The world’s oceans absorb about a quarter of the CO2 and more than 90% of the heat that accumulates in the atmosphere because of human activity, modulating the climate changes we see at the surface. But they do so at huge cost.
The excess heat and CO2 alters the physics, chemistry and ecology of the oceans, as well as affecting valuable ecosystem services such as fisheries, coastal tourism and coastal protection.
Some impacts are already visible, with reef-building corals and bivalves in mid-latitudes at particularly high risk. How serious will the impacts of climate change be on the oceans by the end of the century? The answer to this question strongly depends on the pathway we choose for our greenhouse gas emissions between now and then.
In our new study, published today in Nature Climate Change with colleagues from The Oceans 2015 Initiative, we examine what the world’s current level of commitment to tackling climate change is likely to mean for the oceans.
On current pledges alone, our work suggests that present day risks to the ocean and society posed by climate change will more than double by 2100.
Christmas Tree Worms on Bleached Coral Head, French Reef, Key largo. Credit: Matt Kieffer/Flickr.
Falling short
The Paris Agreement, adopted last December, committed the world to holding global average atmospheric temperature rise by the end of this century to “well below 2C” above pre-industrial levels, or even to 1.5C.
Prior to the negotiations, 185 countries submitted a plan for how they intended to curb their own national emissions in line with this global goal. These were known as Intended Nationally-Determined Contributions (INDCs), and are now just referred to as Nationally-Determined Contributions (NDCs).
These country-level pledges don’t go far enough, however. Aggregations of the NDCs by organisations such as Climate Action Tracker and Climate Initiative imply an average increase in global mean temperature by 2100 of 2.7C and 3.5C, respectively.
Although imperfect, these estimates are the only ones, to our knowledge, that combine the NDCs in order to derive the possible increase in global mean temperature by 2100 – a key piece of information for assessing the potential impacts on the ocean.
Risk multiplier
According to our study, 2.7-3.5C of global warming by 2100 would cause sea surface temperatures to rise by 2.0-2.6C, relative to 1870-1899.
A very ambitious mitigation strategy consistent with a 2C pathway (RCP2.6) would warm the ocean surface by 1.1C, rising to 3.2C if emissions stay as high as they currently are (RCP8.5).
Surface ocean pH will also be affected, dropping 0.26 units for 2.7C global warming by 2100 and by 0.34 units for 3.5C, relative to 1870-1899.
Under a high mitigation pathway (RCP2.6), the expected change is 0.15 units by 2100. But with continued high emissions (RCP8.5), that increases to 0.41.
Based on these changes in the ocean’s basic physical and chemical characteristics, we analysed the knock-on effects for organisms, ecosystems and services to human societies.
Moving from a 2C scenario (RCP2.6) to 2.7C of warming sees the risk to mangroves increase from undetectable to moderate, as you can see in the graph below taken from our paper. The risk to mid-latitude seagrass, bivalve fisheries and aquaculture moves from moderate to high, and the risk to warm water corals rises from high to very high, for example.
> Risks to the ocean and society from climate change: present day (black solid line), RCP2.6 (white solid line), 2.7C warming above preindustrial by 2100 (long dashes), same for 3.5C (medium dashes) and RCP8.5C (short dashes). Colours denote very high risk (purple) through to white (undetectable). Source: Magnan et al., (2016).
Overall, we show that the current level of global ambition for tackling climate multiplies the present-day risk to the oceans by a factor of 2.2-2.5 by 2100.
This carries both good and bad news.
The good news is that world seems to be on track to avoid the “worst case” scenario (RCP8.5), which would multiply today’s risk to the oceans by almost a factor of three.
The not-so-good news is that NDCs are insufficient to bring us in line with the “well below” 2C trajectory called for in the Paris Agreement.
Ratchet-up
We must be aware, however, that these projections are speculative. First, because INDCs only reflect countries’ intentions to act. The real-world trajectory that follows will be defined by whether or not the NDCs are implemented as they stand.
Second, NDCs only target the 2030 time horizon, lacking information on what countries plan to do after that. The information we have available today is insufficient, therefore, to estimate end-of-century warming and the associated impacts with any great degree of certainty.
Fortunately, the Paris Agreement establishes a five-year revision cycle for the NDCS, starting in 2020. This provides an opportunity for countries to increase their 2030 ambition. It is also an opportunity to answer two important questions on what happens after 2030.
First, how far do countries’ existing commitments avoid “lock in” to high emissions pathways in the future? Second, what is the potential for unlocking deeper mitigation in the longer term, with emissions ultimately turning negative in the second half of the century.
Given the importance of the oceans in our climate system, we argue that the scientific community has an important role to play in helping to move away from speculation about the impacts of future climate change on the oceans, towards a greater level of certainty.
The science community must continue the efforts we made in the run-up to Paris: keep contact with the negotiating teams and provide them with updated information on the consequences of political decisions for the ocean.
This will be key for supporting the five-year revision process and the ultimate goal of achieving more ambitious greenhouse gas emission reductions. Let us remember here that even the RCP2.6 scenario, which corresponds to 2C warming in 2100, multiplies the present-day risk to the oceans by a factor of 1.4 by 2100.
The clarification by countries of their post-2030 mitigation strategies will help scientists refine our estimates of the potential risks facing the oceans from climate change. In turn, we can then provide negotiators with more realistic risk assessments.
In other words, we need to move away from theoretical visions of the future risks of climate change to the oceans towards scenarios that are better rooted in the real-world.
Main image: Clouds over the Atlantic Ocean. Credit: Tiago Fioreze, Own work, CC BY-SA 3.0.
