2 Degrees and Dangerous Sea Level Rise

So while negotiations continue to crack on in Paris for the climate deal (that may never happen but my fingers are crossed!), it's time to go back to my investigation into the potential deadly effects of changes to the carbon cycle leading to 2°C of warming!

Today I'm focusing on all things related to the sea and how 2°C of warming can render parts of Earth uninhabitable. There is obviously A LOT of different studies and things I could talk about so this is just a handful I've found particularly interesting.

There's been some really cool visualisations of sea level rise from 2 degrees of warming recently that I think have made huge leaps in making such a scientific and at times difficult to engage with topic accessible to the public.

Mashable, a popular digital media blog, shared a series of photographs that simulate what 8 locations in the US would like if sea level continues rising (by 5, 12 and 25 feet). Below is one example, using the incredibly iconic view of the Statue of Liberty.

The photoshopped scenes were created when Nickolay Lamm saw an article in the New York Times, that depicted sea level rise on maps of different areas in the US. Again, this article helps people understand the local effects of sea level rise, and the descriptions point to specific areas people would actually care about. For example, for New York City, the description reads "Large portions of all five boroughs are gone, including much of Manhattan below 34th Street". This is why I strongly believe this kind of scientific communication is vital for getting public and political support on climate change mitigation.

Another great sign is that this communication isn't slowing down. Climate Central, albeit more scientific than Mashable or national newspapers, is a real-life example of scientific communication geared towards the general public. (See their article for a drag comparison of 2°C and 4°C warming on sea level in major cities around the world, also created by Nickolay Lamm) I particularly love their use of Google Earth to create fly-over videos of sea level rise in many coastal cities around the world. As I grew up in Hong Kong, definitely a coastal city at major risk of high sea level rise, I found it really scary to see buildings I know become inaccessible due to water under 4°C and 2°C scenarios. I think it conveyed a sense of urgency and disaster that perhaps numbers and model predictions just don't quite do.

So how convincing are the arguments that 2°C will result in these sea level rises that are enough to seriously damage coastal cities?

There's been studies on future sea level (rise or even fall) for many years now but I'd like to just talk about a few recent ones as they use new technology (such as laser altimetry) and build upon historic studies.

The main focus of sea level studies is on ice sheets and the temperatures required to cause dangerous melting of ice sheets (such as the Antarctic or Greenland ice sheets). First, this very new study from Golledge et al. (2015) focuses on the Antarctic ice sheet and uses an ice sheet model to estimate sea level rise under 1.5 to 2°C warming. The study (like many others) uses the IPCC's representative concentration pathways (RCPs) as the input variable for greenhouse gas concentration trajectories. Across all 4 RCPs, global mean sea level is projected to rise by 0.26 to 0.82 m by the late-21st century. However, Golledge et al. argue that this underestimates sea level rise in the long term and doesn't take into account polar amplification, which can "can lead to surface air temperatures, and ocean temperatures, that may be as much as double the magnitude of the global mean perturbation". 

This study argues that 3 out of the 4 RCPs (excluding the lowest warming projection RCP2.6), "the future commitment to a rise in sea level from Antarctica is substantial", and loss of major ice shelves within 100-300 years. Sea level rises are predicted to be substantial, such as 0.1–0.39 m by 2100 under RCP8.5 (mean warming of 2°C by 2065). Scarily, they predict that rates of sea level rise are 5.5-15 mm per year by 2300 under RCP8.5, but even under the lesser RCP4.5 and RCP6.0, reach 3-5 mm per year by 2300. Golledge argues that there is a "commitment" to sea level rise due to "the collapse of buttressing ice shelves" that creates an ice-sheet response that greatly increases grounded ice discharge for centennial or millennial timescales, even if greenhouse gas emissions are reduced and temperatures stabilise. These findings, although based on computer models, which are bound to have any number of uncertainties, really hit home the phrase that has been thrown around a lot around COP21 this past week: "We are the first generation to feel the impact of climate change, and the last generation that can do something about it".

Secondly, I'd like to share a study I read about Greenland ice sheet and sea level rise. Now the Greenland ice sheet, much like the Antarctic ice sheet, still has a lot of uncertainty attached to how it will react to temperature but has the potential to cause huge fluxes in sea level. NASA claims it contains 8% of all freshwater on Earth, and if it melted in its entirety, would single-handedly cause a sea level rise of 23 feet (7 m). So a slightly important geographical feature.... 

A study by Csatho et al. (2014) published just over a year ago, using the new technology of laser altimetry, focuses on measuring ice thickness to understand the complex dynamics of the Greenland ice sheet. The study period was from 1993-2012 and used nearly 100,000 points to determine elevation and changes. The main finding of their study was an estimated mean annual ice sheet mass loss of 243 ± 18 Gt per year, which resulted in 0.68 mm sea level rise per year for 2003–2009. Another, slightly older study by Robinson et al. (2012), estimates that the temperature warming threshold for a "monostable, essentially ice-free state" is 1.6°C, not 3.1°C as previously estimated. They also suggest that for "sufficiently high initial temperature anomalies", losing the entire ice sheet becomes irreversible.

Again, there are huge uncertainties regarding the future of ice sheets and their impact on sea level rise but there is a general consensus that preventing huge loss of ice from both the Antarctic and Greenland ice sheet starting as early as possible is important for combating the threat of dangerous sea level rise.

Most recently, a summary report from Climate Central, suggests that 2°C of warming would "lock-in" 4.7 m of sea level rise by 2100 and therefore submerge land home (in 2010) to 280 million people globally. This translates to 5 out of the 10 megacities with the highest populations being affected (and all in Asia), including my home Hong Kong. This is scary, but hopefully will motivate Asian nations (particularly China, who is the biggest emitter of greenhouse gases in the world and in which 2 of the 5 vulnerable cities reside) to stay within 2 degrees and reduce damages as much as possible.

Finally, I wanted to include this counter-argument that came around a few months ago from NASA, that the Antarctic ice sheet has overall seen a gain rather than a loss. The results claim that Antarctica is not currently contributing to sea level rise, but is taking 0.23 mm per year away, which means the IPCC's estimate of 0.27 mm per year of sea level rise attributed to Antarctica is being removed elsewhere.While it is horrifying that people use this one study as an argument that climate change is a hoax, it presents some interesting challenges in accurately measuring and modelling changes to complex climatic features such as ice sheets.

Thanks for reading and next time I'll be looking at some other effects of emission levels high enough to create 2°C of warming!