Course:EOSC270/2023/The Pacific Northwest Heat Dome of 2021
Overview
Between June 25 and July 1, 2021, an intense high-pressure system settled over the Pacific Northwest, creating a “heat dome,” a stagnant air mass that trapped heat and pushed temperatures to record-breaking levels. British Columbia experienced unprecedented highs, including 49.6°C in Lytton, the hottest temperature ever recorded in Canadian History.[1]
This extreme heat coincided with afternoon low tides, exposing intertidal habitats to direct solar radiation during peak temperatures. Marine organisms in the rocky intertidal zone, already adapted to fluctuating but not extreme conditions, were suddenly subjected to lethal thermal stress.
The scale of mortality was staggering: UBC researchers estimate over one billion intertidal marine animals died along BC’s coast.[2] These mass die-offs represented a profound disruption to ecosystem structure, function, and resilience.
Timeline of events
June 25, 2021
Formation of the heat dome begins over British Columbia and the Pacific Northwest. Shallow marine habitats begin to experience elevated temperatures.
June 27-29, 2021
Temperatures peak; Lytton hits 49.6°C on June 29. This coincides with afternoon spring tides, causing extended low-tide exposure during the hottest part of the day.
June 30th, 2021
First reports of intertidal die-offs emerge from the BC coastline.
Early July 2021
Public and scientific documentation begins. Mass mortality events are confirmed by researchers.[2]
July 9, 2021
UBC’s Dr. Chris Harley and colleagues publicly share research detailing widespread intertidal losses.[3]
July–December 2021
Follow-up ecological assessments begin, focusing on recovery.
Impact on the Intertidal
Affected Species
During the 2021 heat dome, organisms in the intertidal zone were subjected to extreme heat and prolonged air exposure during midday low tides. This exposure caused widespread mortality, especially among species that are sessile or slow-moving and unable to escape the heat.
Severely affected species included:
- Blue mussels (Mytilus trossulus) – Sessile filter-feeders highly sensitive to desiccation and heat. Mortality reached ~74% in south-facing zones, which received the most direct sun exposure.[3]
- Acorn barnacles (Balanus glandula) – Died in large numbers due to prolonged aerial exposure and lack of thermal regulation.
- Ochre sea stars (Pisaster ochraceus) – Suffered physiological stress, reducing their effectiveness as predators and impacting broader intertidal food web dynamics.
- Other taxa – Limpets, clams, snails, oysters, and sea anemones also experienced mass die-offs due to the intensity and duration of the heat event.[4]
Algal species were also affected, particularly in exposed upper intertidal areas. Rockweed (Fucus spp.), a dominant brown alga, showed signs of scorching and canopy thinning, reducing the shade and moisture it typically provides for invertebrates.[4] In contrast, green algae like sea lettuce (Ulva spp.) increased in some disturbed zones, potentially taking advantage of reduced competition and altered nutrient conditions.
Microhabitat and orientation played a critical role: north-facing surfaces retained more moisture and had higher survival rates, while south-facing zones experienced almost complete loss of adult mussel stands.[5] These mortality patterns underscore the importance of exposure, mobility, and physiology in determining organism vulnerability to climate-driven events.
In some areas, mortality reached over 1,000 dead mussels per square metre.[3] These losses not only represent individual species impacts but may also lead to long-term disruptions in food webs, competition, and nutrient cycling. While early signs of recovery have emerged, the resilience of these ecosystems will be tested by the increasing frequency and intensity of extreme heat events.
Amplifying Factors
Vulnerability of the Intertidal
The intertidal zone is a marine ecosystem that for some portion of each day is submerged by the oceans tides and other times of the day is exposed to the air. The high intertidal is exposed for most of the day and only submerged for short periods while the low intertidal is underwater nearly all day and only exposed for shorter periods of time. When exposed, marine life has to contend with lots of stressors throughout the day such as temperature, drying out, UV light and force from waves crashing. Species that live in the high intertidal are adapted to these harsh conditions and will settle as high up as they can survive.[6] This is a tradeoff though, so many species that are adapted to high environmental stress will be outcompeted by species with a bit lower tolerance for stress and more qualities that make it easier to reproduce and eliminate the competition moving into the mid and lower intertidal.[6] Moving deeper into areas that are nearly always underwater, species are very competitive and species with more adaptations for stress with be completely outcompeted in lower areas. This dynamic means there are thin horizontal zones where each species lives and they are very strongly adapted for the amount of stress the face on a regular basis and they can only tolerate environmental conditions within the normal range.[6] These narrow zones that are already close to the maximum stress they can handle is what makes the intertidal ecosystem particularly vulnerable to extreme weather events.[6]
Unfortunately, the heat dome temperatures were very high beyond normal historical conditions for these marine organisms which led to very high mortality rates up and down the intertidal.[3] High and Mid intertidal organisms have heat shock proteins, so organisms like mussels can survive temperatures in the high 30°Cs for short periods of time, but these adaptations can only help so much, especially given that some UBC researchers measured some temperatures along rocky shorelines of 40 to 50°C.[7] Low intertidal organisms weren't safe either as they had maximized competitive advantage with reduced heat tolerance, so even though they only get uncovered by the tides for short periods of time each day, that is enough to kill these less stress resistant species.[3]
Tidal Conditions
Unfortunately for marine species, the alignment of the sun, moon and earth just happened to be nearly the worst they could possibly be during the heat dome leading to very low tides and removing protection and relief from hot temperatures during the hottest times of the day.
The daily flow of the tides is dictated by the earth spinning and the moon remaining relatively stationary, so that the moon appears to be rotating around the earth about once per day from the perspective of someone on earth. When the moon is directly overhead and directly underneath from the perspective of someone on earth, the moon's gravity cause the tides bulge up towards the moon causing 2 daily high tides and when the moon is along the horizon it causes 2 daily low tides. Due to the moon rotating around the earth, this daily cycle isn't 24 hours long, it's actually 24 hours and 50 minutes, so the time of each high tide and low tide drifts by 50 minutes each day, so if today the low tide is at noon, in a week it will be at 6pm.[8] In the Pacific Northwest, like many places around the world we actually experience mixed tides, so we have one really high tide, one really low tide and a bit lower high tide and bit higher low tide each day. It is therefore really bad luck that the lowest low tides of the day during the heat dome (as seen in the adjacent table) just happened to coincide with the mid afternoon, which is the most sunny and hottest part of the day.[9]
Not only was the time of day of the low tide essentially the worst it could be in leaving marine organisms exposed during the hottest hours of the day, the heat dome also occurred during the lowest tides of the year. As the moon turns around the earth, once every 29.5 days the moon and sun align to give new moons.[10] The sun's gravity also pulls the tides to a lesser extent than the moon, so when the sun and moon align in new and full moons, tides are more extreme (higher high tide and lower low tide) and during quarter moons the sun and moon pull the tides in opposite directions, so tides are less extreme.[10] It just happens that the heat dome occurred a couple days after the full moon, so the extra boost from the alignment of the sun caused a lower low tide than it could have been. The tilt of the earth also affects the tides so that on the summer and winter solstice the tides are more extreme in the north and south since the sun is more directly overhead whereas the spring and fall equinox have the most extreme tides for the equator since the sun is directly overhead the equator at those times.[11] The heat dome falling just a few days after the summer equinox on June 21st means that the tides for during the heat dome were almost the lowest tides of the whole year. Without the heat dome, marine organisms would be experiencing the highest levels of stress they experience all year due to the lowest tides of the year, but when you add the heat dome temperatures on top of that it is no wonder the mortality of marine organisms was so high.
If the tide had not been so extremely low, or the daily low tide had fallen at a more favourable time of day, like the evening or early morning, these organisms would have had a chance to recover in the cool sea or been insulated by the seawater during the hottest temperatures of the day and that could have potentially dramatically reduced mortality of these organisms. The extremely bad luck of these tides means this is probably the most devastating event this ecosystem will face in the near future, but we should take this as a warning of how dramatic the effects of climate change can be and take steps to mitigate future damage to these ecosystems.
Anthropogenic Factors
While periods of extreme heat are a part of our climate system, we see a pattern of them becoming much longer, and hotter.[12][13] Using data from climate model simulations and weather observations, it was seen that this heat wave would have been virtually impossible to have occurred without human-induced (anthropogenic) climate change contributing.[1]
The massive mortality within the intertidal is an example of how badly climate change can affect a community.[3] Sharing the effects of this event could hopefully encourage the world to reduce greenhouse gas emissions to a net-zero (or even net-negative).
Quantification
A 2024 study[14] looked at 40 years of heat wave and wildfire data in order to quantify the effect that human-influenced climate change had on the heat wave. They applied widely-used algorithms[15][16][17][18][19] which allowed them to remove the warming trend of the lower atmosphere and perform an "event-level analysis."[14] As a partial explanation, there is an implicit relationship between warming and geopotential height that they were able to utilize.
After processing the observed heat wave data, they quantified what would have been expected without the extra warming within the lower atmosphere. The researchers concluded that global heating by humans caused the heat dome to be 34% larger, and last 59% longer (27 days).
While this analysis could be done post-heat wave, more research is needed in order to be able to quantify, understand, and prepare for how climate change can worsen these heat events.[3]
Post-Event Recovery and the Future
Future Actions
Across researchers, there is a common theme of encouraging others to continue monitoring the intertidal zones as changes occur over time.[14][3][5] The extreme nature of this event was unprecedented, thus it is difficult to quantify the extent of the negative effects. This is being framed/used as a learning experience, and can hopefully inform and prepare regions that have not experienced something like this yet.[3] The largest concern for the Pacific Northwest intertidal is whether another extreme heat event occurs in the close years following, and scientists are determined to gather more data on the intertidal community in the interim. [2]
Preventing future heat waves is a difficult and complex problem to tackle, as it involves addressing the greater issue of climate change and carbon emissions. Researchers expect the pattern of more extreme heat waves to become more intense and frequent,[5][12] therefore a combination of observation and mitigating human-induced climate change is the goal.
Recovery as of 2024
Initially, there was a mortality rate of 74% for the mussels on the intertidal, with mussels on south-facing surfaces having near-total mortality, and mussels on the north-facing surfaces being mostly unaffected. This was likely due to the angle of the sun resulting in increased solar radiation upon the south-facing surfaces, and algal cover protecting the north-facing surfaces.[3] The mortality was equivalent to an average of 1000 dead mussels per m2. In June of 2024, three years after the intense heat wave, a survey was done to see how the mussel communities had changed over time.[5]
Because the north-facing surfaces were largely unaffected in 2021, those regions consisted of dense, mature stands of adult mussels. Mussels older than three years were common, with barnacles covering their shells.
The south-facing surfaces were the ones that faced the devastating mortality in 2021, but some recovery was indeed observed. The stands were monolayered, and consisted of juvenile mussels with growth rings indicating they were three years old or less.
While it is promising that we can see these mussel populations actively recovering, there are still concerns about future extreme heat events. These communities need more time to recover.[3][5] Most mussels on the south-facing surfaces died during the heat wave, and are only juveniles as of 2024.
References
- ↑ 1.0 1.1 Philip, S. Y., Kew, S. F., van Oldenborgh, G. J., Anslow, F. S., Seneviratne, S. I., Vautard, R., Coumou, D., Ebi, K. L., Arrighi, J., Singh, R., van Aalst, M., Pereira Marghidan, C., Wehner, M., Yang, W., Li, S., Schumacher, D. L., Hauser, M., Bonnet, R., Luu, L. N., … Otto, F. E. L. (2022). Rapid attribution analysis of the extraordinary heat wave on the Pacific coast of the US and Canada in June 2021. Earth System Dynamics, 13(4), 1689–1713.
- ↑ 2.0 2.1 2.2 CBC. (2022, June 23). The 2021 heat dome in B.C. had wide-ranging impacts on marine life, scientists say. CBC News.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 White, R. H., Anderson, S., Booth, J. F., Braich, G., Draeger, C., Fei, C., Harley, C. D. G., Henderson, S. B., Jakob, M., Lau, C.-A., Mareshet Admasu, L., Narinesingh, V., Rodell, C., Roocroft, E., Weinberger, K. R., & West, G. (2023). The unprecedented Pacific Northwest heatwave of June 2021. Nature Communications, 14, 727
- ↑ 4.0 4.1 Whalen, M. A., Starko, S., Lindstrom, S. C., & Martone, P. T. (2023). Heatwave restructures marine intertidal communities across a stress gradient. Ecology, 104(5), e4027.
- ↑ 5.0 5.1 5.2 5.3 5.4 Scrosati, R. A. (2024). Recovery of Intertidal Mussel Stands Three Years after the Severe 2021 Heatwave in British Columbia, Canada. Diversity, 16(7), Article 7.
- ↑ 6.0 6.1 6.2 6.3 Helmuth, Brian; Mieszkowska, Nova; Moore, Pippa; Hawkins, Stephen J. (August 04, 2006). "Living on the Edge of Two Changing Worlds: Forecasting the Responses of Rocky Intertidal Ecosystems to Climate Change". Annual Reviews. Retrieved April 29 2025. Check date values in:
|access-date=, |date=(help) - ↑ Migdal, Alex (Jul 05, 2021). "More than a billion seashore animals may have cooked to death in B.C. heat wave, says UBC researcher". CBC News Canada. Retrieved April 29 2025. Check date values in:
|access-date=, |date=(help) - ↑ National Oceanic and Atmospheric Administration (April 29 2025). "Tides and Water Levels: Frequency of Tides - The Lunar Day". National Ocean Service. Check date values in:
|date=(help) - ↑ Flater, David (April 29 2025). "Tidal Heights at Vancouver, British Columbia". https://www.dairiki.org/tides/. Check date values in:
|date=(help); External link in|website=(help) - ↑ 10.0 10.1 National Oceanic and Atmospheric Administration (April 29 2025). "Perigean Spring Tide". National Ocean Service. Check date values in:
|date=(help) - ↑ "Why are equinox tides stronger than normal tides?". Ocean Clock. April 29 2025. Check date values in:
|date=(help) - ↑ 12.0 12.1 Fischer, E. M.; Sippel, S; Knutti, R (2021). "Increasing probability of record-shattering climate extremes". Nature Climate Change. 11(8): 689–695.
- ↑ Seneviratne, S.I., X. Zhang, M. Adnan, W. Badi, C. Dereczynski, A. Di Luca, S. Ghosh, I. Iskandar, J. Kossin, S. Lewis, F. Otto, I. Pinto, M. Satoh, S.M. Vicente-Serrano, M. Wehner, and B. Zhou, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1513–1766
- ↑ 14.0 14.1 14.2 Jain, P., Sharma, A. R., Acuna, D. C., Abatzoglou, J. T., & Flannigan, M. (2024). Record-breaking fire weather in North America in 2021 was initiated by the Pacific northwest heat dome. Communications Earth & Environment, 5(1), 1–10.
- ↑ Sharma, A. R., Jain, P., Abatzoglou, J. T. & Flannigan, M. Persistent positive anomalies in geopotential heights promote wildfires in western North America. J. Clim. 35, 2867–2884 (2022).
- ↑ Dole, R. M., & Gordon, N. D. (1983). Persistent Anomalies of the Extratropical Northern Hemisphere Wintertime Circulation: Geographical Distribution and Regional Persistence Characteristics. Monthly Weather Review, 111(8), 1567-1586.
- ↑ Parsons, S., Renwick, J. A. & McDonald, A. J. An assessment of future Southern Hemisphere blocking using CMIP5 projections from four GCMs. J. Clim. 29, 7599–7611 (2016).
- ↑ Gibson, P. B. et al. Ridging associated with drought across the western and southwestern United States: Characteristics, trends, and predictability sources. J. Clim. 33, 2485–2508 (2020).
- ↑ Renwick, J. A. Persistent positive anomalies in the Southern Hemisphere circulation. Mon. Weather Rev. 133, 977–988 (2005).