# Climate Zone Shift Layer - Technical Specification v1.0

## Purpose
Quantify long-term shifts in bioclimatic and aridity regimes without confusing annual weather extremes with climate-zone change.

## Core design
Two independent direct classification layers are maintained:
1. Köppen-Geiger (temperature + precipitation climatology).
2. Aridity Index AI = P/PET (water-balance-sensitive dryness).

Proxy indicators (drought, snow, soil moisture, fire weather, extreme precipitation, vegetation) are used only for validation.

## Time model
- External benchmark: Beck et al. (2023), 30-year historical periods 1901-1930, 1931-1960, 1961-1990, 1991-2020.
- Operational layer: ERA5-Land, rolling 30-year windows stepped annually.
- Latest fully closed window as of August 2026: 1996-2025.
- 2026 is not promoted to a completed 30-year climatology before year-end.

## Spatial model
- Operational grid: ERA5-Land 0.1° CDS grid.
- Weight by actual cell area and country polygon intersection.
- Regions: Europe, DEU, AUT, CZE, POL.
- Optional DEM coupling for altitudinal centroid drift.

## Direct metrics
See `zone_shift_metric_definitions.csv`.

## Validation
Proxy families must remain independent from direct classifier inputs. This prevents circular confirmation.

## Status
This specification is reproducible and implementation-ready. It does not contain fabricated country-specific shift values; these appear only after the gridded source data have actually been processed.