A group of Italian scientists recently published a paper in which they critically assessed extreme [weather] event trends. This has received quite a lot of attention amongst some “skeptics” since it concludes that&
…on the basis of observational data, the climate crisis that, according to many sources, we are experiencing today, is not evident yet.
Before addressing what was presented in this paper, it’s worth making some general comments. Not only is there overwhelming agreement that humans are causing global warming (Cook et al. 2013; Cook et al. 2016), the latest IPCC report went so far as to say that it is now unequivocal that human influence has warmed the atmosphere, ocean and land since pre-industrial times (Eyring et al. 2021). However, whether or not this implies a climate crisis, or a climate emergency, is a judgement that cannot be decided by a scientific analysis alone.
What’s also become clear is that stopping global warming, and the associated changes to the climate, will require getting human emissions of greenhouse gases, mainly carbon dioxide, to (net) zero (MacDougall et al. 2020). Hence, a judgement of whether or not we’re in a climate crisis should not really depend only on an assessment of the human influence to date, but also on an assessment of what could happen in the future and what might be required so as to limit how much more is emitted before reaching (net) zero.
The paper also focussed primarily on what is referred to as Detection and Attribution. This is a two-step process in which a change in some climatic variable is first detected, and then an analysis is carried out to assess if that change can be attributed to an anthropogenic influence. However, as this recent Realclimate article highlights, attribution doesn’t necessarily require first detecting some change. It is possible to determine an anthropogenic influence for an individual extreme event, and there are now many examples of extreme events that have been linked to human-caused climate change.
Additionally, the paper only considered 5 types of extreme events, ignoring that there are many other potential impacts of climate change. For example sea level rise, changing weather patterns, wildfires, ocean acidification, impacts on ecosystems, and even the possibility of compound events. As Friederike Otto points out in this article, they don’t even consider heatwaves, “where the observed trends are so incredibly obvious.” In fact, in the same article, the experts who were quoted regarded the cherry-picking and manipulation as so egregious that some called for the paper to be retracted.&
These criticisms have now been acknowledged by the Journal, which has added an Editor's note to the paper saying "Readers are alerted that the conclusions reported in this manuscript are currently under dispute. The journal is investigating the issue."
The authors of the paper also submitted their paper four weeks after the release of the Intergovernmental Panel on Climate Change’s (IPCC) AR6 WGI report. The paper does mention this report, but then largely ignores what it presents. For example, they failed to note that it is stated in the Summary for Policymakers that “Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5“.&
The first type of extreme event considered in the paper is hurricanes, often referred to as tropical cyclones (TCs). There is still a lot of debate about whether or not there has been detectable change in TC activity that can be attributed to a human influence (Knutson et al. 2019). Some work does indeed detect an increase in TC intensity over the last few decades (Kossins et al. 2020), which is consistent with what would be expected in a warming world (Emanuel 2020). Other work, however, suggests that - in some cases - the trends are a consequence of observational biases, rather than being real (Vecchi et al. 2021). However, even the latter paper concludes that “climate variability and aerosol-induced mid-to-late-20th century major hurricane frequency reductions have probably masked century-scale greenhouse-gas warming contributions to North Atlantic major hurricane frequency.”
One complication with attributing a human influence to TCs is that human-induced warming is expected to reduce the total number of TCs, but cause an increase in the frequency and intensity of the strongest ones. Consequently, metrics that consider all TCs can show little in the way of trends, even if there has been an anthropogenic influence (Kang & Elsner 2016). If one considers the strongest TCs, then there are indeed indications that the strongest ones are getting more frequent.
Additionally, there are other ways in which TCs can be influenced by human-driven warming. Global warming could lead to an increase in the frequency of rapid intensification events (Emanuel 2017) and a poleward migration of the latitude of maximum TC intensity (Kossins et al. 2016). Sea level rise, which can definitively be attributed to human-driven warming, will also lead to enhanced storm surge. Similarly, a warmer atmosphere can hold more water vapour, intensifying the precipitation associated with a TC and increasing the risk of severe flooding.
So, even if there is still debate about whether or not there is an attributable trend in TC activity, there are many others ways to assess an anthropogenic influence, and there is now plenty of evidence that TCs have already been influenced by human-caused warming, and little doubt that this will continue as humans continue to emit greenhouse gases into the atmosphere.
The paper also looked at droughts and floods, tornadoes, extreme precipitation events, and global greening and agricultural production. Droughts are complex phenomena that are not easy to study, but there is clear evidence that climate change is making drought worse, and there is evidence that anthropogenic influences are impacting global drought frequency, duration, and intensity (Chiang et al. 2021). Similarly, both observations and models indicate that total precipitation from intense precipitation events doubles for each degree of global warming (Myhre et al. 2019). The latest IPCC WG1 report says “The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver”.&
On the other hand, our current understanding of how global warming might influence tornadoes is so uncertain that we don’t really know if there is a link between human-induced climate change and tornadoes. It’s certainly the case that CO2 from human emissions has led to global greening and has influenced agricultural production. However, there are already signs that nutrient constraints and water stress is counteracting this effect. For example, drought and tree mortality in the tropics is offsetting enhanced plant productivity in the Arctic.
Clearly, analysing how human-driven global warming is influencing extreme events is very challenging and there is still ongoing debate about the link between climate change and these extreme events. However, there is clear evidence that human-induced climate change is already impacting many extreme events. The link will almost certainly become clearer with time and the expectation is that climate change will make many extreme events more intense and more frequent.
This understanding isn't challenged by a simplistic, and selective, analysis that ignores many other lines of evidence that demonstrate a link between extreme events and human-caused warming. Also, as mentioned above, we cannot determine if we’re in a climate crisis, or not, using a scientific analysis alone. It requires a judgement about the impacts to date, the potential future impacts, and what we might need to do to limit the overall impact. If the authors feel that the label 'climate crisis' is not appropriate, then they are making a subjective choice that - today - is at odds with the views of many other experts, organisations, and even some governments.
Notes:
This post was drafted by Ken Rice, with help from @TheDisproof, who has been very active on Twitter debunking various climate myths.
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