Sky News claims that a recent Alaskan tsunami was caused by a climate change-induced landslide. This claim is speculative at best, since it is difficult to say whether the particular tidewater glacier is retreating because of warming or because of other factors that impact glacial movement. Climate change and glacial retreat have always occurred, but media overuse of the former term conflates natural shifts with supposed human-caused change. This has made it difficult, if not impossible, to discuss natural hazards.
The Sky News article, “Alaskan megatsunami bigger than Empire State Building triggered by climate change,” tries to convince people that using fossil fuels is causing megatsunamis. Sky News writes “[t]he wave at the Tracy Arm Fjord in the Tongass National Forest was triggered by a rock landslide which was driven by climate change,” and the “climate change” link goes to a list of Sky News articles connecting natural phenomena to human use of fossil fuels. This is not true. Recent warming is not all human-driven, except locally in the case of the urban heat island effect.
The amount of warming that humans contribute by industry and other activities releasing carbon dioxide is a question of ongoing debate. It’s true that industrialization has increased the amount of carbon dioxide in the atmosphere, but ice core data show that carbon dioxide was gradually rising even before that, most likely due to outgassing from the oceans as the world warmed after the end of the Little Ice Age, as discussed in Climate at a Glance: Natural vs. Human Contributions to Greenhouse Gases and Global Average Temperatures. Human contributions to greenhouse-gas related warming are very small, probably around 0.28 percent, because the vast majority of the greenhouse effect comes from water vapor, not carbon dioxide.
Modern warming is likewise not unprecedented, in fact global average temperatures are still lower today than there were during the Holocene Climate Optimum.
In the case of this tsunami, glacial retreat is said to have destabilized a section of the fjord walls and a massive landslide resulted, which caused the second-tallest tsunami wave on record (that is to say, that we are aware of) at 1,578 feet high. The record is still held by the 1958 Lituya Bay landslide-caused tsunami, which was an incredible 1,720 feet high. No one was blaming climate change back then.
Gradual retreat of glacier ice can destabilize valley walls like those of the Tracy Arm fjord, but whether climate change (human or otherwise) is the main cause of this specific glacial retreat is unknown.
NASA reports that moderate rainfall was a contributing factor to destabilizing the slope, as is the case with many landslides.
Tidewater glaciers like the South Sawyer Glacier undergo hundred-plus-year-long retreat and advance cycles, and are unique in that they lose ice primarily through calving, or breaking off massive chunks, rather than gradual melting. It is notable that tidewater glaciers are not as sensitive to climate during their retreat and advance cycles as other kinds of glaciers are. Calving is impacted by water depth (which changes as the ice retreats or advances), along with other physical conditions like mass imbalances. Today, there are tidewater glaciers in Alaska that have advanced, not retreating, amid the modest warming of the past century has occurred. The Johns Hopkins Glacier is one of them; it has advanced a mile since 1948. It is unclear how global climate change could be causing one tidewater glacier to collapse while others, in the same climatic region, are expanding.
Because there are a lot of factors that influence tidewater glacier cycles, and some Alaskan glaciers’ advances are unaffected by recent modest warming, it is unclear whether blame climate change – natural or otherwise—for the recent megatsunami, and even more specious for Sky News to blame human activity for it by extension.
