What is the Climate Shift Index®?
The Climate Shift Index (CSI) 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 CSI of level 5 means that a temperature is occurring 5 times more frequently. This would be very difficult to encounter in a world without climate change–not necessarily impossible, just highly unlikely.
What's the Scale?
The Climate Shift Index is a categorical scale, defined by the ratio of how common a temperature is in today’s altered climate vs. how common it would be in a climate without human-caused climate change. For the positive conditions (which occur much more often than the negative), we assigned a simple descriptor to these events:
| CSI Level |
Interpretation |
| 0 No effect |
The influence of climate change is not detectable. |
| 1 Moderate |
Climate change made the conditions at least 1.5 times more likely. |
| 2 Strong |
Climate change made the conditions at least 2 times more likely. |
| 3 Very Strong |
Climate change made the conditions at least 3 times more likely. |
| 4 Extreme |
Climate change made the conditions at least 4 times more likely. |
| 5 Exceptional |
Climate change made the conditions at least 5 times more likely. |
| -1 Moderate |
Climate change made the conditions at least 1.5 times less likely. |
| -2 Strong |
Climate change made the conditions at least 2 times less likely. |
| -3 Very Strong |
Climate change made the conditions at least 3 times less likely. |
| -4 Extreme |
Climate change made the conditions at least 4 times less likely. |
| -5 Exceptional |
Climate change made the conditions at least 5 times less likely. |
What’s happening in the hatched “Currently unavailable” places?
Climate Shift Index levels are evaluated each day using temperatures from a state-of-the-art numerical weather model. Places and seasons with relatively small temperature variability combined with relatively strong temperature trends (e.g., the tropics, especially overnight) are likely to have greater sensitivity to differences between the forecast and observations. We account for these uncertainties by masking out regions where forecast errors could lead to a change in the CSI of two levels (or greater), labeling them as “Currently unavailable.” Masked regions are filled when observational data become available.
