Opinion: What the IPCC actually says about climate change and droughts
The following is from the prepared remarks of Roger Pielke Jr. before the budget committee of the U.S. Senate, 22 May 2024. Pielke is a professor in the College of Arts and Sciences at the University of Colorado (Boulder).
For almost 30 years, along with many colleagues, I have studied extreme weather and climate and associated impacts. Our work has been cited in the most recent assessment of the Intergovernmental Panel on Climate Change, or IPCC.
The IPCC is comprised of hard-working and intelligent people who reflect a spirit of public service. They are also humans, and the IPCC is of course fallible. Conclusions of IPCC reports are snapshots in time reflecting the evolution of scientific understandings. Individual experts … may have legitimate views that are at odds with the IPCC, and that is … expected in a diverse scientific landscape.
The IPCC Working Group 1 assessments of the literature on extreme events in my areas of expertise have, with few exceptions, done an overall excellent job accurately reflecting the scientific literature.
Today, I summarize what the most recent IPCC report concluded about the detection and attribution of trends in drought at the global scale and also for the United States.
I start with some key IPCC terminology:
Detection: “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”
Attribution: “the process of evaluating the relative contributions of multiple causal factors to a change or event with an assessment of confidence.”
Drought: “periods of time with substantially below-average moisture conditions, usually covering large areas, during which limitations in water availability result in negative impacts for various components of natural systems and economic sectors”
It is more challenging to achieve detection and attribution of trends in drought than, say, hurricanes or tornadoes, because drought can be defined and measured in many ways in the context of significant natural climate variability. Detecting and attributing trends in drought impacts is even more challenging.
It is easy to identify drought trends over various time periods in various places that are the result of internal variability rather than indicative of a change in climate. Often, detection and attribution are confused, and so too is climate variability with climate change.
The IPCC finds with high confidence (i.e., an eight-in-10 chance) that human-caused climate change influences the global hydrological cycle and thus drought.
(Let me make) four main points.
The IPCC focuses on three types of drought: meteorological, hydrological, and soil moisture deficits (which the IPCC calls agricultural/ecological drought).
At the global scale, the IPCC has not detected and attributed trends in any of the three types of drought for any region with high confidence (i.e., an eight-in-10 chance). For the United States, the IPCC has only low confidence (i.e., two-in-10 chance) in detected or attributed trends in all three types of drought for all regions, except Western North America where it has medium confidence (i.e., five-in-10 chance) in the detection and attribution of trends in agricultural/ecological drought.
Looking to 2100, at the global scale the IPCC does not expect that a signal of trends in drought will emerge in any region with high confidence (i.e., eight-in-10 chance). For the United States, the IPCC has only low confidence (i.e., two-in-10 chance) that a signal of trends in drought will emerge from the background of natural variability in all three types of drought for all regions, except Western and Central North America for agricultural/ecological drought and also hydrological drought in Western North America — both at medium confidence (i.e., five-in-10 chance)…
In plain English, the IPCC concludes that changes to the climate system resulting from human activity, notably the emission of carbon dioxide from the burning of fossil fuels, changes the hydrological cycle and thus affects drought. At the same time, the IPCC does not have high confidence that research has detected the signal of a change in past drought at the global scale or in the United States. Nor does the IPCC expect with high confidence such a signal to emerge beyond internal variability, even under its most extreme scenario, to 2100.
Such uncertainties and areas of ignorance can inform both mitigation and adaptation policies and planning.
To be clear, I emphasize explicitly and unequivocally that human-caused climate change poses significant risks to society and the environment, and that various policy responses in the form of mitigation and adaptation are necessary and make good sense.
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