An attribution study by World Weather Attribution (WWA) concludes that the heatwaves in Europe and North America this July would have been "almost impossible" without climate change. Over the past few weeks, southern Europe, parts of the United States, Mexico and China have experienced severe heatwaves with temperatures exceeding 45 °C. The WWA report notes that in China the heatwave was at least 50 times more likely due to the climate crisis.
A man cools off in a fountain in central Barcelona on July 18, as temperatures soared in Catalonia. EFE/Quique García.
The paper highlights and quantifies the role played in the July heat episodes over southern Europe, parts of the United States/Mexico and China by climate change caused by greenhouse gas emissions resulting from the massive use of fossil fuels. To do so, they make use of a well-established protocol developed by WWA for rapid, near-real-time attribution studies using pre-calculated simulations. It is a widely demonstrated fact that such extreme heat events are going to devastate areas of the planet more and more frequently and with episodes of greater intensity, geographic extent and duration depending on the level of global warming.
However, the paper does not mention a rather extraordinary fact that many researchers have pointed out, namely the exceptional marine heat wave in the North Atlantic Ocean that has been occurring since last April.
This marine heat wave could be due to a combination of both background warming due to continued greenhouse gas emissions and the intrinsic natural variability of the climate system, mainly represented by the warming and cooling phases of El Niño and La Niña, respectively.It appears that the extremely rare duration of three consecutive years of the La Niña phase may be, according to some researchers, co-responsible - together with global warming - for this marine heat wave in the North Atlantic. The role of this marine heat wave, at least in the heat episodes over southern Europe, remains to be further investigated.
The magnitude and frequency of warm anomalies occurring globally this year, both in the lower atmospheric air and in the surface water of the oceans, as well as some anomalous weather patterns observed, are attracting the attention of climate scientists. Extreme heat domes in different regions of the Earth, such as North America, southern Europe and northwest China, have led to extraordinary temperature records, with heat records being broken once again. It seems clear that this circumstance cannot be dissociated from global warming, but to prove it requires an attribution study, such as the one just published by the WWA (World Weather Attribution).
The objective of this type of study is to directly link a particular extreme weather event (or set of events, such as the heat domes this July) and climate change. They also have the added value of being able to be carried out quickly, providing the results just a few days after the extreme weather event. This latest WWA attribution study has shown that without global warming of mainly anthropogenic origin it would have been very unlikely (extremely rare in the case of China and virtually impossible in the cases of the USA/Mexico and southern Europe) that such simultaneous extreme heat domes would have occurred in the targeted land regions with the magnitude they have reached, leading to such extreme heat spikes.
In parallel to the appearance of this attribution study, the hypothesis that the extraordinary amount of water vapour that the violent eruption of the submarine volcano Hunga Tonga -occurring in the Pacific Ocean on 14 January 2022- launched into the stratosphere (even reaching the mesosphere) could explain, at least in part, the large warm anomalies that we are experiencing so far this year and that are falling short of the projections made last year, is beginning to gain strength among the scientific community.
In addition, as the El Niño event that began in late spring is expected to gain in intensity, it is also expected to contribute to the increase in global temperature.
Heatwaves are the most unmistakable sign of climate change; in all the trends observed in the past and also in future projections they have worsened and will worsen. The WWA attribution study finds this particularly true for the July 2023 heat waves experienced in southern Europe, China, and also the US. Such heat would have been virtually impossible to occur in the US/Mexico region and southern Europe without anthropogenic climate change.
Although heatwaves are part of our climate, every wave we experience today is already amplified in intensity, extent, frequency, and duration due to global warming.
Heatwaves are becoming more frequent and intense in mid-latitudes, such as southern Europe and northern Africa. The increase in frequency and intensity can be attributed to climate change due to human action using well-established methodologies that mix statistics, observations, and climate simulations. These methodologies have been used to analyze to what extent the heatwaves registered simultaneously in various parts of the planet (China, southern United States, northern Mexico and southern Europe) during this month of July can be attributed to climate change. In particular, for southern Europe the results indicate that there is no doubt that the heatwave would have been practically impossible without the human contribution, and if it had occurred it would have been almost more than two degrees less intense.
The authors' conclusions are a serious wake-up call to reduce and eliminate greenhouse gas emissions due to human action and quickly address the adaptation of vulnerable societies to hitherto unknown environmental conditions. The authors provide some useful recommendations for adapting to heatwaves with examples of actions that are already taking place.
Heatwaves are increasingly breaking temperature records around the globe, with the concurrent heat extremes across North America, Europe and China in July 2023 adding another, and certainly not the last, chapter to this story.
The study uses a so-called rapid-attribution approach, which has been similarly applied to other recent extreme events as well. These rapid-attribution analyses can give a first-order estimate of the human contribution in altering the probability of such heat extremes, but may not be robust as it comes to processes more specific to the event and how for example specific atmospheric circulation patterns or the land surface amplified the heat.
Due to this generalised approach, I would not put too much emphasis on the specific quantification such as the event being at least 50 times more likely due to human influences. Accounting for the limitations of the method it could equally be 100 times or only 30 times more likely. But the exact numbers probably do not matter that much in this context, as the important message is that human activities are strongly increasing the probability and intensity of such heat extremes. And these extremes will further increase in the future if humans do not take consequent action to limit the further increase of Greenhouse gases from burning fossil fuels.
An important value of such rapid attribution studies is to assess the role of climate change in such extreme events while the events are still in the memory of people (or even still ongoing), which hopefully helps to raise awareness and highlights the urgency of actions to mitigate dangerous levels of global warming.
- Research article
- Observational study