Climate Shift Index®
Source: Climate Central
The Climate Shift Index® ("CSI" or "Index") reveals how much climate change influences the temperature on a particular day. The index ranges from -5 to +5 with positive levels indicating temperatures that are becoming more likely due to climate change (negative scores indicate conditions that are becoming less likely).
For levels at 2 or above, the Index is a multiple of how frequently a particular temperature will occur due to climate change. For example, a Climate Shift Index of level 5 means that a temperature is occurring 5 times more frequently when compared to a world without human-caused carbon pollution. This temperature would be very difficult to encounter in a world without climate change-not necessarily impossible, just highly unlikely.
To compute the Index, Climate Central averages together two complementary ways of estimating how climate change has altered the frequency of a temperature. The first method uses 22 climate models each run with, and without, historical greenhouse gas emissions. We then calculate the frequency of the temperature in the modeled climates. This gives us 22 pairs of frequencies. We then take the ratio of the frequency in the runs with observed CO2 to the frequency in the run without extra greenhouse gasses. The second method calculates the frequency of daily temperatures for the past 30 years. We then calculate how much the temperature change of a particular location is in response to an increase in global mean temperature. This relationship allows us to remove the contribution of human-caused climate change, giving us the frequency of the particular temperature in a climate without global warming. We then compute the ratio of the frequency of the temperature in the current climate to the frequency in the climate without climate change. The final Climate Shift Index is built from the average of the observation-based and model-based estimates.
The CSI does not show a shift in temperature. It's a shift in how frequently certain temperatures occur. For example, suppose 90°F used to occur on average one day each year in June where you live. If climate change has altered the conditions so that 90°F now occurs on average three days each June, then its frequency (or likelihood) has increased by 3 times. Following this estimate, whenever we see a 90°F day in June in your location, it is assigned a value of 3.
Does a high CSI mean thst climate change caused the hot weather? Not entirely. Any weather event has multiple causes. The Index tells us how much climate change has boosted the odds of a particular temperature. Events where the Index reaches level 5 would be very difficult to encounter in a world without climate change - not necessarily impossible, just very, very unlikely.