What this lesson teaches
I can explain why present and future climate impacts are uncertain.
Syllabus 9173, Topic 3.2, 2. Possible Effects of Climate Change. How certain are we of the present and future impacts of contemporary climate change?
Make a guess
Scientists are confident the world will keep warming, but unsure how much rain one district will get. How can both be true?
- Because uncertainty means anything could happen.
- Because global warming is not really happening.
- Regional detail is harder to project than global trends.
- They cannot both be true; it is a contradiction.
Show the answer
Regional detail is harder to project than global trends.
Global warming rests on well-understood physics. Local rain depends on clouds, winds and patchy data, so it carries more uncertainty.
Climate uncertainty comes from several sources, so identify which one matters for a claim instead of treating all knowledge as equally uncertain.
Some physical processes are not fully understood. Clouds, ice sheets, carbon feedbacks and regional winds make the size and location of future change uncertain. Models simplify small-scale processes, so different models can give different results.
Data are also patchy. Direct measurements on the ground or at sea are sparse over oceans, deserts and polar regions, especially in the past. Satellites help, but need checking. So regional estimates are less certain than global trends. Satellites also do not measure every variable directly.
Every measurement has some error, from instruments, sampling and processing. Scientists estimate errors, compare independent records and correct known biases. This reduces, but does not remove, uncertainty.
Future emissions are uncertain because population, technology and policy choices are not fixed. A scenario describes one possible future. A projection shows the climate if that scenario happens. So the range of projections partly reflects human choices.
Natural ups and downs add noise, especially locally and over short periods. But uncertainty about details does not mean doubt that greenhouse gases cause warming. Decisions can use ranges, robust measures and monitoring, instead of waiting for an exact forecast that may never come.
- Process uncertainty
- Clouds, ice sheets and feedbacks are hard to model.
- Data gaps
- Few direct measurements over oceans, deserts and poles.
- Scenario uncertainty
- Future emissions depend on human choices.
Step by step
State the claim
A global trend and a local flood forecast differ.
Find the uncertainty
Process, data, measurement or future emissions?
Choose a response
Use ranges, scenarios and flexible plans.
Worked example: Two scenario lines are not two failed forecasts
A high-emissions projection and a low-emissions projection split apart later this century. The gap mainly reflects different assumed choices. It does not mean models cannot tell whether greenhouse gases cause warming.
Watch out for this
Uncertainty means every outcome is equally likely.
Evidence narrows the likely outcomes. Scenarios are not all equally likely.
Check your understanding
A town's projections differ because they assume different future fossil fuel use. Which uncertainty is this?
- Natural year-to-year variability.
- Instrument measurement error.
- Scenario uncertainty.
Show the answer
Scenario uncertainty.
Yes. The difference comes from assumed human choices.