EBMs add latitudinal structure and meridional heat transport. Useful for understanding fundamental climate behavior, ice-albedo instability, and sensitivity analysis.
General Circulation Models (GCMs)
Full 3D models solving the Navier-Stokes equations on a rotating sphere. Atmosphere-only GCMs (AGCMs) are coupled to ocean GCMs (OGCMs) to form Atmosphere-Ocean GCMs (AOGCMs). Typical resolution: ~100 km horizontal, 30-80 vertical levels. Subgrid-scale processes (convection, turbulence, cloud microphysics) are parameterized. These are the workhorses of climate projection, with ~40 modeling centers worldwide contributing to CMIP.
Earth System Models (ESMs)
ESMs extend AOGCMs by incorporating interactive biogeochemical cycles (carbon, nitrogen), dynamic vegetation, atmospheric chemistry, aerosol microphysics, ice sheet dynamics, and sometimes interactive ice sheets. ESMs can project atmospheric CO\(_2\) concentrations given emission scenarios (concentration-driven vs. emission-driven mode). Examples: CESM2, UKESM1, GFDL-ESM4, MPI-ESM1.2.
High-Resolution and AI-Enhanced Models
Emerging approaches include convection-permitting models (<5 km resolution), which explicitly resolve deep convection rather than parameterizing it. Machine learning is increasingly used for parameterization development, bias correction, downscaling, and emulation of expensive ESM components. Foundation models for weather (e.g., GraphCast, Pangu-Weather) are being extended to climate timescales.
4.2 Shared Socioeconomic Pathways (SSPs)
The SSP framework combines socioeconomic narratives (SSP1-5) with Representative Concentration Pathways (RCPs) to create integrated scenarios for CMIP6. The SSP number describes the socioeconomic storyline, and the suffix indicates the approximate radiative forcing level in 2100:
SSP1-1.9 (Very Low Emissions)
Sustainability-focused development. CO\(_2\) emissions reach net-zero by ~2050, slight net-negative thereafter. Warming peaks at ~1.5°C before declining. Requires rapid transformation of energy, transport, land use, and industrial systems. Strong international cooperation. Radiative forcing declines to ~1.9 W/m\(^2\) by 2100.
SSP1-2.6 (Low Emissions)
Sustainability pathway with less aggressive mitigation. CO\(_2\) emissions reach net-zero by ~2075. Warming stabilizes at ~1.8°C by 2100 (best estimate). Broadly consistent with the Paris Agreement goal of "well below 2°C." Radiative forcing stabilizes near 2.6 W/m\(^2\).
SSP2-4.5 (Intermediate Emissions)
"Middle of the road" scenario. Moderate mitigation efforts. CO\(_2\) emissions roughly stable until mid-century, then declining. Atmospheric CO\(_2\) reaches ~600 ppm by 2100. Warming of ~2.7°C by 2100 (best estimate, range 2.1-3.5°C). Close to the trajectory implied by current policies and pledges.
SSP3-7.0 (High Emissions)
Regional rivalry narrative. Nationalism, slow economic development, high population growth. Emissions double by 2100. CO\(_2\) reaches ~850 ppm. Warming of ~3.6°C (range 2.8-4.6°C). High aerosol emissions partially offset GHG warming. Used as a high-forcing scenario distinct from the fossil-fuel intensive SSP5-8.5.
SSP5-8.5 (Very High Emissions)
Fossil-fueled development with rapid economic growth and energy-intensive lifestyles. CO\(_2\) exceeds 1000 ppm by 2100. Warming of ~4.4°C (range 3.3-5.7°C). Now widely regarded as implausible given current energy transitions, but remains useful as a worst-case benchmark and for studying high-end risks.
4.3 CMIP6 Projected Warming Ranges
Global Mean Temperature Change by 2081-2100 (relative to 1850-1900)
| Scenario | Best Estimate | Very Likely Range | Sea Level (m) |
|---|---|---|---|
| SSP1-1.9 | 1.4°C | 1.0 - 1.8°C | 0.28 - 0.55 |
| SSP1-2.6 | 1.8°C | 1.3 - 2.4°C | 0.32 - 0.62 |
| SSP2-4.5 | 2.7°C | 2.1 - 3.5°C | 0.44 - 0.76 |
| SSP3-7.0 | 3.6°C | 2.8 - 4.6°C | 0.55 - 0.90 |
| SSP5-8.5 | 4.4°C | 3.3 - 5.7°C | 0.63 - 1.01 |
CMIP6 multi-model ensemble results, assessed by IPCC AR6 WG1. Sea level includes low-confidence ice sheet contributions.
4.4 Regional Patterns of Change
4.5 Tipping Points and Cascading Risks
Climate tipping elements are large-scale components of the Earth system that may undergo abrupt, self-reinforcing, and potentially irreversible transitions once a critical threshold is crossed. McKay et al. (2022) assessed 16 tipping elements:
Critical Tipping Elements and Estimated Thresholds
Video: Climate Science - What You Need To Know
Comprehensive overview of climate change science, models, and projections
5. Impacts and Adaptation
Climate change impacts are already being observed across all continents and oceans. Future impacts scale with warming magnitude, and every increment of warming matters. This chapter covers physical impacts, sectoral vulnerabilities, extreme event attribution, and adaptation strategies.
5.1 Sea Level Rise Projections and Coastal Impacts
5.2 Extreme Event Attribution
Attribution science has matured to the point where the influence of climate change on individual extreme events can be quantified rapidly, often within days. The standard metric is the Fraction of Attributable Risk (FAR):
$$\text{FAR} = 1 - \frac{P_0}{P_1}$$
where \(P_0\) is the probability of the event in a counterfactual world without human influence, and \(P_1\) is the probability in the actual (factual) world. FAR = 0.9 means human influence made the event 10x more likely.
A related metric is the probability ratio (risk ratio): \(\text{RR} = P_1 / P_0 = 1/(1 - \text{FAR})\). An equivalent approach examines changes in event intensity at fixed return periods.
Notable Attribution Studies
5.3 Food Security and Agriculture
5.4 Water Resources
5.5 Ecosystems and Biodiversity
5.6 Human Health Impacts
Heat-Related Mortality
Heat-related deaths have increased ~68% since 2000. Outdoor labor capacity declining in tropics. At 2°C, 37% of the global population exposed to severe heat waves at least once every 5 years. Wet-bulb temperatures approaching human survivability limits (35°C) in parts of South Asia and Persian Gulf.
Vector-Borne Disease
Warming expands the geographic range and season length for mosquito-borne diseases. Dengue risk population doubles by 2080. Malaria expanding to higher elevations in East Africa. Lyme disease range expanding northward. Vibrio infections increasing in warming coastal waters.
Air Quality
Higher temperatures increase ground-level ozone formation. Wildfire smoke exposure increasing dramatically. Allergenic pollen seasons lengthening and intensifying with warming and CO\(_2\) fertilization.
Mental Health and Displacement
Climate anxiety increasing, particularly among youth. Extreme events cause PTSD, depression, and displacement. By 2050, climate change may force 200+ million people to migrate within their countries.
5.7 Climate Adaptation Strategies
Adaptation reduces climate risks by adjusting practices, processes, and structures to actual or expected climate change. Effective adaptation is place-specific, iterative, and integrates with development planning:
Infrastructure and Engineering
Sea walls, storm surge barriers (e.g., Thames Barrier, MOSE), elevated construction, improved drainage systems, building codes for extreme heat and wind, cooling infrastructure, urban green spaces to reduce heat island effects.
Agricultural Adaptation
Heat- and drought-tolerant crop varieties (CRISPR-edited and conventional breeding), shifting planting dates, improved irrigation efficiency (drip, deficit irrigation), diversified cropping systems, agroforestry, index-based crop insurance, climate-informed agricultural extension services.
Ecosystem-Based Adaptation
Mangrove and wetland restoration for coastal protection, urban forests for cooling, watershed protection for water security, biodiversity corridors to facilitate species migration, coral reef restoration and assisted evolution.
Early Warning and Preparedness
Multi-hazard early warning systems, heat action plans, flood forecasting and evacuation planning, climate-resilient health systems, social protection programs linked to climate triggers. The UN targets early warning systems for all by 2027.
Adaptation Limits and Maladaptation
"Hard" limits exist beyond which adaptation is no longer possible (e.g., lethal wet-bulb temperatures, coral thermal tolerance, small island inundation). "Soft" limits are defined by financial, institutional, and social constraints. Maladaptation -- actions that increase vulnerability -- must be avoided (e.g., coastal development behind sea walls, fossil fuel lock-in).
6. Mitigation and Policy
Mitigation encompasses actions to reduce the sources or enhance the sinks of greenhouse gases. The goal is to limit global warming to levels that avoid the most dangerous impacts. This chapter examines the policy frameworks, carbon budgets, technological pathways, and economic instruments available for climate mitigation.
6.1 The Paris Agreement and the 1.5°C Target
Adopted in December 2015 and entered into force in November 2016, the Paris Agreement is the landmark international treaty on climate change. Nearly all nations (196 parties) have ratified it.
Key Elements of the Paris Agreement
6.2 Remaining Carbon Budgets
The carbon budget is the total amount of CO\(_2\) that can still be emitted while limiting warming to a given temperature target with a stated probability. The approximately linear relationship between cumulative CO\(_2\) emissions and temperature change underpins this concept:
$$\Delta T \approx \text{TCRE} \times \sum E_{CO_2}$$
where TCRE is the Transient Climate Response to Cumulative Emissions (~0.45°C per 1000 Gt CO\(_2\), likely range 0.27-0.63°C), and \(\sum E_{CO_2}\) is cumulative emissions
Remaining Carbon Budgets (from January 2020, IPCC AR6)
| Temperature Limit | Probability | Budget (Gt CO\(_2\)) | Years at Current Rate |
|---|---|---|---|
| 1.5°C | 50% | 500 | ~12 years |
| 1.5°C | 67% | 400 | ~10 years |
| 2.0°C | 50% | 1,350 | ~33 years |
| 2.0°C | 67% | 1,150 | ~28 years |
Note: ~200 Gt CO\(_2\) have been emitted since Jan 2020, reducing remaining budgets accordingly. The 1.5°C (50%) budget may be exhausted by ~2030.
6.3 Mitigation Pathways and Technologies
Achieving net-zero CO\(_2\) emissions -- a requirement for stabilizing global temperature -- demands a comprehensive transformation of energy, industry, transport, and land use systems:
Energy System Transformation
Solar PV and wind are now the cheapest forms of new electricity generation in most regions. For 1.5°C pathways: renewables must supply 60-80% of electricity by 2030 and virtually 100% by 2050. Coal phase-out by ~2040 globally. Natural gas use peaks by 2025 and declines rapidly. Nuclear power provides low-carbon baseload. Energy storage (batteries, green hydrogen, pumped hydro) addresses intermittency. Grid modernization and expansion are critical enablers.
Carbon Dioxide Removal (CDR)
Most 1.5°C pathways require CDR to compensate for residual emissions in hard-to-abate sectors:
Hard-to-Abate Sectors
Heavy industry (steel, cement, chemicals), long-haul aviation, and shipping require specific solutions: green hydrogen for steel (direct reduced iron); CCS for cement process emissions; sustainable aviation fuels (SAF) and eventually electric/hydrogen aircraft; ammonia or methanol for shipping. These sectors contribute ~30% of global CO\(_2\) emissions and cannot be easily electrified.
6.4 Net-Zero Emissions Timelines
6.5 Climate Economics: Social Cost of Carbon
The Social Cost of Carbon (SCC) estimates the economic damages from emitting one additional tonne of CO\(_2\). It is used in cost-benefit analyses and regulatory impact assessments:
6.6 The Kaya Identity
The Kaya Identity decomposes total CO\(_2\) emissions into four driving factors, providing a framework for understanding mitigation levers:
$$CO_2 = P \times \frac{GDP}{P} \times \frac{E}{GDP} \times \frac{CO_2}{E}$$
where \(P\) = population, \(GDP/P\) = per-capita income, \(E/GDP\) = energy intensity of the economy, and \(CO_2/E\) = carbon intensity of energy
Decomposition of Emission Drivers (2010-2019, IPCC AR6 WG3)
6.7 Policy Instruments
Carbon Pricing
Carbon taxes and emissions trading systems (ETS) now cover ~23% of global emissions. EU ETS prices reached ~100 EUR/t in 2023. Effective when prices are high enough ($50-100+/t CO\(_2\)) and revenue is recycled equitably. Carbon border adjustment mechanisms (CBAM) address competitiveness concerns.
Regulations and Standards
Vehicle emission standards, building energy codes, appliance efficiency standards, renewable portfolio standards, methane emission regulations, phaseout dates for coal power and ICE vehicles. Often more politically feasible than carbon pricing.
Industrial Policy and Subsidies
US Inflation Reduction Act ($369B clean energy incentives), EU Green Deal, China's renewable manufacturing dominance. Feed-in tariffs, tax credits, and loan guarantees have driven dramatic cost reductions in solar (~90% decline since 2010), wind (~70%), and batteries (~90%).
International Cooperation
Paris Agreement ratchet mechanism, Global Methane Pledge, Kigali Amendment (HFCs), Just Energy Transition Partnerships (JETPs), climate finance flows, technology transfer agreements, loss and damage funding.
The Bottom Line
The 1.5°C target is still technically feasible but requires immediate, unprecedented action: emissions must peak before 2025, decline 43% by 2030, and reach net-zero CO\(_2\) by 2050. Every fraction of a degree matters: impacts at 2°C are substantially worse than at 1.5°C. The technologies exist; the challenge is political will, financial mobilization, and equitable implementation. Global climate finance must increase 3-6x from current levels ($630 billion/yr in 2020) to meet needs ($3-6 trillion/yr by 2030).
Summary
Part VIII has provided a comprehensive synthesis of climate change science, from the observational evidence through the physical mechanisms, carbon cycle dynamics, model projections, impacts, and societal response options:
- Chapter 1 -- Observed Changes: Multiple independent datasets confirm +1.1°C warming since preindustrial, with accelerating trends in temperature, sea level rise (3.7 mm/yr), ocean heat content, ice sheet mass loss, glacier retreat, permafrost thaw, and hydrological cycle intensification. Detection and attribution methods confirm human activities are the dominant cause.
- Chapter 2 -- Radiative Forcing and Sensitivity: Anthropogenic ERF is +2.72 W/m\(^2\), dominated by CO\(_2\) (+2.16 W/m\(^2\)) and partially offset by aerosols (-1.1 W/m\(^2\)). ECS is assessed at 3.0°C (likely 2.5-4.0°C). Key feedbacks include water vapor (+1.8), lapse rate (-0.6), ice-albedo (+0.4), and cloud (+0.5 W/m\(^2\)K\(^{-1}\)).
- Chapter 3 -- Carbon Cycle: Total emissions ~41 Gt CO\(_2\)/yr with ~47% remaining in the atmosphere. Ocean and land sinks absorb the remainder but may weaken. Ocean acidification threatens marine calcifiers. Methane and N\(_2\)O are significant additional forcing agents. Carbon cycle feedbacks (permafrost, weakening sinks) may amplify warming.
- Chapter 4 -- Models and Projections: CMIP6 models project 1.4-4.4°C warming by 2100 depending on emissions pathway. Tipping points (ice sheets, Amazon, AMOC, permafrost) present risks of abrupt, irreversible change, with cascading effects possible above 2°C.
- Chapter 5 -- Impacts and Adaptation: Climate impacts are already severe and will intensify with every increment of warming. Extreme event attribution quantifies human influence on individual events. Adaptation is essential but has limits. Compound and cascading risks are an emerging concern.
- Chapter 6 -- Mitigation and Policy: The Paris Agreement provides the framework; remaining carbon budgets define the constraint. Net-zero CO\(_2\) by ~2050 is required for 1.5°C. The Kaya Identity clarifies that decarbonizing energy supply is the primary lever. Carbon removal technologies are needed but not a substitute for rapid emission reductions.
Climate change is the defining scientific and societal challenge of our time. The physical science is unequivocal: human activities have warmed the planet, and continued emissions will cause further warming and associated impacts. The tools to limit warming exist -- what remains is the collective will to deploy them at the speed and scale required. Understanding the science is the foundation for informed decision-making at individual, institutional, national, and global scales.
Video: How Do We Know Humans Are Causing Climate Change?
Carbon Brief explainer on detection and attribution of climate change