Climate projections use models to estimate how climate will change under different emissions scenarios. Understanding how projections are developed, how they are tested, and how confident scientists are in their predictions is a core skill in VCE Environmental Science.
General Circulation Models (GCMs) — also called Earth System Models (ESMs) — are complex computer programs that:
- Simulate the atmosphere, ocean, land surface and sea ice as interacting systems
- Represent physical, chemical and biological processes
- Divide Earth into a 3D grid and calculate conditions at each grid point over time
- Run forward from an initial state to project future conditions under different emissions scenarios
Modern climate models typically operate at ~50–200 km horizontal resolution for the atmosphere and 1° for the ocean.
Climate models are validated by comparing their simulated output against observed historical records:
Key validation result (IPCC AR6): Multiple independent climate models reproduce observed global temperature increase when human GHG forcing is included. Natural factors alone (solar, volcanic) cannot explain the observed warming trend since ~1950.
Projections depend on future emissions trajectories, quantified as Shared Socioeconomic Pathways (SSPs):
| Scenario | Description | Projected Warming by 2100 |
|---|---|---|
| SSP1-1.9 | Aggressive mitigation; net zero by ~2050 | ~1.5°C |
| SSP2-4.5 | Intermediate; some mitigation | ~2.0–3.5°C |
| SSP3-7.0 | High emissions; limited mitigation | ~2.8–4.6°C |
| SSP5-8.5 | Very high emissions; no mitigation | ~3.3–5.7°C |
The range of outcomes underscores why emissions reduction decisions matter now.
The IPCC uses a structured framework for expressing the degree of confidence in its findings, based on:
- Strength of the evidence (quantity, quality and consistency of studies)
- Degree of agreement among scientists
| Level | Description |
|---|---|
| Very high confidence | ~9 in 10 chance of being correct |
| High confidence | ~8 in 10 chance |
| Medium confidence | ~5 in 10 chance |
| Low confidence | ~2 in 10 chance |
| Very low confidence | ~1 in 10 chance |
| Projection | Confidence Level | Reason |
|---|---|---|
| Global average temperature will increase | Very high | Multiple lines of evidence; well-understood physics |
| Sea level will continue to rise | Very high | Direct measurements; physical understanding |
| Heatwaves will be more frequent | High | Observed trend consistent with models |
| Changes to global average rainfall patterns | High | Physics well understood |
| Regional rainfall changes (e.g. southern Australia drying) | Medium | Greater model uncertainty at regional scale |
| Changes to individual extreme events | Medium–low | Many interacting factors; short observational records |
| Rates of sea level rise > 2 m by 2100 | Low | Ice sheet dynamics poorly constrained |
Key principle: Confidence is higher for larger spatial scales and more direct physical relationships. Projecting specific changes to a small region’s rainfall is more uncertain than projecting global temperature trends.
For Australia (from CSIRO Climate Change in Australia, 2015 update):
- Very high confidence: Further warming; more hot days; sea level rise; ocean acidification
- High confidence: More intense extreme rainfall; more severe fire weather in south
- Medium confidence: Reduced rainfall in southern Australia (already observed trend continuing); changes to tropical cyclone intensity
- Lower confidence: Exact rainfall changes in northern Australia
VCAA FOCUS: VCAA may ask you to explain what an IPCC confidence rating means and how it is determined. Always link confidence to the strength and agreement of evidence, not just to ‘how sure scientists are’. Regional projections always have lower confidence than global projections — explain why (natural variability, model resolution, fewer observational records).
