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131 lines
3.9 KiB
Python
131 lines
3.9 KiB
Python
"""Calculate probability distributions for IPCC sea level rise forecasts.
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This will calculate the values required to generate triangular distributions,
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i.e. 'min', 'mode', and 'max' in the `numpy.random.triang()` function.
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Reads:
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'IPCC AR6.xlsx'
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Writes:
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'triang-values.csv'
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D. Howe
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d.howe@wrl.unsw.edu.au
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2022-05-05
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"""
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import os
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import re
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import numpy as np
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import pandas as pd
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from scipy import stats, optimize
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import matplotlib.pyplot as plt
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PLOT = False
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def norm_cdf(x, loc, scale):
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"""Calculate cumulative density function, using normal distribution."""
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return stats.norm(loc=loc, scale=scale).cdf(x)
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def triang_cdf(x, loc, scale, c):
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"""Calculate cumulative density function, using triangular distribution."""
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return stats.triang(loc=loc, scale=scale, c=c).cdf(x)
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# Read data
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df = pd.read_excel('IPCC AR6.xlsx', index_col=[0, 1, 2, 3, 4])
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df = df.sort_index()
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dff = df.loc[838, 'total', 'medium', 'ssp585'].T
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dff.index.name = 'year'
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percentiles = dff.columns.values / 100
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for i, row in dff.iterrows():
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values = row.values
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# Fit normal distribution
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loc, scale = optimize.curve_fit(norm_cdf, values, percentiles)[0]
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p_norm = {'loc': loc, 'scale': scale}
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# Fit triangular distribution
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loc, scale, c = optimize.curve_fit(triang_cdf,
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values,
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percentiles,
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p0=[values[0] - 0.1, 0.5, 0.5])[0]
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p_triang = {'loc': loc, 'scale': scale, 'c': c}
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# Get triangular distribution parameters
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left = p_triang['loc']
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centre = p_triang['loc'] + p_triang['scale'] * p_triang['c']
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right = p_triang['loc'] + p_triang['scale']
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dff.loc[i, 'min'] = left
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dff.loc[i, 'mode'] = centre
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dff.loc[i, 'max'] = right
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if PLOT:
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fig, ax = plt.subplots(1, 2, figsize=(10, 3))
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x_min = stats.triang.ppf(0.01, **p_triang) - 0.2
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x_max = stats.triang.ppf(0.99, **p_triang) + 0.2
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x = np.linspace(x_min, x_max, num=1000)
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ax[0].plot(x, 100 * stats.norm.cdf(x, **p_norm))
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ax[0].plot(x, 100 * stats.triang.cdf(x, **p_triang))
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ax[0].plot(values, 100 * percentiles, '.', c='#444444')
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ax[1].plot(x, stats.norm.pdf(x, **p_norm), label='Normal')
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ax[1].plot(x, stats.triang.pdf(x, **p_triang), label='Triangular')
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ax[1].plot([], [], '.', c='#444444', label='IPCC data')
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ax[1].legend()
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ax[1].axvline(x=left, c='C3')
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ax[1].axvline(x=centre, c='C3')
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ax[1].axvline(x=right, c='C3')
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ax[0].set_ylabel('Percentile', labelpad=10)
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ax[0].set_title('Cumulative distribution')
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ax[1].set_title('Probability density')
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ax[0].annotate(i, (-0.3, 1),
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xycoords='axes fraction',
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clip_on=False,
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size=14)
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for a in ax:
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a.set_xlabel('SLR (m)', labelpad=10)
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a.spines['top'].set_visible(False)
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a.spines['right'].set_visible(False)
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plt.show()
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# Make SLR relative to 2020 level (at the 50th percentile)
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dff -= dff.loc[2020, 'mode']
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# Save distribution parameters
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dff[['min', 'mode', 'max']].to_csv('triang-values.csv', float_format='%0.3f')
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if PLOT:
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# Plot all triangular distributions
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fig, ax = plt.subplots(1, 1, figsize=(8, 4))
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cmap = plt.cm.get_cmap('RdBu_r', len(dff))
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c = list(cmap(range(cmap.N)))
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j = -1
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for i, row in dff.iterrows():
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j += 1
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ax.plot(row[['min', 'mode', 'max']], [0, 1, 0], c=c[j])
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if j % 2 == 0:
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ax.annotate(f' {i}', (row['mode'], 1),
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ha='center',
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va='bottom',
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rotation=90)
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ax.set_xlabel('SLR (m)', labelpad=10)
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ax.set_ylabel('Probability density (-)', labelpad=10)
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ax.spines['top'].set_visible(False)
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ax.spines['right'].set_visible(False)
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plt.show()
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