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What's the CSI scale?

The CSI is a categorical scale, with the categories defined by the ratio of how common (or likely) 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 CSI conditions (which occur much more often than the negative), we assigned a simple descriptor to these events (see table).

Descriptor

CSI level

Interpretation

CSI level

Interpretation

No effect

0

The influence of climate change on the conditions (i.e. the daily high or low temperature) is not detectable. These conditions could occur about as often with or without climate change.

0

The influence of climate change on the conditions (i.e. the daily high or low temperature) is not detectable. These conditions could occur about as often with or without climate change.

Moderate

1

Climate change made the conditions at least 1.5 times more likely. A CSI level of 1 indicates a detectable climate influence.

-1

Climate change made the conditions at least 1.5 times less likely.

Strong

2

Climate change made the conditions at least twice as common. CSI levels of 2 and higher indicate a dominant climate influence.

-2

Climate change made the conditions at least 2x less common.

Very strong

3

Climate change made the conditions at least 3x more likely

-3

Climate change made the conditions at least 3x less likely

Extreme

4

Climate change made the conditions at least 4x more common. These conditions would be extremely rare without climate change.

-4

Climate change made the conditions at least 4x less common. These conditions are becoming extremely rare with climate change.

Exceptional

5

Climate change made the conditions at least 5x more likely, potentially far more. This is an exceptional event driven by climate change.

-5

Climate change made the conditions more than 5x less likely. These conditions are becoming exceptionally rare with climate change.

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