Ada Palmer

But by the late 1950s, cracks appeared.

A single missing observation could cause huge errors two days later.

Then came Lorenz's 1961 discovery.

Rounding 0.506127 to 0.506 caused his weather simulation to diverge completely, proving that precise long-range forecasts were impossible.

Chaos theory explains why long-range deterministic forecasting fails.

But it doesn't tell you what to do about it.

It took 30 years to achieve a breakthrough.

It came from changing the question.

Instead of asking what will the weather be 10 days from now, ask instead what it could possibly be.

Run the model not once, with your best guess initial conditions, but many times, with slightly different starting points that reflect measurement uncertainty.

Each run produces a different forecast.

Together, they map the range of possible futures.

This is ensemble prediction.

Instead of a single forecast you generate an ensemble of forecasts.

If all ensemble members agree confidence is high.

If they diverge into different patterns uncertainty is high.

You cannot predict which specific future will occur, but you can map the probability distribution across possible futures.

Since becoming used in practice in the early 1990s, the results have vindicated the approach.

Ensemble forecasts consistently outperform single deterministic forecasts.

They provide not just predictions but measures of confidence.