def METHOD_NAME() -> pd.DataFrame:
    """Load OWID population dataset, and add historical regions to it.
    Returns
    -------
    population : pd.DataFrame
        Population dataset.
    """
    # Load population dataset.
    population = catalog.Dataset(DATA_DIR / "garden/owid/latest/key_indicators/")["population"].reset_index()[
        ["country", "year", "population"]
    ]
    # Add data for historical regions (if not in population) by adding the population of its current successors.
    countries_with_population = population["country"].unique()
    missing_countries = [country for country in HISTORIC_TO_CURRENT_REGION if country not in countries_with_population]
    for country in missing_countries:
        members = HISTORIC_TO_CURRENT_REGION[country]["members"]
        _population = (
            population[population["country"].isin(members)]
            .groupby("year")
            .agg({"population": "sum", "country": "nunique"})
            .reset_index()
        )
        # Select only years for which we have data for all member countries.
        _population = _population[_population["country"] == len(members)].reset_index(drop=True)
        _population["country"] = country
        population = pd.concat([population, _population], ignore_index=True).reset_index(drop=True)
    error = "Duplicate country-years found in population. Check if historical regions changed."
    assert population[population.duplicated(subset=["country", "year"])].empty, error
    return cast(pd.DataFrame, population)