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Main source of uncertainty in Numerical Weather Prediction (NWP)

Uncertainty in the initial conditions of the atmosphere.

Main source of uncertainty in long-term climate projections

Uncertainty in boundary conditions, particularly future greenhouse gas emissions.

Reason emissions scenarios do not significantly influence weather forecasts

Weather forecasts operate over short timescales, during which long-term emissions trajectories exert negligible influence.

Core Earth system components simulated by a climate model

The atmosphere, ocean, land surface, and cryosphere, along with their mutual interactions.

Two ways increased computing power enhances climate modeling

It enables the inclusion of additional physical processes/components and allows simulations at finer spatial resolutions.

Primary temporary climate effect of explosive volcanic eruptions

Temporary global cooling caused by reflective aerosol plumes in the stratosphere.

Polar amplification mechanism

A phenomenon where polar regions warm faster than the global mean due to positive feedbacks, including declining snow and ice extent and lower surface albedo.

Comparison of land versus ocean warming rates

Land surfaces warm more rapidly than oceans due to lower heat capacity and differences in evaporative cooling.

Coexistence of short-term variability and long-term climate trends

Internal natural climate variability causes year-to-year fluctuations superimposed on an underlying multi-decadal warming trend.

Shared Socioeconomic Pathway (SSP)

A framework describing plausible societal development trajectories and associated greenhouse gas emissions based on variables such as population, trade, and inequality.

Projected warming difference by 2100 between SSP5-8.5 and SSP1-2.6

Approximately $3\,^{\circ}\text{C}$.

Internal climate variability

Natural fluctuations that originate from internal dynamical processes and energy transfers within the climate system, such as ENSO.

External climate forcings

Drivers originating outside the internal climate system, including solar irradiance variations, orbital shifts, volcanic eruptions, and anthropogenic emissions.

Reasons climate models diverge under the same emissions pathway (SSP)

Differences in internal climate variability and differing structural assumptions, parameterizations, and physics between models.

Three main structural differences among climate models

Spatial resolution and gridbox size, parameterization of aerosols, and mathematical representations of sub-grid physical processes.

Climate sensitivity

The equilibrium rise in global mean surface temperature resulting from a sustained doubling of atmospheric $CO_2$ concentration.

Likely range of equilibrium climate sensitivity

$2.5\text{ to }4\,^{\circ}\text{C}$ of warming per doubling of $CO_2$.

Very likely range of equilibrium climate sensitivity

$2\text{ to }5\,^{\circ}\text{C}$ of warming per doubling of $CO_2$.

Primary benefit of narrowing uncertainty in climate sensitivity

It increases precision and confidence in future global warming projections.

Dominant source of uncertainty in late-21st-century climate projections

The chosen trajectory of future greenhouse gas emissions (the emissions scenario or SSP).

Dominant sources of uncertainty in near-term (early-21st-century) projections

Internal natural climate variability and differences among climate model structures.

IPCC multi-model assessment strategy

Synthesizing results across an ensemble of independent global models tested under standardized experiment protocols and common SSP scenarios.

Consensus scientific attribution of recent long-term warming

Human activities are unequivocally the dominant driver of observed long-term global warming.

Method for testing human attribution using climate simulations

Scientists execute simulations with natural-only forcings versus combined natural and anthropogenic forcings, comparing the outputs against real-world observations.

Model behavior when human forcings are included

Climate models accurately reproduce observed 20th- and 21st-century warming only when anthropogenic greenhouse gas and aerosol forcings are included.

Climate attribution

The scientific process of determining the degree to which observed climate shifts or specific events can be attributed to distinct causal factors.

Key empirical evidence demonstrating human-driven climate change

Observed temperature records track simulations containing human-induced radiative forcings and diverge sharply from simulations driven solely by natural factors.

Six physical indicators of climate change beyond surface temperature

Sea level rise, shrinking sea ice extent, glacial retreat, ocean coral bleaching, shifting precipitation distributions, and changing extreme weather frequencies.

Scientific utility of global average surface temperature

It provides a single diagnostic metric reflecting the overall thermodynamic state and net radiative imbalance of the Earth.

Importance of evaluating regional and local climate changes

Societies and ecosystems experience climate locally, and geographic manifestations of warming differ substantially from the global average.

Distinction between climate observation and climate attribution

Observation measures and tracks physical changes in the climate system; attribution evaluates the causal mechanisms responsible for those changes.

Primary determinant of future global warming levels

The cumulative volume and trajectory of future anthropogenic greenhouse gas emissions.

Observed global mean sea level rise since the mid-19th century

Approximately $20\,\text{cm}$.

Projected sea level rise through the 21st century and beyond

Approximately $40\text{ to }70\,\text{cm}$.

Projected trends in heatwaves versus cold extremes

Heatwaves are projected to become more frequent and severe, while cold extremes will decline in frequency and severity.

Ratio of new record-high to record-low temperatures in Australia (2000–2015)

Approximately $12:1$ in favor of record highs.

Clausius–Clapeyron scaling in atmospheric physics

The physical principle establishing that the moisture-holding capacity of the air increases by roughly $7\%$ per $1\,^{\circ}\text{C}$ of atmospheric warming.

Mechanism linking air temperature to extreme precipitation intensity

Higher temperatures allow the air to hold more water vapor ($\sim 7\%\,\text{per}\,1\,^{\circ}\text{C}$), supplying larger moisture volumes when storm dynamics trigger precipitation.

Relationship between climate change and individual extreme rainfall events

Climate change alters event probability and precipitation intensity, but individual rainfall events cannot be attributed exclusively to climate change.

Mechanism driving increased drought risk under climate change

Altered rainfall distributions accompanied by heightened evaporative demand resulting from higher ambient temperatures.

Physical explanation for coexisting increases in flash flood and drought risks

Precipitation patterns concentrate rainfall into heavier, episodic downpours, while intervening periods experience lengthened dry spells and elevated evaporation.

Projected long-term trend for Arctic sea ice

A sustained decline in seasonal ice extent, volume, and multi-year ice cover.

Projected timeframe for a seasonally ice-free Arctic Ocean

Likely to occur during the summer seasonal minimum prior to $2050$ under all major SSP trajectories.

Direct consequence of marine heatwaves on coral reefs

Thermal stress causes the expulsion of symbiotic algae, precipitating coral bleaching and widespread coral mortality.

Impact of the 2016–2017 bleaching event on the Great Barrier Reef

Severe bleaching across most surveyed reefs, culminating in the death of roughly half of all shallow-water corals.

Core warming targets established by the 2015 Paris Agreement

Keeping warming well below $2\,^{\circ}\text{C}$ above pre-industrial levels while pursuing efforts to limit warming to $1.5\,^{\circ}\text{C}$.

Primary climatic benefit of limiting warming to $1.5\,^{\circ}\text{C}$ over $2\,^{\circ}\text{C}$

Substantial reductions in the frequency, geographic extent, and intensity of extreme heat events and dangerous ecological tipping points.