How to Understand the New IPCC Report: Part 2, Extreme Events

Contrary to what you've been reading, the massive new IPCC report offers grounds for optimism on climate science and policy

This is a long post on what the new IPCC Working Group 1 report says about the detection and attribution of trends in the frequency and intensity of climate and weather extremes. I first explain this terminology of the IPCC and then I systematically go through what the report says phenomenon by phenomenon. This is a topic I’ve published widely on for over 25 years (see this short book for a recent summary).

For those readers just wanting the conclusions, let’s start there (ignoring here levels of certainty and confidence and focusing at the global scale).

These conclusions are substantially similar to those of the previous IPCC WG1 report of 2013, which is not surprising, as a few more years of data on trends observed over many decades or longer wouldn’t be expected to change detection.

These conclusions of the IPCC, and presented in the language of the IPCC below, indicate that it is simply incorrect to claim that on climate time scales the frequency or intensity of extreme weather and climate events has increased for: flooding, drought (meteorological or hydrological), tropical cyclones, winter storms, thunderstorms, tornadoes, hail, lightning or extreme winds (so, storms of any type).

Surprised?

Let’s get to the details.

Detection and Attribution

Understanding the IPCC’s latest conclusions on extreme weather and climate requires a brief introduction to some technical terminology. The IPCC has long used a two-part framework to (a) identify changes in climate, called detection, and (b) to explain why identified changes may have occurred, called attribution.

Here is how the IPCC defines detection:

Detection of change is defined as the process of demonstrating that climate or a system affected by climate has changed in some defined statistical sense, without providing a reason for that change. An identified change is detected in observations if its likelihood of occurrence by chance due to internal variability alone is determined to be small, for example, <10%.

And here is how the IPCC defines attribution:

Attribution is defined as the process of evaluating the relative contributions of multiple causal factors to a change or event with an assessment of confidence.

Let’s look at an example showing how the IPCC employs this framework, with a focus on global average surface temperatures. The figure below (3.4b) shows change global average surface temperature for several methods from 1850 to 2020.

You don’t have to be a statistician to see that temperature since 1900 has increased as compared to an earlier reference period of 1850 to 1900. This indicates a change in climate for this metric. In the vernacular of the IPCC, detection has been achieved.

Once a change in climate has been detected, the next step is to explore why. It is important to understand that just because a weather or climate variable exhibits change over climate time scales (typically 30 years or longer) does not tell us why that change has occurred.

The IPCC uses computer models to explain observed changes in global temperature. The figure below (SPM.1b) shows global average surface temperatures from computer models that are run with and without human contributions to climate. The black line shows the historical observations, as above.

You can see that the computer models run with human factors much better matches historical observations. This is one (of several) bases for the IPCC’s conclusion that the detected change in temperatures can be attributed to human factors.

OK, thanks for sticking with the introductory lesson. We can now get to the main action, and follow along with the IPCC’s use of its detection and attribution framework to summarize what the report says about weather and climate extremes.

The IPCC on Detection and Attribution of Weather and Climate Extremes

That is it for Part 2. The next installment will take a close look at so-called “event attribution” — a methodology developed to support climate lawsuits as it became apparent that the IPCC’s conventional approach to detection and attribution was failing to find changes in many types of extreme events. Stay tuned!