1. Introduction
In the October Newsletter, we reported that the sixth assessment report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) Working Group I 1, published in August, reiterated the global temperature rise and noted that the scale of recent changes across the climate system due to the temperature rise are unprecedented over many centuries to many thousands of years. The report also discusses several impacts of the global temperature rise such as sea-level rise and extreme weather events in various regions across the globe.
While the temperature rise is clear and the role of human-produced greenhouse gases in causing the rise is unequivocal, the resultant impacts have been debated. Climate extremes have been reported in every region across the globe but the confidence level on human causes of observed changes is not uniform across the outcomes. This is illustrated by the three weather outcomes studied in AR6, i) change in hot extremes, ii) change in heavy precipitation, and iii) change in agricultural and ecological drought. For each outcome, the report assigns a confidence level– high, medium, or low, based on assessing trend detection and attribution, and associated literature. The confidence level in extreme precipitation and agricultural/ecological drought is much less than the confidence levels of extreme heat, across regions.
- Weather Outcomes
- a. Hot Extremes: Figure 1 displays the observed change in hot extremes in various regions of the world and associated confidence levels.
Figure 1. Observed change in hot extremes and confidence in human contribution indicated by the number of dots: three dots for high confidence, two dots for medium confidence and one dot for low confidence (filled: limited agreement; empty: limited evidence). (See Figure SPM.3(a) of Summary for Policy Makers, AR6.)
Note from the figure that most regions, including South Asia (SAS) display an increase in hot extremes with a high confidence level in human contribution. However, Central North America (CNA) and Eastern North America (ENA) show a low agreement in the type of change. Southern South America (SSA) and Central Africa (CAF) also display a low confidence due to limited evidence.
- b. Heavy precipitation: Figure 2 displays the observed change in heavy precipitation and associated confidence levels for human contribution. None of the regions show an increased high confidence. Of the 19 regions showing an increase, only one (Central North America (CAN) shows an increase with medium confidence level. The rest, including South Asia (SAS) show an increase with low confidence due to limited agreement between observation and model, and among models. None of the regions show a decrease in heavy precipitation. Eight regions show low agreement in the type of change and 18 regions have limited data and/or literature.
Figure 2. Observed change in heavy precipitation and confidence in human contribution. (See Figure SPM.3 of Summary for Policy Makers, AR6.)
- c. Agricultural and ecological drought: 2Figure 3 displays the change in agricultural and ecological drought and the level of confidence on human contribution to the changes. From the figure we see an increase in agricultural and ecological drought in 12 regions with only one, Western North America (WNA), depicting medium confidence. The increase in the remaining 11 regions indicates a low confidence. 28 regions, including South Asia (SAS), show low agreement in the type of change. Four regions show limited data and/or literature. Only one region, Northern Australia, shows a decrease and that too with low confidence due to limited agreement.
Figure 3. Observed change in Agricultural and ecological drought and confidence in human contribution. (See Figure SPM.3 of Summary for Policy Makers, AR6.)
Comments: The term ‘confidence level’ may cause confusion in understanding the impacts of global temperature rise. So, it should be considered carefully. AR6 notes that while increasing confidence level would correlate with the evidence and degrees of agreement, a low confidence level does not necessarily mean that confidence in its opposite is high, nor does it imply distrust in the finding. It simply is the best conclusion based on the current knowledge on the impact. Thus, new knowledge vetted by research may change the level of confidence in any finding in future assessments, AR6 argues correctly.
However, a low confidence level in human contribution variability adds to the uncertainty in the climate change debate and in steps needed to combat it. For example, it is often cited in the media that climate change is causing a larger number of wildfires in the US. While there have been several fires in the past three years with spectacularly devastating effects such as those in California, a recent compilation of data on natural disasters by Oxford University shows that there were fewer forest fires in the US in the decade 2010-2020 than were in the previous decade 3 However, the greater propensity for hot extremes noted in AR6 for western North America (WNA) region (Figure 1 above) likely led to dryer conditions and thus a lower moisture content in the air making it harder to fight fires resulting in the tragic effects reported in the news.
Consequently, continuing research is imperative to further narrow the uncertainty in confidence levels in human contribution to extreme weather outcomes of global warming and a more nuanced interpretation of these outcomes is needed to arrive at more definitive actions to combat the effects of climate change.
Dr. Ahmed Badruzzaman, a nuclear physicist by education, has spent over 40 years studying multiple energy technologies and related issues at various US organizations. He is a professional faculty at Haas School of Business in University of California, Berkeley and chairs the BEN Energy Panel. He is an SME Consultant to the US Department of Energy.
- IPCC Group I (WGI) deals with the Physical Science Basis of Climate Change.
- Droughts in general are caused when precipitation falls below normal for extended periods. Many factors can cause droughts and there are several types of droughts based on their impact (https://www.drought.unl.edu/Education/DroughtIn-depth/TypesofDrought.aspx.) Agricultural droughts impact agriculture crop yield and agricultural profitability and are caused by reduced moisture in soils. Human activity can determine their severity – irrigation of croplands reducing their socioeconomic impact while depletion of aquifers used for irrigation can make droughts worse. Ecological drought is an episode of deficit in water availability that stresses ecosystems. These can arise from a combination of evaporation and low soil moisture.
- https://ourworldindata.org/natural-disasters; Accessed September 2021, in connection with the course UGBA 193B, Energy and Civilization, the author co-teaches at University of California, Berkeley
