What we are doing to the oceans
Human-induced global warming is exacerbating marine heatwaves and is threatening the survival of coral reefs (and most other life)
As a fundraiser for the Australian Conservation Fund (ACF) in 2018, my pitch started with “Have you ever been to the Great Barrier Reef, or would you like to go sometime? Well, you might no longer be able to do so soon, with mass coral bleaching and mortality events degrading coral reefs at a devastating pace”. Sadly, now in 2023, I am still singing the same chant, this time with even more urgency as marine heatwaves (MHWs) are increasing in frequency and intensity under human-induced global warming.
With continuing high carbon emissions, wide-spread climate change impacts are and will be unavoidable. While some impacts might be more visible, e.g., droughts, floods and wildfires, the impact of MHWs is less noticeable in most people’s everyday lives. MHWs are prolonged periods (≥ five days) of warming in the ocean that can substantially affect entire marine ecosystems. While driven by natural processes like air-sea feedback mechanisms, changes in heat transport, and persistent high-pressure systems, humans are greatly exacerbating the occurrence of MHWs.
Studies show that during the last decade MHWs have increased by more than twenty times due to human impact, attributing 87% of the heat events to anthropogenic warming. Alarmingly, if we continue our current climate track (which means 3.5°C warming by the end of this century), MHW occurrence will further increase by 41 times, expand its range by 21 times and its duration by 112 days. It’s safe to say that this would be disastrous for all marine life.
The Australian Great Barrier Reef (GBR) especially has been a MHW hotspot. Between 2015 and 2016, the GBR recorded more than 250 (!!) days of MHWs, and the area also suffered several extensive MHWs between 2017 and 2019. Under 2°C atmospheric warming, extreme events like the 2015-2016 MHW will be twice as frequent in the GBR compared to now. This will be catastrophic for the extremely climate-vulnerable coral reef.
While being able to withstand some warming, corals have heat thresholds – the breaching of which leads to mass coral bleaching and if persistent, oftentimes mortality. Not only will this be a huge loss for the local coastal communities and reef-dependent industries, it would be devastating for the entire marine ecosystem. As corals are ecosystem-engineers, the degradation of coral reef habitat directly impacts the abundance and diversity of marine species that live in it (e.g. fish, marine megafauna, invertebrates, shellfish). Additionally, it interferes with the coral’s essential regulating ecosystem services like coastline storm protection, sediment and water current regulation, and nutrient and carbon cycles facilitation.
Recently, the Intergovernmental Panel on Climate Change (IPCC) reported that there will be 90% coral mortality under 1.5°C atmospheric warming, and 97-98% mortality under 2°C. These numbers are shocking and greatly stress the importance of addressing human-driven climate change. We need to drastically reduce carbon emissions to prevent coral reefs from declining into the heartbreaking third stage of the picture above.
While tackling climate change should be the core focus of the long-term risk management and adaptation strategy, we must start implementing comprehensive short-term strategies now to mitigate the immediate effects of MHWs. Forecasting and early warning systems can enable proper employment of response strategies like opening protected marine areas – even if just temporarily like in Thailand in preparation for the 2016 extreme bleaching event – or conservation of rare specimens by relocating samples to onshore, climate-controlled nurseries. Other strategies include assisted migration, habitat conservation and rehabilitation, and coral shading to minimise heat exposure.
Circling back to my fundraising pitch opener, there is thus indeed a scary but real possibility that the GBR will move even further away from the once vibrant, lively ecosystem to a marine deadzone. While the above-mentioned interventions might buy time for the reefs, they are merely addressing the symptoms of MHWs rather than the root cause. To truly protect the GBR, we need to drastically mitigate global warming and limit further intensification of MHWs.
This was a short summary (focused only on corals) of the systematic review two classmates and I did on MHWs for our course Science and Politics of Climate Change. It was interesting to research as the impacts of climate change in the oceans are sooo visible and alarming, yet often forgotten as we personally rarely see the effects.
The ocean covers more than 70% of our earth and the truth is, there is still so much we don’t know about it, that we don’t even truly know the full extent of the feedback mechanisms between the ocean and the climate. Nevertheless, science shows that so far, events like MHWs, marine species depletion, and ocean acidification, have worsened the earth’s resilience. It is thus extremely important that we do not forget and neglect the effects of global warming on the oceans, even if we don’t see or feel the degradation of marine ecosystems in our everyday life.
It’s simple really, to have a healthy planet we need a healthy ocean.
A few days ago I came across this post on Instagram (perfect timing, one day after we finished our report) and while it was slightly saddening, I do appreciate dark humour haha.