Detection and Attribution

There is growing evidence supporting the conclusion that AGW is already having an effect on extreme weather events, and those impacts are not beneficial to humanity or to the earth's ecosystems. But the process of arriving at those conclusions is complex, and different types of extreme weather are affected by AGW in different ways. While there is a clear (and obvious) link between AGW and extreme heat, heatwaves, droughts, and flooding, the effects of AGW on other extreme weather are far more difficult to determine. For instance, it may be that AGW is only affecting which parts of the US are more likely to be affected by tornadoes without impacting their frequency.

The effect of AGW on Tropical Cyclones (TCs) has been extensively studied, and it appears that there is growing evidence that there are clear connections between AGW and hurricane activity, but I see many reported connections (or lack of connections) misstated by the media, politicians and "think tanks." In order to explain those connections accurately, we have to get into some terminology, specifically the terminology of "detection" and "attribution." Here's how the IPCC defines these:

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%.

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

Simply stated, "detection" answers the question, is there an identifiable climate trend in observations? while "attribution" answers the question, to what extent can we determine that human activity is responsible for those observations? 

Normally, we think of detection as Step 1 and attribution as Step 2. That is to say, there's no point assigning blame to a trend that may not even exist. First we determine if there's a trend; second, we decide if AGW is responsible for a portion of that trend. However, I came across a post by Gavin Schmidt recently that points out that there is no reason why this process has to happen in this order. We don't always have to wait for an anthropogenic signal to be detectable above the noise (natural variability) in order to show that AGW is having an effect on observations. With enough understanding of climate physics, we can still distinguish the pattern of variability that exists from the pattern of variability that would exist in the absence of AGW. This point is nicely illustrated in a post by Patrick Brown. 

So even if a signal in extreme events has not yet fully separated from natural variability, it may still be possible to say that AGW is making that extreme event worse than it otherwise would have been. Schmidt summarizes this as, "by the time a slowly growing signal is loud enough to be heard, it has been contributing to the noise for a while already." With sufficient knowledge of climate physics, scientists can assess attribution even when there is not yet a trend detectable from natural variability.

For example, tropical cyclone activity is largely affected by internal variability, and "noise" from internal variability can swamp the AGW signal. But that doesn't mean that the signal doesn't exist,a nd it doesn't itself mean that scientists can't attribute some portion of tropical cyclone intensity to AGW. So it's important not to infer from what is stated that which is not implied. For instance, NOAA writes, "it is premature to conclude with high confidence that human-caused increases in greenhouse gases have caused a change in past Atlantic basin hurricane activity that is outside the range of natural variability." That statement does not mean that 1) AGW has not caused any change in Atlantic basin hurricane activity or that 2) it's not possible to attribute some fraction of some aspects of hurricane activity to AGW.  Both of these statements are non sequiturs from NOAA's statement above. NOAA's statement means simply that to whatever extent AGW is having an impact on hurricane activity, it's not yet detectable above natural variability - the AGW signal is not yet louder than the noise. 

At the same time, attribution studies that show AGW is making some extreme weather worse should not be interpreted as saying that AGW is causing these events. AGW is responsible for a certain portion of the severity of an event, or it's responsible for making events of a specified severity more frequent. Extreme events happen with or without AGW. AGW simply "loads the dice" in favor of making some extreme events more frequent or more severe.

References:

[1] IPCC. "SR1.5 Glossary." https://www.ipcc.ch/sr15/chapter/glossary/

[2] Gavin Schmidt, "Watching the detections." RealClimate. September 25, 2022. https://www.realclimate.org/index.php/archives/2022/09/watching-the-detections

[3] Patrick T. Brown, PhD "Signal, Noise, and Global Warming's Influence on Weather." https://patricktbrown.org/2018/09/11/signal-noise-and-global-warmings-influence-on-weather/

[4] Tom Knutson. "Global Warming and Hurricanes: An Overview of Current Research Results." NOAA: Geophysical Fluid Dynamics Laboratory. October 3, 2022. https://www.gfdl.noaa.gov/global-warming-and-hurricanes/

[5] NOAA. International Best Track Archive for Climate Stewardship (IBTrACS).  https://www.ncei.noaa.gov/products/international-best-track-archive.

[6] NOAA. Tropical Cyclone Climatology. https://www.nhc.noaa.gov/climo/

[7] Vecchi, G.A., Landsea, C., Zhang, W. et al. Changes in Atlantic major hurricane frequency since the late-19th century. Nat Commun 12, 4054 (2021). https://doi.org/10.1038/s41467-021-24268-5

[8] Emanuel, K. Atlantic tropical cyclones downscaled from climate reanalyses show increasing activity over past 150 years. Nat Commun 12, 7027 (2021). https://doi.org/10.1038/s41467-021-27364-8

[9] A Chan, Duo, Vecchi, Gabriel A, Yang, Wenchang, Huybers, Peter. Improved simulation of 19th- and 20th-century North Atlantic hurricane frequency after correcting historical sea surface temperatures. 2021. Science Advances. eabg6931 7 26. doi:10.1126/sciadv.abg6931. https://www.science.org/doi/abs/10.1126/sciadv.abg6931

[10] Chand, S.S., Walsh, K.J.E., Camargo, S.J. et al. Declining tropical cyclone frequency under global warming. Nat. Clim. Chang. 12, 655–661 (2022). https://doi.org/10.1038/s41558-022-01388-4

[11] Yan, X., Zhang, R. & Knutson, T.R. The role of Atlantic overturning circulation in the recent decline of Atlantic major hurricane frequency. Nat Commun 8, 1695 (2017). https://doi.org/10.1038/s41467-017-01377-8

[12] Murakami, Hiroyuki. Delworth, Thomas L. Cooke, William F.. Zhao, Ming. Xiang, Baoqiang. Hsu, Pang-Chi. Detected climatic change in global distribution of tropical cyclones. Proceedings of the National Academy of Sciences. 117.20 (2020). 10706-10714. doi:10.1073/pnas.1922500117

https://www.pnas.org/doi/abs/10.1073/pnas.1922500117

[13] Ken Rice. "No, a cherry-picked analysis doesn’t demonstrate that we’re not in a climate crisis." And Then There's Physics.... https://andthentheresphysics.wordpress.com/2022/10/07/no-a-cherry-picked-analysis-doesnt-demonstrate-that-were-not-in-a-climate-crisis/

[14] Alimonti, G., Mariani, L., Prodi, F. et al. A critical assessment of extreme events trends in times of global warming. Eur. Phys. J. Plus 137, 112 (2022). https://doi.org/10.1140/epjp/s13360-021-02243-9 https://link.springer.com/article/10.1140/epjp/s13360-021-02243-9

[15] Klotzbach, P. J., Wood, K. M., Schreck, C. J., Bowen, S. G., Patricola, C. M., & Bell, M. M. (2022). Trends in global tropical cyclone activity: 1990–2021. Geophysical Research Letters, 49, e2021GL095774. https://doi.org/10.1029/2021GL095774